Device for measuring an electric field in a conducting medium

A device for measuring an electric field in a conducting medium, including: an insulating enclosure; first, second, and third pairs of electrodes, the electrodes of a same pair being arranged on opposite external walls of the enclosure, and the electrodes of the first, second, and third pairs being centered on first, second, and third orthogonal axes; a first conductive coil; a first pair of switches enabling to alternately connect the first coil between the electrodes of the first, of the second, and of the third pair of electrodes; and a single magnetometer.

This application claims the priority benefit of French Patent application number 15/51296, filed on Feb. 17, 2015, the contents of which is hereby incorporated by reference in its entirety to the maximum extent allowable by law.

BACKGROUND

The present application relates to a device for measuring an electric field in a conducting medium. It particularly applies to undersea electric field measurements.

DISCUSSION OF THE RELATED ART

An offshore electric field measurement device, or sea electrometer, comprising two immersed electrodes distant by a distance1, connected to a voltage measurement device, has already been provided. To measure an electric field E in the area located between the two electrodes, voltage U between the electrodes is measured. Field E can be deduced from this measurement by formula E=U/d. Electrometers of this type are called “voltage measurement” electrometers.

A disadvantage of voltage measurement electrometers is that only a very small portion of the current propagating in the conducting medium formed by the sea water is deviated in the voltage measurement device, which raises metrology problems. Further, voltage measurement electrometers are generally bulky. Indeed, to obtain a satisfactory signal-to-noise ratio, distance d separating the two electrodes should be relatively large, typically in the range from a few meters to a plurality of kilometers.

To overcome these disadvantages, another type of device of electric field measurement in a conducting medium, called “current measurement” device, which determines the density of current generated, under the effect of the electric field, in a fixed conducting medium volume, is provided.

A device of this type is for example described in French patent N°9102273 of the applicant (filed on Feb. 26, 1991). Devices of this type, which will be called current measurement electrometers, may be more compact than voltage measurement electrometers, and may further have a better sensitivity.

The present application more specifically aims at current measurement electrometers. It is indeed needed to improve certain aspects of existing current measurement electrometers.

SUMMARY

Thus, an embodiment provides a device for measuring an electric field in a conducting environment, comprising: an insulating enclosure; first, second, and third pairs of electrodes, the electrodes of a same pair being arranged on opposite external walls of the enclosure, and the electrodes of the first, second, and third pairs being centered on first, second, and third orthogonal axes; a first conductive coil; a first pair of switches enabling to alternately connect the first coil between the electrodes of the first, of the second, and of the third pair of electrodes; and a single magnetometer.

According to an embodiment, the device further comprises: second and third conductive coils; and second and third pairs of switches respectively enabling to alternately connect the second coil between the electrodes of the first, of the second, and of the third pair of electrodes, and of alternately connecting the third coil between the electrodes of the first, of the second, and of the third pair of electrodes, wherein the magnetometer is capable of measuring a magnetic field induced by the flowing of currents through the first, second, and third coils.

According to an embodiment, the first, second, and third coils are respectively series-connected to first, second, and third variable resistors.

According to an embodiment, the device comprises a control circuit capable of successively implementing the steps of: controlling the switches so that the first, second, and third coils are not connected to the electrodes, and measuring the module and the direction of the ambient magnetic field; and controlling the variable resistors so that the flowing of a current in the parallel association of a first branch comprising the first variable resistor and the first coil, of a second branch comprising the second variable resistor and the second coil, and of a third branch comprising the third variable resistor and the third coil, induces, at the level of the magnetometer, a magnetic field substantially parallel to the ambient magnetic field.

According to an embodiment, the control circuit is further capable of successively implementing the steps of: controlling the switches so that the first, second, and third branches are connected in parallel between the electrodes of the first pair of electrodes and are not connected to the second and third pairs of electrodes, and then measuring the module of the magnetic field at the level of the magnetometer; controlling the switches so that the first, second, and third branches are connected in parallel between the electrodes of the second pair of electrodes and are not connected to the first and third pairs of electrodes, and then measuring the module of the magnetic field at the level of the magnetometer; and controlling the switches so that the first, second, and third branches are connected in parallel between the electrodes of the third pair of electrodes and are not connected to the first and second pairs of electrodes, and then measuring the module of the magnetic field at the level of the magnetometer.

According to an embodiment, the first, second, and third coils have non-parallel longitudinal axes, for example, substantially orthogonal.

According to an embodiment, the magnetometer is a scalar magnetometer.

According to an embodiment, the device comprises a control circuit capable of successively implementing the steps of: controlling the switches so that the first coil is not connected to the electrodes, and performing a vector measurement of the ambient magnetic field; controlling the switches so that the first coil is connected between the electrodes of the first pair of electrodes and is not connected to the second and third pairs of electrodes, and then performing a vector measurement of the magnetic field at the level of the magnetometer; controlling the switches so that the first coil is connected between the electrodes of the second pair of electrodes and is not connected to the first and third pairs of electrodes, and then performing a vector measurement of the magnetic field at the level of the magnetometer; and controlling the switches so that the first coil is connected between the electrodes of the third pair of electrodes and is not connected to the first and second pairs of electrodes, and then performing a vector measurement of the magnetic field at the level of the magnetometer.

According to an embodiment, the magnetometer is a vector magnetometer.

According to an embodiment, the electrodes have substantially the same surface area, and a same distance separates the two electrodes of each of the three pairs of electrodes.

DETAILED DESCRIPTION

The same elements have been designated with the same reference numerals in the different drawings and, further, the various drawings are not to scale. Further, only those elements which are useful to the understanding of the described embodiments have been detailed. In particular, the control and/or analysis and processing circuits of the described electrometers have not been detailed, the forming of these circuits being within the abilities of those skilled in the art having read the present description. Further, in the following description, when reference is made to terms qualifying absolute positions, such as terms “front”, “rear”, “top”, “bottom”, “left”, “right”, etc., or relative positions, such as terms “above”, “under”, “upper”, “lower”, etc., or to terms qualifying directions, such as terms “horizontal”, “vertical”, etc., it is referred to the orientation of the drawings. Further, in the following description, unless otherwise specified, expressions “approximately”, “substantially”, and “in the order of” mean to within 10%, preferably to within 5%.

FIG. 1schematically shows an example of a current-measurement electrometer100intended to be placed in a conducting medium where an electric field is desired to be measured, for example, sea water. Electrometer100comprises a tightly closed enclosure101, shown in dotted lines in the drawing, delimiting a volume103of an insulating medium, for example, air or vacuum. Two electrodes105and105′ are arranged outside of enclosure101, on opposite surfaces of the enclosure, so that each electrode has a surface in contact with the conducting medium when the electrometer is submerged. In this example, the two electrodes105and105′ are approximately planar and parallel to each other, have substantially the same surface area S, and are separated by a distance D. A current-measurement device107connects the two electrodes.

In operation, electrometer100is submerged in a conducting medium submitted to an electric field E which is desired to be measured. Under the effect of electric field E, a current of density J=α*σe*E flows in the conducting medium, σedesignating the electric conductivity of the conducting medium, and α being a coefficient which is a function of the ratio of electric conductivity σcof the sensor to electric conductivity σeof water, with σc=D/(S*Zc), Zcdesignating the impedance of the electrometer, Zc=Zelec+Zint, Zelecbeing the impedance of the electrodes of the electrometer, and Zintbeing the impedance of the element(s) connected between the electrodes, including at least current-measurement device107.

Electrodes105and105′ enable to channel current density J generated in the conducting medium under the effect of field E. Channeled current I flowing through electrodes105and105′ is equal to I=S*J. Current I may be measured by current-measurement device107.

Conductivity σeof the conducting medium where electrometer100is used is assumed to be known and, knowing the characteristics of the electrometer, coefficient α may be determined. Electric field E can then be deduced from the measurement of current I by formula E=I/(S*α*σe). The electrometer may comprise processing means, not shown, capable of determining electric field E from the measurement of current I.

FIG. 2shows in more detailed fashion an example of a current-measurement electrometer described in above-mentioned French patent N°9102273. In this example, the measurement of the current channeled by the electrodes is performed by measuring the magnetic field generated by a coil having the channeled current flowing therethrough. The device ofFIG. 2comprises two electrodes12and14placed on opposite surfaces of an insulating enclosure10, and connected to a measurement device internal to enclosure10. A first coil16is series-connected with electrodes12and14. The device ofFIG. 2further comprises a second coil18of same longitudinal axis and of same geometric center C as coil16. As then conduct an identical current, coils16and18generate identical magnetic fields, of opposite directions. Coil18is connected via a switch20to a variable current source22. Two switches24and26enable to connect coils16and18in parallel. A magnetometric sensor28is arranged inside of coils16,18. This sensor is connected to control means30which provide, among others, the excitation signals necessary to the sensor operation, and with analysis and processing means32which enable to determine an electric field based on the measurement of the magnetic field.

To determine an electric field in a conducting medium, insulating enclosure10is plunged into the conducting medium and positioned along the axis of the field which is desired to be measured. The axis connecting the electrodes determines the direction of the measured electric field component.

First, switch20is off, and switches24and26are on. Coils16and18are thus connected in parallel and the current originating from electrodes12and14is equal in each of them. The coils being identical and conducting identical currents of opposite directions, the magnetic fields generated by each of them are identical but of opposite direction. Magnetometer28thus only measures the local magnetic field which is recorded and stored in the memory.

In a second step, switches24and26are off while switch20is on. Coil16, conducting a current originating from electrodes12and14, generates a magnetic field which superposes to the local magnetic field. As it is controlled by analysis and processing means32, current source22delivers a current having an intensity such that the magnetic field generated by coil18cancels the magnetic field generated by coil16under the effect of the current channeled by the electrodes. When the magnetic field measured by sensor28is equal to the previously-recorded value of the local magnetic field, the intensity of the current output by current source22is equal to the intensity of the current flowing through coil16. Knowing the value of the current flowing through coil16, the electric field in the axis connecting electrodes12and14of the device can then be determined.

FIG. 3of above-mentioned French patent N°9102273 shows a variation of the electric field measurement device ofFIG. 2, enabling to determine two perpendicular components of the electric field. However, this device is relatively complex. Indeed, the measurement device internal to insulating enclosure10is formed by duplicating the architecture of a unidirectional measurement device. Thus such a device comprises at least one magnetometer per electric field measurement direction.

An object of the described embodiments is to provide a device for measuring the electric field in a conducting medium, or electrometer, enabling to measure the three components of the electric field, such a device having a simpler architecture than known devices capable of measuring a plurality of components of the electric field. In particular, an object of the described embodiments is to provide a device comprising a single magnetometer.

FIGS. 3A and 3Bschematically show an embodiment of a current-measurement electrometer300intended to be placed in a conducting medium where an electric field is desired to be measured, for example, sea water.FIG. 3Ais a cross-section view along plane A-A ofFIG. 3B, andFIG. 3Bis a cross-section view along plane B-B ofFIG. 3A.

Electrometer300comprises a tightly closed enclosure301, shown in dotted lines in the drawings, delimiting a volume303of an insulating medium, for example, air or vacuum. Three pairs of electrodes305,305′;307,307′; and309,309′ are arranged on external walls of the enclosure, so that each electrode has a surface in contact with the conducting medium. The electrodes of a same pair are preferably parallel to each other, and are arranged on opposite surfaces of enclosure301. In this example, electrodes305,305′ of the first pair, electrodes307,307′ of the second pair, and electrodes309,309′ of the third pair are respectively centered on first, second, and third substantially orthogonal axes crossing a same point located substantially at the center of enclosure301. Enclosure301for example has the general shape of an assembly of three cylinders of orthogonal axes crossing a same point located substantially at the center of each of the cylinders. The electrodes may then have the shape of disks placed on the cylinder surfaces, the electrodes of a same pair being arranged on the two opposite surfaces of a same cylinder. The described embodiments are however not limited to this specific configuration. As an example, enclosure301may have the general shape of an assembly of three portions of a tube having a square cross-section with orthogonal axes crossing a same point located substantially at the center of each of the tube portions. The electrodes may then have the shape of square plates placed on the transverse surfaces of the tube portions. As a variation, enclosure301may have the shape of a cube, the six electrodes respectively coating the six surfaces of the cube. Electrodes305,305′,307,307′,309, and309′ for example have substantially the same surface area S. A same distance D separates, for example, the two electrodes of each of the three pairs of electrodes.

A measurement device located inside of enclosure301will now be described. The measurement device comprises a first branch comprising a coil L1in series with a variable resistor R1, a second branch comprising a coil L2in series with a variable resistor R2, and a third branch comprising a coil L3in series with a variable resistor R3. The first branch comprises, connected to the end of resistor R1opposite to coil L1, a switch SW1with four positions enabling to connect resistor R1either to electrode305(position 1), or to electrode307(position 2), or to electrode309(position 3), or to none of the electrometer electrodes (position 0) and, connected to the end of coil L1opposite to resistor R1, a switch SW1′ with four positions enabling to connect coil L1either to electrode305′ (position 1), or to electrode307′ (position 2), or to electrode309′ (position 3), or to none of the electrometer electrodes (position 0). The second branch comprises, connected to the end of resistor R2opposite to coil L2, a switch SW2with four positions enabling to connect resistor R2either to electrode305(position 1), or to electrode307(position 2), or to electrode309(position 3), or to none of the electrometer electrodes (position 0) and, connected to the end of coil L2opposite to resistor R2, a switch SW2′ with four positions enabling to connect coil L2either to electrode305′ (position 1), or to electrode307′ (position 2), or to electrode309′ (position 3), or to none of the electrometer electrodes (position 0). The third branch comprises, connected to the end of resistor R3opposite to coil L3, a switch SW3with four positions enabling to connect resistor R3either to electrode305(position 1), or to electrode307(position 2), or to electrode309(position 3), or to none of the electrometer electrodes (position 0) and, connected to the end of coil L2opposite to resistor R2, a switch SW2′ with four positions enabling to connect coil L2either to electrode305′ (position 1), or to electrode307′ (position 2), or to electrode309′ (position 3), or to none of the electrometer electrodes (position 0).

Coils L1, L2, and L3have longitudinal axes having different directions. As an example, coils L1, L2, and L3have substantially orthogonal axes, crossing at a same point located substantially at the center of enclosure301. As an example, each of coils L1, L2, and L3has as a longitudinal axis the axis connecting at their centers the electrodes of the first, second, and third pairs of electrodes, respectively.

Electrometer300further comprises a single magnetometer311placed inside of enclosure301, capable of measuring the magnetic field generated by the flowing of a current in any of coils L1, L2, and L3. As an example, and as shown inFIGS. 3A and 3B, each of coils L1, L2, and L3is divided into two series-connected identical coil portions of same axis, arranged on either side of a central region of enclosure301. Magnetometer311may then be arranged in the central region which is not occupied by the coil spirals.

In this example, magnetometer311is an absolute scalar magnetometer, that is, a magnetometer capable of delivering an absolute measurement of the module of the magnetic field. As an example, magnetometer311is a nuclear magnetic resonance (NMR) magnetometer or, preferably, a helium magnetometer4(based on the atomic spectrometry of helium4) which has the advantage over NMR magnetometers of providing more accurate measurements, including for very low frequency fields.

The operation of electrometer300ofFIGS. 3A and 3Bwill now be described.

In a first step, switches SW1, SW1′, SW2, SW2′, SW3, and SW3′ are controlled to position 0 by a control circuit, not shown. Electrodes305,305′,307,307′,309, and309′ are then disconnected from the measurement circuit, and coils L1, L2, and L3conduct no current and thus generate no magnetic field. The only magnetic field seen by magnetometer311is the ambient magnetic field. Module B0of the ambient magnetic field is measured by magnetometer311, and stored by an analysis and processing circuit (not shown) of the electrometer. During this step or at a previous step, the orientation of the ambient magnetic field is further determined. A method of determining the direction of a magnetic field by means of a helium magnetometer4is for example described in the article entitled “On the calibration of a vectorial4He pumped magnetometer” of O. Gravant et al. (Earth Planets Space, 53, 949-958, 2001). As a variation, the orientation of the ambient magnetic field may be previously determined by a measurement device external to the electrometer, and transmitted to the analysis and processing circuit of the electrometer.

At a second step, switches SW1, SW1′, SW2, SW2′, SW3, and SW3′ are controlled to position 1. Electrodes307,307′,309, and309′ are then disconnected from the measurement circuit, and current Ixcollected by electrode pair305,305′ is distributed in coils L1, L2, and L3according to a distribution which depends on the values of resistors R1, R2, and R3. The values of resistors R1, R2, and R3are set by a control circuit so that general magnetic field Bxgenerated by the parallel association of coils L1, L2, and L3is substantially collinear to the ambient magnetic field. Module B1of the magnetic field seen by magnetometer311, which corresponds to the sum of ambient magnetic field B0and of the magnetic field Bxgenerated by the coils under the effect of current Ix, is then measured by magnetometer311. The analysis and processing circuit can deduce therefrom the module of magnetic field Bx, according to formula Bx=B1−B0. Knowing the electric/magnetic transfer rate of each of coils L1, L2, and L3, current Ixcan be deduced. Current Ixcorresponds to the current induced by component Exof the electric field along an axis x orthogonal to electrodes305and305′. Thus, similarly to what has been explained hereabove in relation withFIG. 1, one can deduce from current Ixcomponent Exof the electric field according to formula Ex=Ix/(S*α*σe).

At a third step, switches SW1, SW1′, SW2, SW2′, SW3, and SW3′ are controlled to position 2. Electrodes305,305′,309, and309′ are then disconnected from the measurement circuit, and current Iycollected by electrode pair307,307′ is distributed in coils L1, L2, and L3. As for the previous step, the values of resistors R1, R2, and R3are set so that general magnetic field Bygenerated by the parallel association of coils L1, L2, and L3under the effect of current Iyis substantially collinear to the ambient magnetic field. Module B2of the magnetic field seen by magnetometer311is then measured by magnetometer311. The analysis and processing circuit can deduce therefrom the module of magnetic field Byaccording to formula By=B2−B0and, similarly to what has been described for the second step, component Eyof the electric field according to an axis y orthogonal to electrodes307and307′.

At a fourth step, switches SW1, SW1′, SW2, SW2′, SW3, and SW3′ are controlled to position 3. Electrodes305,305′,307, and307′ are then disconnected from the measurement circuit, and current Izcollected by electrode pair309,309′ is distributed into coils L1, L2, and L3. As for the previous step, the values of resistors R1, R2, and R3are set so that general magnetic field Bzgenerated by the parallel association of coils L1, L2, and L3under the effect of current Izis substantially collinear to the ambient magnetic field. Module B3of the magnetic field seen by magnetometer311is then measured by magnetometer311. The analysis and processing circuit can deduce therefrom the module of magnetic field Bzaccording to formula Bz=B3−B0and, similarly to what has been described for the second step, component Ezof the electric field according to an axis z orthogonal to electrodes309and309′.

Thus, the above-mentioned steps enable to determine the three components Ex, Ey, and Ezof the electric field in the conducting medium having electrometer300plunged into it.

Preferably, the four above-mentioned steps are repeated periodically (except for the phase of determining the ambient magnetic field, which may be carried out only once), at a sampling frequency (switching frequency of switches SW1, SW1′, SW2, SW2′, SW3, and SW3′) at least twice greater than the maximum frequency of the electric field which is desired to be measured (Nyquist-Shannon criterion), for example, at a sampling frequency in the range from 1 to 1,000 Hz. It should be noted that impedance Zelecof the electrodes, used to calculate electric conductivity σcof the sensor, then is the impedance at the sampling frequency of the device, which may be lower than the DC impedance.

FIGS. 4A and 4Bschematically show an example of another embodiment of a current-measurement electrometer400.FIG. 4Ais a cross-section view along plane A-A ofFIG. 4B, andFIG. 4Bis a cross-section view along plane B-B ofFIG. 4A.

Electrometer400ofFIGS. 4A and 4Bcomprises elements common with electrometer300ofFIGS. 3A and 3B. These elements will thus not be described again. In the following, only the differences between electrometers300and400will be detailed.

Electrometer400differs from electrometer300essentially by its measurement device internal to insulating enclosure301.

The measurement device of electrometer400comprises a single coil L. It further comprises, connected to a first end of coil L, a switch SW with four positions enabling to connect the first end of the coil either to electrode305(position 1), or to electrode307(position 2), or to electrode309(position 3), or to none of the electrometer electrodes (position 0), and, connected to a second end of coil L opposite to the first end, a switch SW′ with four positions enabling to connect the second end of the coil either to electrode305′ (position 1), or to electrode307′ (position 2), or to electrode309′ (position 3), or to none of the electrometer electrodes (position 0).

Electrometer400further comprises a single magnetometer411placed inside of enclosure301, capable of measuring the magnetic field generated by the flowing of a current through coil L. In the shown example, magnetometer411is placed inside of coil L, substantially at the center thereof.

Magnetometer411is a vector magnetometer, that is, a magnetometer capable of providing a measurement of the three components of the magnetic field. As an example, magnetometer411is a helium vector magnetometer4of the type described in the above-mentioned article of O. Gravant et al. As a variation, magnetometer411is a fluxgate magnetometer, or an induction magnetometer. It should be noted that such a magnetometer does not necessarily provide an absolute measurement of the three components of the magnetic field, but provide at least a relative measurement.

The operation of electrometer400ofFIGS. 4A and 4Bwill now be described.

In a first step, switches SW and SW′ are controlled to position 0 by a control circuit, not shown. Electrodes305,305′,307,307′,309, and309′ are then disconnected from the measurement circuit, and coil L conducts no current and thus generates no magnetic field. Ambient magnetic field B0is then vectorially measured by magnetometer411, and stored by an analysis and processing circuit (not shown) of the electrometer.

At a second step, switches SW and SW′ are controlled to position 1. Electrodes307,307′,309, and309′ are then disconnected from the measurement circuit, and current Ixcollected by electrode pair305,305′ flows through coil L, inducing a magnetic field Bxparallel to the axis of coil L. A magnetic field B1, corresponding to the sum of magnetic field Bxand of ambient field B0, is then vectorially measured by magnetometer411. By vector subtraction (component by component), an absolute value of field Bxcan be deduced therefrom. As in the example ofFIGS. 3A and 3B, current Ixand component Exof the electric field can be deduced from this measurement according to formula Ex=Ix/(S*α*σe).

At a third step, switches SW and SW′ are controlled to position 2. Electrodes305,305′,309, and309′ are then disconnected from the measurement circuit, and current Iycollected by electrode pair307,307′ flows through coil L, inducing a magnetic field Byparallel to the axis of coil L. A magnetic field B2, corresponding to the sum of magnetic field Byand of ambient field B0, is then vectorially measured by magnetometer411. Similarly to what has been described for the second step, the analysis and processing circuit can deduce the module of magnetic field By, the intensity of current Iy, and the value of component Eyof the electric field.

At a third step, switches SW and SW′ are controlled to position 3. Electrodes305,305′,307, and307′ are then disconnected from the measurement circuit, and current Izcollected by electrode pair309,309′ flows through coil L, inducing a magnetic field Bzparallel to the axis of coil L. A magnetic field B3, corresponding to the sum of magnetic field Bzand of ambient field B0, is then vectorially measured by magnetometer411. Similarly to what has been described from the second step, the analysis and processing circuit can deduce therefrom the module of magnetic field Bz, the intensity of current Iz, and the value of component Ezof the electric field.

Thus, the above-mentioned steps enable to determine the three components Ex, Ey, and Ezof the electric field in the conducting medium having electrometer400plunged into it.

Preferably, the four above-mentioned steps are periodically repeated at a sampling frequency (switching frequency of switches SW, SW′) at least twice greater than the maximum frequency of the electric field which is desired to be measured (Nyquist-Shannon criterion). As an example, the sampling frequency is in the range from 1 to 1,000 Hz. Impedance Zelecof the electrodes, used to calculate electric conductivity σcof the sensor, then is the impedance at the sampling frequency of the device, which may be lower than the DC impedance.

An advantage of the described embodiments is that they enable to provide a vector measurement (3D) of the electric field, while preserving a relatively simple architecture, and in particular comprising a single magnetometer.

The embodiment ofFIGS. 3A and 3Bhas the additional advantage of providing an absolute measurement of the ambient magnetic field.

The architecture of the embodiment ofFIGS. 4A and 4Bis simpler than that of the embodiment ofFIGS. 3A and 3B, since it comprises a single coil (against three in the embodiment ofFIGS. 3A and 3B), and does not comprise variable resistors R1, R2, and R3.

It should be noted that vector magnetometers generally have a 1/f noise, which may be disturbing to perform electric field measurements in low frequency bands. The architecture ofFIGS. 4A and 4Benables to do away with this disadvantage, since the switching frequency of switches SW, SW′ may be selected to be sufficiently high (for example, higher than 200 Hz) to take the 1/f noise down to an acceptable level.

The architecture ofFIGS. 3A and 3Bfurther has the advantage of being less sensitive to alignment defects than the architecture ofFIGS. 4A and 4B. Indeed, in the embodiment ofFIGS. 4A and 4B, it should be ascertained that the magnetic orientation of the magnetometer relative to the terrestrial field remains stable all along the measurement. Further, the alignment of magnetometer411relative to coil L should remain stable during the measurement.