METHODE ET DETECTION ET DE CORRECTION DU MAGNETISME PARASITE EN DIAGRAPHIE DU SONDAGE

METHOD FOR THE DETECTION AND CORRECTION OF MAGNETIC INTERFERENCE IN THE SURVEYING OF BOREHOLES

Abrégé anglais

METHOD FOR THE DETECTION
AND CORRECTION OF MAGNETIC INTERFERENCE
IN THE SURVEYING OF BOREHOLES
Abstract of the Disclosure:
A method is presented, for use in borehole
drilling, for correcting errors in azimuth
determination resulting from variations in the
earth's magnetic field to which a measuring
instrument is exposed. The method distinguishes
between variations in the earth's magnetic field
caused by the drillstring and variation caused by
external sources, and the method corrects only for
errors caused by the drillstring. If the error is
caused by the drillstring, an azimuth correction is
made based on dip angle data, thereby reducing errors
caused by scale factor variations since dip angle is
a ratio quantity of magnetic field components.

Revendications

The embodiments of the invention in which an exclusive
property or privilege is claimed are defined as follows:
CLAIM 1. The method of determining a correction to
be made to an azimuth measurement of an instrument in
a borehole to compensate for magnetic interference,
including the steps of:
determining the measured azimuth angle of the
instrument;
determining the measured inclination angle of the
instrument;
determining the measured dip angle;
ascertaining the true dip angle at the location
of the borehole; and
calculating the error in azimuth measurement
caused by magnetic interference from the difference
between said measured dip angle and said true dip
angle and a factor determined from said measured
azimuth angle, said measured inclination angle and
said true dip angle.
19

CLAIM 2. The method of claim 1, further including:
determining the measured azimuth angle at the
downhole location of the instrument and transmitting
said azimuth angle measurement to the surface;
determining the measure dip angle at the downhole
location of the instrument and transmitting said dip
angle measurement to the surface;
determining the strength of the measured magnetic
field and transmitting said magnetic field strength
to the surface;
determining the strength of the measured
component of the magnetic field along the axis of the
instrument, and transmitting said measured component
strength to the surface;
determining the inclination angle of the
instrument at the downhole location of the instrument
and transmitting said inclination measurement to the
surface;
calculating the expected value of the component
of the magnetic field along the axis of the
instrument from the measured values of azimuth,
inclination, magnetic field strength and dip angle;
and
comparing said calculated and measured values of
the component of the magnetic field along the axis of
the instrument as a check on the consistency of
transmission of data to the surface.

CLAIM 3. The method of claim 1, wherein said error,
dAZ is determined by:
<IMG>
where:
AZ = azimuth (measured)
dAZ = azimuth error
? = dip angle (measured)
?o = dip angle (true)
INC = inclination
CLAIM 4. The method of determining a correction to
be made to an azimuth measurement of an instrument in
a drillstring in a borehole to compensate for
magnetic interference, including the steps of:
determining the measured azimuth angle of the
instrument;
determining the measured inclination angle of the
instrument;
determining the measured dip angle:
ascertaining the true dip angle at the location
of the borehole;
determining whether the source of magnetic
interference is from the drillstring or from an
external source; and
calculating the error in azimuth measurement
caused by magnetic interference from the difference
between said measured dip angle and said true dip
angle and a factor determined from said measured
azimuth angle said measured inclination angle and
said true dip angle only in the case where the
magnetic interference is determined to be from the
drillstring.
21

CLAIM 5. The method of claim 4 wherein said step of
determining the source of the magnetic interference
includes:
determining the measured value of the component
of the magnetic field perpendicular to the axis of
the instrument; and
determining the expected value of the component
of the earth's magnetic field perpendicular to the
axis of the instrument: and
determining the difference between said expected
and measured values of the component of the earth's
magnetic field perpendicular to the axis of the
instrument to indicate the source of magnetic
interference.
CLAIM 6. The method of claim 5 wherein:
the difference between said measured and expected
values of the component of the earth's magnetic field
perpendicular to the axis of the instrument is used
as a measure of the magnitude of the magnetic
interference arising from an external source.
22

CLAIM 7. The method of claim 4, further including:
determining the measured azimuth angle at the
downhole location of the instrument and transmitting
said azimuth angle measurement to the surface;
determining the measured dip angle at the
downhole location of the instrument and transmitting
said dip angle measurement to to the surface;
determining the strength of the measured magnetic
field and transmitting said magnetic field strength
of the surface;
determining the strength of the measured
component of the magnetic field along the axis of the
instrument, and transmitting said measured component
strength to the surface;
determining the inclination angle of the
instrument at the downhole location of the instrument
and transmitting said inclination measurement to the
surface;
calculating the expected value of the component
of the magnetic field along the axis of the
instrument from the measured values of azimuth,
inclination, magnetic field strength and dip angle;
and
comparing said calculated and measured values of
the component of the magnetic field along the axis of
the instrument as a check on the consistency of
transmission of data to the surface.
23

CLAIM 8. The method of claim 4, wherein said error,
in azimuth measurement (dAZ) is:
<IMG>
where:
AZ = azimuth (measured)
dAZ = azimuth error
? = dip angle (measured)
?o = dip angle (true)
INC = inclination
24

CLAIM 9. The method of checking the consistency of
data transmission from an instrument in a downhole
location in a borehole to the surface, including the
steps of:
sensing a plurality of components of the earth's
magnetic field at a downhole location in a borehole;
sensing a plurality of components of the earth's
gravitational field at the downhole location in the
borehole;
determining the measured azimuth angle at the
downhole location of the instrument from a plurality
of said sensed magnetic components and said sensed
gravitational components and transmitting said
azimuth angle measurement to the surface;
determining the measured dip angle at the
downhole location of the instrument from a plurality
of said sensed magnetic components and said sensed
gravitational components and transmitting said dip
angle measurement to the surface;
determining the strength of the measured magnetic
field and transmitting said magnetic field strength
to the surface;
determining the strength of the measured
component of the magnetic field along the axis of the
instrument, and transmitting said measured component
strength to the surface;
determining the inclination angle of the
instrument at the downhole location of the instrument
from a plurality of said sensed gravitational
components and transmitting said inclination
measurement to the surface;
calculating the expected value of the component
of the magnetic field along the axis of the
instrument from the measured values of azimuth,

inclination, magnetic field strength and dip angle;
and
comparing said calculated and measured values of
the component of the magnetic field along the axis of
the instrument as a check on the consistency of
transmission of data to the surface.
CLAIM 10. The method of determining a correction to
be made to an azimuth measurement of an instrument in
a borehole to compensate for magnetic interference,
including the steps of:
sensing a plurality of components of the earth's
magnetic field at a downhole location in a borehole;
sensing a plurality of components of the earth's
gravitational field at the downhole location in the
borehole;
determining the measured azimuth angle of the
instrument from a plurality of said sensed magnetic
components and said sensed gravitational components;
determining the measured inclination angle of the
instrument from a plurality of said sensed
gravitational components;
determining the measured dip angle from a
plurality of said sensed magnetic components and said
sensed gravitational components;
ascertaining the true dip angle at the location
of the borehole; and
calculating the error in azimuth measurement
caused by magnetic interference from the difference
between said measured dip angle and said true dip
angle and a factor determined from said measured
azimuth angle, said measured inclination and said
true dip angle.
26

CLAIM 11. The method of claim 10, further including:
determining the measured azimuth angle at the
downhole location of the instrument and transmitting
said azimuth angle measurement to the surface;
determining the measured dip angle at the
downhole location of the instrument and transmitting
said dip angle measurement to the surface;
determining the strength of the measured magnetic
field and transmitting said magnetic field strength
to the surface;
determining the inclination angle of the
instrument at the downhole location of the instrument
and transmitting said inclination measurement to the
surface;
calculating the expected value of the component
of the magnetic field along the axis of the
instrument from the measured values of azimuth,
inclination, magnetic field strength and dip angle;
and
comparing said calculated and measured values of
the component of the magnetic field along the axis of
the instrument as a check on the consistency of
transmission of data to the surface.
27

CLAIM 12. The method of claim 10, wherein said
correction, dAZ is determined by:
<IMG>
where:
AZ = azimuth (measured)
dAZ = azimuth error
? = dip angle (measured)
?o = dip angle (true)
INC = inclination
CLAIM 13. The method of claim 10, including the
steps of:
determining whether the source of magnetic
interference is from drillstring or from an external
source; and
determining the error in azimuth measurement
caused by magnetic interference as a function of the
difference between said measured dip angle and said
true dip angle only in the case where the magnetic
interference is determined to be from the drillstring.
28

CLAIM 14. The method of determining a correction to
be made to an azimuth measurement of an instrument in
a drillstring in a borehole to compensate for
magnetic interference, including steps of:
sensing a plurality of components of the earth's
magnetic field at a downhole location in a borehole;
sensing a plurality of components of the earth's
gravitational field at the downhole location in the
borehole;
determining the measured azimuth angle of the
instrument from a plurality of said sensed magnetic
components and said sensed gravitational components;
determining the measured inclination angle of the
instrument from a plurality of said sensed magnetic
components and said sensed gravitational components;
ascertaining the true dip angle at the location
of the borehole;
determining whether the source of magnetic
interference is from the drillstring or from an
external source; and
calculating the error in azimuth measurement
caused by magnetic interference from the difference
between said measured dip angle and said true dip
angle and a factor determined from said measured
azimuth angle, said measured angle inclination and
said true dip angle only in the case where magnetic
interference is determined to be from the drillstring.
29

CLAIM 15. The method of claim 14 wherein the step of
determining the source of the magnetic interference
includes:
determining the measured value of the component
of the magnetic field perpendicular to the axis of
the instrument;
determining the expected value of the component
of the earth's magnetic field perpendicular to the
axis of the instrument; and
determining the difference between said expected
and measured value of the component of the earth's
magnetic field and perpendicular to the axis of the
instrument to indicate the source of magnetic
interference.
CLAIM 16. The method of claim 15 wherein:
the difference between said measured and expected
values of the component of the earth's magnetic field
perpendicular to the axis of the instrument is used
as a measure of the magnitude of the magnetic
interference arising from an eternal source.

CLAIM 17. The method of claim 14, further including:
determining the measured azimuth angle at the
downhole location of the instrument and transmitting
said azimuth angle measurement to the surface;
determining the measured dip angle at the
downhole location of the instrument and transmitting
said dip angle measurement to the surface;
determining the strength of the measured magnetic
field and transmitting said magnetic field strength
to the surface;
determining the strength of the measured
component of the magnetic field along the axis of the
instrument, and transmitting said measured component
strength to the surface;
determining the inclination angle of the
instrument at the downhole location of the instrument
and transmitting said inclination measurement to the
surface;
calculating the expected value of the component
of the magnetic field along the axis of the
instrument from the measured values of azimuth,
inclination, magnetic field strength and dip angle;
and
comparing said calculated and measured values of
the component of the magnetic field along the axis of
the instrument as a check on the consistency of
transmission of data to the surface.
31

CLAIM 18. The method of claim 10, wherein said
correction in azimuth measurement (dAZ) is:
<IMG>
where:
AZ = azimuth (measured)
dAZ = azimuth error
? = dip angle (measured)
?o = dip angle (true)
INC = inclination
32

Description

METHOD FOR THE DETECTION
AND CORRECTION OF MAGNETIC INTÆRFERENCE
IN THE SURVEYING OF BOREHOLES
Back~round of the Invention:
This invention relates to the field of borehole
surveying or measurement. More particularly, this
invention relates to a method for determining the
S directional parameter of borehole a2imuth and
correcting the azimuth for errors caused by
perturbations in the earthls magnetic field.
The general class of such instruments used for
borehole directional measurement use a three-axis
magnetometer and a two- or three-axis accelerometer
to determine the components of the earth's magnetic
and gravitational fields in a coordinate system
centered on the instrument. A straightforward
geometric trans~ormation is employed to determine the
desired parameters defining the tool's orientation,
namely the azimuth, inclination and tool face
reference. For a prior art reference which describes
this art and technique by means of a programmable
Calculator, see ~Hand-Held Calculator Assists in
Directional Drilling Control", ~.L. Marsh, Petroleurn
Engine,er ~ntexnational, ~uly & September 1982.
~'
...

--2--
A~imuth is defined as the anyle between magnetic
north and the horizontal projection of the borehole
axis. Measurement of the earth's magnetic field is
commonly employed in determining azimuth. One common
feature of any surveying device relying on the
earth's magnetic field for the determination of
azimuth is that a perturbation of the earth's
magnetic field may result in an error in the measured
azimuth. Such perturbation will hereinafter be
referred to as magnetic interference. One source of
magnetic interference may be within the drilling
apparatus itself; i.e., it may arise from the
presence of permeable, and possibly magnetized,
materials in the drillstring. Another source of
`magnetic interference may be from an external source
; such as a ferrous ore body, or an adjacent well.
The existence of this source of error in azimuth
measurement and the need to correct for the error has
been recognized in the art, and attempts have been
made to solve the problem. However, prior attempts
to solve the problem have been deficient, and in some
cases could actually result in greater errors in or
greater unreliability of azimuth measurement; and the
need for an accurate and reliable azimuth error
correction system still exists.
The most relevant prior art known to the present
inventors is disclosed in U.S. Patent 4,163,324 to
Russell et al (hereinafter the Russell et al
patent). In the Russell et al patent, it is assumed
that all interference is caused by magnetic material
in the drillstring and is, therefore, axial (i.e.,
along the drillstring axis). No means are provided
for verifying the validity of this assumption. If
the assumption is wron~, then the correction made to

~ Z~
~ _ 3 _
azimuth measurement is also wrong; and this rnay
actually lead to worsening of the results of the
directional measurement sys-tem.
The system of the ~ussell et al patent also
introduces another potential source of error in that
it uses absolute values of the local magnetic field
and absolute values of the earth's magnetic field in
carrying out its azimuth correction procedure. The
use of absolu-te values increases the sensitivity of
the method to scale factor errors to the procedure,
thus reducing or impairing the accuracy and
reliability of the error correction.
Summary of the Invention:
The above discussed and other deficiencies
of the prior art are overcome or alleviated by the
system and method of the present invention.
In accordance with an embodiment of the
inven-tion, a method of determining a correction to be
made to an axlmuth measurement of an instrument in a
borehole to compensate for magnetic interference
~ includes the steps of determining the measured
; azimuth angle of the instrument and determinlng the
measured inclination angle of the instrument. The
measured dip angle is determined and the true dip
angle at the location of the borehole is ascertained.
The ai;- in azimuth measurement, caused by magnetic
int-erference from the difference between the measured
dip angle and the true dip angle and a factor deter-
mined from the measured azimu-th angle, the measured
inclination angle and the true dip angle are then
calculated.
The present invention provides a means for
deterrnining the presence of magne-tic interference;
and lt also provides a means for d:istin~uishing
between internal in-terference (from the drilling
,~ .
.~ `"
_,"",~ s~

~2~
- 3a -
itself) and external interference. In the case of
internal interference, the value of the azimuth error
introduced by this interference is determined and
used -to correct -the measured azimuth. In the present
invention -the correction is based on dip angle
quanti-ties which are functions of ra-tios of measured
or known values. The use of dip angle reduces the
problem of errors in sensor scale factors. If the
interference is from an external source, azimuth
correc-tion is not made. However, this system is more
reliable than the prior art because the driller knows
(l)azimuth measuremen-ts are unreliable, (2) azimuth
error correction cannot be made, and (3) there is an
. j .
,~ .i~ ~, ',

_a, _
external so~lrce of magnetic interference. In this
sit~ation, alternative means, such as a gyroscopic
survey may be employed for azimuth measurement.
As previously indicated, in the present
invention, no assumptions are made regarding the
source or magnitude of the perturhing field.
~easurements are made of three components of the
ambient magnetic field and at least two components of
the gravitational field in coordinate axes fi~ed
relative to the tool. It is customary that these
axes be the same for both sets of measurements, that
they be orthogonal, and, furthermore, that one of
these axes (generally designated as the z-axis) be
along the tool axis and another (the y-axis) be in
the direction of a reference or scribe line. Based
on these readings, the three ~drillers' angles~,
azimuth, inclination, and tool face reference may be
determined, either at the surface or by a downhole
microprocessor. In the presence of magnetic
interference (and, in particular, the east-west
component of such interference), the measured azimuth
will be in error. In the present invention, at least
two, and in the preferred embodiment, three,
; quantities are determined which are characteristic of
the measured magnetic field of the tool. If three
quantities are determined, one of these will be
redundant (i.e., an algebraic combination of the
others). When the determination is made downhole
(eOg., in a measurement-while-drilling (MWD) system
employing a downhole microprocessor), this redundancy
allows a check of the data transmission and decoding
by verifying the consistency of all of the results.
The differences between the measured values of the
earth's magnetic field and thé nominal (i.e,,

--5--
charted) values for the particular region of the
earth allow one to determine the magnitude of the
interfering field along the tool axis. Rather than
assume that this is the only interference, as is done
in the pricr art, the present invention tests the
validity of this hypothesis by checking for
self-consistency among all of the measurements. If
the measured values are not consistent with the
existence of purely axial interference, an estimate
of the magnitude of the external interference is
generated. If the test determines that only internal
interference exists, a determination is then made of
the azimuth error resulting from this interference.
This error determination is based upon differences
between measured and nominal dip angles, which angles
are all derived from ratios of measurements, thereby
reducing one source of potential error, i.e., a
variation in scale factors in the downhole sensor.
Brief Description of the Drawings:
Referring to the drawings, wherein like elements
are numbered alike in the several FIGURES:
~IGURE 1 is a generalized schematic view of a
borehole and drilling derrick showing the environment
of the present invention.
FIGURE 2 is a view of a section of a drillstring
of FIGURE 1 showing, in schematic form, the
drillstring environment of the present invention.
FIGURE 3 is a perspective view of a drillstring
segment showing the relationship of various axes,
angles and vectors of interest in the present
invention.

~4~3
--6--
Descri~tlon of the Preferred Embodiment
-
The peesent invention will be described with
reference to and in the environment of a
measurement-while-drilling (MWD) system. ~owever, it
will be understood that the invention is not limited
to an MWD system; rather the invention may be
employed in a wire line or other directional
measurement system.
Referring first to FIG~RES 1 and 2, the general
environment of the present invention is shown. It
will, however, be understood that these generalized
showings are only for purposes of showing a
representative environment in which the present
invention may be used, and there is no intention to
limit applicability of the present invention to the
specific configuration of FIGURES 1 and 2.
The drilling apparatus shown in FIGURE 1 has a
derrick 10 which supports a drillstring or drill stem
12 which terminates in a drill bit 14. As is well
known in the art, the entire drillstring may rotate,
or the drillstring may be maintained stationary and
only the drill bit rotated, either of which may be
the environment of the present invention. The
drillstring 12 is made up of a series of
interconnected segments, with new segments being
added as the depth of the well increases. The
drillstring is suspended from a movable block 16 of a
winch 18, and the entire drillstring may be driven in
rotation by a square kelly 20 which slidably passes
through but is rotatably driven by the rotary table
22 at the foot of the derrick. ~ motor assembly 24
is connected to both operate winch 18 and rotatably
drive rotary table 22.

3~
--7--
The lower part of the drillstring may contain one
or more segments 26 of larger diameter ~nd thicker
walls than other se~ments of the drillstring (known
as drill collars). ~s is well known in the art,
these drill collars may contain sensors and
electronic circuitry for sensors, and power sources,
such as mud driven turbines which drive drill bits
and/or generators and, to supply the electrical
energy for the sensing elements.
Drill cuttings produced by the operation of drill
bit 14 are carried away by a mud stream rising up
through the frçe annular space 28 between the
drillstring and the wall 30 of the well. That mud is
delivered via a pipe 32 to a filtering and decanting
system, schematically shown as tank 34. The filtered
mud is then sucked by a pump 36, provided with a
pulsation absorber 38, and is delivered via line 40
under pressure to a revolving injector head 42 and
thence to the interior of drillstring 12 to be
delivered to drill bit 14 and the mud turbine if a
mud turbine is included in the system.
The mud column in drillstring 12 may ~lso serves
as the transmission medium for carrying signals of
downhole parameters to the surface. This signal
transmission is accomplished by the well known
technique of mud pulse generation whereby pressure
pulses are generated in the mud column in drillstring
12 representative of sensed parameters down the
well. The drilling parameters are ~ensed in a sensor
unit 44 (see FIGURE 2) in a drill collar 26 near or
ad~acent to the drill bit. Pressure pulses are
established in the mud stream within drillstring 12,
and these pressure pulses are received by a pressure
transducer 46 and then transmitted to a signal

--8--
receiving unit 48 which may record~ display and/or
perform computations on the signals to provide
information of various conditions down the well.
Referring brieEly to FIGUR~ 2, a schematic system
is shown of a drillstring segment 26 in which the mud
pulses are generated. The mud flows through a
variable flow orifice S0 and is delivered to drive
turbine 52. Turbine 52 powers a generator 54 which
deli~ers electrical power to the sensors in sensor
unit 44 (via electrical lines 55)O The output from
sensor unit 44, which may be in the form of
electrical or hydraulic or similar signals, operates
a plunger 56 which varies the size of variable
orifice 50, plunger 56 having a valve driver 57 which
may be hydraulically or electrically operated.
Variations in the size of orifice 50 create pressure
pulses in the mud stream which are transmitted to and
sensed at the surface to provide indications of
various conditions sensed by sensor unit 44. Mud
flow is indicated by the arrows.
Since sensors in sensor unit 44 are magnetically
sensitive, the particular drillstring segment ~6
which houses the sensor elements must be a
non-magnetic section of the drillstring, preferably
of stainless steel or monel. Sensor unit 44 is
further encased within a non-magnetic pressure vessel
59 to protect and isolate the sensor unit from the
pressure in the well.
~hile sensor unit 44 may contain other sensors
for directional or other measuremet, it will include
a triaxial magnetometer 58 (having 3 orthogonal "X",
"y" and nZ~ windings), and a two (X, Y) or three (X,
Y, Z) axis accelerometer 60. The sensitive axes of
sensors 58 and 60 are aligned so that they coincide,

9~
g
with the Z axes being along or parallel to the Z axes
of the drillstriny and the Y axls perpendicular to
the Z axi.s in the direction of a reference or scribe
: mark 62 on the drillstring. The X axes are ortho-
gonal to Y and Z in a direction -to make a right-
handed coordinate system. Unit 44 contains a means
of sensing rotation which may be a rotation sensor
(as in U.S. Patent 4,013,945 or a software means
using a downhole processor); and directional
measurements are taken only in the nonrotation state.
The sensor unit 44 also contains a temper-
atuxe sensor 64 to provide temperature compensation
for outputs of sensors 58 and 60, an analog to
digital converter 68 (ADC) and a microprocessor 66
for analyzing the outputs from sensors 58 and 60 (as
well as from other sensors). ADC 68 receives the
signals from sensors 58 and 60 and delivers those
signals in digital form to microprocessor 66 where
the signals are also temperature compensated by the
Olltput from sensor 64. Microprocessor 66 then
calcula-tes various values, such as drillers' angles
: (azimuth, inclination, gravity tool face reference
(GTF) or magnetic tool face reference (MTF) (see
FIGURE 3) and the parameters that charac-terize the
measured magnetic field. The outputs from micro-
processor 66 are then delivered to valve driver 57 to
operate valve 56 to create the mud pulse signals for
eventual display and/or computation at unit 48.
In the following discussion, an explana-tion
will be presented of the method of the present
invention whereby (1) the na-ture of magnetic inter-
ference is determined and (2) azirnuth error
correction is efEected if the lnterference is alon~
the ~ axis. To
~",~
. ~,"; ~... .
......

~z~
--10--
facilitate an understanding of that discussion,
various texms will first be defined, sometimes with
reference to FIGURE 3. Notations used herein
correspond to those of the articles by Marsh.
The term H means the magnetic field. HX, Hy, Hz
are components of H in the coordinate system of the
tool and correspond to the three outputs of triaxial
magnetometer 58. G refers to the force o~ qravity.
Gx, Gy, Gz are components of G in the coordinate
system of the tool and correspond to the three
outputs of triaxial accelerometer 60. In all cases,
the subscript ~o" means an unperturbed, i.e., no~inal
value (such as from available charts). The absence
of a subscript indicates a measured value. A sy~bol
with a bar (e.g./ H) refers to a vector; that sy~bol
without the bar (e.~., H) refers to the magnitude of
the vector.
Referring to FIGURE 3, one can see the
relationship between the tool-related axes and those
fixed to the earth. For clarity, the origin of the
tool fixed axis has been displaced from 0 to 0' and
the Z (tool) axis is shown as a double line. The
inclination angle INC is defined as the angle between
the vertical line OD and the tool axis OZ. The
gravity tool face reference angle GTF is defined as
the angle between the vertical plane containing OD
and OZ and the plane containing O'Z and O'Y. At low
values of inclination, the magnetic tool face angle
MTF (not shown) is employed, this being the angle
3Q between the vertical plane through OD and ON and the
plane through OIZ and O'Y. The azimuth angle AZ is
defined as the angle between the vertical plane
through O~ and ON and the vertical plane through OD
and OZ ~ The relationships between the sensor
readings and the angles ~NC~ AZ and GTF (or MTF) are
well known in the literature.

The following relationships exist:
(l) INC = T~N~~ x2 ~ ~y2)l/2/ ~ )
(0 ~- INC ~ l80 )
(2) GTF = TAN l ( Gx / Gy )
(0 ~ GTF -360 )
(3) MTF = TAN l ( Hx / Hy )
(0 ~ MTF c 3360)
(4) AZ = TANl ( G*( Hx Gy_- Hy Gx~ )
Hz* (Gx + Gy ) + Gz* ( Hx Gx ~ Hy Gy)
(0 AZ ~ 360)
Where:
G = (Gx2 + Gy2 ~ Gz2)l/2
In evaluating these equations, the value INC is taken
between 0 and 180, and the values of GTF, MTF and AZ
lie between 0 and 360.
It should be noted that~ although the gravitational
vector G lies along one of the earth-fixed coordinate axes
OD, the magnetic field H will not, in general coincide
with the axis ON (i.e., the magnetic field will not be in
the horizontal plane of ON, OE). The angle that the
magnetic field makes with the horizontal plane containing
ON and OE is the dip angle ~ . This angle is positive
in the northern hemisphere (i.e., the vertical component
of H is downward~ and negative in the southern
hemisphere. In the preferred embodiment of this
; invention, three quantities which characterize the local
magnetic field are determined by the downhole
microprocessor 18. These are, the dip angle ~ , the
magnetic field vector magnitude H, and the axial field
strength Hz. The equations for these ~uantities, in terms
of the six sensor readings, are as follows:
.

-12-
(6) ~ = SIN-l ~ Gx Hx -~ Gy Hy + Gz HzJ
( _goO ~: ~ c 90)
(7) H = ( Hx2 + Hy2 ~ Hz2 )1/2
(8) Hz - Hz
In the absence of magnetic interference, the
first two of these quantities are independent of the
orientation of the toolO Furthermore, equation ~4)
given above for azimuth angle is not dependent upon
either the dip angle or total field strength. It
only depends upon the assumption that the horizontal
component of the earth's field points "North~. The
nominal values for the total field strength, dip
angle and magnetic declination (i.e., the difference
in heading between true geographic North and
geomagnetic North) are tabulated for any latitude and
longitude. In the following discussions, the term
North will refer to the direction to the ~orth
geomagnetic pole; any correction for magnetic
declination can be made after the fac~.
In the presence of magnetic intererence, any or
all of the above magnetic quantities le.g., (~)-(8))
may be affected. The only component of the
interfering field which will influence the measured
azimuth is that in the east-west direction, i.e.,
along the axis OE. The presence of such a component
will violate the assumption that the local field
points North. In the preferred embodiment, the
current invention deals with such interference in the
following steps, after determining the three
quantities as above in the downhole microprocessor,
and transmitting them to the surface.
1) The expected value of the axial field Hzc is
determined, based on the measured values of AZ, INC,
H and ~ from the following:

-13-
(9) Hzc = H * ( sin ~ cos INC -~ cos ~ cos AZ
sin INC).
This quantity should agree with the measured value of
Hz, regardless of the nature and magnitude of the
interference, since it represents only the geometric
relationship amo~g the various measurements and
derived quantities. Since all of the quantities ~lere
determined downhole based upon the same set of sensor
outputs, any difference between Hz and Hzc, beyond
those introduced by the least count of the digitized
signals, can be attributed to an error in signal
coding, transmission or decoding. Thus, transmission
of a redundant quantity, derivable from the other
five parameters, permits a consistency check on the
transmission process~ It should be particularly
noted that this consistency check is useful and can
be performed independent of whether azimuth
correction is desired or performed. If, in another
embodiment, the output of the individual sensors are
transmitted (by cable or MWD telemetry~ such a check
is not possible, since the sensor outputs are
linearly independent.
2~ The expected value Hzo of the axial field is
determined based upon the tabulated values of the
total field and dip angle and the measured values of
azi~uth and inclination. ~he difference between this
value Hzo and the measured value of Hz will give, to
first order, the value of the axial component of the
magnetic interference dHz. Thus:
(10) Hzo = Ho * ( sin~ o cos INC + cos~ o cos AZ
sin INC )
(11) d~z = Hz - Hzo

-14-
Some error will be introduced due to the fact that
the measured, rather than the true (but unknown),
azimuth is used in the calculation. In the case of
purely internal interfexence, the calcu;Lation can be
repeated after the first-order correction is applied
to the azimuth reading, giving a better estimation of
the azimuth error. For most levels of interference
normally encountered, at most one such iteration will
be necessary.
3) If the interference is due solely to the
magnetic material in the drillstring, and is
therefore axial in nature, the component (Hp) of the
earth's magnetic field perpendicular to the borehole
axis will be unaffected by it. Such will not be the
case if the interference is from an external source.
The current invention determines the nature (i.e.,
internal or external) of the interference by
comparing the magnitude of the measured perpendicular
field to that expected from the nominal values of the
geometric field. Any difference, beyond that
attributable to the resolution of the sensors and the
transmission system, is taken to be a result of
external interference. Thus:
(12) Hp = ( Hx2 ~ Hy2 )1/~
( H2 _ Hz2 )1/2
(13) Hpo = ( Ho - Hzo2 Jl/2
(14) dHp = Hp - Hpo
The value of Hp (Equation 12) is determined from
measured values. The value of Hpo (Equation 13~ is
derived using the measured value of azimut~, and is
therefore also only a first-order approximation.
Also~ equation 14 for dHp represents a lower limit on
external interference, since the geomagnetic field
and the external interference are vector quantities

~2~
-15-
which can combine in different orientations. In
certain cases, a finite external interference can
combine with the geomagnetic field to give the
measured value of Hp while still having a component
perpendicular to the borehole. Thus, the actual
perpendicular interference dHp must be equal to or
greater than Hp - Hpo.
One way of avoiding such uncertainties is to
compare the measured values of Hx and Hy individually
to those predicted from the nominal geomagnetic
field. In the embodiment which transmits the
individual sensor outputs ts the surface, the values
of Hx and ~1y are available directly~ When the
drillersi angles are transmitted, Hx and Hy are
calculated as follows:
(lS) Hx = H * (cos ~ (cos AZ cos INC sin GTF + sin
A~ cos GTF)
-sin ~ sin INC sin GTF )
(16) Hy = H * (cos ~ (cos AZ cos INC cos GTF - sin
AZ sin GTF)
-sin ~ sin INC cos GTF )
(171 Hxo = Ho * (cos ~o(cos AZ cos INC sin GTF + sin
AZ cos GTF)
-sin~ o sin INC sin GTF )
(l8) Hyo = Ho * (cos ~ o(cos AZ cos INC cos GTF - sin
AZ sin GTF)
-sin~ o sin INC cos GTF )
(l9) dHx = Hx - Hxo
(20) dHy = Hy - Hyo
(21) dHp = (dHx2 + dHy2)l/2
Equations 17 and 18 are predicted values based on
tabulated fields and measured angles. While the
method given above gives a determination, rather than

-16~
just a lower limit, for the perpendicular (and
therefore external) interference, the quantities
calculated are all quite sensitive to errors in the
values of azimuth and tool face reference.
Therefore, for typical values of the external
interference, the lower limit derived above may be
more accurate than this calculation.
4) If the above tests indicate that the
perpendicular component of the interfering field is
negligible, the effect of the axial interference upon
the measured azimuth, i.e., the azimuth error dAZ is
then determined.
To first order, the change in measured azimuth
can be related to the difference d ~ between the
measured dip angle ~ and the tabulated value ~ o.
(22)
dAZ = d ~ sin_INC sin AZ
cos ~o ( sin INC cos AZ sin ~ o - cos INC cos ~o )
Since, in equation 22, dAZ is taken to represent the
difference between the measured azimuth AZ and the
true azimuth AZo, the corrected azimuth AZ' is given
by:
(231 AZ' = AZ - dAZ.
Since the measured azimuth appears in the
equation for dAZ, this value will be slightly in
error. This error can be reduced by replacing AZ in
the equation by AZ'; the process can be repeated
until a consistent value for dAZ is generated. In
most cases, no iteration will be necessaryr since the
value of dAZ will be small.

-17-
5) If the axial magnetic interference results
from the remanent, rather than induced, magnetization
of components in the drillstring, the magnitude of
dHz may remain constant during drilling. This will
be true if there are not any violent shocks to the
drillstring, such as jarring or rotary drilling in
hard rock. ~.n examination of the equations reveals
that, even for constant dHz, the measured values of
H, Hz and ~ will vary with azimuth and inclination.
~here there is no apparent large discontinuity in
dHz, the values for a bit run may be averaged to
obtain a more accurate estimation of the interference.
Once such as estimation is made, it may be
possible to refine the calculation of the azimuth
error. The equation employed in the current
invention to determine the azimuth error is strongly
dependent upon the accuracy of the tabulated values
for the geometric field. In particular, an error of
a tenth of a degree in the nominal dip angle ~ o can
result in an error of several tenths of a degree in
the azimuth error (equation 22), at particular
inclinations and azimuths. Vsing the average value
of dHz for a given bit one can calculate the expected
value of d ~ .
(24)
d ~= dHz ~cos INC cos~ o -sin INC cos AZ sin ~o~l80
llo
~y comparing the calculated value of d~ for each
survey point ~ith the measured value, one may find a
small correction to ~ o which will give consistent
values for the entire bit run.

-18~
Following the steps set forth above/ the nature
of ~he interference is determined (i.e., whether it
is caused by the drillstring or by external
sources). If the source is the drillstring, the
azimuth error dAZ determined and the corrected
aæimuth AZ' is established. No correction is made if
the source of the interference is determined to be
external.
The correction determination process of the
present invention can be carried out manually or by
computer.
While preferred embodiments have been shown and
described, various modifications and substitutions
may be made thereto without departing from the spirit
and scope of the invention. Accordingly, it is to be
understood that the present invention has been
described by way of illustrations and not limitation.