Note: Descriptions are shown in the official language in which they were submitted.
CA 022~4490 l99X-11-25
24.74~/53
METHOD AND APPAI~ATUS FOR IN~rESTIGATING
EART~ FOF~lATIONS
FIELD OF THE INVENTION
This invention relates to the field of well logging and,
more particularly, to well logging apparatus for determining
earth formation resistivity and sending the information to the
earthls surface. A form of the invention has general application
to the well logging art, but the invention is particularly useful
for logging-while-drilling (also called measurement-while-
drilling).
BACKGROUND OF THE INVENTION
Resistivity logging, which measures the electrical
resistivity of formations surrounding an earth borehole, is a
commonly used technique of formation evaluation. For example,
porous for~ations having high resistivity generally indicate the
presence of hydrocarbons, while porous formations having low
resistivity are generally water saturated. In so-called
"wireline" well logging, wherein measurements are taken in a well
. . ~.. . ~ . . .
CA 022~4490 1998-11-25
bore (with the drill string removed) by lowering a logging device
in the well bore on a wireline cable and taking measurements with
the device as the cable is withdrawn, there are several
techniques of resistivity logging which use elements such as
electrodes or coils. Various arrangements of electrodes, on the
logging device and at the earth's surface, have been utilized to
measure electrical currents and/or potentials from which
formation resistivity can be derived. For example, button
electrodes have been employed on a pad which is urged against the
borehole wall. These electrodes have been used to obtain
azimuthal resistivity measurements, and focusing techniques have
been employed to obtain resistivity measurements that have
substantial lateral extent into the formations and provide
relatively high vertical resolution resistivity information.
Various techniques for measùring resistivity while drilling
have also been utilized or proposed. Techniques employed in
wireline logging may or may not be adaptable for use in
measurement-while-drilling equipment. The borehole presents a
difficult environment, even for wireline logging, but the
environment near the well bottom during drilling is particularly
hcstile to measuring equipment. For logging-while-drilling
applications, the measuring devices are housed in heavy steel
drill collars, the mechanical integrity of which cannot be
compromised. Measurement approaches which require a substantial
surface area of electrically insulating material on the surface
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CA 022~4490 1998-11-2~
of a drill collar housing are considered impractical, since the
insulating material will likely be damaged or destroyed. This is
particularly true for measuring structures that would attempt to
attain intimate contact with the newly drilled borehole wall as
the drill string continues its rotation and penetration, with the
attendant abrasion and other stresses.
One resistivity measuring approach is to utilize a plurality
of toroidal coil antennas, spaced apart, that are mounted in
insulating media around a drill collar or recessed regions
thereof. A transmitting antenna of this nature radiates
electromagnetic energy having a dominant '.ransverse magnetic
component, and can use the electrically conductive body of the
drill collar to good advantage, as described next.
In U.S. Patent No. 3,408,561 there is disclosed a logging-
while-drilling system wherein a receiving toroidal coil is
mounted in a recess on a drill collar near the drill bit and a
transmitting toroidal coil is mounted on the drill collar above
the receiver coil. The drill collar serves as part of a one-turn
"secondary winding" for the toroidal antennas, the remainder of
such "secondary winding" including a current return path through
the mud and formations. The voltage induced in the receiver
toroidal coil provides an indication of the resistivity of
formations around the drill bit. U.S. Pa~ ~t No. 3,305,771
utilizes a similar princip le, but employs a pair of spaced-apart
transmitting toroidal coils and a pair of spaced-apart receiving
toroidal coils between the transmitting toroidal coils.
CA 022~4490 1998-11-2~
As generally described in the prior art, a transmitter
toroidal coil mounted on a drill collar induces current in the
drill collar which can be envisioned as leaving the drill collar,
entering the formations below the transmitter coil, and returning
to the drill string abov'e the transmitter coil. Since the drill
collar below the transmitter coil is substantially an
equipotential surface, a portion of the current measured by a
lower receiver toroidal coil mounted near the drill bit tends to
be laterally focused. This can provide a "lateral" resistivity
measurement of formations adjacent the drill collar. Also, a
portion of current leaving the drill stem below the receiver coil
provides a "bit resistivity" measurement; that is, a measurement
of the resistivity of the formations instantaneously being cut by
the bit. [See, for example, the above-identified U.S. Patent
No.s 3,408,561 and 3,305,771, and publications entitled "A New
Resistivity Tool For Measurement While Drilling", SPWLA Twenty-
Sixth Annual Logging Symposium (1985) and "Determining The
Invasion Near The Bit With The MWD Toroid Sonde", SPWLA Twenty-
Seventh Annual Logging Symposium (1986).~ Thus, the prior art
indicates that a measurement-while-drilling logging device using
toroidal coil transmitting and receiving antennas can be employed
to obtain lateral resistivity measurements and/or bit resistivity
measurements.
Reference can also be made to the following which relate to
measurement-while-drilling using electrodes and other
CA 022~4490 1998-11-2~
transducers: U.S. Patent No. 4,786,874, U.S. Patent No.
5,017,778, and U.S. Patent No. 5,130,950.
Resistivity measurements obtained using transmitting and
receiving toroidal coils on a conductive metal body are useful,
particularly in logging-while-drilling applications, but it would
be desirable to obtain measurements which can provide further
information concerning the downhole formations; for example,
lateral resistivity information having improved vertical
resolution, azimuthal resistivity information, and multiple
depths of investigation for such resistivity information. It is
among the objects of the present invention to devise equipment
which can provide such further resistivity measurement
information. It is among the objects hereof to devise techniques
of resistivity logging which exhibit improved performance in the
presence of formation bedding having substantial resistivity
contrasts.
In logging-while-drilling applications, various schemes have
been proposed for transmitting the measurement information to the
surface of the earth. A number of these schemes involve using a
toroidal coil antenna to radiate electromagnetic energy having a
transverse magnetic component from downhole to the earth's
surface, or to repeaters along the drill string which receive,
boost, and re-transmit the signals using further toroidal coil
transmitters. As in the systems first described above which
utilize toroidal ccils for obtainment of resistivity
CA 022~4490 1998-11-2~
measurements, the drill string is used as a current carrier.
Reference can be made, for example, to U.S. Patent No.s
3,186,222, 3,967,201, 4,578,675, 4,725,837, ~,739,325, and
4,839,644. In the U.S. Patent No. 4,578,675 there is disclosed a
logging-while-drilling apparatus which utilizes toroidal coil
antennas to obtain bottom-hole resistivity measurements and
employs one of these antennas, on a time-sharing basis, for two-
way communication with equipment at the surface of the earth. The
communication may be via passive or active repeater units further
uphole. In general, downhole/surface electromagnetic telemetry
approaches which use the drill string as a current carrying
component (and, typically, the mud and the formations as a return
current path) have intrinsic limitations. The mud conductivity
and the conductivity and heterogeneity of the surrounding
lS formations will affect the signal, and the need for boosters or
repeaters is inconvenient and expensive.
For various reasons, the approach that has been the most
successful for logging-while-drilling communication between the
well bottom and the earth's surface has been so-called mud pulse
I telemetry. Briefly, pressure pulses (or acoustic pulses)
modulated with the information to be conveyed, are applied to the
mud column [typically downhole, for com~unication to the surface,
although two-way communication is also used], and received and
demodulated uphole.
CA 022~4490 1998-11-2~
A downhole mud telemetry subassembly typically includes the
equipment for controlling data communication with the surface and
for applying modulated acoustic pulses to the mud. When a
measurement subassembly (e.g. one measuring formation parameters
and/or other parameters concerning drilling such as downhole
weight on bit or direction and inclination of the borehole) is
housed in a drill collar~that is mounted adjacent the downhole
mud telemetry subassembly, a wiring connector can be provided for
electronic connection between these subassemblies. The nature of
the drill collar sections housing these units, the typical
threaded mechanical connections therebetween, and the stresses to
which the connections are subjected, render the connection of
wires somewhat inconvenient, but such connections are commonly
implemented. A larger problem arises, however, when a desired
bottom hole arrangement of telemetry equipment, measurement
collars, stabilizer collars, etc. involves separation between the
mud telemetry subassembly and one or more measurement
subassemblies that are intended to communicate therewith. Under
such circumstance, wiring buses and connectors may be provided
for local electronic communication between the measurement
subassembly and the downhole mud telemetry subassembly, but the
requirement for crossing other drill collar sections and joints
is disadvantageous. The problem is exacerbated when the relative
placements of a particular measurement subassembly (or
subassemblies) with respect to the downhole mud telemetry
CA 022~4490 1998-11-2~
subassem~ly is not known a-priori and is decided spontaneously at
the well site, as is often the case in modern drilling
operations.
It is therefore among the further objects of the present
invention to provide improvement in the efficiency and
flexibility of communications in logging-while-drilling systems.
CA 022~4490 1998-11-2~
SUMMARY OF THE INVENTION
In accordance with the present invention, there is
provided apparatus for determining the resistivity of formations
surrounding an earth borehole, comprising: an elongated
electrically conductive body that is movable through the bore-
hole; first transmitter means for establishing a first current
in the body from a first transmitter position on the body, said
first current travelling in a path that includes the body and
the formations; an electrode on said body having a surface that
is electrically isolated from the surface of the body; means
for measuring at said electrode a first electrical signal
resulting from said first current; second transmitter means for
establishing a second current in the body from a second trans-
mitter position on the body that is spaced from the first
transmitter position, said second current travelling in a path
that includes the body and the formations; means for measuring
at said electrode a second electrical signal resulting from
said second current; current monitor means for measuring the
axial current passing a monitor position on the body to obtain
a monitor current value; and means for deriving an indication
of formation resistivity as a function of said first electrical
signal, said second electrical signal, and said monitor current
value.
In accordance with another aspect of the invention,
there is provided a method for determining the resistivity of
formations surrounding a borehole, comprising the steps of:
providing an elongated electrically conductive body that is
movable through the borehole; establishing a current in said
71511-39D
CA 022~4490 1998-11-2~
body that travels in a path that includes the body and the
formations; measuring an electrical signal resulting from said
current at an electrode on the body that has a surface which is
electrically isolated from the surface of the body, to obtain a
measured electrode signal; deriving a compensation signal from a
measurement of axial current in the body; and producing a
compensated electrode signal as a function of said measured
electrode signal and said compensation signal, the compensated
electrode signal being indicative of formation resistivity.
In accordance with a further aspect of the invention,
there is provided a method for determining the resistivity of
formations surrounding a borehole, comprising the steps of:
providing an elongated electrically conductive body that is
movable through the borehole; providing a first source to
establish a first current in the body that travels in a path
that includes the body and the formations; providing a second
source to establish a second current in the body that travels
in a path that includes the body and the formations such that
said second and first currents travel in opposite directions in
the body; measuring axial current in the body, and controlling
at least one of said sources as a function of the measured
axial current; and measuring an electrical signal at an
electrode on the body that has a surface which is electrically
isolated from the surface of the body, said signal being
indicative of formation resistivity.
In accordance with a still further aspect of the
invention, there is provided for use in conjunction with earth
borehole drilling apparatus that includes a drill string, a
- 9a -
71511-39D
CA 022~4490 1998-11-2~
drill bit coupled to an end thereof; a logging-while-drilling
system, comprising: a first subassembly mountable in said
drill string near said bit, said first subassembly including
a first electrically conductive body; means disposed in said
first body for measuring a physical parameter relating to said
drilling; a first toroidal coil antenna disposed on said first
body; means for generating a local communication signal which
depends on said measured physical parameter, and for coupling
said local communication signal to said first toroidal coil
antenna; a second subassembly mountable in said drilling string
near said bit, said second subassembly including a second
electrically conductive body; a second toroidal coil antenna
disposed on said second body; means coupled with said second
toroidal coil antenna for receiving said local communication
signal and generating a surface communication signal which
depends on said local communication signal; an acoustic trans-
mitter for transmitting an acoustic surface communication
signal; and an acoustic receiver at the earth's surface for
receiving said acoustic surface communication signal.
As is generally known in the art, one or more toroidal
coil receiving antennas can be mounted, in an insulating medium,
on the drill collar to obtain the types of measurements
described in the Background hereof. A form of the present
invention expands on the toroid-to-toroid type of measurement
to obtain further useful information about the
- 9b -
71511-39D
CA 022~4490 1998-11-2~
The electrode(s) can be mounted in a drill collar or, in
accordance with a feature hereof, on a stabilizer blade attached
to or integral with the drill collar. In an embodiment hereof,
button-type electrode(s) are utilized, as well as a ring-type of
electrode.
In accordance with an embodiment of the invention, an
apparatus is disclosed for determining the resistivity of
formations surrounding an earth borehole. [In the present
application, any references to the determination or use of
resistivity are intended to generically mean conductivity as
well, and vice versa. These quantities are reciprocals, and
mention of one or the other herein is for convenience of
description, and not intended in a limiting sense.] An
electrically conductive metal body is movable through the
borehole. A toroidal coil antenna is disposed on the body.
[Throughout the present application "disposed on" and "disposed
in" are both intended to generically include "disposed on or in",
and "mounted on" and "mounted in" are both intended to
generically include "mounted on or in".] Means are provided for
energizing the transmitting toroidal coil antenna to induce a
current which travels in a path that includes the body and the
formations. An electrode is disposed on the body, and means are
provided for measuring the electrical effect of the current on
the electrode, said electrical effect being an indication of the
resistivity of the formations. In a preferred embodiment of the
CA 022~4490 1998-11-2~
invention, the means for measuring the electrical effect on the
electrode comprises means for measuring the cu~rent flow in the
electrode. Also in this embodiment, the eléctrode is
electrically coupled to said body, either directly or via
circuitry used to measure the current flow in the electrode, and
the surface of said electrode is electrically isolated from the
surface of said body.
Applicant recognizes that the measurement at an electrode in
the described type of system is, at least to some degree,
determined by the total current distribution into the overall
body of the apparatus which, in the described system, is the
drill collar and the conductive drill string coupled therewith.
The total current distribution, in turn, depends to some extent
on the formation resistivity along the entire length of the drill
string. A problem arises when the current measured at the
previously described electrode(s) is affected to a substantial
degree by formations a meaningful distance from the region of the
electrode, and such formations have resistivities that are
different than the resistivity of the formations in the region of
the electrode(s). For example, a problem occurs in the logging-
while-drilling apparatus when the measuring electrode(s) is
traversing a resistive bed and the drill bit cuts into a more
conductive bed. When this happens the current being emitted from
the electrode decreases, falsely indicating a more resistive
formation in the region of the electrode. [This occurs when the
11
CA 022~4490 1998-11-2~
electrode is below the transmitter. Conversely, if the electrode
is above the transmitter, the current being emitted from the
electrode increases, falsely indicating a ~ore conductive
formation in the region of the electrode.] A form of the present
invention greatly reduc~es this and other problems.
In accordance with an embodiment of this form of the
invention, a first transmitter means is provided for establishing
a first current in the body from a first transmitter position on
the body, said first current traveling in a path that includes
the body and the formations. Means are provided for measuring at
the electrode a first electrical signal resulting from the first
current. A second transmitter means is provided for establishing
a second current in the body from a second transmitter position
on the body that is spaced from the first transmitter position,
the second current traveling in a path that includes the body and
the formations. Means are provided for measuring at the
electrode a second electrical signal resulting from the second
current. Current monitor means are provided for measuring the
axial current passing a monitor position on the body to obtain a
monitor current value. Means are then provided for deriving an
indication of formation resistivity as a function of the first
electrical signal, the second electrical signal, and the monitor
current value.
In one embodiment of this form of the invention, the current
~5 monitor means comprises means for obtaining a first monitor
12
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CA 022~4490 1998-11-2~
current value when the first transmitter means is operative, and
means for obtaining a second monitor current value when the
second transmitter means is operative, the deriving means being
operative to derive said indication of formation resistivity as a
function of the first eiectrical signal, the second electrical
signal, said first monitor current value, and said second monitor
current value. In a form of this embodiment, a further current
monitor means is provided at a further monitor position for
measuring the axial current passing a further monitor position on
the body to obtain a further monitor current value, and the
deriving means is operative to derive the indication of formation
resistivity as a function of the further monitor current value.
As described further in detail hereinbelow, this form of the
present invention operates to effectively reduce or eliminate the
deleterious effects that resistivity bedding contrasts in the
general vicinity of the tool can have on the intended measurement
of the resistivity of formations surrounding the measuring
electrode.
In accordance with a further form of the invention,
advantages in the efficiency and flexibility of communication are
attained by utilizing both electromagnetic transmission and
acoustic transmission in communicating information between bottom
hole subassemblies and the earth's surface. Local downhole
electromagnetic communication (e.g. toroid-to-toroid), e.g.
between several sections of drill collar, is an effective means
13
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CA 022~4490 1998-11-2~
of communication over a relatively short distance, and the need
for hard wiring communication between bottom hole subassemblies
is reduced or eliminated, while reliable acoustic communication
with the earth's surface is retained. This is particularly
advantageous in situations where a measurement subassembly is
non-adjacent to the surface communications subassembly in a
bottom hole arrangement,~ or where the relative placements of
these subassemblies in the bottom hole arrangement are not known
a priori.
In accordance with an embodiment of a form of the invention,
a first subassembly is mountable in the drill string near the
bit, the first subassembly including: a first electrically
conductive body; means disposed in the first body for measuring a
physical parameter relating to said drilling; a first toroidal
coil antenna disposed on the first body; and means for generating
a local co~munication signal which depends on said measured
physical parameter, and for coupling said local communication
signal to said first toroidal coil antenna. [As used herein, a
physical parameter relating to the drilling is intended to
generically include measurements of the properties of formations
near the drill bit and measurements relating to the drilling
operation and the drill bit itself.] A second subassembly is
mountable in the drill string near the bit, the second
subassembly including: a second electrically conductive body; a
second toroidal coil antenna disposed on the second body; means
14
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CA 022~4490 1998-11-2~
coupled with the second toroidal coil antenna for receiving said
local communication signal and generating a surface communication
signal which depends on said local communication signal; and an
acoustic transmitter for transmitting an acoustic surface
communication signal. ~n acoustic receiver is provided at the
earth's surface for receiving the acoustic surface communication
signal.
Further features and advantages of the invention will become
more readily apparent from the following detailed description
when taken in conjunction with the accompanying drawings.
CA 022~4490 1998-ll-2~
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic diagram, partially in block form, of a
logging-while-drilling apparatus in accordance with an embodiment
of the invention, shown attached to a drill string that is
suspended in a borehole by a conventional drilling rig.
Fig. 2 is a cross-sectional view of a measuring and local
communications subassembly in accordance with an embodiment of
the invention.
Fig. 3 is a cross-sectional view of the subassembly of Fig.
2, in greater detail.
Fig. 4 is a front view of a stabilizer blade, with
electrodes mounted therein, in accordance with an embodiment of
the invention.
1~ Fig. 5 is a cross-sectional view, as taken through a section
defined by section line 5-5 of Fig. 4, of an embodiment of an
electrode in accordance with a form of the invention.
Fig. 6 is a schematic diagram of an equivalent circuit of
the Fig. 5 embodiment.
Fig. 7 is a cross-sectional view, partially in schematic
form, of an embodiment of an electrode and associated circuitry
in accordance with a further form of the invention.
Fig. 8 is a cross-sectional view of an embodiment of a ring
electrode used in a form of the invention.
16
. .
CA 022~4490 1998-11-2~
Fig. 9 is a representation of the type of current pattern
obtained when the transmitting toroidal coil of Fig. 2 is
energized.
Fig. 10 is a block diagram, partially in schematic form, of
the antennas, electrodes, and circuitry utill~ed in an embodiment
of the invention.
Fig. 11 is a flow diagram of an embodiment of a routine for
programming the processor of the Fig. 10 embodiment.
Fig. 12 is a simplified diagram of a drill collar, upper and
lower toroidal transmitters, and a ring electrode, which is
useful in understanding a form of the invention.
Fig. 13 is a graph of resistivity versus depth in a series
of conductive beds for measurement at the ring electrode of Fig.
12, with transmission by the upper and the lower transmitters
thereof.
Fig. 14 illustrates a structure like that of Fig. 12, but
for an em~odiment with a monitor toroidal receiver adjacent the
ring electrode.
Fig. 15 illustrates a structure like that of Fig. 14, but
for an embodiment with a further monitor toroidal receiver
adjacent the lower transmitter.
Fig. 16 is a graph of resistivity versus depth in a series
of conductive beds for an improvement in accordance with an
embodiment of the invention.
17
CA 022~4490 1998-11-2~
Figures 17 and 18 are graphs of resistivity versus depth for
uncompensated and compensated logs in resistive beds.
Fig. 19 illustrates current path lines for the device of
Fig. 12 adjacent a conductive bed, with transmission by the upper
transmitter.
Fig. 20 illustrates current path lines for the device of
Fig. 12 adjacent a conductive bed, with transmission by the lower
transmitter.
Fig. 21 illustrates current path lines for the same
situation as in Figures 19 and 20, with currents from the two
transmitters superposed.
Fig. 22 is a graph of resistivity versus depth for a series
of conductive beds, with resistivity obtained using the
compensated ring current of equation (2) in accordance with an
embodiment of the invention.
Fig. 23 is a graph of resistivity versus depth for a series
of resistive beds, with resistivity obtained using the
compensated ring current of equation (2) in accordance with an
embodiment of the invention.
Fig. 24 is a variation of the embodiment of Fig. 15 wherein
a further monitor toroidal antenna is located adjacent the upper
transmitter.
Fig. 25 illustrates a further embodiment having two button
electrodes adjacent a monitor toroidal antenna.
18
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CA 022~4490 1998-11-2~
Fig. 26 is a block diagram of a portion of the electronics
in accordance with an embodiment of the invention.
Fig. 27 is a flow diagram of a routine for controlling a
processor in accordance with an embodiment of the invention.
Fig. 28 is a block diagram of a portion of the electronics
in accordance with another embodiment of the invention.
Fig. 29 is a schematic diagram, partially in block diagram
form, of a further embodiment of the invention.
Fig. 30 is a diagram of the surface/local communications
subassembly of the Fig. l bottom hole arrangement.
Fig. 31 is a block diagram, partially in schematic form, of
the antenna and circuitry used in an embodiment of the local
communications portion of the surface/local communications
subassembly of Fig. 30.
Fig. 32 is a flow diagram of an embodiment of a routine for
programming the processor of the Fig. 31 embodiment.
Fig. 33 is a diagram of another e~bodiment of the invention
which utilizes electrodes mounted in a drill collar.
19
CA 022~4490 1998-ll-2~
DETAILED DESCRIPTION
Referring to Fig. 1, there is illustrated a
measuring-while-drilling apparatus in which embodiments of the
invention can be employed; [As used herein, and unless otherwise
specified, measurement-while-drilling (also called measuring-
while-drilling or logging-while-drilling) is intended to include
the taking of measurements in an earth borehole, with the drill
bit and at least some of the drill string in the borehole, during
drilling, pausing, and/or tripping.] A platform and derrick 10
are positioned over a borehole 11 that is formed in the earth by
rotary drilling. A drill string 12 is suspended within the
borehole and includes a drill bit 15 at its lower end. The drill
string 12 and the drill bit 15 attached thereto are rotated by a
rotating table 16 (energized by means not shown) which engages a
kelly 17 at the upper end of the drill string. The drill string
is suspended from a hook 18 attached to a travelling block (not
shown). The kelly is connected to the hook through a rotary
swivel 19 which permits rotation of the drill string relative to
~o the hook. AlternatiVely, the drill string 12 and drill bit 15
may be rotated from the surface by a "top drive" type of drilling
rig. Drilling fluid or mud 26 is contained in a pit 27 in the
earth. A pump 29 pumps the drilling fluid into the drill string
via a port in the swivel 19 to flow downward (arrow 9) through
~5 the center of drill string 12. The drilling fluid exits the
. . ~
CA 022~4490 1998-11-2~
drill string via ports in the drill bit 15 and then circulates
upward in the region between the outside of the drill string and
the periphery of the borehole, commonly refe~red to as the
annulus, as indicated by the flow arrows 32 The drilling fluid
thereby lubricates the bit and carries formation cuttings to the
surface of the earth. The drilling fluid is returned to the pit
27 for recirculation. An optional directional drilling assembly
(not shown) with a mud motor having a bent housing or an offset
sub could also be employed.
Mounted within the drill string 12, preferably near the
drill bit 15, is a bottom hole assembly, generally referred to by
reference numeral 100, which includes capabilities for measuring,
processing, and storing information, and co~municating with the
earth's surface. [As used herein, near the drill bit means
within several drill collar lengths from the drill bit.] The
assembly 100 includes a measuring and local communications
apparatus 200 which is described further hereinbelow. In the
example of the illustrated bottom hole arrangement, a drill
collar 130 and a stabilizer collar 140 are shown successively
above the apparatus 200. The collar 130 may be, for example, a
pony collar or a collar housing measuring apparatus which
performs measurement functions other than those described herein.
The need for or desirability of a stabilizer collar such as 140
will depend on drilling parameters. Located above stabilizer
collar 140 is a surface/local communications subassembly 150.
21
CA 022~4490 1998-11-2~
The subassembly 150, described in further detail hereinbelow,
includes a toroidal antenna 1250 used for local communication
with the apparatus 200, and a known type of acoustic
communication system that communicates with a similar system at
the earth's surface via signals carried in the drilling fluid or
mud. The surface communication system in subassembly 150
includes an acoustic t.ransmitter which generates an acoustic
signal in the drilling fluid that is typically representative of
measured downhole parameters. One suitable type of acoustic
transmitter employs a device known as a "mud siren" which
includes a slotted stator and a slotted rotor that rotates and
repeatedly interrupts the flow of drilling fluid to establish a
desired acoustic wave signal in the drilling fluid. The driving
electronics in subassembly 150 may include a suitable modulator,
such as a phase shift keying (PSK) modulator, which
conventionally produces driving signals for application to the
mud transmitter. These driving signals can be used to apply
appropriate modulation to the mud siren. The generated acoustic
mud wave travels upward in the fluid through the center of the
drill string at the speed of sound in the fluid. The acoustic
wave is received at the surface of the earth by transducers
represented by reference numeral 31. The transducers, which are,
for example, piezoelectric transducers, convert the received
acoustic signals to electronic signals. The output of the
~5 transducers 31 is coupled to the uphole receiving subsystem 90
22
CA 022~4490 1998-11-2~
which is operative to demodulate the transmitted signals, which
can then be coupled to processor 85 and recorder 45. An uphole
transmitting subsystem 95 is also provided, and can control
interruption of the operation of pump 29 in a manner which is
detectable by the transdùcers in the subassembly 150 (represented
at 99), so that there is two way communication between the
subassembly 150 and the uphole equipment. In existing systems,
downward communication is provided by cycling the pump(s) 29 on
and off in a predetermined pattern, and sensing this condition
downhole. This or other technique of uphole-to-downhole
communication can be utilized in conjunction with the features
disclosed herein. The subsystem 150 may also conventionally
include acquisition and processor electronics comprising a
microprocessor system (with associated memory, clock and timing
circuitry, and interface circuitry) capable of storing data from
a measuring apparatus, processing the data and storing the
results, and coupling any desired portion of the information it
contains to the transmitter control and driving electronics for
transmission to the surface. A battery may provide downhole
power for this subassembly. As known in the art, a downhole
generator (not shown) such as a so-called "mud turbine" powered
by the drilling fluid, can also be utilized to provide power, for
immediate use or battery recharging, during drilling. It will be
understood that alternative acoustic or other techniques can be
~5 employed for communication with the surface of the earth.
23
CA 022~4490 1998-11-25
As seen in Fig. 2, the subsystem 200 includes a section of
tubular drill collar 202 having mounted thereon a transmitting
antenna 205, a receiving antenna 207, and receiving electrodes
226, 227, 228 and 235. In the present embodiment the
transmitting antenna 205 comprises a toroidal antenna (see also
Fig. 3) having coil turns wound on a ferromagnetic toroidal core
that is axially coincident~with the axis of the drill collar 202.
The core may have a circular or rectangular cross-section,
although other shapes can be used. The purpose of this toroidal
transmitter is to induce a voltage along the drill collar. The
drill collar and the formations correspond to a one turn
secondary winding. If the transmitter is excited with a drive
voltage VT and the transmitter toroid has NT turns, then the
voltage induced along the drill collar will be VT/NT. That is,
; the voltage difference between the drill collar above the
transmitter and the drill collar below the transmitter will be
VT/NT. The resultant current travels in a path that includes the
drill string and the formations (as well as the borehole fluid
which is assumed to have substantial conductivity). The
receiving electrodes 226, 227 and 228 are button electrodes
mounted in a stabilizer 220, and electrode 235 is a ring
electrode. The receiving antenna 207 is another toroidal coil
antenna. The toroidal receiver measures the axial current
flowing through the drill collar. If the receiver toroid
contains N~ turns and the current in the drill collar is I, then
24
CA 022~4490 1998-11-2~
the current flowing through the receiver winding into a short
circuit will be I/NR.
Referring now also to Fig. 3 as well as Fig. 2, there are
illustrated further details of the structure of the measurement
and communication subsystem 200 that is housed in the drill
collar 202. An annular chassis 290, which contains most of the
electronics, fits within ~the drill collar 202. In this
embodiment, the drilling mud path is through the center of the
chassis, as illustrated by arrows 299 (Fig. 2). The chassis 290
has a number of slots, such as for containment of batteries (at
position 291, see Fig. 2) and circuit boards 292. In the
disclosed embodiment, the circuit boards are in the form of
elongated thin strips, and can accordingly be planar. Other
circuit board configurations or circuit packaging can be
utilized. The transmitting toroidal antenna 205 [which can also
be utilized in a communications mode as a receiver] is supported
in a suitable insulating medium, such as "VITON" rubber 206. The
assembled coil, in the insulating medium, is mounted on the
collar 202 in a subassembly which includes a protective tapered
metal ring 209, that is secured to the collar surface by bolts
(not shown). The antenna wiring, and other wiring, is coupled to
the annular circuit assembly via bulkhead feed-throughs, as
represented at 261 (for wiring to antenna 205), 266, 267, 268
(for wiring to electrodes 226, 227 and 228, respectively), and
263 (for wiring to electrode 235 and antenna 207). The receiving
......... .. . ..
CA 022~4490 1998-11-2~
toroidal coil antenna 207 is constructed in generally the same
way, although with more coil turns in the present embodiment, in
insulating medium 211, and with protective ring 213. The
receiving ring electrode 235 is also mounted in an insulating
medium such as a fiberglass-epoxy composite.236, and is held in a
subassembly that includes tapered ring 237, which can be
integrated with the protective ring for the receiving antenna
207.
The three button electrodes 226, 227 and 228 are provided in
stabilizer blade 220 which may have, for example, a typical
straight or curved configuration. [The electrodes can
alternatively be mounted in the drill collar itself.] Two of
four (or three) straight stabilizer blades 219 and 220 are
visible in Fig.s 2 and 3. The stabilizer blades are formed of
steel, integral with a steel cylindrical sleeve that slides onto
the drill collar 202 and abuts a shoulder 203 formed on the drill
collar. The stabilizer is secured to collar 202 with lock nuts
221. The blades can be undersized to prevent wear of the
electrodes. The button electrode faces have generally round (in
this case, circular) peripheries which will be generally adjacent
the borehole wall. The button faces can have generally
cylindrical curvatures to conform to the stabilizer surface or
can have flat faces with surfaces that are slightly recessed from
the stabilizer surface shape. These electrodes span only a small
fraction of the total circumferential locus of the borehole and
26
CA 022~4490 1998-11-2~
provide azimuthal resistivity measurements. Also, these
electrodes have a vertical extent that is a small fraction of the
vertical dimension of the stabilizer on which they are mounted,
and provide relatively high vertical resolution resistivity
measurements. In the illustrated embodiment, thé surfaces of
electrodes 226, 227 and 228 have diameters of about 1 inch (about
2.5 cm.), which is large enough to provide sufficient signal, and
small enough to provide the desired vertical and azimuthal
measurement resolution. The electrode periphery, which can also
o be oval, is preferably contained within a circular region that is
less than about 1.5 inches (about 3.8 cm.) in diameter. In the
present embodiment, the top portion of each electrode is
surrounded by an insulating medium, such as "VITON" rubber, which
isolates the electrode surface from the surface of the stabilizer
blade 220. A fiberglass epoxy composite can be used around the
base of the electrode. The electrodes 226, 227 and 228 (see also
Fig. 4) provide a return path from the formations to the collar
202 (of course, when the AC potential reverses the current path
will also reverse), and the current is measured to determine
0 lateral resistivity of the region of the formation generally
opposing the electrode. The electrodes 227 and 228 are
respectively further from the transmitter than the electrode 226,
and will be expected to provide resistivity measurements that
tend to be respectively deeper than the measurement obtained from
electrode 226. The electrodes are mounted in apertures in the
27
CA 022~4490 1998-11-2~
stabilizer 220 that align with apertures in the drill collar 202
to facilitate coupling of the electrodes to circuitry in the
annular chassis 290.
In one electrode configuration, the electrode body is
directly mounted, in the manner of a "stud", in the stabilizer
body. As seen in Fig. 5 (and also in Fig. 3), the metal button
electrode (226, for example) is mounted in an insulating medium
251, such as "VITON" rubber, and its neck portion engages
threading 252 in collar 202. A small toroidal coil 253 is seated
in an insulating medium 255, which can also be "VITON" rubber, in
a circular recess in the collar surface. The toroidal coil 253
is used to sense current flow in the electrode 226. The leads
from coil 253 pass through a bulkhead feed-through (see Fig. 3)
to circuitry shown in Fig. 5. In particular, one conductor from
the current sensing toroidal coil 253 is coupled to the inverting
input of an operational amplifier 256. The other conductor from
toroidal coil 253, and the non-inverting input of operational
amplifier 256, are coupled to ground reference potential; e.g.
the body of drill collar 202. A feedback resistor Rl is provided
between the output and the inverting input of operational
amplifier 256. The circuit equivalent is illustrated in Fig. 6
which shows the button electrode stud as a single turn through
the core of toroidal coil 253, the number of turns in the coil
being N. The gain of operational amplifier 256 is very high, and
VA, the voltage difference between the inverting and non-
28
CA 022~4490 1998-11-2~
inverting input terminals is very small, virtually zero. The
input impedance of the operational amplifier is very high, and
essentially no current flows into either input terminal. Thus,
if the current flow in the electrode 226 is IB, and the current
flow in the toroidal coil '"secondary" is IB/N., the current IB/N
flows through the feedback resistor Rl, making the amplifier
output voltage RlIB/N.
Referring to Fig 7, there is shown a diagram of a further
embodiment of a button electrode that can be utilized in a form
LO of the present invention. In this embodiment, the electrode body
(e.g. 226') is supported on an insulating mounting frame 271
formed of a material such as epoxy fiberglass composite, and is
sealed with "VITON" rubber insulating material 273. The
electrode is coupled, via a bulkhead feed-through, to one end of
the primary coil of a transformer 275, the other end of which is
coupled to ground reference potential (e.g., the collar body).
The secondary winding of transformer 275 is coupled to the inputs
of an operational amplifier 256' which operates in a manner
similar to the operational amplifier 256 of Fig.s 5 and 6. A
0 feedback resistor R2 is coupled between the output of the
operational amplifier 256' and its inverting input, and the
output is designated VB. Derivation of the output voltage as a
function of the electrode current IB is similar to that of the
circuit of Fig. 6, except that in this case the turns ratio,
secondary to primary, is n2/nl, and the expression for the output
29
CA 022~4490 1998-11-2~
voltage is VB = R2IBnl/n2. An advantage of this electrode
arrangement and circuit is that nl can be increased to increase
the output voltage sensitivity to the current being measured.
Fig. 8 illustrates a form of the ring electrode 235 utilized
in the Fig. 2 embodiment. The ring electrode, which can be
welded into a single piece, is seated on fiberglass-epoxy
insulator 236, and is sealed with viton rubber 239. A conductor
238 that can be brazed or welded to the ring electrode 235, is
coupled, via a feed-through, to circuitry similar to that of Fig.
7, with a transformer 275, an operational amplifier 256, a
feedback resistor R2, and an output VB. The current sensing
operation of this circuit is substantially the same as that of
the Fig. 7 circuit.
Apparent resistivity of the formation is inversely
proportional to the current I measured at the electrode. If the
voltage at the electrode relative to the voltage of the drill
collar surface above the toroidal coil transmitter coil 205 is V,
the apparent resistivity is Rapp = kV/I, where k is a constant
that can be determined empirically or by modeling. If desired, a
correction can be applied to compensate for electromagnetic skin
effect.
Fig. 9 shows a general representation of the known type of
current pattern that results from energizing the transmitter
toroidal coil in a well being drilled with mud having substantial
~5 conductivity. The pattern will, of course, depend on the
CA 022~4490 1998-11-2~
formations' bed pattern and conductivities, the example in Fig. 9
being for the simplified case of uniform conductivity.
Fig. 10 shows a block diagram of an embodiment of downhole
circuitry in subassembly 200 for implementing measurements and/or
for transmitting information to the surface/local communications
subassembly 150. The button electrodes 226, 227 and 228 and ring
electrode 235 are each coupled, via the previously described
sensing and amplification circuits (e.g. Fig.s 5-8, now referred
to by reference numerals 1011-1014, respectively), to a
multiplexer 1020. The output of the receiver toroidal coil 207
is also coupled, via a sensing and amplification circuit 1015, to
the multiplexer 1020. The multiplexer 1020 is under control of a
computer or processor 1025, as represented by the line 1020A.
The processor 1025 may be, for example, a suitable digital
microprocessor, and includes memory 1026, as well as typical
clock, timing, and input/output capabilities (not separately
represented). The processor can be programmed in accordance with
a routine illustrated in Fig. 11. The output of multiplexer 1020
is coupled, via a bandpass filter 1030, to a programmable gain
amplifier 1033, the gain of which can be controlled by the
processor 1025 via line 1033A. The output of amplifier 1033 is
coupled to a rectifier 1035, a low-pass filter 1036, and then to
an analog-to-digital converter 1037, the output of which is
coupled to the processor 1025 via a buffer 1039 that is
controlled by the processor. [This and other buffers can be part
31
CA 022~4490 1998-11-2~
of the processor memory and control capability, as is known in
the art.] The bandpass filter 1030 passes a ba~d of frequencies
around the center frequency transmitted by the transmitter
toroidal coil 205. The processor 1025 controls the multiplexer
1020 to select the different receiver outputs in sequence. The
gain of programmable amplifier 1033 can be selected in accordance
with the receiver bein~J interrogated during a particular
multiplexer time interval and/or in accordance with the received
signal level to implement processing within a desired range. The
amplified signal is then rectified, filtered, and converted to
digital form for reading by the processor 1025.
In the present embodiment, the transmitter of subassembly
200 can operate in two different modes. In a first mode, the
transmitter toroidal coil 205 transmits measurement signals, and
the signals received at the electrodes and the receiver toroidal
coil are processed to obtain formation measurement information.
In a second mode of operation, the transmitter toroidal coil 205
is utilized for communication with the transmitter/receiver in
the surface/local communications subassembly 150 (Fig. 1).
A sinewave generator 1051, which may be under control of
processor 1025 (line 1051A) is provided and has a frequency, for
example, of the order of 100 Hz to lM Hz, with the low kilohertz
range being generally preferred. In one operating embodime~t,
the frequency was 1500 Hz. The generated sinewave is coupled to
a modulator 1053 which operates, when the system is transmitting
32
CA 022~4490 1998-11-2~
in a communications mode, to modulate the sinewave in accordance
with an information signal from the processor 1025. The
processor signal is coupled to modulator 1053 via buffer 1055 and
digital-to-analog converter 1057. In the illustrated embodiment
the modulator 1053 is a phase modulator. Tpe output of
modulator 1053 is coupled to a power amplifier 1060, which is
under control of processor 1025 (line 1060A). The output of
power amplifier 1060 is coupled, via electronic switch 1065, to
the transmitter toroidal coil antenna 205. Also coupled to the
toroidal coil antenna 205, via another branch of electronic
switch 1065, is a demodulator 1070 which, in the present
embodiment is a phase demodulator. The output of demodulator
1070 is, in turn, coupled to analog-to-digital converter 1072
which is coupled to the processor 102S via buffer 1074. The
processor controls electronic switch 1065, depending on whether
the toroidal coil antenna 205 is to be in its usual transmitting
mode, or, occasionally, in a receiving mode to receive control
information from the surface/local communications subassembly
150.
Referring to Fig. ll, there is shown a flow diagram of a
routine for programming the processor 1025 in accordance with an
embodiment of the invention. In the example of the routine set
forth, functions are performed or controlled in a repetitive
sequential fashion, but the program may alternatively be set up
with a routine that handles the indicated tasks on a prioritized
33
CA 022~4490 1998-11-2~
basis, or with a combination of sequential and prioritized
functions. Also, the processor may be multi-ported or multiple
processors may be used. The routine has two basic modes; a
"measurement" mode wherein the toroidal coil antenna 205 is
S transmitting for the purpose of obtaining measurement signals at
the receiving electrodes 226-228 and 235 and the receiving
toroidal coil antenna 207, and a "local communications" mode
wherein the toroidal coil antenna 205 is utilized to transmit
and/or receive modulated information signals to and/or from a
toroidal coil antenna located in the surface/local communications
subassembly 150 (Fig. 1), for ultimate communication with
equipment at the earth's surface via mud pulse telemetry
equipment which is part of the subassembly 150. The block 1115
represents the initializing of the system to the measurement
mode. Inquiry is then made (diamond 1118) as to which mode is
active. Initially, as just set, the measurement mode will be
active, and the block 1120 will be entered, this block
representing the enabling of the sinewave generator 1051 and the
power amplifier 1060 (Fig. 10). The electronic switch 1065 is
then set to the measurement/send position (block 1122) [i.e.,
with the toroidal coil antenna 205 coupled to the power amplifier
1060], and the multiplexer 1020 is set to pass information from
the first receiver (block 112S), for example the closest button
electrode 226. The data is then read (block 1128) and the
resistivity, as measured by the electrode from which the data has
34
CA 022~4490 1998-11-2~
passed, is computed [for example in accordance with the
relationships set forth above in conjunction with Fig.s 5-8] and
stored (block 1130), and can be sent to outpUt buffer 1055 (block
1132). Inquiry is then made (diamond 1140j as to whether the
last receiver has been interrogated. If not, the multiplexer
1020 is set to pass the output of the next receiver (for example,
the button electrode 227), as represented by the block 1143. The
block 1128 is then re-entered, and the loop 1145 continues until
data has been obtained and processed from all receivers. When
this is the case, the operating mode is switched (block 1150),
and inquiry is made as to which mode is active. Assuming that
the local communications mode is now active, the block 1160 is
entered, this block representing the transmission of the latest
frame of data to the main communications subassembly. In
particular, data from the processor 1025 (or from the optional
buffer 1055) is coupled to the modulator 1053 to modulate the
sinewave output of generator 1051 for transmission by antenna
205. At the end of a frame of data, a "ready to receive" signal
can be transmitted (block 1165). The sinewave generator and
power amplifier are then disabled (block 1168), and the
electronic switch 1065 is set to the "receive" position. [i.e.,
with the toroidal coil antenna 205 coupled to the demodulator
1070] (block 1170). A frame of information can then be received
via buffer 1074, as represented by the block 1175. During this
time, as represented by the arrows 1176 and 1177, other processor
CA 022~4490 1998-11-2~
computations can be performed, as desired. The block 1150 can
then be re-entered to switch the operating mode, and the cycle
continues, as described. The information ~eceived from the
surface/local communications subassembly can be utilized in any
desired manner.
It will be understood that the routine set forth is
illustrative, and other~suitable routines will occur to those
skilled in the art. Also, other suitable communications
techniques can be employed, if desired. For example,
simultaneous measurement and communication, such as at different
frequencies, could be employed while still using a single
transmitting antenna. Of course, local communication by wire
conductor may be preferred in some situations, if available, and
an output port 1029 (Fig. 10) can be provided for this purpose,
lS or as a general read-out port. Further detail of the
communications system is provided in conjunction with Figures 30-
33 below.
In general, the resistivity obtained from the electrodes in
the previously described manner is an accurate indication of the
resistivity of formations in the region immediately surrounding
the electrode, but under certain conditions this may not be the
case. As noted above, Applicant recognizes that the measurement
at an electrode in the described type of system is, at least to
some degree, determined by the total current distribution into
the overall body of the apparatus which, in the described system,
36
CA 022~4490 1998-11-2~
is the drill collar and the conductive drill string coupled
therewith. The total current distribution, in turn, depends to
some extent on the formation resistivity along the entire length
of the drill string. A problem arises when the current measured
at the indicated electrode(s) is affected to a substantial degree
by formations a meaningful distance from the region of the
electrode, and such formations have resistivities that are
different than the resistivity of the for~ations in the region of
the electrode(s). For example, a problem occurs in the logging-
while-drilling apparatus when the measuring electrode(s) is
traversing a resistive bed and the drill bit cuts into a more
conductive bed. When this happens the current being emitted from
the electrode decreases, falsely indicating a more resistive
formation in the region of the electrode. As described further
below, other conditions can give rise to errors in resistivity
indications.
Consider the arrangement of Fig. 12, which has a toroidal
transmitter Tl and a ring electrode R on a conductive body 1202
which is like the drill collar 202 in a logging-while-drilling
setup of the general type shown in Figures 1-3. A further
toroidal transmitter T2, also called a lower transmitter, is
located near the drill bit. For this example, the lower
transmitter T2 is about 24 inches from the end of the bit 15, the
upper transmitter Tl is about 84 inches from the end of the bit,
~5 and the ring electrode is equidistant from the transmitters,
37
CA 022~4490 1998-11-2~
i.e., about 54 inches from the end of the bit. The logging
device is assumed to be in a formation of resistivity 2000 ohm-m
having a bed of resistivity 20 ohm-m and a specified thickness.
Simulated resistivity logs for five such bed thicknesses [8, 4,
2, 1 and 0.5 feet] are shown left-to-right in Fig. 13. [This and
other simulated logs hereof are computed without consideration of
borehole effect, which will be small if the transmitter-to-
electrode spacing is larger compared to the standoff between the
electrode and the borehole wall.] The simulated resistivity
logs, as a function of the depth of ring R, are computed for
transmission by the upper transmitter Tl (solid line) and by the
lower transmitter T2 (dotted line). [As above, resistivity is
inversely proportional to the measured ring current.~ For
operation with the upper transmitter, relatively large horn-
shaped artifacts labelled Al through A5 can be observed to occur
when the logging tool enters the bed; that is, in this case, when
the bit first cuts into the bed. The length of this artifact is
approximately equal to the distance from the ring to the bit.
There is also an artifact on the lower side of the bed for the
thin beds, labelled Bl through B3. This artifact has a length
approximately equal to the transmitter-ring spacing minus the bed
thickness and so is absent for thick beds greater in extent than
the transmitter receiver distance. The simulated ring
resistivity computed when the lower transmitter is active (dotted
line) has initial artifacts which roughly oppose Al through A5,
and other serious distortions.
38
CA 022~4490 1998-11-2~
Consider next the arrangement of Figure 14, which is like
that of Figure 12, but also has a receiver (or monitor) toroid MO
at about the position of the ring electrode.R to monitor the
axial current flowing up or down through the conductive body at
the position of the ring R. The axial current which is induced
by Tl is linear with respect to the voltage induced on the drill
collar and inverse to the~resistivity of the earth formation
surrounding the tool. The axial current which is induced by T2
is linear with respect to the voltage induced on the drill collar
by T2 and inverse to the resistivity of the earth formation
surrounding the tool. Assume that the excitation voltage of the
upper transmitter is fixed while the excitation voltage of the
lower transmitter is adjustable. The net axial current which
flows along the drill collar at any point is the linear
superposition of the induced current from Tl and T2. Assume that
the voltage of T2 can be adjusted so that the net axial current
flowing through the monitor toroid MO is zero. This will require
that the current induced by T2 be approximately opposite in phase
to the current induced by Tl, so that when the upper transmitter
~O is driving current down the tool, the lower transmitter is
driving the current up, and vice versa. All of the current
leaving the tool between the lower transmitter and the monitor
returns to the tool below the lower transmitter while all of the
current leaving the tool between the upper transmitter and the
~5 monitor returns to the tool above the upper transmitter. This
39
CA 02254490 1998-11-25
has the effect of isolating the reglon of the tool above the
monitor from the region of the tool below the monitor since no
current flows between them, either on the collar or through the
formation. As a result, the resistivity determined from the ring
current more accurately rep'resents the resisti~ity of formations
surrounding the ring R.
A similar result can be obtained by energizing the
transmitters separately and computing a compensated ring current.
In the arrangement of Fig. 14 [or that of Fig. 15, which includes
a lower receiving or monitoring toroid M2, to be subsequently
considered], the upper position is designated 1, the lower
position is designated 2, and the center position is designated
0. The ring and toroid currents when the upper transmitter is
operated at an arbitrary but fixed voltage are Rl, Mol, and M
and the ring and toroid currents when the lower transmitter is
operated at the same voltage are R2, Mo2~ and M12. Consider a
compensated current of the form:
Rc = M (Mo2Rl + ~olR2) ( 1)
or
c 1 Mo2 2 ( la)
CA 022~4490 1998-11-2~
In equation (la), the ratio Mol/Mo2 is the adjustment factor for
the lower transmitter to achieve the condition of zero axial
current at M0. The expression Rl+ M0l/MO2R2 is the ring current
for the condition of zero axial current.
The two terms in eq~ation (la) add. This is due to the fact
that operating the two transmitters in opposition in order to
achieve a zero axial current at the monitor toroid causes an
increase in the ring current. That is, when the upper
transmitter drives a current down the mandrel, current flows out
of the ring. Similarly when the lower transmitter drives current
up the mandrel, it also causes current to flow out of the ring.
The implication of this processing on the noise is that, since
the terms add, the noise in the output is not amplified as would
be the case if one took a small difference between two large
1~ numbers.
Fig. 16 shows the response [resistivity, inversely
proportional to compensated ring current~ for the same beds as in
Fig. 13. The artifact Al through A5 (of Fig. 13), which occurs
as the tool enters the bed, is greatly reduced. The smaller
artifact Bl through B3 on the downside of the thinner beds is
almost unchanged. The shape of the log within the bed is
improved, but could still stand improvement.
Figures 13 and 16 illustrate performance when relatively
conductive beds are encountered. Figures 17 and 18 illustrate
performance in thin beds that are more resistive than the
41
CA 022~4490 1998-11-2~
formations in which they are located. The contrast ratio is
again 100 to 1 (2000 ohm-m beds in 20 ohm-m formations). Fig.-17
shows that in this case the uncompensated log using the upper
transmitter is reasonably good without compensation, and Fig. 18
shows the improvement usi'ng the indicated compensation of
equation (1).
To better understand the compensation approach, reference
can be made to Figures 19, 20, and 21, which illustrate current
path lines for a tool just above an 8 foot thick conductive bed.
In Fig. 19 transmission is from the upper transmitter (at 84
inches on the depth scale) and the ring electrode is just above
the bed boundary (the ring electrode being at 54 inches on the
depth scale), the resistivity contrast ratio being 10 to 1. [A
lower contrast ratio than before is used to facilitate
visualization of the current line plots.] It is seen in Fig. 19
that the current lines emerge from the bed and curve upward into
the more resistive shoulder. This distortion accounts for the
horn-shaped artifacts such as Al in Fig. 13. Fig. 20 shows the
same situation, but with transmission from the lower transmitter,
T2. Fig. 21 illustrates the compensated situation, with current
from the two transmitters superposed. The current paths near the
borehole at about the position of the ring are substantially
parallel to the bed and are not distorted by the presence of the
bed. The fact that the current paths are substantially
independent of the presence of the bed below the ring electrode
- 42
CA 022~4490 1998-11-2
explains the improved response.
The monitor toroid M0 should preferably be at substantially
the same position as the ring electrode R to obtain excellent
compensation. [An arrangement of ring, toroid, and protective
covering ring, as first shown above in Fig. 3, will effectively
put a ring and toroid receiver at substantially the same receiver
position.] However, even if there is some distance between them,
improvement will be realized from the indicated compensation.
The condition of zero axial current at the monitor M0 fixes
the ratio of the voltages generated by transmitter Tl and
transmitter T2 or, equivalently, the ratio of factors to be
applied to the respective ring currents Rl and R2. It does not
set the overall level of the transmitter voltages. Choice of the
prefactor as l/Mo2 corresponds to a fixed voltage at Tl and a
voltage at transmitter T2 of Mol/Mo2. This can be seen from
equation (la). If instead of l/Mo2 one uses a prefactor of
l/Mol, this corresponds to the lower transmitter producing a
fixed voltage, while the upper transmitter produces a voltage
Mo2/Mol times as large. In this case, one is trying to
electrically remove the effect of the upper portion of the tool.
This produces an inferior log from what corresponds to a short
asymmetrical tool with poor response.
At present, the most preferred multiplying factor is l/M21,
where M21 is the current produced by the upper transmitter
~5 measured at the lower monitor toroid M2 (which is at
43
CA 022~4490 1998-11-2~
substantially the same position as the lower toroidal transmitter
T2 - see Fig. 15). [By reciprocity, M21 is equal to the current
M12 that would be produced by the lower transmitter measured at
an imaginary monitor Ml located at the position of the upper
transmitter.] Logs produ'ced using this compensation, that is,
with the compensated ring current
RC = ~ (~02R1 + ~o1R2) (2)
are shown in Figures 22 and 23. Figures 22 and 23 are for the
tool arrangement previously described, with Fig. 22 having
conductive beds and Fig. 23 having resistive beds. In both
cases, as in previous logs, the contrast ratio is 100 to 1. The
logs match well to the bed patterns, wlth only small artifacts.
There is another way to visualize the compensation
represented by equation (2), in terms of both transmitters
operating at adjustable levels. Transmitter Tl operates at a
relative level of Mo2/M21. This is the ratio of the current from
the lower transmitter measured at the central monitor divided by
the current measured at the upper transmitter. Thus, it is
sensitive to the leakage of current between the upper transmitter
and the ring. Similarly, the lower transmitter operates at a
relative level Mol/M21 which is the ratio of currents from the
44
.
CA 022~4490 1998-11-2~
upper transmitter measured at the central monitor divided by the
current measured at the lower transmitter. This is sensitive to
the leakage of current between the lower transmitter and the
ring. Thus both transmitters are operated to compensate for
leakage between that transmitter and the monitor.
The foregoing assumes that the ratio Mo2/M12 precisely
compensates for the effect of "shielding" by a conductive bed
between the upper transmitter and the ring electrode. The tacit
assumption is that the conductive regions have the same effect on
the leakage of current between the ring electrode and the upper
transmitter from the lower transmitter as they do on the
"shielding" of the ring from the upper transmitter. This is true
to first order.
A general expression can be set forth in which one term
contains a factor times Rl and the second term contains a factor
times R2. These factors are measures of leakage and are
functions of the ratios of monitor currents as follows:
RC = Fl ( M ' M ' M ) R, + F2 ( Ml ' M ' M ) R2 ~ ( 3 )
~O
where Fl and F2 are compensation functions, and the requirement
of zero axial current can be relaxed. Further generalization can
CA 022~4490 1998-11-2~
be achieved by adding additional transmitters and monitors and so
making the functions Fl and F2 more general.
It can be noted that leakage can be practically eliminated
by covering the region between the transmitters and the electrode
with an insulating material. When combined with the described
compensation technique, this can provide an excellent resistivity
log. A drawback is the fragility of the insulating material in a
logging-while-drilling application. Also, the resultant
measurement has more response close to the tool, and a far larger
borehole effect.
Fig. 24 illustrates an embodiment which is similar to that
of Fig. lS, but wherein the toroidal monitor M2 at the lower
transmitter position is replaced by the toroidal monitor Ml at
the upper transmitter position. By reciprocity, M12 (signal at
Ml with transmitter T2 energized) can be measured and will
provide substantially the same value as M21, for use in equation
(2). The principle of reciprocity would also permit the reversal
of position of other transmitter/receiver combinations from which
signals are obtained. For example, a further transmitter T0
could be provided adjacent the ring electrode to be used in
conjunction with a monitor toroidal antenna at the bottom
position, and the monitor signal obtained from this
transmitter/receiver would be equivalent, by reciprocity, to the
previously indicated Mol.
Fig. 25 illustrates an embodiment like that of Fig. 15, but
46
CA 022~4490 1998-11-2~
with a button electrode B (which may be of the type previously
described) replacing the ring electrode R. As described above,
azimuthal resistivity information can be obtai~ed from the button
electrode. Typically one or more button elëctrodes and/or one or
more ring electrodes may be employed, in con~unction with one or
more monitor toroids such as M0. A second button electrode, B',
is shown in Fig. 25. The described type of compensation can
improve the vertical response of multiple electrodes (which
provide different depths of investigation) and make their
vertical responses more similar. Different depths of
investigation can also be obtained by providing additional
transmitters and monitors which are spaced different distances
from the electrodes. These can be operated either sequentially
or at different frequencies. The longer transmitter/electrode
spacings will generate responses that are relatively deep, while
the shorter transmitter/electrode spacings will provide
relatively shallow responses.
The electronics for the foregoing embodiments can be of the
type set forth in the block diagram of Fig. 10 having the further
~o features shown in the block diagram of Fig. 26. In particular,
the switch 1065 (Fig. 10) is under control of the processor 1025
(Fig. 10) and couples an energizing signal to either transmitter
Tl or T2. [If desired, these transmitters can be operated
simultaneously out of phase, as previously described.] The
~5 multiplexer 1020 (Fig. 10), which is also under control of
47
CA 022~4490 1998-11-2~
processor 1025, in this case receives inputs from the ring R via
amplifier 2611, from one or more further rings represented at R'
via amplifier 2612, from the button B via amplifier 2613, from
one or more further buttons represented at B' via amplifier 2614,
from receiver (monitor)'toroid M0 via amplifier 2615, and from
receiver (monitor) toroid M2 via amplifier 2616.
Fig. 27 is a flow diagram of a routine for programming a
processor, such as the processor 1025 of Fig. 10 (as modified by
Fig. 26) to implement operation of the e~bodiment of Fig. 15 in
lo accordance with a form of the invention. The block 2710
represents the enabling of transmission from transmitter Tl, this
being implemented by control of switch 1065. The blocks 2715,
2720 and 2725 respectively represent the measurement and storage
of signal data received at receivers R, M0 and M2, the functions
being initiated by controlling the multiplexer 1020 in sequence
to obtain these measurements. In particular, the block 2715
represents the reading and storage of data from the ring
electrode R to obtain Rl, the block 2720 represents the reading
and storage of data from monitor toroid M0 to obtain Mol/ and the
block 2725 represents the reading and storage of data from
monitor toroid M2 to obtain M21. The transmitter T1 is then
turned off and the transmitter T2 is enabled, as represented by
the block 2735. The blocks 2740 and 2745 respec'ively represent
the measurement and storage of signal data received at receivers
R and M0, these functions again being initiated by controlling
48
CA 022~4490 1998-11-2~
the multiplexer 1020. In particular, the block 2740 represents
the reading and storage of data from ring elect~ode R to obtain
R2, and the block 2745 represents the reading and storage of data
from monitor toroid M0 to obtain Mo2. The transmitter T2 is then
turned off (block 2750) ànd the corrected ring current is
computed (block 2755) from equation (2). The apparent
resistivity can then be obtained from the ring current as
previously described, in accordance with Rapp = kV/I.
In the flow diagram of Fig. 27, currents are generated and
measured by operating transmitters Tl and T2 alternately. A
similar result can be achieved utilizing frequency multiplexing,
where both transmitters are operated simultaneously, but at
different frequencies. This generates a current in the tool
which has components at two frequencies. The current at any of
the sensors (monitor, ring, or button) which comes from either
transmitter can be determined by separating the received signal
by frequency, such as with bandpass filters.
Fig. 28 illustrates a portion of the electronics that can be
employed when the upper and lower transmitters are to be operated
simultaneously and the monitor current is utilized to "balance"
the resultant currents to contain a substantially zero axial
current condition at the monitor toroid, such as the previously
described monitor toroid M0. The AC source 2810 (which may, for
example, be coupled to the transmitters through a switch, as in
~5 prior embodiments), is coupled to each of the transmitter
49
CA 022~4490 1998-11-2~
toroidal antennas Tl and T2 via respective amplitude modulators
2820 and 2830. [Only one such amplitude modulator is strictly
necessary, the general case being shown in this diagram.] The
toroidal antennas are wired in phase opposition, so that they
generate respective axia~ currents in the conductive body that
travel in opposite directions. The amplitude modulators 2820 and
2830 are under control of processor 1025. The processor also
receives a sample of the AC output via flip-flop 2850 which is
switched to a different binary output state as the AC signal
changes polarity, so that the processor knows the phase of the AC
signal. In this case, the multiplexer 1020 is shown as receiving
the output of the ring electrode R and the monitor toroidal
antenna M0. In operation, when the current received at the
monitor toroid M0 is above a predetermined threshold, amplitude
control is sent to modulator 2820 and/or 2830 to reduce the
current sensed by the monitor toroid in the manner of a
conventional closed loop control. This embodiment is presently
considered less preferred as it does not make use of the M21 (or
M12) type of prefactor that is obtained by considering the effect
of current from a transmitter independently.
The principles of the this form of invention are also
applicable to logging in an earth borehole with the drill string
removed. Fig. 29 illustrates a logging device 2940 for
investigating subsurface formations 2931 traversed by a borehole
2932. The logging device is suspended in the borehole 2932 on an
CA 022~4490 1998-11-2~
armored cable 2933, the length of which substantially determines
the relative depth of the device 2940. The cable length is
controlled by suitable means at the surface such as a drum and
winch mechanism (not shown). Electronic signals indicative of
the information obtained by the logging device can be
conventionally transmitted through the cable 2933 to electronics
2985 and recorder 2995 located at the surface of the earth.
Alternatively, some or all processing can be performed downhole.
Depth information can be provided from a rotating wheel 2996 that
L0 is coupled to the cable 2933.
The logging device 2940 includes elongated generally
cylindrical sections 2951, 2953, 2955 and 2957 which may be
formed, for example, of conductive metal pipe. Electrically
insulating isolators, 2961 and 2963 are located at the
intersection of sections 2951 and 2953 and the intersection of
sections 2955 and 2957, respectively. The isolators 2961 and
2963 may comprise, for example, threaded annular fiberglass pipe
couplers. Located between the sections 2953 and 2955 is an
electrode 2970, which is illustrated in the present embodiment as
o being a ring electrode, but could also be one or more button
and/or ring electrodes. In the embodiment of Fig. 29, the ring
2970 is a conductive metal ring that is mounted between two more
isolators 2973 and 2975.
In the present embodiment, the electronics, shown adjacent
the logging device for ease of illustration, are located in the
51
CA 022~4490 1998-11-2~
central hollow portion of one or more of the pipe sections of the
device, although it will be understood that the electronics could
alternatively be located in an adjacent module coupled above or
below the device 2940, such as by a further coupler that may be
conductive or non-conductive, with wiring that passes through the
hollow sections and the annular couplers, as necessary. Power
and computer processor control can be provided, for example, from
the uphole electronics via cable 2933, it being understood that
some of these functions, including communications capability, can
be provided downhole, as is well known in the art. In the
embodiment of Fig. 29, AC energizing sources for transmitters Tl
and T2 are shown at 2958 and 2959, respectively. [Although
separate energizing sources are illustrated, it will be
understood that a common transmitter energizing source could
alternatively be utilized.] The AC frequency may be, for
example, in the range 100 Hz to 1 ~Hz. Conductor pair 2941 is
coupled across pipe sections 2951 and 2953 and to one side of a
switch 2942, the other side of which is coupled to either the
transmitter Tl [switch position (a)] or to a low impedance
~o winding of a current sense transformer 2943 [switch position (b)]
which, in the illustrated embodiment, has a turns ratio l:Nl.
The transformer secondary is coupled across the inverting and
non-inverting inputs of an operational amplifier 2944, the non-
inverting input of which is coupled to ground reference potential
~5 by resistor Rl. A feedback resistor R2 is coupled between the
52
CA 022~4490 1998-11-2~
output of operational amplifier 2944 and the inverting input
thereof. The output of operational amplifier 2944 is designated
VMl ~
A conductor pair 2945 is coupled across the pipe sections
2955 and 2957, and to one end of a switch 2946, the other end of
which is coupled to either the transmitter T2 [switch position
(b)] or to the short circuit 2947 [switch position ta)]. The
switches 2942 and 2946 are under common control.
A further conductor pair 2981 couples pipe sections 2953 and
2955 via a low impedance winding of current sense transformer
2983 having an indicated turns ratio of N3:1. The secondary
winding of transformer 2983 is coupled across the inverting and
non-inverting inputs of an operational amplifier 29~34, the non-
inverting input of which is coupled to ground reference potential
by resistor R3. A feedback resistor R4 is coupled between the
output of operational amplifier 2984 and the inverting input
thereof. The output of operational amplifier 2984 is designated
VMo.
A further conductor pair 2991 couples the ring electrode
2970 to the pipe section 2955 (which is, in turn, effectively
shorted to pipe section 2953 by the low impedance winding of
transformer 2983) via a low impedance winding of current sense
transformer 2993 having an indicated turns ratio of N2:1. The
secondary winding of transformer 2993 is coupled across the
inverting and non-inverting inputs of an operational amplifier
53
CA 022~4490 1998-11-2~
2994, the non-inverting input of which is coupled to ground
reference potential by resistor R5. A feedback resistor R6 is
coupled between the output of operational amplifier 2994 and the
inverting input thereof. The output of operational amplifier
2994 is designated VR.
Operation is similar to the previously described case for
sequential energizing of the transmitters Tl and T2, and the
obtainment of measurement signals at the ring electrode and the
monitor positions. [In this case, the second monitor is at the
same position as the upper transmitter, as in the analogous
arrangement of Ml in Fig. 24.] The electronics control can be
similar to that described in conjunction with Figures 10, 11, 26
and 27. In particular, with the switches 2942 and 2946 at
position "a", the transmitter Tl is operative and pipe sections
2955 and 2957 are shorted. Measurements are taken to obtain the
voltages VRl and VMOl. The ring current IR is obtained from
VR = (Rs + R6)IR/N2
that is:
IR = VRN2/(R5 + R6)
Similarly, the monitor current Mo is obtained from
VM0 (R3 + R4)Mo/N3, (6)
that is:
54
,
CA 022~4490 1998-11-2~
Mo - VMON3/(R3 + R4)- (7)
Using the same convention as above, VRl and VM0l are the voltages
obtained with transmitter Tl enabled. Next, the switches 2942
and 2946 are put in position (b), the transmitter T2 is enabled,
and the pipe sections 2951 and 2953 are effectively shorted.
Measurements are taken to~obtain the voltages VM02 (from which
the current Mo2 is obtained in accordance with equation (7)
above) and VM12. The monitor current Ml is obtained from
VMl = (Rl + R2)Ml/Nl, (8)
that is:
Ml = VMlNl/ (Rl + R2 ) -
Again, using the same convention as above, VM02 and VM12 are the
voltages obtained with transmitter T2 enabled. The compensatedring current, IRC/ is
IRC = (M02/M12)IRl + (Mol/Ml2)IR2 (10)
~0 Apparent resistivity in the region surrounding the ring electrode
is inversely proportional to the corrected ring current.
The embodiment illustrated in Fig. 29 uses isolators in
establishing electrical potential differences between conductive
sections of the body of the logging device, and to electrically
~5 isolate the electrode 2970. Also, currents are measured with
CA 022~4490 1998-11-2~
direct current flow occurring through a low impedance winding of
a current sense transformer. It will be understood that, for
example, the voltage gaps at isolators 2961 and/or 2963, and
their associated sources could alternatively be toroidal
transmitters and their àssociated sources as in previous
embodiments, and that currents can be measured using toroidal
receivers. Also, the electrode may be, for example, a button or
ring electrode of the type previously described. Similarly, it
will be understood that the techniques illustrated in Fig. 29
could be employed, in whole or in part, in a measurement-while-
drilling embodiment, but are presently considered less preferred
for such application from at least the standpoint of construction
ruggedness.
It will be understood that while separate toroidal antennas
are shown for transmitting and receiving from substantially the
same position (there being certain practical advantages to having
different antennas for use as transmitter and receiver), a single
toroidal antenna can be shared for this purpose using suitable
switching.
Fig. 30 illustrates an embodiment of the surface/local
communication subassembly 150 of the Fig. l embodiment. As
previously described, this subassembly can include a conventional
type of mud communications equipment, including a mud transmitter
(or mud siren), a mud receiver, and associated circuitry. This
equipment, which is not, of itself, a novel feature of the
56
CA 022~4490 1998-11-2~
present invention, is represented in Fig. 30 as being contained
in a section of drill collar represented at 1210. Connected
thereto, and housed in a section of drill collar 1220, is the
local communications portion of the subassembly 150. In general,
the collar 1220 is constructed in a manner similar to a portion
of the measurement and local communications subassembly 200 as
previously described, bu~t the collar 1220 can be much shorter
since only a single toroidal coil antenna, and no receiving
electrodes, are utilized in the present embodiment. In
particular, the collar 1220 has an inner annular chassis 1225
through which the drilling mud passes, and which has slots for
circuit boards and batteries (not shown). The toroidal coil
antenna 1250 can be constructed and mounted, in an insulating
medium, in the manner previously described in conjunction with
Fig.s 2 and 3, the coil again communicating with the electronics
via a bulkhead connector (not shown).
Fig. 31 illustrates an embodiment of the electronics in the
local communications portion of surface/local communications
subassembly 150. The electronics can be similar to the portion
of the electronics utilized for local communication in the
previously described measurement and local communications
subassembly 200. In particular, the toroidal coil antenna 1250
is coupled, via electronic switch 1365, to a demodulator 1370 and
to the output of a power amplifier 1325. The switch 1365 is
under control (line 1365A) of a suitably equipped processor 1350,
57
CA 022~4490 1998-11-2~
which includes memory 1355 and typical clock, timing, and
input/output capabilities (not separately represented). A
sinewave generator 1351 and a modulator 1353 are provided, as are
a digital-to-analog converter 1357 and buffer 1356, these units
operating in a manner similar to their counterparts in Fig. 10.
Also, as in the Fig. 10 embodiment, the output of demodulator
1370 is coupled, via analog-to-digital converter 1372 and buffer
1374, to the processor 1350. In the present embodiment, the
processor 1350 is also coupled with the mud communications
equipment (1210) via buffer 1380 and wiring 1386 that is coupled
between the equipments 1210 and 1220. It will be understood that
the equipments 1210 and 1220 (which comprise the subassembly 150)
can be formed as a single unit within a single drill collar or
housing, or can be separately formed, with provision for
electrical coupling of the wiring at the interface. If desired,
the circuitry for the two parts of the subassembly 150 can be
shared; for example, a single processor or processor system could
be utilized for the entire subassembly 150.
Referring to Fig. 32, there is shown a flow diagram of an
embodiment of a routine for programming the processor 1350 in
accordance with a form of the invention. The block 1420
represents the setting of the electronic switch 1365 to its
"receive" position. The block 1422 represents a processor state
defined by waiting until a synchronization signal is recognized,
at which point, the processor knows that information will follow.
58
... .. . .. . . .
CA 022~4490 1998-11-2~
The data is then received and stored into a buffer (block 1424)
continuously until an "end of frame" signal is recognized (block
1426). This signal indicates that the subassembly 200 is
prepared to receive data and/or commands, if any. Block 1430
S represents the reading and storage of data via the receiver
buffer 1374. The data is then loaded into the output buffer 1380
(block 1440) which is coupled to the processor in the mud
communications equipment of unit 1210. The mud telemetry may
transmit fresh data or the previous data if no new data has
arrived since the last mud telemetry frame was sent.
Inquiry is then made (block 1460) as to whether there is any
information to transmit to the measurement and local
communications subassembly 200. If not, block 1422 is re-
entered. If data is to be transmitted, the sinewave generator is
enabled (block 1465), the switch 1365 is set to its transmit
position (block 1470), and the transmission of data is
implemented (block 1475). When data transmission is complete,
the switch 1365 is set back to the receive position (block 1420)
and block 1422 is entered to wait for the next synchronization
signal. As noted above with regard to the flow diagram of Fig.
11, various alternative techniques for implementing and
controlling the processor can be employed.
Fig. 33 illustrates an embodiment wherein electrodes such as
button electrodes 1526, 1527 and 1528 are mounted in the drill
collar 202 instead of in a stabilizer or an undersized
stabilizer. In other respects, the structure of the electrodes,
59
.
CA 022~4490 1998-11-2~
the toroidal coil antennas, and their associated circuitry, can
be as described above. This embodiment can be useful under
conditions where use of a slick collar is indicated or
beneficial.
The azimuthal resistivity obtained using the electrodes
hereof can be correlated with the rotational orientation of the
drill collar housing (or with respect to other reference) in
various ways. For example, assume that the subassembly 130
(Fig. 1) of the bottom hole assembly includes conventional
direction and inclination ("D and I") measuring equipment that
provides the direction and inclination of the borehole and
provides the rotational azimuth of the subassembly 130 with
respect to magnetic north (known as "magnetic toolface") and with
respect to the high side of the borehole (known as "gravitational
toolface"). This equipment produces signals that can be coupled
with the processor 1025, stored locally, and or communicated
uphole for ultimate correlation, such as by using clock
synchronization, of all acquired signals. If desired, azimuthal
orientation can be obtained during rotation.
The invention has been described with reference to
particular preferred embodiments, but variations within the
spirit and scope of the invention will occur to those skilled in
the art. For example, it will be understood that functions which
are illustrated as being implemented using a processor control
can alternatively be implemented using hard-wired analog and/or
digital processing.
