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Claims  |
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What is claimed is:
1. A borehole drift-direction probe, comprising an elongate housing of
length and diameter selected for reliable assumption of housing-axis
orientation parallel to the locally applicable borehole-axis orientation,
a rotary spindle with magnetic flux-sensor means mounted theretofor
directional response normal to the spindle axis, means suspending said
spindle with universal action with respect to said housing, said
suspension means including gravitationally sensitive means for maintaining
a constant vertical orientation of the spindle axis, motor means connected
to said spindle for imparting continuous rotation thereto, whereby said
flux-sensor means will develop an electrical output signal which is
cyclically responsive to the horizontal component of the local direction
of the earth's magnetic-field lines, electric-coil means fixedly mounted
within said housing and positioned when excited to establish at said
flux-sensor means a magnetic field of substantially uniformly distributed
straight lines and parallel to the housing axis, means for exciting said
electric-coil means, whereby the electrical output of said magnetic-sensor
means is additionally characterized by an output signal which is
cyclically responsive to instantaneous housing-axis inclination with
respect to the vertical, and means for remotely transmitting said output
signals.
2. The borehole drift-direction probe of claim 1, in which said excitation
means comprises an a-c source of frequency greater than the rotational
frequency of said spindle.
3. The borehole drift-direction probe of claim 2, in which
synchronous-detector means is coupled to the output of said flux-sensor
means, whereby the output signal which is responsive to instantaneous
housing-axis inclination is segregated from the output signal which is
responsive to the horizontal component of the local direction of the
earth's magnetic field.
4. The borehole drift-direction probe of claim 2, in which the a-c source
frequency is in the order of 100 Hz and the spindle-rotation frequency is
very much less than 100 Hz.
5. The borehole drift-direction probe of claim 4, in which the
spindle-rotation frequency is in the order of 1 Hz.
6. The borehole drift-direction probe of claim 1, in which said motor means
is fixedly mounted in said housing with its output shaft facing downwardly
on the housing axis, and universal-coupling means connecting said spindle
to the output shaft of said motor means.
7. The borehold drift-direction probe of claim 6, in which said
spindle-suspension means includes vertically spaced spindle-engaging
bearings in which said spindle has a degree of axial-positioning motional
freedom, whereby said spindle may be vertically erect in its continuous
rotation regardless of any slight changes in the distance between said
spindle and said motor means as a function of inclination of said housing
axis.
8. The borehole drift-direction probe of claim 7, in which said coupling
means is a length of piano wire of such length and flexibility as to
present negligible resistance to the ability of said gravitationally
sensitive means to maintain the vertical orientation of the spindle axis.
9. The borehole drift-direction probe of claim 1, in which said suspension
means includes a pendulum mounted in said housing on a two-axis signal
system, and spindle-erecting bearing means carried by said pendulum.
10. The borehole drift-direction probe of claim 1, in which said coil means
comprises axially spaced turns on opposite axial sides of the axial
location of said flux-sensor means and of diameter at least as great as
the axial spacing of said turns.
11. The borehole drift-direction probe of claim 10, in which said turns
diameter substantially exceeds said axial spacing.
12. The borehole drift-direction probe of claim 1, in which said
flux-sensor means is a Hall-effect transducer.
13. The borehole drift-direction probe of claim 1, in which said
flux-sensor means includes a local carrier-frequency excitation source
therefor, the carrier frequency being substantially greater than the scan
rate imparted by said motor means.
14. The borehole drift-direction probe of claim 13, in which said
coil-excitation means is a source of alternating current at a frequency
intermediate said scan rate and said carrier frequency.
15. The borehole drift-direction probe of claim 14, in which said carrier
frequency is also substantially greater than that of said coil-excitation
means.
16. The borehole drift-direction probe of claim 1, in which said
signal-generator means includes coacting housing-mounted non-rotatable
component means and spindle-mounted rotatable component means for
identifying the instant at which a selected part of the flux-sensor scan
traverses the effective angular location of said non-rotatable component
means.
17. A borehole drift-direction probe, comprising an elongate housing of
length and diameter selected for reliable assumption of housing-axis
orientation parallel to the locally applicable borehole-axis orientation,
a rotary spindle with magnetic flux-sensor means mounted thereto for
directional response normal to the spindle axis, means suspending said
spindle with universal action with respect to said housing, said
suspension means including gravitationally sensitive means for maintaining
a constant vertical orientation of the spindle axis, motor means fixedly
mounted in said housing with its output shaft facing downwardly on the
housing axis, universal coupling means connecting said spindle to said
output shaft for imparting continuous rotation thereto, whereby said
flux-sensor means will develop an electrical output signal which is
cyclically responsive to the horizontal component of the local direction
of the earth's magnetic-field lines, electric-coil means fixedly mounted
within said housing and positioned when excited to establish at said
flux-sensor means a magnetic field of substantially uniformly distributed
straight lines and parallel to the housing axis, means for exciting said
electric-coil means, whereby the electrical output of said magnetic
flux-sensor means is additionally characterized by an output signal which
is cyclically responsive to instantaneous housing-axis inclination with
respect to the vertical, and means for remotely transmitting said output
signals.
18. The borehole drift-direction probe of claim 17, in which said
gravitationally sensitive means is a bottom-heavy float, and in which said
housing includes a liquid-filled chamber in which said float is neutrally
buoyant.
19. The borehole drift-direction probe of claim 18, in which said chamber
includes a central retaining cage in clearance relation with said float
for containing float location, generally to the axially central region of
said probe.
20. The borehole drift-direction probe of claim 18, in which said spindle
is fixed to said float whereby said float and sensor rotate in unison,
said sensor being contained within said float.
21. The borehole drift-direction probe of claim 18, in which said spindle
is rotatable in spaced vertically-orienting spindle bearings in said
float.
22. The borehole drift-direction probe of claim 1, and including
synchronous commutating means comprising commutating square-wave generator
means responsive to the output signal responsive to the horizontal
component of the earth's field to develop a first square wave in phase
with said earth's field signal and a second square wave in 90.degree.
phase-offset from said in-phase signal, first means synchronously
detecting with said in-phase square wave the output signal responsive to
housing tilt to derive the "North" component of the horizontal-plane
component of housing tilt, and second means synchronously detecting with
said 90.degree. phase-offset square wave the output signal responsive to
housing tilt to derive the "East" component of the horizontal component of
housing tilt.
23. The borehole drift-direction probe of claim 1, and including
cable-displacement responsive means responsive to cable pay-out to said
probe at depth in a borehole to be mapped, and display means connected to
said displacement-responsive means and to said "North" and "East"
component signals for correlating such component signals as a function of
pay-out depth.
24. The borehole drift-direction probe of claim 23, in which said display
means includes a chart recorder, with chart drive synchronized by said
cable-displacement-responsive means.
25. The borehole drift-direction probe of claim 22, and including display
means responsive to said "North" and "East" component signals for
vectorially summing the same to determine the resultant horizontal-plane
tilt-vector component and for displaying the same both as to magnitude and
azimuth.
26. The borehole drift-direction probe of claim 17, in which said
gravitationally sensitive means comprises and elongate flexible torsion
wire which additionally comprises said universal coupling means, said wire
being of such length and flexibility as to pendulously assume vertical
orientation of said spindle at its lower end, whereby sensor scan in a
horizintal plane is achieved without constraint imposed upon said spindle
or said wire.
27. The borehole drift-direction probe of claim 17, in which said means
suspending said spindle with universal action comprises a rigid frame
mounted for motor-driven rotation on the probe axis, a two-axis gimbal
system carried by and within said frame, said gravitationally-sensitive
means being suspended by said gimbal system, said spindle being vertically
oriented by said grativationaly sensitive means, whereby said frame and
gimbal system and sensor all rotate in unison in the course of
horizontal-plane scanning by said sensor.
28. The method of continuously tracking local axis inclination in a
borehole using a single magnetic flux sensor within an elongate probe
housing, wherein the housing length and diameter proportions have been
selected for reliable ssumption of housing-axis orientation parallel to
the locally applicable borehole axis orientation, which method comprises
orienting said flux sensor for directional response normal to an axis of
rotation, continuously rotating said flux sensor about said axis of
rotation, gravitationally maintaining a constant vertical orientation of
said axis of rotation, whereby the flux sensor will develop a first
electrical output-signal component which is cyclically responsive to the
horizontal component of the local direction of the earth's magnetic-field
lines, establishing within said housing and in the region of flux-sensor
rotation a magnetic field of substantially uniformly distributed straight
lines parallel to the axis of the probe housing, whereby the electrical
output of the flux sensor will be additionally characterized by a second
component signal which is cyclically responsive to instantaneous
housing-axis inclination with respect to the vertical, segregating said
components, monitoring the amplitude of the second component signal, and
monitoring the phase relationbetween said first and second component
signals.
29. A borehole drift-direction probe for use in a rotated-probe context, as
in conjunction with a rotated boring tool, comprising an elongate housing
of length and diameter selected for reliable assumption of housing-axis
orientation parallel to the locally applicable borehole-axis orientation,
a spindle with magnetic flux-sensor means mounted thereto for directional
response normal to the spindle axis, means non-rotationally suspending
said spindle with universal action with respect to said housing, said
suspension means including gravitationally sensitive means for maintaining
a constant vertical orientation of the spindle axis, whereby in the course
of probe rotation said flux-sensor means will develop an electrical output
signal which is cyclically responsive to the horizontal component of the
local direction of the earth's magnetic-field lines, electric-coil means
fixedly mounted within said housing and positioned when excited to
establish at said flux-sensor means a magnetic field of substatially
uniformly distributed straight lines and parallel to the housing axis,
means for exciting said electric-coil means, whereby the electrical output
of said magnetic flux-sensor means is additionally characterized by an
output signal which is cyclically responsive to instantaneous housing-axis
inclination with respect to the vertical, and means for remotely
transmitting said output signals.
30. A borehole drift-direction probe, comprising an elongate housing of
length and diameter selected for reliable assumption of housing-axis
orientation parallel to the locally applicable borehole-axis orientation,
a rotary element including magnetic flux-sensitive means having
directional response normal to the axis of rotation of said element, means
suspending said element with universal action with respect to said
housing, said suspension means including gravitationally sensitive means
for maintaining a constant vertical orientation of said axis of rotation,
motor means associated with said element for importing continuous rotation
thereto, whereby said flux-sensitive means will develop an electrical
output signal which is cyclically responsive to the horizontal component
of the local direction of the earth.ltoreq.s magnetic-field lines,
electric-coil means fixedly mounted within said housing and positioned
when excited to establish at said flux-sensitive means a magnetic field of
substantially uniformly distributed straight lines and parallel to the
housing axis, means for exciting said electric-coil means, whereby the
electrical output of said magnetic-sensitive means is additionally
characterized by an output signal which is cyclically responsive to
instantaneous housing-axis inclination with respect to the vertical, and
means for remotely transmitting said output signals.
31. A borehole drift-direction probe, comprising an elongate housing of
length and diameter selected for reliable assumption of housing-axis
orentation parallel to the locally applicable borehole-axis orientation, a
rotatable element including magnetic flux-sensitive means having
directional response normal to the axis of rotation of said element, means
suspending said element with universal action with respect to said
housing, said suspension means including gravitationally sensitive means
for maintaining a constant vertical orientation of said axis of rotation,
means associated with said element for imparting rotation thereto, whereby
said flux-sensitive means will develop an electrical output signal which
is responsive to the horizontal component of the local direction of the
earth's magnetic-field lines, electric-coil means fixedly mounted within
said housing and positioned when excited to establish at said
flux-sensitive means a magnetic field of substantially uniformly
distributed straight lines and parallel to the housing axis, means for
exciting said electric-coil means, whereby the electrical output of said
magnetic-sensitive means is additionally characterized by an output signal
which is responsive to instantaneous housing-axis inclination with respect
to the vertical, and means for remotely transmitting said output signals.
32. The borehole drift-direction probe of claim 31, in which the imparted
rotation is oscillatory.
33. The method of determining local axis inclination in a borehole using a
single magnetic flux-sensitive device within an elongate probe housing,
wherein the housing length and diameter proportions have been selected for
reliable assumption of housing-axis orientation parallel to the locally
applicable borehole axis orientation, which method comprises orienting
said flux-sensitive device for directional response normal to an axis of
rotation, rotating said flux sensor about said axis of rotation,
gravitationally maintaining a constant vertical orientation of said axis
of rotation, whereby the flux sensor will develop a first electrical
output-signal component which is responsive to the horizontal component of
the local direction of the earth's magnetic-field lines, establishing
within said housing and in the region of flux-sensor rotation a magnetic
field of substantially uniformly distributed straight lines parallel to
the axis of the probe housing, whereby the electrical output of the flux
sensor will be additionally characterized by a second component signal
which is responsive to instantaneous housing-axis inclination with respect
to the vertical, segregating said components, monitoring the amplitude of
the second component signal, and monitoring the phase relation between
said first and second component signals.
34. The method of claim 33, in which said rotation oscillates as to the
direction of rotation.
35. The method of continuously tracking local axis inclination in a
borehole using a single magnetic flux sensor within an elongate probe
housing, wherein the housing length and diameter proportions have been
selected for reliable assumption of housing-axis orientation parallel to
the locally applicable borehole axis orientation, which method comprises
orienting said flux sensor for directional response normal to an axis of
rotation, continuously rotating said flux sensor about said axis or
rotation, gravitationally maintaining a constat vertical orientation of
said axis of rotation, whereby the flux sensor will develop a first
electrical output-signal component which is cyclically responsive to the
horizontal component of the local direction of the earth's magnetic-field
lines, establishing within said housing and in the region of flux-sensor
rotation a magnetic field of substantially umiformly distributed straight
lines parallel to the axis of the probe housing, whereby the electrical
output of the flux sensor will be additionally characterized by a second
output-signal component which is cyclically responsive to instantaneous
housing-axis inclination with respect to the vertical, segregating said
components, generating from said first output-signal component a first or
in-phase switching signal having in-phase relation to said first
output-signal component and a second or quadrature-phase switching signal
having quadrature-phase relation to said first output-signal component,
using said in-phase switching signal for synchronously detecting said
second output-signal component to derive the "North" component of
instantaneous probe-axis tilt, and using said quadrature-phase switching
signal for synchronously detecting the "East" component of instantaneous
probe-axis tilt.
36. The method of claim 35, in which said probe-housing is paid out via
cable from a monitoring station, and separately recording via said
monitoring station the respective magnitudes of said detected "North" and
"East" components, said magnitudes being concurrently recorded as a
function of the instantaneous length of paid-out cable. |
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Claims |
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Description |
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This invention relates to a method and means for remotely determining the
instantaneous orientation of a body relative to a coordinate system based
on the earth's magnetic and gravitational fields. The invention will be
described in the context of an elongate probe configurated to orient its
axis of exterior symmetry in locally tangent relation to a borehold
drilled into the earth, as for mineral-ore prospecting, oil-well drilling
and the like, and the invention is applicable anywhere except at the
magnetic poles, or where the earth's magnetic field is erratic or
unmapped.
A borehole of the character indicated cannot be relied upon to be straight
and vertical, in that the drilling of the hole is subject to laterally
deflecting obstacles, forces and other asymmetries which can account for
substantial offsets from the point of earth entry, the deeper the
borehole; typically, such drift in a mineral-drilling operation can be as
much as 340 feet per 1000 feet of borehole. A problem is thus presented of
determining a three-dimensional map of the borehole. To this end, a
suitable probe is lowered into the borehole. For mineral-ore prospecting,
the probe is typically in the shape of an elongate cylinder, of 21/4-in.
diameter and 6-ft. length, so that the axis of the cylinder will
essentially lie parallel to the tangent of the borehole. A device located
in the probe to determine the inclination and azimuth of the probe then
necessarily determines the orientation of the tangent to the borehole as a
function of the amount of cable paid out, and this is sufficient
information from which to make a three-dimensional map of the borehole.
A straightforward approach to determining probe azimuth and inclination
would be to use multiple sensors to measure the components of the earth's
magnetic field along three orthogonal axes fixed in the probe, and
inclination relative to two of these axes. Suitable manipulation of
resulting data would then yield azimuth and inclination. A reduction in
the number of independent measurements needed to achieve this result can
be achieved by gimbal-suspension of the magnetic-field sensor to eliminate
sensitivity to the earth's vertical field, and by mounting the
inclinometer on a servo-driven platform which is kept level on one axis.
Even so, multiple sensors are required, and inaccuracies can be introduced
as a result.
A well-known technique which does not have the above-noted disadvantage
employs a small sphere engraved with longitude and latitude lines. The
sphere is bottom-heavy, contains a magnet, and is suspended in a liquid,
with rotational freedom to assume a position of equilibrium (a) in
response to the earth's magnetic-field influence upon the magnet and (b)
in response to gravitational influence upon the bottom-heavy property. The
sphere thus tends to assume a fixed relation to the earth, and by
photographing the sphere as viewed by a camera fixed to the probe, the
orientation of the probe can be deduced. Among the limitations of this
technique are time lag to achieve equilibrium for each photograph, and the
need to remove the probe from the borehole for film development and
evaluation, in that it is not convenient to convert the data to a form
that can be telemetered to the surface; specifically, the technique has
been incapable of providing at the surface a continuous and current record
of probe orientation as the probe is moved down the hole.
It is accordingly an object of the invention to provide an improved method
and means for providing probe orientation data, and not subject to
deficiencies noted above.
A specific object is to provide a method and means for continuously
generating and remotely transmitting data of the character indicated, with
superior accuracy and reliability, and currently, in the course of probe
movement along a borehole.
Another specific object is to achieve the foregoing objects without using
more than a single magnetic-field sensor.
It is also a specific object to provide in a probe of the character
indicated means whereby a single magnetic-field sensor may continuously
generate tilt-azimuth and inclination data, referenced to the azimuth
component of the earth's magnetic field.
A further specific object is to provide a device of the character indicated
which will continuously determine strength of the horizontal component of
the earth's magnetic field, as a means of continuously verifying validity
of derived orientation data.
A still further object is to materially reduce uncertainty in borehole
mapping, to a level in the order of .+-.2.5 feet per 1000 feet of borehole
depth.
It is a general object to achieve the above objects with rugged and
basically simple structure which is reliably operable in an environment of
mechanical shock, as when employed in conjunction with a drilling
operation.
Other objects and various further features of novelty and invention will be
pointed out or will occur to those skilled in the art from a reading of
the following specification, in conjunction with the accompanying
drawings. In said drawings which show, for illustrative purposes only,
preferred embodiments of the invention:
FIG. 1 is a simplified diagram in elevation to show the relation between
surface and probe components of the invention as used in the mapping of a
borehole;
FIG. 2 is an enlarged and somewhat schematic diagram of structural
components of FIG. 1;
FIG. 3 is a simplified view in perspective schematically showing
orientation and suspension system for the single magnetic sensor of the
probe;
FIG. 4 is an enlarged fragmentary longitudinal sectional view of
orientation structure for the single magnetic-field sensor of the probe;
FIG. 5 is a fragmentary plan view of the orientation structure of FIG. 3,
as viewed generally from the elevation 5--5 of FIG. 4;
FIGS. 6a and 6b are two sheets of an electrical block diagram to show
circuit components within the probe and at the remote readout,
respectively;
FIG. 7 is a series of graphs to depict, to the same time scale, electrical
signals within readout circuitry of FIG. 6;
FIGS. 8, 9, 10 and 11 are diagrams similar to FIG. 3 to illustrate
modifications.
Referring initially to FIGS. 1 and 2, the invention is shown in application
to an elongate cylindrical probe 10 suspended via an armored
multiple-conductor electrical cable 11, within a borehole 12 of
undetermined drift beneath a surface-drilling site 13. In mineral
prospecting, such drift is customarily well within 20 degrees of the
vertical, as suggested by legend in FIG. 1. The cable 11 is paid out from
a winch 14 and over a footage wheel 15, serving with readout and recording
equipment contained in a logging van 16. The winch means 14 is equipped
with slip-ring pick-off means 17 for conducting separate signals from
conductors of cable 11 to readout circuitry 18, which in turn provides
outputs as desired for mapping purposes. As shown, segregated "North" and
"East" component signals are independently supplied by such outputs to
separate pen-displacement inputs of a dual-chart moving-strip recorder 19,
having a synchronizing chart-drive connection 20 to the footage wheel 15.
Power for probe operation is supplied from van 16 via cable 11, and
various signal-processing circuits at the probe yield two telemetered
output signals via the cable; these signals are of the same frequency and
are graphically suggested alongside the slip-ring connection to readout
device 18 in FIG. 2, being a first output signal A which is cyclically
responsive to instantaneous probe-axis inclination with respect to the
vertical, and a second output signal B which is cyclically responsive to
the horizontal component of the earth's magnetic field. The legend in FIG.
2 additionally indicates an angle .psi. of phase offset between signals A
and B, and it will later be explained how these signals and the angle
.psi. are utilized to develop the respective North and East components
needed for map recording at 19.
Briefly stated, the invention contemplates use of a single directionally
responsive magnetic-field or flux sensor which is caused to cyclically
scan in a horizontal plane to develop response to the horizontal component
of the earth's magnetic field and which at the same time also scans for
the horizontal component of a locally imposed magnetic field in which the
sensor is immersed and which is characterized by magnetic lines that are
parallel to the probe axis. Since the latter field is locally imposed it
may be on-off or otherwise modulated to provide a basis of segregation
from the earth's field component of sensor output. And in the preferred
form to be described, such segregating, field-generating and other
synchronizing functions and signals are locally generated and processed
within the probe itself, enablingthe indicated A and B signals (or signals
from which the A and B signals can be readily further discriminated) to be
telemetered to the readout device 18. The simplified diagram of FIG. 3 is
useful as an introduction to a description of suspension and orientation
structure to enable such operation of a single flux sensor within the
probe 10. Throughout the description, it will be understood that except
for the flux sensor, all housing and other parts near the flux sensor are
non-magnetic, meaning that they are inherently non-magnetic or that they
have been effectively demagnetized; for example, a stainless-steel probe
housing or case is to be preferred for its strength and durability, and it
is suitably demagnetized prior to probe assembly.
The single magnetic-field sensor used in my invention may be of the
Hall-effect type, the flux-gate type, the rotating-coil type, or an
eddy-current type as disclosed in U.S. Pat. No. 4,013,946; however, it is
my preference to employ the Hall-effect type, due to its directional
sensitivity, small size and weight. The single flux sensor 22 in FIG. 3 is
thus preferably a Hall-effect transducer, mounted to a supporting spindle
23 to that its directional response is normal to the spindle axis, as
suggested by an arrow labeled n. Spindle 23 is suspended by a universal
coupling 24 from the output shaft of motor means 25, the latter being
fixedly mounted to the probe case, as suggested at 26; a length of piano
wire serves admirably as the universal coupling 24, and the necessary
leads to and from transducer 22 may be of fine enameled wire loosely
wrapped about the piano wire to slip-ring means 27 at the motor output
shaft. The motor means 25 may include reduction gearing to provide a
continuous low frequency torsion drive of spindle 23, causing the
directional axis n of the flux sensor to cyclically scan about the spindle
axis, which must be maintained truly vertical for accomplishement of the
desired horizontal-plane scan of the magnetic-response axis n. In the form
shown, a pendulous mass 28 is part of a frame for two vertically spaced
bearings 29 in which spindle 23 is rotatable and with respect to which
spindle 23 has a limited range of vertical displaceability, and the frame
for pendulous mass 28 is suspended from the case by a gimbal system on two
crossed axes 30-31. The system thus far described will be seen to
cyclically cause the response axis n of sensor 22 to scan a horizontal
plane about the pendulum-erected vertical spindle 22, producing a
low-frequency output signal which is maximum when traversing magnetic
North, which is minimum when traversing magnetic South, and which is an
a-c coupled situation changes polarity in opposite directions upon
traversing East and West, respectively. The above-indicated signal B is
thus uniquely defined, and this is true regardless of probe tilt within
the limits of pendulous departure from the probe axis, because scanning is
always in a horizontal plane.
Referring now to FIGS. 4 and 5 for greater detail, the pendulous mechanism
for spindle orientation in the vertical is seen to comprise an annular
gimbal mount 33 of circular outer contour to fit and be secured in a
tubular cylindrical chassis 34, which is later fixedly assembled in the
probe housing or pressure case 35 already mentioned. At each of two
diametrically opposed locating pockets 36 (FIG. 5) an agate bearing block
37 is retained with limited freedom to permit knife-edge suspension of an
outer gimbal 38 on the gimbal axis 30. For each such bearing, a
knife-insert element 39 is clamped to outer gimbal 38 and projects
outwardly for knife-edge engagement at 30' in a V-groove of the associated
agate block 37. Within each pocket 36, a bottom rocker 40 enables
uninhibited full knife-edge bearing engagement, and side-rocker elements
41 provide lateral clearance to further assure such uninhibited bearing
engagement. In similar fashion, an inner gimbal 42 mounts knife-insert
elements 43 to establish the gimbal axis 31 at knife-edge bearing
engagement 31' with a V-groove of an associated one of two diametrically
opposed agate blocks 44 carried by the outer gimbal 38. The inner gimbal
42 is generally U-shaped, with knife inserts 43 clamped to spaced arms 45
which are interconnected below axis 31 by a base 46 which includes a
central downward stud 47 for clamped retention of a pendulum bob 48. A
central bore 59 through base 46 and stud 47 communicates with a deep bore
50 in the bob 48, and the spindle bearings 29 are provided as spaced
annular jewel elements mounted to the upper and lower ends of bore 49. The
spindle 23 is seen to be vertically stabilized by and rotatable in
bearings 29, with ample freedom within bore 50 for such longitudinal
displacement as may be occasioned by pendulum accommodation to a tilted
probe. As shown, the bob 48 is conical and has freedom for accommodation
to probe tilt within the .+-. 20 degree range indicated in FIG. 1. Also,
the bob 48 is shown with an upwardly open annular groove 51, providing
space for affixing such eccentrically mounted balancing weights as may be
necessary to assure a desired precision of vertical-axis orientation for
spindle 23.
The described two-axis gimbal suspension will be seen to place the flux
sensor 22 at the intersection of gimbal axes 30-31. In FIG. 4, this
location is identified at 52, and it is to be understood that the mark 52
also identifies the axis of directional sensitivity of sensor 22, thus
assuring that horizontal-plane scanning by sensor 22 will at all times be
essentially about the center of the chasis 34, i.e., regardless of the
degree of tilt which may be applicable in the course of continuous
horizontal plane scanning.
To locally establish a magnetic field of straight lines parallel to the
probe axis, and with the flux sensor 22 consistently immersed therein,
like axially spaced upper and lower coil windings 53-54 are developed and
suitably potted as annuli fixedly assembled to the gimbal mount 33. These
windings are electrically connected in flux-aiding relation and are
excited by a carrier source local to the probe, as will be later
explained. The effective diameter of windings 53-54 is at least as great
as their axial separation, being preferably twice as great, as shown.
Also, windings 53-54 are spaced equally above and below the axial location
52 of the directional axis n of the flux sensor 22. It will be seen that
on a consideration of sensor rotation in the sole presence of the
parallel-line field established by windings 53-54, the sensor-output
signal will be cyclically varying with a magnitude which directly reflects
the current degree of probe tilt from the vertical, and that the phase
offset of such signal with respect, say, to North, will be a direct
measure of the azimuth component of the tilt. Thus, the sensor-output
signal attributable solely to horizontal-plane scanning of the local field
established upon excitation of windings 53-54, will be that which has
already been identified as signal A (FIG. 2), it being understood that, in
the presence of any periodic modulation of such field excitation (e.g., at
a rate materially in excess of the scan rate), signal A is represented by
the envelope of the modulation.
The left half of FIG. 6, namely up to and including a cable connector 55,
depicts electrical circuitry of the probe 10; to the right of a
corresponding connector 56, FIG. 6 depicts circuitry of the readout device
18, it being understood that various of the conductors of cable 11 provide
intervening connections. Three of the terminals of connector 55 are
committed to reception of externally applied a-c power, with a central
ground, the same being applied to suitable means 57 for providing
regulated power supply voltage at output terminals 58-59 for various
operating uses at the probe. One of these uses involves drive circuitry 60
for powering the motor 25, which it will be recalled operates through
reduction gearing to impart rotation in the order of 1 Hz to the flux
sensor 22, via its supporting spindle 23. In FIG. 6, the phantom outline
24' indicates the motor-driven rotating assembly of wire 24, spindle 23,
sensor 22, and thin-lead connectionsto the slip-ring system 27 of FIG. 3.
Specifically, a first stable current source 61, operating from the power
supply 57, provides carrier current via supply slip rings 27', to the
rotating transducer assembly, an impedance-matching transformer 62 being
employed in the supply-lead connection 63 to the flux sensor 22; legend
suggests a carrier frequency of 5 kHz. In similar fashion, the
sensor-output connection 64 is via an impedance-matching transformer 65
and preamplifier means 66 to output slip rings 27", for further processing
in non-rotating circuitry; a "coarse-offset" potentiometer adjustment at
67 is additionally shown on the rotating structure for balanced treatment
of input vs. output lead connections to the sensor 22. With the components
thus far described in FIG. 6, the output in line 68 to an amplifier 69
will be seen to be a sinusoidal envelope at 1 Hz on the 5 kHz carrier from
source 61, the envelope being the product of cyclical scan of the
horizontal component of the earth's magnetic field.
To additionally scan for tilt, a second stable current source 70, also
operating from the power supply 57, provides sinusoidal carrier current at
100 Hz to the windings 53-54 for establishing the axially oriented local
field in which sensor 22 scans in a horizontal plane. Thus, the output in
line 68 to amplifier 69 will be seen to additionally reflect mixing with
the 100 Hz carrier and its 1 Hz envelope representing the tilt scan.
To demodulate and segregate the signal A and signal B envelopes from the
output mix at amplifier 69, I find it convenient and effective to employ
synchronous-detection techniques wherein square-wave outputs of the
generators 61-70 provide the requisite synchronization control. Thus, the
5 Khz source is shown with square-wave outputs, labelled 5 kI and 5 kQ,
being square waves at 5 kHz; the 5 kI output is In phase with the
sinusoidal output to slip rings 27' for exciting the sensor 22, and the 5
kQ output is in Quadrature relation thereto. Similarly, for 100-Hz
detection purposes, the 100-Hz source 70 provides an In-phase square-wave
output 100 I. The In-phase square-wave outputs are employed to process
signals for telemetering in the separate cable lines, designated V.sub.B
and V.sub.A (i.e., essentially the B and A signals previously referred
to), respectively at the outputs of line drivers 71-72, and destined
ultimately to provide the "North" and "East" recording signals after
processing at the readout device 18; additionally, the In-phase and
Quadrature-phase square waves 5 kI and 5 kQ are used to develop a
continuous corrective signal for automatic suppression of such
zero-frequency envelope offset as may develop in the sensor output, in
spite of the measures taken at 67 to effect at least a coarse correction
of the offset.
For the foregoing purposes, the circuit of FIG. 6 operates upon the output
of amplifier 69 through three parallel circuit lines 73-74-75, the first
two of which serve a circuit 76 for automatic-offset suppression, and the
latter two of which serve the respective line drivers 71-72. In line 73, a
mixer 76 is supplied with the quadrature square wave 5 kQ, and the product
is subjected to low-pass filtering (e.g., 0 to 1 kHz) at 77 to develop an
earth's field horizontal-scan quadrature signal B.sub.Q for integration at
78 to become a first d-c signal component for offset suppression;
similarly, in line 74, a mixer 79 is supplied with the in-phase square
wave 5 kI, and the product is subjected to low-pass filtering at 80 to
develop an earth's field horizontal-scan in-phase signal B.sub.I for
integration at 81 to become a second d-c signal component for offset
suppression. The respective d-c signals are then multiplied (with the 5 kI
and 5kQ square waves) at mixers 82-83, before summing at 84 and filtering
at 85 to pass essentially only an offset-correcting 5 k Hz signal for
corrective summation at 86 with the output signal at 68, produced by
sensor rotation.
For telemetering purposes, the development of the earth's field scan signal
V.sub.B has already been explained. And the tilt-scan signal V.sub.A will
be understood to be similarly produced in line 75, by means of a first
mixer 79' supplied in parallel with mixer 79 with the same In-phase square
wave 5 kI, but additionally subject to second mixing at 87 with the
In-phase square wave 100 I below low-pass filtering at 88 to create an
A.sub.I signal output to the line driver 72.
It may be helpful at this point to provide a generalized catalog of various
frequency components which will be found in the respective telemetered
signal outputs V.sub.B and V.sub.A from the described probe circuitry, as
follows:
Signal V.sub.B :
At zero frequency
very small, due to offset-suppression measures, at 67 and at 76.
At 1 Hz
a strong signal, representing the described horizontal-plane scan for the
horizontal component of the earth's magnetic field, i.e., North.
At 99 Hz
At 100 Hz
At 101 Hz
strong sideband signals (99 Hz and 101 Hz) about a weak 100 Hz center
frequency, representing the described horizontal-plane scan for the
horizontal component of instantaneous probe-axis inclination, i.e., tilt
azimuth.
At 5 and at 10 k Hz
a low-level ripple, most of which has already been removed by the filter
80.
Signal V.sub.A :
At 1 Hz
a strong signal, representing the horizontal-plane scan for tilt azimuth.
At 99 Hz
At 100 Hz
At 101 Hz
strong sideband signals (e.g., 99 Hz and 101 Hz) characterized by sharp
edges, due to the square-wave mixing at 100 Hz and representing the scan
of the earth's field; the center frequency (100 Hz) is de minimis.
The probe-output signals V.sub.A and V.sub.B are telemetered to the surface
as electrical currents, where they are received by buffers 89-90 and
converted back to voltages. The signals then go through filters 91-92 to
remove some of the noise picked up in transmission over the cable, as well
as such ripple as may remain from the modulation-demodulation processes
already described. Typically, these are notch filters tuned to reject the
modulation frequency (100 Hz); thus, output of filter 91 is essentially
the A signal (1 Hz scan for tilt azimuth) and output of filter 92 is
essentially the B signal (1 Hz scan for the horizontal component of the
North-referenced earth's field). The North direction, available from the
B-signal output of filter 92 provides a convenient zero-azimuth reference
against which to develop phase-displacement data pertaining to the
instantaneous tilt azimuth present in the A-signal output of filter 91.
And in the preferred form to be described, I choose to develop such
phase-displacement data in the form of two quadrature components of the
tilt-azimuth vector; these tilt-azimuth components are developed as
"North" and "East" components, being d-c signals in output line 93 to the
"East"--recording stylus of recorder 19 and in output line 94 to the
"North"--recording stylus of recorder 19; a negative polarity for either
of these signals will be indicative of "South" and "West" components, as
the case may be. A phase-locked loop which defines the quadrature
component of the V.sub.B signal is used to generate the basic square waves
by which synchronous detection accurately segregates the indicated "North"
and "East" signals supplied to the chart recorder 19, all as will be more
fully explained.
First, in discussing the phase-locked loop for basic synchronizing
purposes, additional reference will be made to the serie | | |
