Note: Descriptions are shown in the official language in which they were submitted.
- 1 -
T 8270
NUCLEAR MAGNETISM LOGGING TOOL USING HIG~-TEMPERATURE
SUPERCONDUCTING SQUID DETECTORS
This invention relates to apparatus for use in a borehole, and
more particularly, relates to nuclear magnetism logging apparatus
for use in an earthen borehole.
The most accurate open borehole logging device for measuring
5 residual oil saturation and permeability in earth formations is the
Nuclear Magnetism Log (NMI.). This logging tool uses the magnetic
field from a solenoidal coil to polarize the protons in any fluids
contained within the earth formation adjacent the tool. The
solenoidal coil is then turned off and any mobile protons in-
fluenced by the magnetic field of the solenoidal coil precess at
their Larmor frequency about the earth's magnetic field. This
precession may be measured as a damped sinusoidal voltage induced
in a separate detection coil in the logging tool. The induced
voltage decays rapidly, typically of the order of 20-50 milli-
seconds, because of extremely short spin-spin or transverse
relaxation times, T2~, of these fluids.
The initial amplitude of the nuclear magnetism signal is
proportional to the "free-fluid" or producible fluids in the
formation adjacent the tool. If Mn-EDTA is added to the drilling
fluid and allowed to invade the formation, then the only residual
nuclear magnetism signal will be from any oil phase in the
formation ad~acent the tool. Thus, a NML log-inject-log procedure
results in highly accurate measurements of residual oil saturation
in the formation.
The spin-lattice or longitudinal relaxation time Tl of any
fluid in an earth formation is related to the pore size(s) (and
their distribution) containing the fluid; the pore sizes and
distribution may then be related to the capillary pressure curve
~3~
- 2
which may then be related to the permeability of the Eormation in a
well known way.
The NML attempts to measure Tl by repeated polarization cycles
with successively longer polarization times. Since Tl is very much
longer than the time T2* of the damped sinusoidal decay, Tl cannot
be determined directly from the damped sinusoidal decay curve with
only one measurement.
Although a highly useful logging tool, one of the main dis-
advantages of the NML is its poor signal-to-noise ratio, which
limits its accuracy to about 1 unit of porosity during continuous
logging operation. Although this is adequate for measuring a
free-fluid index, residual oil saturations are typically 1/3 or
less of the total porosity, so that stationary NML operation is
required in order to measure the residual oil saturation to
sufficient accuracy for enhanced oil recovery requirements.
Typically, about 15 minutes of data is collected at a borehole
location and then averaged to obtain residual oil saturation to
better than 1 percent of saturation at that location. However,
maintaining the tool stationary for this length of time slows data
acquisition for the entire formation and increases the risk of
sticking the tool in the borehole.
Another disadvantage of the NML is the shallow depth of in-
vestigation into the formation. Again, due to poor signal-to-noise
ratio, only signals from the initial few centimeters of the bore-
hole wall can be detected.
Still another disadvantage is that repeated polarization
cycles are required to obtain a discrete number of measurements of
the Tl decay curve. This requires additional time, and since only
a few measurements are obtained, the shape of the Tl decay curve so
obtained is not very precise. This Tl decay curve then contains
imprecise information on pore sizes and their distribution (and
accordingly any capillary pressure or permeability determined
therefrom is imprecise) in the formation, which is a severe
disadvantage of the present NML tool.
~3~9~
- 3 -
In the prior art the use of a Superconducting ~uantum
Interference Device (SQUID) as a detector in a modified nuclear
magnetism tool has been proposed, with the modification being the
use oE two opposed superconducting magnets instead of the
conventional polarizing solenoid to provide an increased depth of
investigation into the borehole wall. ~lowever, the use of a SQUID
as a detector in a downhole device has not occurred because of the
cryogenic and safety considerations of using liquid helium in the
borehole environment. More specifically, liquid helium expands
over 600 times on vaporizing and cannot be safely vented into the
borehole. This problem, as well as that of providing an adequate
amount of operating time downhole, in the high-temperature
environment of a borehole, has so far prevented -the use of a SQUID
as a detector in a nuclear magnetism tool.
These and other limitations and disadvantages of the prior art
are overcome by the present invention, however, and apparatus are
provided for nuclear magnetism logging with a detector capable of
detecting sinusoidal and slowly varying magnetic fields in an
earthen borehole.
The present invention provides a logging device for measuring
the nuclear magnetism response of earth formations using one or
more detectors capable of essentially simultaneously detecting
sinusoidal and slowly varying magnetic fields resulting from
precession of mobile nuclei about the earth's magnetic field.
~ The apparatus according to the invention thereto comprises
- means for providing a magnetic field that is capable of being
energized or de-energized in response to preselected control
signals,
- detector means for detecting sinusoidal and slowly varying
changes in magnetic field when said means for providing a
magnetic field is de-energized and outputting signals indica-
tive thereof, and
- an electronics package for generating said preselected control
signals and for receiving signals from said detector means.
~33'~
Preferably, the detectors in the apparatus according to the
invention are high-temperature Superconducting Quantum Interference
Device(s) (SQVID). Alternatively, one or more laser-pumped helium
magnetometers may be used AS detectors. The presently preferred
apparatus of the present invention uses a micro-miniature
Joule-Thomson refrigerator, or alternatively, thermoelectri-
cally-cooled Peltier modules, integrally mounted on the same wafer
as the SQUID(s) to maintain any high-temperature SQUID(s) below its
superconducting transition temperature Further, the SQUID(s) may
be flux coupled in an axial gradiometer configuration to reduce any
motion-induced magnetic field noise.
The preferred apparatus of the present invention provides a
means for using a SQUID sensor in a nuclear magnetism logging tool
without the need for any cryogenic liquid cooling. The method
consists of fabricating one or more SQUIDs from a high-temperature
superconducting material, such as for example, but not limlted to
the rare earth-barium- copper oxide materials, such as Yttrium
Barium Copper Oxide (YBa2Cu307), which is superconducting at
temperatures exceeding 90 K. The SQUID(s) is epitaxially deposited
on a high thermal conductivity substrate such as SrTiO3 or MgO.
The wafer substrate in turn is bonded to the cold stage of a
micro-miniature Joule-Thomson refrigerator (or a Peltier module for
higher temperature superconducting materials) which can cool down
to about 80 K. A vacuum casing of non-metallic construction such
as G-lO fiberglass surrounding the SQUID(s) and wafer provides
thermal insulation to reduce the heat load and facilitate keeping
the refrigerator cold stage at temperatures below the super-
conducting transition temperature of the SQUID(s) material.
A SQUID detector and a laser-pumped helium magnetometer
respond to all frequencies down to DC, while a resonant coil
detector as used in the current state-of-the-art NML responds only
to a narrow frequency range and has ~ero response at DC. Thus, a
SQUID detector and a laser-pumped helium magnetometer may
essentially simultaneously and directly measure both a T2* and T
decay in a single NML tool-with a single measurement.
~3~
63293-3015
In addlkion ~o the S~UID element~, the SQUID electronics
may also be thermally anchored to the refrigerator cold ætage.
This provldes an additional pexformance improvement by reduced
noise.
One or more SQUID detectors may be flux coupled to a
superconducting axial gradiometer detection coil. This coil may
also be constructed of a hlgh-temperature superconductor material
and may be thermally anchored to the refrigerator cold stage. The
function of the detection loop is to incraase the flux sen~itlvity
of the detector. The axial gradiometer configuration is preferred
I to reduce flux noise from any motion of the logging tool in the
earth's magnetic field while in the borehole. For this
configuration, ona loop of the gradiometer is cen~ered in~ide the
polarizing coil while the second counterwound loop extends outside
the polarizlng coll. Thus, the inside loop detects the full
signal from any polarized protons in the formation, whereas the
outside loop detects sub~tantially no proton signal. A uniform
field such as the Earth's magnetic field createæ an equal and
opposite flux through each loop so that the Ear~h's magnetic field
i8 effectively cancelled. Similarly, the laser-pumped helium
magnetometers may be used in dif$erential pairs, with one inside
the polarizing coil and the other fixed outside the polarizing
coil.
Thus, the presently preferred apparatus of the present
invention provides a SQUID detection module for use in a downhole
logging tool that obviates the need for liquid cryogens, provides
improved signal-to-noise for nuclear magnetism logging, and makes
possible the direct measurement of the T1 decay curve of the earth
formation.
According to another a~pect, the prevent invention
provides a method for nuclear magnetism logging, comprising~
- generating a magnetic fleld in a borehole,
- removing said magnetic field, and then
- essentially simultaneously detecting the sinusoidal and slowly
varying magnetic fields resulting from precession of mobile nuclei
abouk the earth's magnetic field.
,, ~
63293-3015
It is therefore an object of this lnventlon to provide a
nuclear magnetism logging device for obtainin~ signal-to-noiæe
ratios several orders of mayni~ude above ~hat o~ the presen~ N~L
sta~e of the art.
It is another object of thls inven~lon to provide an NML
device khat may measure the entire Tl decay curve of an earth
formation directly, wi~hout requiriny successive polariza~ion
cycles.
- ' a Sa
,~
~3~
- 6
These and other advantages and objects of the present
invention will become apparent from the following detailed
description, wherein reference is made to the figures in the
accompanying drawings, in which:
- Figure 1 shows a simplified block diagram of a nuclear magnetism
logging tool, the magnetic field vectors associated with
this nuclear magnetism logging tool and their relationship
to the magnetic field of the Earth.
- Figure 2 shows a typical NML signal detected using a resonant
coil tuned to the Larmor frequency.
- Figure 3 shows a typical NML signal detected using a SQUID as a
detector instead of the resonant coil.
- Figures 4A and B show a SQUID detector and micro-miniature
Joule-Thomson refrigerator integrally constructed on the
same wafer substrate, and the details of the SQUID
detector, respectively.
- Figure 5 shows the axial gradiometer configùration of super-
conducting coils flux-coupled to a SQUID that may be employed in
the present invention.
- Figure 6 shows typical measuring electronics for a SQUID
detector.
Referring now to Figure 1, there may be seen a borehole 10
through an earth formation 20 and a nuclear magnetism logging tool
11 disposed Ln said borehole by cable 9. The logging tool 11
contains solenoidal coil 12 which generates a polarizing magnetic
field vector 13 in the formation ad~acent the tool. The earth's
magnetic field, He, 14 is shown at an angle ~ to the magnetization
vector, Mo~ 15 representing the polarized proton spins. After the
polarizing field is turned off, the proton spins 15 precess around
the Earth's field 14 as shown. If the detector is a resonant coil
17 that is tuned to the Larmor frequency, it detects t'he precessing
proton spins M . An instrumentation package 19 provides about 1 Kw
of power to polarizing coil 12 and detects the voltage induced in
resonant coil 17 by the precessing spins 15.
~3~
Figure 2 shows a typical signal 16 expected to be detected in
resonant coil 17 by the precessing proton spins 15. The signal
decays sinusoidally at the l.armor frequency with a time constant
T2* which is much shorter than Tl. T2* represents the time
constant for the decay of the envelope of the sinusoidal signal.
The Larmor frequency of precession is ~ x He, where ~ is 4.26 kHz/g
for the proton. Note the Tl decay is not observed because the Tl
decay is of the order of one second, which is far from the 2000 Hz
Larmor frequency at which the resonant coil is tuned. Figure 2
also shows the measurement delay discussed later herein and
indicates (by dotted line) the extension of the signal envelope
back to time t indicative of ~F~ the porosity occupied by free
fluids in the formation adjacent the tool.
SQUID detectors have been found to have advantages over
conventional resonant coils for detecting NMR signals in the
laboratory. SQUIDs detect the magnetic field (more precisely,
total magnetic flux) linking the detection loop, whereas conven-
tional NMR coils detect a voltage (rate of change of magnetic flux)
induced in the loop. Thus, SQUIDs respond at all frequencies down
to DC (i.e., have a very wide frequency response), whereas resonant
coils respond only to a narrow frequency range (determined by the Q
of the coil) and have zero response at DC. The superiority of a
SQUID as a detector over resonant coils is greatest for low ~R
frequencies, long spin lattice relaxation times Tl, and short
transverse relaxation times T2*. Further, other non-SQUID type
detectors capable of measuring DC or near DC and having a wide
frequency response, such as laser-pumped helium magnetometers may
also be employed as detectors to detect both the T2* sinusoidal
decay and the slowly varying Tl decay.
Long Tl times and short T2* times are substantially the
conditions that exist for downhole nuclear magnetism logging of
earth formations. The Larmor frequency for protons in the Earth's
magnetic field is about 2000 Hz, a very low frequency compared to
the 10-500 M~lz used in laboratory NMR spectrometers. Secondly, the
Tl relaxation time of earth formations is fairly long, typically
~3~
about 1.0 second, while the T2* rel~ation time is short, typically
about 20-50 milliseconds. This high Tl/T2* ratio found in downhole
nuclear magnetism logging is the situation where SQUIDs or
similarly sensitive, wide frequency response detectors sre most
advantageous over conventional resonant coil detectors.
A measure of the improvement in signal-to-noise (S/N) ratio of
SQUID vs. resonant coil detection is given in the paper "New
Technique For Improved Low-Temperature SQUID NMR Measurements" by
R. A. Webb (Rev. Sci. Instum., Vol. 48, No. 12, pp. 1585-1594,
December, 1977). For typical laboratory values of SQUID noise
figure, coil Q, etc.
SQUID S/N 2.2 x 10 (Tl/T2)1/2
COIL S/N (~ )1/2
Using typical earth formation values of Tl = 1000 ms, T2* - 20 ms,
and ~ D 2*~*(2 x 10 ) = 1.26 x 10 , the SQUID S/N is a factor of
139 greater than the coil S/N.
Moreover, for NML logging, the improvement in S/N for a SQUID
detector compared to resonant coil detector should be even greater
because the conventional NML has a deadtime of about 25 milli-
seconds after the polarizing field is turned off. This measurement
delay or deadtime is due to coupling and ringing between the
polarizing coils and its circuitry, and the detection coils and its
circuitry; this delay is illustrated in Figure 2. Because the
delay or deadtime is an appreciable part of the T2* decay, A
majority of the signal has decayed before the NML begins recording.
For example, if T2* ~ 25 milliseconds and the NML deadtime is 25
milliseconds, then l/e of the signal has decayed before detection
even begins.
A SQUID or similar detector does not have this problem.
Unlike the resonant coil, which is sensitive only to the rapid
transverse dephasing of the nuclear spins (T2*) at the Larmor
frequency and has no sensitivity at DC, the SQUID or similar
detector measures the z-component of the magnetization at fre-
quencies down to DC. Thus, even instrument deadtimes of the order
of 25 milliseconds will not significantly reduce the signal level
3~
with SQUID or similar detectors. Such a similar detector that is a
non-SQUID type detector capable of measuring DC or near DC, is a
laser-pumped helium magnetometer; such magnetometers may also be
employed as detectors to essentially simultaneously detect both the
T2* sinusoidal decay and the slowly varying Tl decay.
In generall an optically-pumped magnetometer uses a lamp or
laser containing an alkali metal or helium whose light is passed
through a cell containing the vapor of the same element and
impinging on a photodetector on the other side of the vapor cell.
An optically-pumped magnetometer utilizes a population of electrons
in this alkali vapor gas or in metastable helium to obtain a
continuous measurement of the magnetic field intensity. The output
of the photodetector is amplified and fed to a coil surrounding the
vapor cell. This electro-optic system is an oscillator whose
frequency is directly proportional to the magnetic field intensity.
The magnetometer takes advantage of optical pumping to cause atomic
or electron spin precession similar that employed by an NML to
cause proton precession. Resonance absorption and reradiation of
energy of various or specific resonant lines are a function of
magnetic field intensity.
For downhole nuclear magnetism logging, another very important
advantage of SQUIDs or similar detectors is that they may directly
measure the Tl decay curve. It is not necessary to apply repeated
polarizing cycles to obtain the Tl decay curve needed for pore size
and permeability measurements. In order to measure the Tl decay
curve directly, a detector must respond at very low frequencies
below l Hz, because Tl is typically of the order of l second.
Thus, SQUID or similar detectors are sensitive not only to the
transverse dephasing of the spins but also to the thermal
relaxation Tl. Since Tl is typically 50 times greater than T2 in
earth formations, SQUID or similar detectors also may detect a
signal for the much longer spin lattice relaxation time Tl.
Figure 3 shows a typical signal expected to be detected if
resonant coil 17 of Figure t is replaced by a SQUID detector 18 and
; 35 consists of a sinusoidally-decaying signal at the Larmor frequency
: L2~3~
- 10 -
with time constant T2* which is proportional to MOSin 0, as wel]. as
a non-sinusoidal component decaying wi~h time constant Tl which i~
proportional to MoCos 9. The sinusoidal T2* decay is due to the
component of polari~ed nuclear spins perpendicular to the earth's
field He, while the Tl component is due to the component of nuclear
spin aligned along the earth's Eield He. The Larmor sinusoidal T2*
decay can be separated from the near-DC Tl decay using either an
electronic low-pass filter, a copper eddy-current container as a
shield around the detection coils, or a digital filter applied to
the recorded data by realtime or postprocessing software. The
longer Tl decay will not be measured by coil 17 while SQUID 18
measures essentially simultaneously both the sinusoidal T2* decay
as well as the Tl decay. However, other non-SQUID type detectors
capable of measuring DC or near DC, such as laser-pumped helium
magnetometers may also be employed as detectors to essentially
simultaneously detect both the T2* sinusoidal decay and the slowly
varying Tl decay.
Figure 4A shows a SQUID detector 20 and a cooling module 21,
which is preEerably a micro-miniature Joule-Thomson refrigerator,
on a substrate 30. Although only one SQUID detector 20 is shown in
Figure 4A, more than one such SQUID detector may be located on the
substrate 30. The substrate 30 is itself used as the cold stage of
the Joule-Thomson refrigerator. This provides excellent thermal
contact between the SQUID and cold stage. Such thermal contact is
required to provide the lowest possible temperature for the SQUIDs,
so that their noise figures will be optimal. The SQUIDs are
fabricated onto the substrate as close to the Joule-Thomson
reservoir as possible to ensure maximum cooling. The SQUID may be
of either the RF-biased or DC-biased type, although only the
DC-biased type is discussed herein. The SQUID is made from high
temperature superconducting material, such as, for example, but not
limited to rare earth-barium-copper oxide materials. The term
"high temperature superconducting material" is used herein to mean
a material whose superconducting transition temperature is above
the boiling point of liquid nitrogen (i.e., - 77 K).
~35~
- 11. -
As shown in Figure 4B, the DC ~QUID consists of two granular
weak links formed by epitaxial growth oE a rare earth, barium,
copper oxide material, such as YBa2Cu307 on an appropriate
substrate. The granular weak links are about 15 microns long and 1
micron thick and are defined by insulating gaps 47, 48; the weak
links are said to be granular in that the weak link is actually
between grains of the superconductor that are weakly Josephson-
coupled. If the weak links are too narrow ( 15 microns) the
current around central insulating portion 46 may become too high
and quench the superconductor material causing the SQUID not to
operate Although, a square 46 is shown in Figure 4B, other
geometries may be employed. Similarly, if the weak links are too
wide, too much current fl.ows so that the "staircase" operating
characteristics of a SQUID are not achieved and the SQUID does not
operate properly.
Although YBa2Cu307 is discussed herein as the preferred
material for making the SQUID, any of the high-temperature super-
conductors, such as those with oxygen-deEicient Perovskite
structure may be used. For YBa2Cu3O7, the substrate should have a
molecular structure similar to YBa2Cu3O7 (for any other material
employed for the SQUID, the substrate should have a molecular
structure similar to that material) to achieve epitaxial growth,
and should also be a good thermal conductor near liquid nitrogen
temperatures to achieve good thermal connection to cooling module
21. Examples of such substrates may be, but are not limited to
SrTiO3 or MgO. For example SrTiO3 has an "a" cell dimension of 3.9
Angstroms while YBa2Cu3O7 has a corresponding cell dimension of
3.83 Angstroms. Epitaxial growth o:E oriented crystals is important
in YBa2Cu307 superconductors for achieving the highest possible
critical fields and currents. Since it is preEerable that the
SQUIDs remain supe.rconducting during the polarizing cycle (when
they will be subjected to magnetic fields in excess oE 1
kilogauss), the SQUIDs should preferably have a critical field in
excess of the strength of any anticipated polarizing magnetic
Eield.
3~g~
- 12 -
Epitaxial growth of the SQUID material on the SrTiO3 substrate
may be achieved by several techniques well known in the art, such
as molecular beam epitaxy, electron beam evaporation, sputterine
from single and multiple targets, pulsed excimer laser ablation of
single targets, sol gel, and plasma oxidation. For example, in the
molecular beam epitaxy method, a thin layer (- 20 microns) of
Y-Ba-Cu is first vapor deposited onto the SrTi03 substrate at the
correct stoichiometry and then oxidized to YBa2Cu3O7. Next a thin
layer of gold is deposited on top of the YBa2Cu307 layer. Finally,
a photoresist layer is placed on the gold layer for
photolithography patterning. The photoresist and gold are then ion
milled in the SQUID pattern, i.e. removing the resist and gold
covering central portion 46 and gaps 47, 48 shown in Figure 4B;
that is, the gold and resist are left above the portion of the
layer designed to be superconducting. An oxygen ion beam of 0.3-3
MeV is then used to implant oxygen into the exposed YBa2Cu307 of
central portion 46 and gaps 47, 4S, causing these areas to become
an insulator, while the gold protects the portion of the pattern
designed to be superconducting. Finally, the remaining gold is
removed by ion milling, leaving a perfectly planar structure.
A plurality of SQUID sensors may be so constructed on the
substrate. The advantage of having a plurality oE sensors is that
the SQUID noise of each sensor will be random and uncorrelated, and
therefore, summing the SQUID outputs will result in further
improvement in signal-to-noise ratio.
The micro-miniature refrigerator operates by Joule-Thomson
expansion of a working gas, such as nitrogen. There are no
cryogenic fluids or moving parts, which results in very low
vibration and correspondingly low noise. M:Lcro-miniature
Joule-Thomson refrigerators are commercially available, such as for
example, the System I refrigerator manufactured by MMR Technolo-
gies, Inc. of Mountain View, California. However, these MMR
Technologies refrigerators are manufactured from glass or silicon
and can not be used as an epitaxial substrate ~ YBa2Cu3O7 SQUIDs.
As shown in Figure 4A, a microminiature Joule-Thomson refrigerator
9~
40 has three main parts. Thcse parts are a he.at exchanger 41, an
expansion capillary 42, and a liquid reservoir 43. The refrige-
rator may be constructed from a SrTiO3 wafer 30 using the photo-
fabrication technique. For example, in the photofabrication
technique, a mask for the refrigerator portion such as that shown
in Figure 4A is first prepared. The wafer 30 may then b~ coated
with a thick, water solution of gelatine activated with ammonium
bichromate. The solution on the wafer 30 is then dried, covered by
a mask, exposed to ultraviolet light through openings in the mask
and then developed in hot water. This resist, when dried, forms a
tough resilient coating which can withstand an abrasive etch with
Al2O3 powder. The unexposed gelatine washes off in hot water
leaving a pattern of unprotected SrTiO3.
An abrasive etching is performed by exposing the wafer
partially covered with resist to a blast of A1203 or other powder
of sufficient hardness, in sizes ranging from 10-30 microns; this
blast is scanned across the surface of the waEer. Channels of
precisely controlled depth in the range of 2-100 microns and with
nearly vertical side walls can be etched in the wafer 30 in this
manner. The remaining resist is removed by chemical solution of
the gelatine.
To complete the refrigerator portion, a cover plate is bonded
to the etched wafer, using a variety of possible adhesives, such as
low temperature epoxy. Some care must be taken to assure that the
adhesive does not flow into the micron size channels.
The refrigerant working gas is contained in a pressurized
tank, typically at pressures of 140 bar, and allowed to expand
through the Joule-Thomson refrigerator into a larger low pressure
collection tank (not shown) within the logging tool. The
collection tank is many times (typically at least 10 times) larger
than the pressurized tank so that the expanded gas does not reach a
high pressure and degrade refrigerator operation. Typical
refrigerator operating times are about 12 hours with 1 liter of
nitrogen gas at 140 bar pressure. Alternatively, a closed cycle
compressor could be used for the gas, although such moving parts
~3~
will cause vibrations that may degrade signal-to-noise (S/N~.
Alternatively, a Peltier module may be used as a cooling module 21
when superconducting materials whose transition temperature is
above about 180 K are used Eor SQUIDs, since the temperature down
to which current Peltier modules may cool is about 180 K.
The refrigerator cold stage and SQUID may be surrounded by a
vacuum can (not shown). The vacuum can provides a vacuum space
necessary for reducing the heat leakage into the cold stage and the
SQUID sensor. The vacuum can may be constructed of any non-
metallic and non-magnetic material with low thermal conductivity at
liquid nitrogen temperatures. Thin strips of superinsulation
(i.e., aluminized mylar) can be added within the vacuum space to
improve ther~al isolation of the SQUID. However, the strips must
be cut vertically to reduce eddy currents in the superinsulation.
Figure 5 shows the axial gradiometer coils that may be used in
the practice of this invention. The function of these coils is to
improve flux sensitivity and reduce magnetic noise from tool motion
in the Earth's magnetic field. The pickup coils 51 and 52 are
coaxial and counterwound so that a uniform magnetic field through
the coils is exactly cancelled. One of the picXup coils 51 is
centered inside the polarizing coil 12, while the other coil 52 is
substantially outside the polarizing coil. The pickup coil 51
centered in the polarizing coil 12 will detect the maximum signal
from the earth formation 20, while the pickup coil 52 outside the
polarizing coil will detect substantially no signal from the
formation. These coils may be flux coupled to one or more SQUIDs,
or each SQUID may have its own set of coils.
However, the logging tool may also be operated with the SQUID
itself as the flux sensor and with the tool clamped against the
borehole wall to reduce vibration. If flux-coupled coils are used,
the SQUID sensor is shielded from magnetic fields by a super-
conducting shield of YBa2Cu3O7 surrounding the SQUID. The super-
conducting leads to the coil enter through small holes in the
superconducting shield.
- 15 -
The gradiometer coils may be thin epitaxial Eilms deposited on
a SrTiO3 substrate, similar to the SQUID fabrication. Alter-
natively, the coils may be made from short lengths of super-
conducting YBa2Cu307 wire. This wire may be made from a gold or
silver tube packed with a compressed powder of YBa2Cu307. The tube
i9 sealed, swaged to smaller diameter, then refired above 900C and
allowed to slowly cool (~ 20 hours). Although wire made in this
manner may not have the highest obtainable critical currents,
values in e~cess of 1000 A/cm may be obtained at zero magnetic
field, and 200 A/cm above 1000 gauss.
Similarly, laser-pumped helium magnetometer detectors may be
employed in pairs, with one magnetometer inside the polarizing coil
and one outside the polarizing coil. These two magnetometers are
again differentially coupled to eliminate signals from motion in
the earth's field.
Although a DC granular weak link thin-film type of SQUID has
been described hereinabove, other types of SQUIDS may also be
fabricated according to the teachings of this invention. Among
these are tunnel junction SQUIDS (superconductor-insulator-super-
conductor, superconductor-normal metal-superconductor, and super-
conductor-semiconductor-superconductor), thin film weak links such
as those of Dayem and Grimes, point-contact SQUIDs, and various
hybrid combinations of thin Eilm and point-contact SQUIDs.
Similarly, the SQUIDs may be either single junctlon RF-biased type
or double junction DC SQUIDs.
Figure 6 shows a block diagram of the electronics required for
operating a DC SQUID 620, as is well known in the art. The
electronics must consist of a blas current source 610, an
amplification means 600, 601, 602, a modulation means 603, 604,
605, a feedbflck means 607, 608, a filter means 606, a monitor means
609, and a zero offset 612. At the present time, it is preferred
that a DC SQUID be operated in a feedback mode so that the SQUID is
maintained at a particularly sharp point on the SQUID's staircase
current versus voltage operating characteristics, as selected by
the bias current. The modulation and feedback may be at any
~z53~3~r~L
frequency whi.ch allows the electronics to follow any changes in
magnetic field in the logging tool. Figure 6 shows a 100 kHz
modulation oscillator 604.
More particularly, the detected signal is flux coupled into
the SQUID 620 via the input coil 621. If the SQUID 620 is the
detector itself, then input coil 621 is not present and the
detected flux is the flux passing through the insulating, central
portion of the SQUID (i.e., item 46 of Figure 4B). The SQUID is
modulated by modulation oscillator 604 at a frequency sufficient to
allow the electronics to follow changes in the magnetic field by
modulation and feedback coil 622. Advantageously said frequency is
100 kHz. A phaseshifter 603 is connected with said oscillator. As
the SQUID senses a change in magnetic field, the SQUID trys to move
up or down the vertical portion of the selected operating point
(selected by its bias current) on its staircase operating
characteristic. The amplifier means 600, 601, 602 amplify this
change which is then filtered by filter means 606 to provide
unmodulated feedback, via feedback means 607, 608 and modulation
and feedback coil 622, to the SQUID 620. In particular the
amplifier means comprise an amplifier 600 with amplification xlO0,
a tuned amplifier 601 with amplification xlO0 and quality Q ~ 3,
and a broadband amplifier 602 with amplification x0-300. The
broadband amplifier 602 is also connected with a multiplier 611. As
a further detail it is noted that the filter means 606 contains lO0
and 200 kHz traps. The feedback keeps the SQUID at its selected
operating point. The amount and type of feedback is detected by
monitor means buffer amplifier 609 and provided as an output to
indicate changes in the magnetic field sensed by the SQUID.
Many other variations and modifications may be made in
the apparatus and techniques hereinbefore described, by those
having experience in this technology, without departing from the
concepts of the present invention. Accordingly, it should be
clearly understood that the apparatus and methods depicted in the
accompanying drawings and referred to in the foregoing description
~2~3~g3l
- 17 -
are illustrative only and are not intended as limitations on the
scope of the invention.
.
,, ' , '~ :
. , ' "'~ ~ ' , ;
. ' .
~; , ' '
.
