FR2622977A1 - METHOD AND INSTRUMENT FOR NUCLEAR MAGNETISM DIAGRAPHY - Google Patents

Abstract

The invention relates to a nuclear magnetism logging instrument comprising: - means 12 for generating a magnetic field, which can be activated or deactivated in response to predetermined control signals, - detector means 18, detecting sinusoidal and slowly varying variations of the magnetic field when the magnetic field generating means are deactivated, and outputting signals representative of these variations, and - an electronic unit 19, producing said predetermined control signals and receiving signals originating from the detector means. The detector means 18 can in particular consist of a superconducting quantum interference SQUID component made from a high temperature superconducting material such as YBa2Cu3O7, or else of a laser pumped helium magnetometer.

Description

The present invention relates to an apparatus for use in

  a borehole. It relates more particularly to a nuclear magnetic logging apparatus for use in a ground borehole. The borehole logging technique opened the

  more precise for measuring the saturation of

  duel and the permeability of the terrestrial formations is the

  logging by nuclear magnetism or NMI.

  This logging technique uses the magnetic field from a solenoid-shaped coil to polarize the protons of any fluids within the earth's formations in the vicinity of the instrument. The solenoid-shaped coil is then deactivated, and all mobile protons influenced by the solenoid's magnetic field precede around the Earth's magnetic field at their Larmor frequency. This precession can be measured as a damped sinusoidal voltage induced in a separate sensing coil of the diagrammatic instrument. The induced voltage decreases rapidly, in a time which is typically of the order of 20 to 50 ms, because of the extremely short transverse relaxation or spin-spin times T2 * of

these fluids.

  The initial amplitude of the nuclear magnetism signal is proportional to the amount of "free fluid", or producible fluid, found in the formation in the vicinity of the instrument. If Mn-EDTA is added to the drilling fluid and allowed to penetrate the formation, the residual nuclear magnetism signal threshold will be that from a potential oil phase in the formation in the vicinity of the formation. 'instrument. Thus, an NML Logging / Injection / Logging process yields extremely accurate measurements

  residual oil saturation in the formation.

  The time Ti of longitudinal relaxation or spin-lattice of any fluid found in a terrestrial formation is

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  a function of the size (s) of the pores containing the fluid (and their distribution); the pore dimensions and their distribution can then be related to the capillary pressure curve, which can itself be related to the permeability of the formation, in a well-known manner. In NML, we try to measure T1 by repeated cycles of polarization with successive periods of polarization of longer and longer. Since T1 is much longer than the damped sinusoidal attenuation time T2 *, T1 can be determined directly from the curve

  attenuated sinusoidal attenuation in a single measurement.

  Although it is an extremely useful logging technique, one of the main disadvantages of NML is its poor signal-to-noise ratio, which limits its accuracy to about 1 pore unit for a continuous logging operation. Although this is suitable for measuring a free fluid index, residual oil saturations are typically one-third or less of total porosity, so that when oil is being extreme conditions, a fixed-position NML measurement to determine residual oil saturation with sufficient accuracy. Typically, data are collected at a point in the borehole for about 15 minutes, and then averaged to obtain residual oil saturation at this point better than 1% saturation. However, holding the instrument in a fixed position for this length of time slows data acquisition for the entire training and increases the risk of locking the instrument indoors

from the borehole.

  Another disadvantage of the NML is its low depth of investigation in training. Here again, because of a poor signal-to-noise ratio it is only possible to detect the signals coming from the very first centimeters of the

wall of the borehole.

  Another disadvantage is the fact that repeated polarization cycles are required to obtain a

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  number of measuring points of the weakening curve Ti. This requires additional time, and since only a few measurements are obtained, the shape of the weakening curve Ti thus obtained is not very precise. This weakening curve T1 then contains an inaccurate information on the pore sizes and their distribution in the formation (so that any capillary pressure or permeability determined from these measurements is not very precise), which is an important disadvantage of

current NML techniques.

  In the prior art, the use as a detector of a SQUID (Superconducting Quantum Interference Device) in a modified nuclear magnetism instrument has been proposed, the modification of using two opposed superconducting magnets in place of one another. of conventional polarization solenoid, in order to have a deeper depth

  important investigation in the borehole wall.

  However, the use as a detector of a SQUID in a downhole device was not possible due to the cryogenic and safety constraints associated with the use of liquid helium in the environment of a borehole. probe. In fact, liquid helium increases by more than 600 times of volume when it vaporizes and can not be evacuated safely in the borehole. This problem, together with the need for downhole operation in the high temperature environment of a borehole for a sufficient period of time, has hitherto prevented the use of a SQUID in a instrument to

nuclear magnetism.

  These limitations and disadvantages of the prior art, as well as others, are however overcome by the present invention, which provides a nuclear magnetic logging apparatus with a detector capable of detecting sinusoidal magnetic fields and slowly varying in a

earth borehole.

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  The present invention provides a logging device for measuring the terrestrial magnetism response of terrestrial magnetism, which utilizes one or more detectors capable of substantially simultaneously sensing sinusoidal and slowly varying magnetic fields resulting from precession.

  mobile nuclei around the Earth's magnetic field.

  For this purpose, the apparatus according to the invention comprises: - magnetic field generating means, which can be activated or deactivated in response to predetermined control signals, - detector means, detecting sinusoidal variations and slow variation of the field magnetic field when the magnetic field generating means are deactivated, and outputting signals representative of these variations, and - an electronic block, producing said predetermined control signals and receiving signals from

detector means.

  Preferably, the detectors of the apparatus according to the invention are SQUIDs (Superconducting Quantum Interference Devices). Alternatively, one or more laser pumped helium magnetometers could also be used as the detector. The presently preferred apparatus according to the invention uses a subminiature Joule-Thomson cooler or, alternatively, thermoelectric cooled Peltier modules, made on the same wafer as the SQUID (s), in order to keep below of its superconducting transition temperature each SQUID high temperature. In addition, the SQUID (s) may be flow coupled in an axial gradiometer configuration to reduce any magnetic field noise induced by the

movement.

  The preferred apparatus according to the present invention provides a means for using a SQUID detector in a nuclear magnetic logging instrument without

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  need no cryogenic liquid cooling. The method consists in producing one or more SQUIDs from a high temperature superconducting material, such as, for example, but without limitation, one of copper oxide, barium and rare earth materials, such as copper oxide, barium and yttrium (YBa2Cu307), which is superconducting at temperatures exceeding 90 K. The SQUID (s) is (are) deposited by epitaxy on a substrate with a high thermal conductivity such that SrTiO3 or MgO. The wafer substrate is, in turn, fixed against the cold part of a Joules-Thomson subminiature cooler (or a module

  Peltier for high temperature superconducting materials

  It can be cooled down to about 80 K. A vacuum enclosure constructed of a non-metallic material such as G-10 fiberglass surrounds the SQUID (s) and the wafer and provides thermal insulation for reducing the thermal load and facilitating the maintenance of the cold part of the cooler at temperatures below the superconducting transition temperature

material (s) SQUID (s).

  A SQUID detector and a laser pumping helium magnetometer respond to all frequencies up to continuous, while a resonant coil detector, such as one used in the current state of the art. of the NML, responds only to a narrow frequency band and has a continuous zero response. A SQUID detector and a helium magnetometer with laser pumping make it possible to measure, essentially simultaneously and directly, with an instrument

  NML unique and in a unique way, both the business and the

  T2 * and the T1 loss.

  In addition to the SQUID elements, the SQUID electronics can

  also be anchored to the cold part of the cooler.

  This allows further improvement of the performance

mances, with a reduced noise.

  One or more SQUID detectors can be flow coupled to a superconducting coil for detecting an axial gradiometer. This coil can also be made from

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  of a superconducting material at high temperature and can be thermally anchored to the cold part of the cooler. The function of the detection loop is to increase the flow sensitivity of the detector. Advantageously, an axial gradiometer configuration is used to reduce the flow noise resulting from a possible movement of the survey instrument into the Earth's magnetic field when it is inside the semiconductor. For this configuration, one of the gradiometer loops is centered inside the bias coil, while the second loop, wound in the opposite direction, extends outside the bias coil. Thus, the inner loop detects the entire signal from the eventual polarized protons of the formation, while the outer loop detects virtually no proton signal. A uniform field such as the Earth's magnetic field creates equal and opposite fluxes in each loop,

  which has the effect of canceling the Earth's magnetic field.

  In the same way, the laser pumping helium magnetometers can be mounted in differential pairs, one being disposed within the bias coil and the other being fixed outside the bias coil. Thus, the presently preferred apparatus according to the present invention provides a SQUID detection module for use in a downhole survey instrument which avoids the use of cryogenic liquids, which provides an improved signal-to-noise ratio for the logging by nuclear magnetism and which makes it possible to directly measure the weakening curve T1 of the

land formation.

  One of the aims of the present invention is therefore to propose a nuclear magnetic logging device which makes it possible to obtain signal / noise ratios several orders of magnitude higher than those obtained in

the current state of the art of NML.

  Another object of the present invention is to propose an NML device which can directly determine the totality of

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  of the weakening curve Ti of a terrestrial formation, without needing several successive cycles of polarization. These objects and advantages of the present invention, as well as

  others, will appear on reading the description

  DETAILED DESCRIPTION BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a simplified block diagram of a nuclear magnetic logging instrument, of the magnetic field vector associated with this magnetism diagrammed instrument. Figure 2 shows a typical NML signal collected with a resonant coil tuned to the Larmor frequency, - Figure 3 shows a typical NML signal collected with a SQUID as a detector in place of 4A and 4B respectively show, on the one hand, a SQUID detector and a Joule-Thomson subminiature cooler made in one piece on the same edge of the substrate, and on the other hand the details of the SQUID detector, FIG. shows the axial gradiometer configuration of superconducting coils coupled in flux to a SQUID and usable with the present invention, Figure 6 shows a typical measurement electronics

for SQUID detector.

  Referring now to FIG. 1, there can be seen a borehole 10 in a subterranean formation 20 and a nuclear magnetic logging tool 11, installed in this troudesondeaumoyen of a cable 9. The instrument logging tool 11 contains a solenoid-shaped coil 12 which produces a polarizing magnetic field vector 13 in the formation in the vicinity of the instrument. The He magnetic field 14 is shown at an angle 8 with respect to the magnetization vector Mo 15 representing the spins of the polarized protons. After the polarizing field has been removed, the proton spins precede

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  around the Earth's magnetic field 14, as illustrated. If the detector is a resonant coil 17 that is tuned to the Larmor frequency, it detects proton Mo spin protons. An instrumentation block 19 supplies a power of about 1 kW to the bias coil 12 and detects the induced voltage in the resonant coil 17

by the spins in precession 15.

  Figure 2 shows the typical signal 16 which can be expected to collect in the resonant coil 17 due to the precession of the proton spins. The signal weakens sinusoidally at the Larmor frequency with a time constant T2 * which is significantly shorter

  than T1. T2 * represents the time constant of

  the envelope of the sinusoidal signal. The Larmor frequency of the precession is given by Y x He, with Y = 4.26.104 kHz / T for the proton. It will be noted that the attenuation Ti is not observed because the attenuation T1 is of the order of one second, a value very far from the 2000 Hz Larmor frequency on which the resonant coil is tuned. FIG. 2 also shows the measurement delay, which will be explained later, and indicates (in dashed lines) the extension up to the time of the envelope of the signal, which indicates OF, that is, say the porosity occupied. by free fluids in training at

neighborhood of the instrument.

  SQUID detectors have been found to have some advantages over resonant coils

  classics for detecting NMR signals in the laboratory.

  The SQUIDs detect the magnetic field (more precisely, the total magnetic flux) passing through the detection loop, whereas the conventional NMR coils detect a voltage (rate of change of the magnetic flux) induced in the loop. In this way, the SQUIDs respond to all frequencies up to the continuous (ie they have an extremely wide frequency response), while the resonant coils respond only over a narrow range of frequencies ( determined by the overvoltage factor of the coil) and have a zero response continuously. The superiority of a SQUID as detector compared to the coils

  resonant is particularly important for the weak-

  NMR frequencies, long T1 spin-lattice relaxation times and short T2 * transverse relaxation times. In addition, other detectors that are not of the SQUID type, but which are capable of continuous measurements or which are capable of continuous measurements, may also be used as detectors for detecting both the sinusoidal attenuation T2 * and the slow variation attenuation T1. in quasi-continuous mode with a large frequency response, for example the magnetometers with

helium with laser pumping.

  Long T1 times and short T2 * times generally correspond to the conditions existing in well-ground nuclear magnetic logging. The proton Larmor frequency in the Earth's magnetic field is about 2000 Hz, a very low frequency compared to the 10-500 MHz range used by laboratory NMR spectrometers. In addition, the T1 relaxation time of the earth formations is quite long, typically about 1.0 second, while the T2 * relaxation time is short, typically about 20 to 50 ms. This high T1 / T2 * ratio found in downhole nuclear magnetism prospecting corresponds to the case where SQUIDs or similar sensitive and broad frequency response detectors prove to be particularly advantageous with respect to

  conventional resonant coil detectors.

  R. A. Webb's article entitled New Technique for Improved Low Temperature SQUID NMR Measurements, published in the Rev. Sci. Instrum., Vol. 48, No. 12, Dec. 1977, pages 1585-1594, gives the measurement of the improvement of the signal-to-noise ratio provided by a SQUID over a resonant coil detector. For typical values of signal-to-noise ratio of SQUID, coil overvoltage factor, etc., we have:

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  1o (S / N ratio) SQUID 2.2 x 103 (T1 / T2 *) 1/2

(S / N ratio) Coil (o) 1/2 -

  If we use values, typical for earth formation, of T1 = 1000 ms, T2 * = 20 ms and wo = 2c (2 x 103) = 1.26 x 104, the signal-to-noise ratio of SQUID is higher

  by a factor of 139 to the signal-to-noise ratio of the coil.

  In addition, in NML prospecting, the improvement of the signal-to-noise ratio of a SQUID detector compared to a resonant coil detector will be even greater because, in conventional NML, there is a dead time of about 25 ms after the suppression of the polarizing field. This delay in the measurement or dead time is due to the coupling and the rebound effect between the polarizing coils and their circuits, and between the detection coils and their circuits; this delay is illustrated in Figure 2. Since the delay or dead time is an appreciable fraction of the T2 * loss, most of the signal has weakened before the NML recording is started. For example, if T2 * = 25 ms and the NML dead time is 25 ms, we will already have a 1 / e loss of the signal even before the detection

do not start.

  A SQUID or similar detector does not pose this problem.

  Unlike the resonant coil, which is sensitive only to the rapid transverse phase shift of the nuclear spins (T2 *) at the Larmor frequency and shows no sensitivity to the continuous, the SQUID or similar detector measures the z component of the magnetization at frequencies up to continuous. Thus, even instrument dead times of the order of 25 ms will not significantly reduce the signal level if we have a SQUID or a similar detector. A laser-pumped helium magnetometer is such a similar detector, other than a SQUID type detector but capable of measuring the continuous or nearly continuous; it is also possible to use as detectors such magnetometers to detect, essentially simultaneously, the

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  times T2 * sinusoidal attenuation and attenuation

T1 slow variation.

  In general, an optically pumped magnetometer uses a lamp or laser containing an alkali metal or helium and which is passed through a cell containing a vapor of the same element, the light then striking a photodetector located on the other side of the steam cell. An optically pumped magnetometer uses an electron population of this alkali metal vapor gas or metastable helium to obtain a continuous measurement of the intensity of the magnetic field. The output of the photodetector is amplified and applied to a coil surrounding the vapor cell. This electro-optical system is an oscillator whose frequency is directly proportional to the intensity of the magnetic field. The magnetometer takes advantage of optical pumping to cause atomic precession or electron spin precession similar to that used by an NML system to cause proton precession. The resonance absorption and the energy back-radiation of various or particular resonant lines are a function of

the intensity of the magnetic field.

  Another well-known advantage of SQUIDs or similar detectors for downhole nuclear magnetic logging is that they can directly measure the weakening curve Ti. It is not necessary to apply repeated cycles of polarization to obtain the weakening curve T1 necessary for the measurement of the pore size and the permeability. To measure the attenuation curve T1 directly, a detector must respond to very low frequencies, less than 1 Hz, since T1 is typically of the order of one second. However, SQUIDs or similar detectors are sensitive not only to the transverse phase shift of the spins but also to the thermal relaxation T1a. As T1 is typically, in the terrestrial formations, fifty times greater than T2 *, a SQUID or similar detector can also detect a corresponding signal. at the relaxation time 12 6Zl

  T1 spin-lattice relaxation time.

  FIG. 3 shows a typical signal that can be expected to detect if the resonant coil 17 of FIG. 1 is replaced by a SQUID detector 18; this signal consists of a sinusoidal weakening signal at the Larmor frequency, with a time constant T2 * proportional to Mosin2e, with at the same time a non-sinusoidal component weakening with a time constant T1 proportional to MocoS28. The sinusoidal attenuation T2 * is due to the polarized nuclear spin component perpendicular to the terrestrial field He, whereas the component T1 is due to the component of the nuclear spin aligned with the terrestrial field He. The sinusoidal attenuation T2 * at the Larmor frequency can be separated from the attenuation T1 close to the continuous using either a low-pass electronic filter or an eddy-current copper casing forming a shield around the detection coils, or a digital filter applied by software operating in real time or delayed time to the recorded data. The weakening T1, longer, will not be measured by the coil 17, while the SQUID 18 measures, essentially simultaneously, at the same time

  T2 * sinusoidal attenuation and T1 loss.

  However, it would also be possible to use as a detector, in order to measure, essentially simultaneously, both the sinusoidal attenuation T2 * and the slow variation attenuation T1, other detectors which are not of the SQUID type, but which are capable of continuous or quasi-continuous measurements, as is the case, for example

  helium magnetometers with laser pumping.

  FIG. 4A shows a SQUID detector 20 and a cooling module 21, which is preferably a Joule-Thomson subminiature cooler, formed on a substrate 30. Although only a single detector has been shown in FIG. SQUID 20, one could have on the substrate 30 several of these SQUID detectors. Substrate 30 is itself used as a cool part of the Joule-Thomson cooler. This allows to have an excellent contact

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  between the SQUID and the cold part. Such thermal contact is necessary to give the SQUIDs the lowest possible temperature, so as to optimize their noise characteristics. The SQUIDs are made on the substrate as close as possible to the JouleThomson accumulator, to allow maximum cooling. The SQUID can be of the radio frequency polarization type as well as the continuous polarization type, although only the continuous polarization type is described here. SQUID is made from a high temperature superconducting material such as for example, but not limited to, one of the oxides of copper, barium and rare earths. The term "high temperature superconductive material" as used herein refers to a material whose superconducting transition temperature is above the boiling point of liquid nitrogen (which is

of -77 K).

  As illustrated in FIG. 4B, the continuous polarization SQUID is formed by two granular weak bonds made by epitaxially growing a material such as YBa2Cu307 on a suitable substrate. The small granular bonds are about 15 μm in length and about 1 μm thick and are defined by the insulating gaps 47 and 48; it will be said that the weak bonds are "granular" in that these weak bonds are in fact formed between grains of the superconductor which have a weak Josephson bond between them. If the weak links are too narrow (<< 15.m), the current passing around the central insulating part 46 may become too high and annihilate the superconducting effect of the material, so that the SQUID will no longer work. Although a square 36 has been shown in FIG. 4B, other geometries could be used. Conversely, if the weak links are too wide, it will pass too much current and we will not get the operating characteristics "in stair steps" of a SQUID, so

  that the SQUID will not work properly.

  Although YBa2Cu307 is described here as a material

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  preferential for achieving the SQUID, one could as well use any superconducting material at high temperature, for example those having an oxygen-deficient perovskite structure. In the case of YBa2Cu307, the substrate must have a molecular structure similar to YBa2Cu307 (for any other material used for SQUID, the substrate must have a molecular structure similar to that of this material) in order to allow epitaxial growth, and must also to be a good thermal conductor at temperatures close to those of liquid nitrogen, in order to achieve a good thermal bond with the cooler module 21. SrTiO3 and MgO are nonlimiting examples of such substrates. For example, SrTiO3 has a mesh size of 0.39 nm, while YBa2Cu307 has a mesh size of 0.383 nm. Epitaxial growth of oriented crystals is important with YBa2Cu307 superconductors to obtain the highest possible critical fields and currents. Since it is preferable that SQUIDs remain superconducting during the polarization cycle (when subjected to magnetic fields exceeding 0.1 T), SQUIDs will preferably have a critical field that exceeds the intensity of any

  Predictable polarizing magnetic field.

  The epitaxial growth of the SQUID material on the SrTiO3 substrate can be achieved by various well-known techniques, such as molecular beam epitaxy, electron beam evaporation, sputtering from an isolated target, or multiple targets. removal of material on pulsed excimer laser isolated targets, coagulation of a colloidal solution or plasma oxidation. For example, in the molecular beam epitaxy process, a thin layer (about 20 μm) of Y-Ba-Cu is first vapor deposited on the SrTiO3 substrate with

  the appropriate stoichiometry, and then oxidized to YBa2Cu307.

  A thin layer of gold is then deposited over the layer of YBa2Cu307. Finally, a layer of photoresist is placed on the gold layer, allowing the formation of patterns by photolithography. The photoresist and the gold are then removed by ion etching following the pattern of the SQUID, that is to say by removing the resin and the gold covering the central portion 46 and the intervals 47 and 48 illustrated in FIG-4B; in other words, the gold and the resin remain above the part of the layer that we want to make superconducting. An energy oxygen ion beam of 0.3 to 3 MeV is then used to implant oxygen into exposed YBa2Cu307 of central portion 46 and gaps 47 and 48, causing these areas to become insulating. , while gold protects the

  part of the pattern that one wants to make superconducting.

  Finally, the remaining gold is removed by ion etching,

  leaving a perfectly flat structure.

  In this way, a plurality of SQUIDs sensors can be produced on the substrate. The advantage of having a plurality of sensors is that the SQUID noise of each sensor will be random and uncorrelated so that a further improvement of the signal-to-noise ratio can be obtained by adding the output signals of the sensors.

different SQUIDs.

  The subminiature cooler operates by Joule-Thomson expansion of a working gas such as nitrogen. It does not have any cryogenic fluid or moving parts, which

  allows for very low vibrations and,

  latively, a low noise. Joule-Thomson subminiature chillers are commercially available, for example the System I chiller manufactured by

  MMR Technologies, Inc. in Mountain View, California.

  However, these MMR Technologies coolers are made from glass or silicon and can not be used as an epitaxial substrate for SQUIDs at YBa2Cu307. As illustrated in FIG. 4A, a Joule-Thomson subminiature cooler 40 has three main elements. These elements are: a heat exchanger 41,

  an expansion capillary 42 and a liquid reservoir 43.

  The cooler can be made from a slice of SrTiO3 using a photographic technique of

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  manufacturing. For example, in this technique photo-

  graph of manufacture, we begin by preparing a mask for the cooler portion illustrated in Figure 4A. The wafer can then be coated with a thick aqueous solution of ammonium dichromate-activated gelatin. The extended solution is then dried on wafer 30, covered with a mask, exposed to ultraviolet light through openings in the mask and then developed in hot water. This resin, when dried, forms a soft and hard coating that can withstand abrasive etching by alumina powder. The unexposed gelatin can be removed by washing in hot water,

  leaving a pattern of unprotected SrTiO3.

  Abrasive etching is performed by exposing the partly resin covered wafer to sandblasting alumina or other powder of sufficient hardness with particle sizes of 10 to 30 μm; we sweep the entire surface of the slice by sandblasting. In this way, it is possible to engrave in the slice 30 of precisely controlled depth channels between 2 and 100 μm and with virtually vertical side walls. The remaining resin is removed by chemical dissolution of the gelatin. To complete the cooler portion, a closure plate is glued to the etched edge using one of many possible adhesives, such as a low temperature epoxy adhesive. Some precautions must be taken to ensure that the adhesive does not overflow into

the micron size channels.

  The working coolant gas is contained in a pressurized tank, typically at pressures of the order of 140 bar, and is caused to expand in the Joule-Thomson cooler inside a low-level collecting tank (not shown). pressure and larger dimensions, located inside the survey instrument. The collecting tank is several times larger (typically, at least ten times larger) than the pressurized tank, so that the expanded gas does not reach

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  not a high pressure, which would degrade the cooling operation. Typical operating times of the cooler are about 12 hours with one liter of nitrogen gas at a pressure of 140 bar. Alternatively, a closed cycle compressor could be used for the gas, although the moving parts thereof would cause vibrations which could degrade the signal-to-noise ratio. In

  Alternatively, one could use as a cooling module

  21 a Peltier module when SQUIDs are used superconducting materials whose transition temperature is greater than about 180 K, because the temperature up to which the usual Peltier modules can achieve cooling is about 180 K. The cold part of the cooler and the SQUID can

  be surrounded by a vacuum chamber (not shown).

  The vacuum chamber provides the volume of vacuum necessary to reduce thermal leakage of the cold part and SQUID sensor. The vacuum chamber may be made from any non-metallic and non-magnetic material having low thermal conductivity at liquid nitrogen temperatures. Thin strips of super-insulating material (eg, aluminized Mylar) may be added to the void volume to improve the thermal insulation of the SQUID. However, the strips must be cut vertically to reduce the

  Eddy currents in the super-insulator.

  Figure 5 shows the coils of the axial gradiometer that can be used to implement the present invention. The function of these coils is to improve the flux sensitivity and reduce the magnetic noise from the movement of the instrument in the Earth's magnetic field. The pickup coils 51 and 52 are coaxial and wound in opposite directions so as to cancel

  exactly any uniform magnetic field in the coils.

  One of the pickup coils 51 is centered inside the polarizing coil 12, while the other coil 52 is

  essentially outside the Polish coil.

  RISANTE. The sensor coil 51 centered in the coil

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  polarizing 12 will detect the maximum signal of the earth formation 20, while the sensing coil 52 out of the polarizing coil will essentially detect no signal from the formation. These coils can be flow coupled to one or more SQUID (s), or each SQUID

  can have his own reel game.

  However, the instrument can also be operated

  prospecting with SQUID itself as a flow sensor, the instrument being blocked against drilling to reduce vibration. If flux-coupled coils are used, the SQUID sensor is shielded from the magnetic field by means of a YBa2Cu307 superconducting shield surrounding the SQUID. The superconducting wires leading to the coil enter through small holes

  practiced in the superconducting shield.

  The gradiometer coils may be thin epitaxial films deposited on a SrTiO3 substrate, made in the same manner as SQUID. Alternatively, the coils can be made from small lengths of YBa2Cu307 superconducting wire. This wire can be made from a gold or silver tube filled with a compressed powder of YBa2Cu307. The tube is sealed, stamped at a smaller diameter then annealed above 900 C and slowly cooled (in about 20 hours). Although a wire made in this way may not have the highest critical currents obtainable, it is possible to obtain values exceeding 1000 A / cm 2 for a zero magnetic field and 200 A / cm 2 above 0.1 T. In the same way, laser pumped helium magnetometer detectors can be used in pairs, one of the magnetometers being located in the polarizing coil and the other outside the polarizing coil. These two

  Magnetometers are again connected in different

  in order to eliminate signals from movement in

the terrestrial field.

  Although a type of granular-bonded thin-film continuous detection SQUID has been described above

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  low, one could also realize SQUIDs of other types in accordance with the teachings of the present invention. In particular, tunnel junction SQUIDs (superconductor / insulator / superconductor, superconductor / normal metal / superconductor and superconductor / semiconductor / superconductor) could be made, thin thin-film bonds such as Dayem and Grimes, contact SQUIDs. punctual or various hybrid combinations of SQUIDs thin-film and-point contact. Similarly, SQUIDs could be SQUIDs either of the radio-frequency polarized single-junction type or the

double junction continuously.

  FIG. 6 shows a block diagram of the electronics, well known in the art, necessary for the operation of a continuous SQUID 620. This electronics must consist of a source of polarization current 610, amplifying means 600, 601 and 602, modulator means 603, 604 and 605, servo means 607 and 608, filter means 606, monitor means 609 and a zero shifter 612. Currently, it is preferred to operate a SQUID in Continuous in slave mode, so as to maintain the SQUID on a very precise point of the operating characteristic of the current / voltage SQUID in stair steps, this point being selected by the bias current. Modulation and slaving can be done at any frequency allowing the electronics to track any changes in the magnetic field in the survey instrument. Figure 6 shows a

  604 modulation oscillator operating at 100 kHz.

  More precisely, the detected signal is coupled in flux to the SQUID 620 via the input coil 621. If the SQUID 620 itself constitutes the detector, there is no input coil 621 and the flux detected is the flux passing through the insulating central portion of the SQUID (i.e., element 46 of FIG. 4B). The SQUID is modulated by a modulation oscillator 604 at a frequency sufficient to allow

  electronics to follow the variations in the magnetic field.

  by the modulation and servo coil 622.

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  Advantageously, this frequency is 100 kHz. A phase shifter 603 is connected to this oscillator. When the SQUID detects a variation of the magnetic field, the SQUID attempts to move up or down the vertical portion of the selected operating point (selected by its bias current) on the characteristic of

  functioning in stair steps. The means

  The amplifiers 600, 601 and 602 amplify this variation, which is then filtered by the filtering means 606 in order to provide the SQUID 620 with unmodulated servocontrol via the servocontrol means 607 and 608 and the modulation and servo coil 622. In particular, the amplifying means comprise an amplifier 600 amplifying in a ratio x 100, a tuned amplifier 601 amplifying in a ratio x 100 and with an overvoltage factor of approximately 3, and a broadband amplifier.

  602 amplifying by a factor between 0 and 300.

  The broadband amplifier 602 is also connected to a multiplier 611. An additional detail to note is that the filtering means 606 includes traps at 100 and kHz. The servo keeps the SQUID at its selected operating point. The magnitude and type of servo are detected by a buffer amplifier of the monitor means 609, and are output as an output signal indicating the field variations

magnetic detected by the SQUID.

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Claims (13)

RVENDICATIONS   1. A nuclear magnetic logging instrument, characterized in that it comprises: - magnetic field generating means (12), which can be activated or deactivated in response to predetermined control signals, - detector means, detecting sinusoidal variations and slow variation of the magnetic field when the magnetic field generating means are deactivated, and outputting signals representative of these variations, and - an electronic block (19; 600-612), producing said predetermined control signals and receiving signals in from the detector means.   The nuclear magnetic logging instrument of claim 1, wherein the detector means is a pumping helium magnetometer. laser.   The nuclear magnetic logging tool of claim 1, wherein the detector means   are constituted by a superconducting   quantum ferrific SQUID (18; 20; 620).   The nuclear magnetic logging instrument of claim 3, further comprising means (21) for cooling the SQUID detector means (20) to maintain them at a temperature below   superconducting transition of their supracon-   ducer. The nuclear magnetic logging instrument of claim 1, wherein the magnetic field generating means (12) is a coil solenoid form. 22 2622977   The nuclear magnetic logging instrument of claim 3, wherein the SQUID uses a   superconducting material at high temperature.   The nuclear magnetic logging instrument of claim 4, wherein the cooling means (21) employs a Joule-Thomson refrigeration cycle. The nuclear magnetic logging instrument of claim 6, wherein the high temperature superconducting material is substantially   consisting of a perovskite oxygen deficient.   The nuclear magnetic logging instrument of claim 8, wherein the high temperature superconducting material is substantially consisting of YBa2Cu307.   The nuclear magnetic logging instrument of claim 3, further comprising a pair of axial gradiometer superconducting detecting coils (51, 52) flux-coupled to the SQUID, one (51) of which is arranged to inside the magnetic field generating means (12) and the other coil (52) being arranged at   the outside of the magnetic field generating means (12).   The nuclear magnetic logging tool of claim 10, further comprising a shield   superconductor arranged around the SQUID.   12. The nuclear magnetic dyadography instrument of claim 4, wherein the SQUID '(20) and the cooling means (21) are mounted adjacent one of   the other, on a thermally conductive substrate (30).   The nuclear magnetic logging instrument of claim 12, wherein the substrate (30) is in 23 2622977   SrTiO3. 15. A method of nuclear magnetic logging, characterized by the following steps: - production of a magnetic field in a borehole, - removal of this magnetic field, then - detection, essentially simultaneously, of the sinusoidal magnetic fields and with slow variation resulting from the precession of the mobile nuclei around the Earth's magnetic field.