Note: Descriptions are shown in the official language in which they were submitted.
1235179
--2--
Background of the Invention
l. Field of the Invention
The present invention relates generally to an apparatus and
method for measuring formation parameters by transmitting and
receiving electromagnetic signals by means disposed in recesses
in a tubular housing member and including means for reducing the
capacitive coupling of noise from conducting elements located
adjacent the recess and housing means. More particularly, a
method and apparatus is disclosed employing electrostatic
shielding of the antennas and ground loop isolation to reduce
noise and minimum spacing of antennas from conductive elements
adjacent the recesses and inductive current coupling means to
enhance the signal level at the receiving means whereby the
system is capable of use in a drill string to make measurements
while drilling.
2. Description of the Background
It is desirable for many reasons to transmit electrical
signals through the earth as a propagating medium, and receive
the signals at a location spaced from the transmitter. Such a
signal propagation system is, for example, used both for the
determination of various parameters associated with the prop-
grating medium and for communication purposes. These systems are
often used in the investigation of the environment surrounding a
Barlow, and in particular, the surrounding earth formations.
25 Various types of Barlow logging systems are available to per-
form these investigations. A class of these systems utilize
electromagnetic field phenomena to obtain data from the environ-
mint surrounding the Barlow.
One type of electromagnetic logging is electrode logging
I which utilizes an electric field in the surrounding formation to
produce a measure of the conductivity of the formation. A con-
ductile mud is necessary for proper use of this system, thus non-
during the system inoperative with oil base muds. Inductive
logging is another type of electromagnetic logging which uses a
I
~23S~9
magnetic field in the formation to produce a secondary current flow
in the formation. The secondary current flow sets up a second mug-
netic field which induces current in receiving coils positioned in
the Barlow. The induced current in the receiving coil or oils
is proportional to the secondary current flow in the formation and
thus is directly proportional to the conductivity or inversely pro-
portion Al to the resistivity of the surrounding formation. Electron
magnetic wave propagation affords still another logging system for
investigating the environment around a Barlow and is the subject
of the present invention.
An electromagnetic logging system of the wave propagation type
is disclosed in Gould et at, U.S. Patent No. 3,551,797. This
patent discloses a wire line system having a transmitter and no-
severs for measuring formation parameters, and utilizing phase
comparison and amplitude. However, the Gould wire line system
is not usable in a measuring while drilling (hereinafter, Mud
configuration. The Gould patent discloses a noninductive
Sunday of insufficient strength to operate in a drill string kirk-
terraced by a mass of steel and more particularly drill dollars in
the vicinity of the drill bit and measurement apparatus. U.S.
Patent Nos. 4,107,597 and 4,185,238 also show electromagnetic wave
propagation systems for use in wire line apparatus. U.S. Patent No.
4,107,597 describes the wire line Sunday as being constructed of a
non-conductive material which is customary in such devices in order
to aooommcdat~ the use of electromagnetic transmitting and receiving
apparatus. The U.S. Patent No. 3,079,550 shows an induction logging
system for measuring similar formation parameters, utilizing lower
frequencies and requiring a conductive mud in the Barlow.
Both the electrode and induction systems, heretofore the prim
many methods for measuring formation resistivity, as well as thewireline systems using wave propagation, have certain drawbacks,
particularly for application in a MUD configuration. An elect
123~ 79
--4--
trove system requires insulation of the drill string from the
several transmitting and receiving electrodes in the system.
This normally requires a special insulation coating to be applied
over the steel drill string in the vicinity of the electrodes.
This coating is expensive to maintain and is of questionable
reliability. An induction logging system normally operates at 20
KHz and requires large diameter coils to obtain the necessary
coupling. In a MUD configuration, inductive logging coils must
be mounted in or about a drill collar in a drill string and that
portion of the collar must be non-conductive. Non-conductive
collars are difficult to build while maintaining the structural
integrity and strength necessary to their use in a drill string.
In this regard thicker collar walls and improved mechanical
strength characteristics are obtainable in a collar by reducing
the size of the coils. In order, however, to achieve the
necessary coupling between spaced coils which are small, the
operating frequency of the system must be increased. As the ire-
quench is raised from 20 KHz, wave propagation begins and stank
dart induction is no longer effective. The wire line electromag-
netic wave propagation devices described above do not use a Sunday
assembly having sufficient structural integrity and strength to
be incorporated in a drill string or noise reduction and signal
enhancement techniques which enable reliable performance in an
MUD environment.
The unsuitability of the above systems for incorporation into
a drill string to measure formation parameters using electron
magnetic signals in a measurement while drilling configuration is
clear. The electrode system discussed above is limited to use
with electrically conducting, water base muds. Induction logging
systems utilize large coil configurations which require too much
space on a drill collar to maintain the strength and fluid come
monkeyshine characteristics described above. Smaller coils no-
quite the use of higher frequencies to insure proper coupling be-
tweet coils, and the higher frequencies propagate in the format
1235179
--5--
lion. Thus, standard induction measurements cannot be utilized
in MUD systems. The lack of structural integrity and ineffec-
live signal reception are problems associated with these
systems. The art has long sought a means of overcoming these
disadvantages useful in providing an effective apparatus using
electromagnetic signals to measure formation parameters while
drilling.
Summary of the Invention
The present invention is directed to a method and appear-
tusk for reducing the coupling of noise from conducting elements
near the receiving means and enhancing electromagnetic signal
reception suitable for measuring formation parameters about a
drill hole and particularly useful for making such measurements
while drilling by incorporation of the apparatus in the drill
string. The apparatus of the present invention is character-
iced by a tubular, electrically conductive housing means
threaded at each end for threaded engagement in a drill string
above the drill bit and including a longitudinal passage suit-
able for conveying drilling fluids there through; means for
transmitting electromagnetic energy into the formation sun-
rounding said housing means, said transmitting means insulated
from but disposed about the exterior of said housing means;
first receiving means for receiving electromagnetic energy
from said formation, said first receiving means longitudinally
spaced a first distance from said transmitting means and
insulated from but disposed about the exterior of said housing
means; first shield means substantially surrounding said first
receiving means for electrically, but not magnetically,
shielding said first receiving means by reducing the coupling
of the electric field component of the received signal,
including noise, to said first receiving means, said first
shield means comprising a pair of first conductive shield
tubes substantially surrounding said first receiving means and
within which said first receiving means is disposed, said
first tubes each having a first end, said first ends spaced
from one another with a first gap to prevent eddy currents
therein, each tube having a second end, said second ends
Jo
~23~ 9
-pa-
joined to a first conductive case at a pair of opposed open-
ins in the side walls of said first case through which said
first receiving means extends, said first shield tubes and
said first conductive case insulated from said housing means;
means for electrically connecting said first shield tubes and
said first conductive case to ground potential while insulate
in them from said conductive housing to reduce electrical
noise from ground loop currents therein; and first inductive
coupling means mounted within said first conductive case for
inductively coupling the signal from said first receiving means
to an electrical conductor leading to signal handling means.
The present invention provides a new and improved Barlow
logging system for use in a MUD configuration and utilizing
electromagnetic wave propagation while maintaining the motion-
teal strength and fluid flow characteristics of the drill
string.
A presently preferred embodiment of the invention includes
a tubular housing member having the essential mechanical and
strength characteristics of a drill collar and which is likely
to be capable of conducting an electrical current. The
transmitting, receiving and noise reducing means of the present
system are arranged on or about the drill collar member. One
or more recesses are formed in the wall of the drill collar to
house the transmitting and receiving components, leaving a
full longitudinal path through the drill collar to thereby
permit unobstructed flow of the drilling fluids through the
pipe string. The transmitting and receiving antennas are
preferably arranged in a generally circular array about the
exterior of the tubular
~Z3S~79
--6--
member. Preferably the receiving antenna comprises a low impel
dance antenna electrically insulated from the tubular member.
Most preferably, an antenna having an impedance of less than
about lo ohms, such as a single loop antenna, is disposed annum
S laxly about the tubular member. Transmitting and receiving antennas of the system are arranged in one or more recesses
formed in the peripheral surface of the drill collar.
Molded sleeves of insulative and wear resistant material
encapsulate the antennas in the recesses and fill the spaces bet-
wren the outer surface profile of the collar and the coils. Two receiving antennas are preferably employed and are located on the
same axial side of the transmitting antenna. More preferably,
the longitudinal distance between each pair of antennas, in-
eluding transmitting and receiving antennas, is different.
The capacitive coupling of noise from conducting elements
located adjacent the recess means and housing means, e.g., from a
conducting drilling fluid, the drill string or a conducting
housing is reduced, preferably minimized, by use of a low impel
dance receiver antenna, electrostatic but not magnetic shielding
of the antenna, ground loop current isolation and inductive
current coupling of the received signal from the antenna to the
data handling means. Preferably, the low impedance antenna is
substantially shielded, i.e., about ninety percent or more
shielded.
The inductive current coupling means is preferably an
impedance matching towardly ferrite core transformer and compare-
sues a secondary coil used to pick up the signal from the receiver
antenna. The transformer coil is wound on a ring of low magnetic
permeability, preferably ferrite, which in turn encircles an
annular antenna loop about the collar. Each secondary trays-
former coil couples its respective antenna coil to a receiver
circuit. Transmitter and receiver circuits as well as power
supplies are arranged in the wall of the collar.
In an exemplary system employing the present invention, a
. .
~Z3S~79
--7--
transmitting means and two spaced receiving means with associated
circuitry detect phase change in a wave propagated through the
formation surrounding the Barlow between the spaced receiver
antennas. The required comparison circuitry is disposed in the
wall of the tubular housing member in this system. The present
apparatus is also capable of measuring amplitude ratios as well
as phase changes relative to spaced receiver antennas.
The method of the present invention comprises transmitting
and receiving electromagnetic signals with a device as described
above and preferably incorporated into a drill string for
measurement while drilling. The method further comprises
reducing the capacitive coupling of noise into the system
and enhancing the signal at the receiving means with the
shielded, low impedance antenna and inductive current
coupling means disclosed.
The method and apparatus of the present invention have many
advantages. Capacitive coupling of signals transmitted along
the tubular housing member or through adjacent conducting eye-
mints, e.g., the drill string or the drilling mud, is reduced,
thus enhancing reception of the signal from the surrounding for-
motion. A device having sufficient structural integrity and
strength for incorporation into a drill string is provided.
Accordingly, the method and apparatus of the present invention
make possible the measurement of formation parameters using
electromagnetic signals in a measurement while drilling con-
figuration. These and other meritorious features and advantages
of the present invention will be more fully appreciated from the
following detailed description and claims.
Brief Description of the Drawings
Other features and intended advantages of the invention will
be more readily apparent by reference to the following detailed
description in connection with the accompanying drawings where-
in:
Fig. l is a schematic view of a drill string in a Barlow
1235~79
--8--
with a section of drill collar incorporating a measurement while
drilling logging system in accord with one embodiment of the pro-
sent invention included in the drill string; also illustrated is
a schematic analysis of wave propagation in a surrounding for-
motion viewed relative to the transmitting and receiving means
spacing on the collar;
Figs. PA and 2B combined provide a schematic side elevation Al
view in cross-section of a drill collar including a logging
system in accord with one embodiment of the present invention,
Fig. PA illustrating the top portion of the apparatus and Fig. 2B
the bottom portion;
Figs. 2C, ED and YE, respectively, illustrate, in more
detail, schematic side elevation Al views of the transmitting
antenna, the first receiving antenna and the second receiving
antenna;
Fig. 3 is a schematic illustrative cross-sectional view taken
along line 3-3 of Fig. PA, illustrating a transmitter antenna
coil in accord with the present invention;
Fig. 4 is a schematic illustrative cross-sectional view taken
in the direction of line 4 of Fig. 2B, showing a portion of the
I shielding means and a coupling for connecting a receiving antenna
coil to an associated circuit;
Fig. 5 is a schematic illustrative partial cross-sectional
view of a receiver antenna coil, shield and coupling means in
accord with the present invention;
Fig. 6 is a schematic view of an alternative embodiment of a
logging system in accord with the present invention; and
Fig. 7 is a schematic block diagram of a circuit for pro-
cussing a signal received in accord with the present invention;
While the invention will be described in connection with a
I presently preferred embodiment, it will be understood that it is
not intended to limit the invention to that embodiment. On the
contrary, it is intended to cover all alternatives, modifications
and equivalents as may be included within the spirit of the
~,~
1235179
g
invention as defined in the appended claims.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In drilling wells, the hourly cost of a drilling rig is very
large and, therefore, it is desirable to minimize the time during
which the equipment is not drilling due to mechanical failure of
the drill string. Thus, maintenance of the mechanical integrity
of the drill string is of primary concern in any measuring while
drilling (MUD) logging system.
By definition, an MUD system seeks to provide realtime
measurement of formation parameters and, therefore, measuring
instrumentation should be positioned in the drill string near the
bit, an extremely hostile physical environment. During drilling
operations there are two characteristics of the drill string
which are of prime importance: the integrity of the longitudinal
conduit through the string for the flow of drilling fluids, and
the physical strength of the components of the string to resist
failure under stress. MUD logging systems include very delicate
instrumentation which must be designed to function reliably and
very precisely as an integral part of the mass of steel hardware
required to survive the tremendous heat, pressures and stresses
near a working drill bit. MUD instrumentation must be placed in
steel drill collars which form part of the drill string near the
bit. Such collars are basically cylinders of solid steel having
an axial tubular conduit for the passage of circulating drilling
fluids down the string to the bit. Such an instrumentation
carrier provides many design constraints on both the packaging
and the function of MUD systems. This is especially true in the
case of the system of the present invention where the instrument
station is a radio frequency transmitter and receiver which must
broadcast and receive and measure low-power electromagnetic
signals in a very noisy electrical environment. Transmit and
receive antennas are part of the present invention and are
required to be mounted very close to a highly conductive mass of
solid steel. These design constraints require a careful con-
~Z3~179
--10--
side ration of all the factors which affect the function of an
electromagnetic logging tool to produce an accurate measurement
of the formation parameters of interest.
There are three formation parameters which affect an electron
magnetic wave in an earth formation, whether the wave commune-
gates from one point to another by induction or by propagation.
These three formation parameters are the conductivity
(resistivity) the magnetic susceptibility and dielectric
constant. Conductivity provides an indication of the energy
absorbing characteristics of the medium, while magnetic suscep-
tabulate and dielectric constant each give a measure of the
energy storing capacity of a material. Conductivity, which is` a
commonly measured parameter in well logging operations, exhibits
wide variations in value for earth materials and strongly affects
all electromagnetic waves. The magnetic susceptibility of most
earth materials has relatively little effect on electromagnetic
waves. The dielectric constant shows considerable variation in
earth formations and has a large influence on high frequency pro-
pagating waves.
Amplitude and phase are the two fundamental characteristics
of a wave. As the wave passes, or propagates, through a medium
the amplitude and phase change. In a Wilbur, the wave begins
at some source, or transmitting point, and radiates away from
that point by propagation, as indicated in Fig. l. As the wave
passes one receiver point the wave has a certain amplitude and
phase character which reflects the effects of the Wilbur and
the formation through which the wave traveled. As the wave
passes a second receiver point the amplitude and phase are
generally changed, reflecting the effects of that same Wilbur,
the same formation and the additional formation through which the
wave passed. By comparing the amplitude and phase of the wave as
it passes the two receivers, propagation changes due to the for-
motion may be studied. The difference in amplitude and phase
between the two received wave signals can be measured and related
Jo
'I-' `
I. .,
.
~Z35179
to the wave propagation parameters, i.e., travel time and cite-
nation.
The slower a wave travels in a medium, the greater will be
the measured phase difference between the two spaced receiver
points. Thus, phase difference can be measured and related to
propagation velocity, i.e., the larger the measured phase dip-
furriness the slower the wave is traveling. The multiple graphs in
Fig. l schematically illustrate an electromagnetic wave as a
function of time as the wave is propagated from a transmitter
antenna T, and is received by a receiver antenna R2 relatively
near to the transmitter antenna, and by a receiver antenna R
relatively far from the transmitter antenna. The wave is
received at the far receiver antenna Al a time delay "D" after
reception at the near receiver antenna R2.
From the foregoing, it can be seen that different media
affect a propagating wave through two mechanisms. One such
mechanism is travel time and the other is attenuation. Since the
dielectric constant and the conductivity are the only parameters
of the media that affect wave propagation, they should be deter-
I mixable by a measurement of phase difference (travel time amplitude ratio (attenuation), or both.
One of the major parameters in the design of an electromagnet
tic logging system is the frequency of operation. The higher the
frequency of electromagnetic energy propagating in a medium, the
more the signal is affected by the dielectric constant of the
medium rather than the conductivity of the medium. Because of
the well-developed relationship between formation lithology and
conductivity, it is this parameter which is of the most general
interest to the drilling community. Thus, a frequency is
I selected to produce data closely correlated to the formation con-
ductility, however, many of the aspects of the present invention
could be included in a dielectric logging tool if desired. In
the present preferred embodiment, a frequency has been used so
that the signal is still predominantly influenced by the conduct
^ ::
I. . .
12351~9
-12-
tivity of the formation and not very strongly affected by
dielectric constant variations.
Another aspect of frequency selection is that the lower the
frequency of operation, the closer are the correlations between
received amplitude and phase variations and the conductivity of
the formation. However, the lower the frequency, the longer the
wave-length and the greater are the effects of other physical
factors such as the spacing between the transmitting and
receiving antennas and any conductive surfaces at ground potent
trial. A factor, d/A , where d is the distance between an antenna and a conductive mass, such as a steel drill collar, and
is the wave-length of the transmitted wave, is very important.
The lower the frequency, the larger the wave-length and, there-
fore, the smaller the d/A value. The smaller the d/A ratio the
more power there is required in order to transmit sufficient
energy into the formation to produce a measurable received signal
due to increased coupling of the signal into the conductive mass
and the signal degrading losses produced thereby. Power to
operate instrumentation systems Donnelly in an MUD environment is
at a premium and efficiency of operation is a major consideration
of design. Thus, frequencies high enough to produce acceptable
do ratios for acceptable power consumption levels are required.
The other aspect of ensuring a minimum do ratio is that
d, the minimum distance between the antennas and the steel drill
collar, involve system packaging factors which directly affect
the structural reliability of the drill string. While the anion-
nay could be mounted outside the periphery of a drill collar and
protected against damage from interaction with the Barlow by
stabilizers, this is not practical. In order to mount these come
pennants within the peripheral borders of a collar, a recess must be formed in the outer surface of the collar. These
recesses must be deep enough to surround the antenna for physical
protection and still provide a sufficient spacing, d, between the
antenna and the nearest surface of the conductive steel collar
.
;
,
~2351~79
-13-
for an acceptable do ratio. However, the depth of the recess
( also affects the physical strength of the collar and, therefore,
the structural reliability of the drill string. In a drill
collar, there is both a twisting torque force component as well
as a bending moment force which requires great strength to
withstand. Cutting recesses and grooves into the surface of a
collar invariably affects its strength. Thus, for a given
diameter drill collar both the axial length and radial depth of
the antenna receiving recesses in the collar must be carefully
selected.
The environment within which the present MUD electromagnetic
logging system must operate is extremely noisy from an electrical
standpoint. Other electrical equipment operating Donnelly ire-
quaintly produces spurious- signals which have a strong electric
component and a weak magnetic field component. The system of the
present invention is configured to reduce the effect of noise
from outside sources by two techniques: The use of low impedance
antennas which are more sensitive to magnetic field components
and less sensitive to electric fields, and the use of highly
efficient electrostatic shielding means. In the preferred embo-
dominate a single turn small loop antenna lying in a plane normal
to the axis of the drill collar is used to simplify the matte-
mattes of interpreting the received signal, however, other con-
figurations of low impedance antennas could be employed. Single
turn loop antennas are used to eliminate inter winding capacitance
and reduce stray capacitance coupling between the antennas and
the drill collar. The loops are also electrostatically shielded
by a cylindrical shield open at the electrical center of the loop
and antenna couplings are shielded by conductive cases. The
electrostatic shield is also electrically isolated from the body
of the drill collar to minimize ground currents in the shielding.
Fig. 1 shows schematically drill string 11 positioned in
Barlow 12 traversing earth formations 13. The drill string
includes sections of drill pipe and, at the lower end of the
123~7~
string, drill collars to provide weight to the system. A MUD
logging apparatus in accord with the present invention and
designed to make the measurements discussed above is housed in
drill collar or logging sub 14 or the like which is illustrated
positioned in the drill string above drill bit 16. The logging
sub 14 is illustrated having longitudinal passage 17 for pro-
voiding drilling fluid or mud flow through the system and access
to the bottom of the drill string. Such access is vital for a
number of reasons which are known to those skilled in the art
and, therefore, will not be discussed here. Power supply and
electronic circuit components for operating the system are housed
in circumferential compartments 18 in the wall of sub 14 arranged
about the passage 17. Transmitter antenna 19 and receiver anion-
nay 21 and 22 having an annular configuration are illustrated
arranged about collar 14 and spaced apart to provide the come
prison of travel time and attenuation as described above.
By comparison of the arrangement of components of the MUD
logging sub 14 with the graph of Fig. 1, it will be appreciated
that an electromagnetic wave from the transmitter antenna 19,
corresponding to transmitter antenna T, penetrates surrounding
formations 13. The wave propagated within the formation in the
vicinity of sub 14 is first received at receiver antenna 22 which
corresponds to the near receiver antenna R2 spaced relatively
closer along the sub to transmitter antenna 19 than receiver
; 25 antenna Al. The propagated wave is received later by a delay
time "D" at receiver antenna 21 which corresponds to the far
receiver antenna Al, spaced relatively farther along the sub from
transmitter antenna 19.
Referring next to Figs. PA and 2B of the drawings, sub 14 is
schematically illustrated in greater detail. The compartment 18
is located within a recessed bore 31 and houses a power supply
and data handling means such as the electronic components for the
transmitter and receiver circuits as well as other circuitry, if
desired.
: ;
.
~2351~79
-15-
In Figs. PA and 2B and the details of 2C, ED and YE, MUD
apparatus is illustrated with transmitter antenna 19 just below
the electronic compartments 18, and with receiver antenna 21,
which corresponds to the far receiver Al positioned toward the
bottom of sub 14.
Beginning at the lower end of recessed bore 31, as viewed in
Fig. PA, passageways 34 and 35 provide openings to pass electric
eel wires 36 and 37 from the transmitter and receiver circuitry
in 18 to transmitter antenna 19 and receiver antennas 21 and 22,
respectively.
Transmitter antenna 19 coil consists of an antenna coil 119
positioned in an annular recess 38 which is formed in the outer
surface of drill collar 14. Receiver antennas 21 and 22 consists
of coils 121 and 122 located in elongate annular recess 39 in the
outer surface of drill collar 14 below and axially spaced from
first recess 38. A durable, electrical insulator material 23
encapsulates the transmitter and receiver antenna coils to pro-
vise electrical insulation of the coils, to protect the coils
from wear and to maintain the hydrostatic integrity of the
drilling fluid system. One example of a useful insulator
material 23 is nitrite rubber. Generally, coils 119, 121 and
122 and the additional components described below located within
the recesses 38 and 39 are constructed and arranged to allow the
depths of the recesses to be minimized while completely accom-
modeling the coils and other components within the outer surface profile of the drill collar 14 and having the antenna coils
spaced a minimum distance from the nearest surface of the collar.
Thus, the structural strength characteristics of drill collar
14 are preserved since relatively little material is removed
from the wall of the collar to form recesses 38 and 39.
The construction of transmitter antenna coil 119 and its
mounting in recess 38 may be appreciated by reference to Figs.
PA, 2C and 3. The coil 119 is made up of one or more wire win-
dings 42 which are wound about the collar in recess 38. Coil 119
~2~35179
-16-
is of low impedance, e.g., four or less turns 42 of multi filament
insulated wire. The coil 119 may be mounted in recess 38 by
various means, such as suspension within an encapsulation
material 23 filling recess 38 or by being positioned about an
annular insulative sleeve received into recess 38. In any case a
minimum space "d" between the wire windings 42 of the transmitter
coil 119 and the bottom of recess 38 is maintained to preserve
the signal strength of the transmitted signal. As discussed
above, unless a minimum value of d/A is maintained for a par
titular operating frequency too much of the signal will be lost
into the conductive steel collar to obtain a measurable signal
level in the formation for practical power input values.
Windings 42 are enclosed within copper tube 43 which serves
as an electrostatic shield. See Figs. 3 and 2C. Electrostatic
shield 43 is split in two halves midway along its accurate
length, with the halves being mechanically connected by insular
live connector 44.
A copper coated steel case 46 is also mounted in the recess
38 and has openings in each side thereof to receive and electric
gaily connect the ends of shield 43 and enclosed coil windings
42. Copper coated steel case 46 is electrically isolated from collar 14. A direct connection (not illustrated) between the
ends of windings 42 and lead wires 36 extending from the
transmitter circuitry is made in case 46 so that the lead lines
continue a loop as the windings. Copper coated steel tube 47
extends from the end of passageway 34 to an opening in the top of
case 46 to provide a conduit for lead wires 36. Molded insular
live material 23 fills recess 38 and encapsulates case 46, tube
47, shield 43 and coil 42 to provide a protective and electric
I gaily insulating cover for transmitter antenna 19 and to maintain the hydrostatic integrity of the drilling system.
Tube 47 is in mechanical and electrical contact with the
steel body of drill collar 14 at its upper end toward passageway
34. However, tube 47 is isolated from electrical contact with
: .
~23~79
-17-
case 46 by an electrically insulating annular spacer 4B. Thus,
transmitter lead lines 36 are electrostatically shielded by tube
47 and collar 14, which is electrically grounded at the
transmitter circuitry, thereby grounding the tube.
Transmitter lead lines 36 may comprise a pair of coaxial
cables, with the center lead of the cables continuing as wire
windings 42 of transmitter antenna coil 119. The shield lead
(not illustrated) of each cable, grounded at the transmitter air-
quoter, is in electrical contact with each of tube shield halves
43 but is insulated from drill collar 14. Thus, windings 42
are also electrostatically shielded by case 46 and tube 43, which
are at ground potential.
The transmitter antenna shield 43 aids in reducing the
generation of noise in the transmitted signal while the split
in tube shield 43 prevents circulating eddy currents from flowing
about the tube which would interfere with the magnetic field of
the propagating electromagnetic wave. Also, since grounded tube
47 is insulated from direct contact with grounded case 46, no
ground currents are able to be generated in the shielding of lead
wires 36 and coil windings 42. Therefore, transmitter antenna
19, including lead wires 36 and coil windings 42, is electricity-
tidally shielded, but is not magnetically shielded.
- Both the receiver antenna coils 121 and 122 are mounted about
the body of drill collar 14 in the same way and in a manner semi-
z5 far to that of transmitter coil 119.
Annular recess 39 is sized in depth to preserve the strength
of the drill collar 14 and to accommodate the receiver antenna
coils and other components within the outer surface profile of
the collar while having the antenna coils spaced the minimum
I distance "d" from the nearest conductive surface of the collar.
To enhance the sensitivity of the receiver antenna coils 121
and 122 to magnetic field components and reduce their sensitivity
to the noisy electric field components low-impedance antenna
coils are preferred. Although single turn loop antennas are used
lZ35179
-18-
in the present system to reduce inter winding capacitance and
capacitive coupling of noise into the antenna, other types of low
impedance antennas could be employed. Also, axially symmetrical
loops simplify the mathematics of interpreting the received
signals but with proper analysis techniques other configurations
could be used.
The receiver coil 122 is enclosed within a copper tube 55
which serves as an electrostatic shield, as shown in Figs. 2B, 4
and 5. The shield 55 is split in two halves midway along its
arcuate length with the halves being mechanically connected by an
insulative connector 56. As in the case of transmitter coil 119
and conductive case 46, a copper coated steel case 57 is mounted
in but electrically isolated from the annular recess 39 and has
openings in each side thereof to receive the ends of shield 55
and enclosed coil 122. Mounted within the case 57 is a trays-
former for inductively coupling the receiver antenna coil 122 to
the receiver circuitry.
To maximize the energy coupled from the receiver antenna to
the receiver input circuitry, a high efficiency transformer such
as a ferrite towardly transformer is preferred. AS shown in
Figs. 4 and 5, a ring 58 of low magnetic permeability material,
which may be a ferromagnetic material such as ferrite or powdered
iron, is positioned about the receiver coil 121 and wound with a
towardly coil 59. The output of the coil 59 couples the received
signal from the antenna coil 121 to the transmission line con-
netted to the receiver circuit components (not shown). The impel
dance of the coil 59 is selected to match the impedances between
the antenna coil 121 and the receiver input for maximum signal
transfer. The coil 121 impedance may be on the order of a few
ohms while the transmission line leading to the receiver circuit
may be on the order of 50 ohms.
Receiver lead wires 37 include two coaxial cables extending
from the receiver circuitry through a sub passageway 35 and a
copper coated steel shielding tube 49 to connector case 57. As
.
~23~7~
--19--
in the case of transmitter antenna coil 119, the outer shielding
leads of coaxial cables 37, which are grounded at the receiver
circuitry, are electrically connected to conductive case 57 and,
therefore, to tube shield halves 55. The center leads of
receiver coaxial cables 37 are connected to opposite ends of
towardly coil 59. As illustrated in Fig. 4, shielding tube 49 is
mechanically connected to case 57 but is electrically isolated
therefrom by insulating spacer 60. As in the case of shielding
tube 47, shielding tube 49 is mechanically and electrically con-
netted to the wall of drill collar 14 at the upper end of tube 49
toward the bottom of recess 31. Consequently, the receiver
antenna leads and antenna coil 122 and 121 are electrostatically
shielded by separately grounded tubing shield 49 and 67 enclosing
lead cables 37 and the combination of cases 57 and tube halves
55, enclosing antenna coils 122 and 121, respectively. The split
in copper shielding tube halves 55 prevents eddy currents from
circulating about the tube which would interfere with reception
of magnetic field components of the propagating waves by coils
122 and 121. Further, since grounded shield tubes 49 and 67
(enclosing lead cables for receive antenna coil 121) are ins-
fated from direct contact with grounded cases 57, no ground
currents can be generated in the shielding of lead wires 37. The
receiver antenna coils 121 and 122 and their coupling components
are carefully electrostatically shielded from noise, while
remaining exposed to the magnetic field components of the
electromagnetic waves generated by transmitter antenna 19 and
propagated through the surrounding formations 13.
Electromagnetic waves generated by transmitter antenna 19 and
30~ propagated through surrounding formations 13 are incident on
receiver antennas 22 and 21, whereby the varying magnetic fields
of the waves interact with coils 122 and 121 of respective
receiver antennas 22 and 21 to induce electric currents in the
windings. Low impedance antennas, such as the relatively large
wire, single loop preferred construction of receiver coils 122
. .
~Z35~79
-20-
and 121 exhibit minimized impedance to such magnetic field
induced current flow while remaining less susceptible to noise
due to varying electric fields. The current flow in receiver
coil 122 generates electromagnetic fields in the plane of the
corresponding ferritomagnetic ring 58 and towardly coupling coil
59, thus providing a highly efficient inductive transformer
coupling of the towardly coil to the antenna coil. Similarly,
a transformer towardly coil is coupled to antenna coil 121.
Voltages varying in accord with the field variations of the
electromagnetic wave incident on the receiver antenna coil 121
are thus generated across the output of its coupling coil 59, and
transmitted therefrom to receiver input circuitry. Similarly,
voltages are generated due to electromagnetic waves incident upon
receive antenna coil 122. The pair of received voltages are
amplified and converted to lower frequencies for ease of handling
in the electronics section.
In one preferred embodiment of the system of the invention, a
drill collar having an outside diameter of 7 inches was formed
with cylindrical annular recesses having a diameter of 5.75
inches. Antenna coils having an inner diameter of 6.2 inches
were arranged in the recesses to produce a minimum distance bet-
wren antenna and drill collar of about 0.22 inches. This system
performed well Donnelly at an operating frequency of about 2
MHz.
Next referring to Fig. 6 of the drawings, an alternative
arrangement is illustrated for the transmitting and receiving
antenna coils. The upper portions of sub 80, housing circuitry
; component and power supply section 83, are similar to those
of sub 14 set out with respect to Fig. PA. Transmitter antenna
30 coil 84 and receiver antenna coils 85 and 86, however, are
illustrated in eccentric longitudinal recesses 81 and 82, respect
lively, on the outer wall surface of drill collar 80. Each coil
84, 85 and 86 typically includes a plurality of turns wound about
a longitudinal core snot illustrated) and arranged parallel to
:
.
1 Z35179
the longitudinal axis of drill collar 80. The cores and coils
are made as small as possible to limit the depth of recesses 81
and 82 needed to house and protect the coil assemblies. The
coils are spaced from the steel body of drill collar 80 a minimum
5 distance "d" to preserve an adequate d/A ratio for efficient
operation and covered with an encapsulating medium 87 to allow
magnetic fields to encircle the coils, to protect the coils and
to maintain the hydrostatic integrity of the drilling fluid
system. Appropriate electrostatic shielding (not illustrated) is
also provided in the system, preferably as described above with
respect to Figs. PA and 2B.
Referring now to Fig. 7 of the drawings, a schematic circuit
diagram is illustrated for providing a system to make formation
resistivity measurements using the apparatus heretofore
described. Such data handling means are included in the electron
nits section in drill collars 14 or 80, although the embodiment
of Figs. PA and 2B will be referred to hereafter. The output of
the circuit shown in Fig. 7 may be recorded in the electronics
section or may be Ted by a telemetry system (not illustrated) to
the surface for concurrent processing and readout at the surface.
In any event, the transmitter is operated by power supply 24 at a
frequency preferably in the range of 500 KHz to 10 MHz to provide
an electromagnetic wave for output from the transmitter antenna
coil lit. This wave is propagated through the environment,
including earth formations 13 surrounding the Barlow, to
receiver antenna coils 122 and 121 of receivers 22 and 21,
respectively, located at spaced longitudinal distances on the
collar. The receiver antenna coils are coupled to the respective
electronic receiver components which are driven by local oscilla-
ion 90 operating at a frequency within several KHz of the
transmitted frequency to generate lower frequency output signals
which are more easily handled. For example, transmitter 19 is
illustrated operating at 2 MHz and oscillator 90 is operating at
1.998 MHz. The outputs from receivers 22 and 21 are then fed to
,::.
:'
'
:
lZ~5~79
-22-
a phase comparator 91 and/or amplitude measuring circuits 92 and
93 which are in turn coupled to a ratio circuit 94 to measure
signal phase shift and attenuator all in accordance with the
teaching of the aforementioned prior art patents.
yin the operation of the system heretofore described, the
signal emanates from a source, transmitter antenna 19, and ire-
vets away from it. The steel of drill collar 14 has a high per-
mobility and is located to the inside of the source so that the
steel only causes small effects on the signal as long as a mini-
mum distance d from antenna to drill collar is maintained so that
an adequate do ratio is maintained. The signal that does travel
along the steel is highly attenuated, so that the signal received
at the receiver antenna coils must come from the region outside
collar 14, i.e. earth formations 13 as long as the receiver
antennas coils 121 and 122 are spaced the distance "d" from the
surface of the collar.
The foregoing description of the invention has been directed
primarily to a particular preferred embodiment in accordance with
the requirements of the patent statutes and for purposes of
explanation and illustration. It will be apparent, however, to
those skilled in the art that many modifications and changes in
the specifically described and illustrated apparatus and method
may be made without departing from the scope and spirit of the
invention. For example, while the disclosure of the system has
Boone described primarily with regard to resistivity measurements,
particular coil configurations, and specific frequencies and ire-
quench ranges, it may be appreciated from the present descrip-
Sheehan and illustrations that other measurements, coil
configurations, frequencies and the like could be used without
deporting from the present invention in its broadest aspects.
Therefore, the invention is not restricted to the particular form
of construction illustrated and described, but covers all modify-
cations which may fall within the scope of the following claims.
It is Applicants' intention in the following claims to cover
~Z35~
-23-
such modifications and variations as fall within the true spirit
and scope of the invention.
