GB2171523A

GB2171523A - Magnetic gradient detection

Info

Publication number: GB2171523A
Application number: GB08504729A
Authority: GB
Inventors: Philip Extance; Roger Edward Jones
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1985-02-23
Filing date: 1985-02-23
Publication date: 1986-08-28
Also published as: GB2171523B

Abstract

A magnetic field gradient detector is comprised by a plurality of magnetic field sensors disposed in a planar arrangement such that the five independent magnetic field gradients, from which all nine gradients of the full gradient tensor can be determined, can be measured without requiring measurements to be taken at a position spaced apart from the planar arrangement in a direction orthogonal thereto. A planar construction can be more robust and of lower volume than one requiring all three axes.

Description

SPECIFICATION Magnetic gradient detection This invention relates to magnetic gradient detection.

Magnetic anomaly detection (MAD) systems are used for submarine detection and geophysical survey systems. A convenient way of detecting an anomaly is to measure the gradient of the magnetic field. The Earth's field will be distorted by an anomaly so that a gradient of the ambient field results. A fibre optic magnetic gradient detector for detecting two orthogonal gradient components simultaneously is described in our co-pending Application No. 8504730 (Serial No ) (P. Extance-R.E. Jones 17-17. An optical fibre magnetic of an alternative construction and a gradient detector employing it is described in our co-pending Application No.

8504731 (Serial No ) (P. Extance-R.E. Jones 20-20).

According to the present invention there is provided a magnetic field gradient detector comprising a plurality of magnetic field sensors disposed in a planar arrangement such as to permit measurement of the five-axis magnetic field gradients from which the full nine axis magnetic field gradient tensor can be determined without requiring measurements to be taken at a position spaced apart from the planar arrangement in a direction orthogonal thereto.

According to another aspect of the present invention there is provided a method of determining the full nine axis magnetic field gradient tensor by measuring five independent magnetic field gradient tensor terms with magnetic field sensors disposed in a planar arrangement and calculating the remaining tensor terms therefrom.

Embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 illustrates schematically one possible sensor arrangement on three axes; Figure 2 illustrates schematically one possible sensor arrangement on two axes, a planar arrangement; Figure 3 illustrates, schematically, a planar arrangement of ring-core fluxgate magnetometers; Figure 4 illustrates, schematically, a planar arrangement of superconducting quantum interference device magnetometers; Figure 5 illustrates an all-fibre Mach-Zehnder interferometer magnetometer for measuring two orthogonal magnetic ngradients, and Figure 6 illustrates, schematically, a planar arrangement of "golf-ball" optical fibre magnetic field sensors.

A full description of the gradient of a magnetic field requires the knowledge of the nine components of the gradient tensor: dB, dB, dB dx dy dz dBv dBy dBV dx dy dz dB, dB, dB, dx dy dz However, from Maxwell's equations div B=O, therefore dB, dBV dB, ~+~+=O dx dy dz so that only two of these gradients need be measured to determine all three. Hence there is no need to measure dB, dz say. In addition, for this system curl B=O, so that the tensor is symmetrical, that is dBx dBz dBy dBx dBz dBy dz dx dz dy dy dz and only five independent components remain.The matrix of the components to be measured can be arranged in several ways, each having implication for the number of magnetic field sensors required and for the geometry of the sensor head. One possible arrangement (Arrangement I) is: dBx dBx dBx dx dy dz dBy dBy - dy dz This requires seven magnetic field sensors on three axes as indicated in Fig. 1 in which each arrow indicates a uniaxial magnetic field sensor with its axis in the direction of the arrow.

Another possible arrangement (Arrangement II) is: dBx dB - dx dy - dBy - dy dB, dB, dx dy This requires eight magnetic field sensors on two axes as indicated in Fig. 2. This second arrangement is interesting because one can calculate the three gradient components along the z axis without measuring along the z axis. Only two axes are necessary since the derivatives are only with respect to x and y. Similar matrix rearrangement can be made to remove the x-axis or y-axis measurements. These arrangements with eight sensors on two axes mean that the sensor head will be of a planar construction which will be advantageous in many applications, since it can have a more robust construction and lower volume than for one requiring all three axes. The present invention is concerned with such planar constructions.A five-axis magnetic field gradient detector of "planar" construction allows the measurement of the full gradient tensor without a baseline separation in one direction.

Various types of magnetic field sensors can be used. In particular the use of ring-core fluxgate magnetometers in the pianar construction of Fig. 2 affords a very simple geometry as indicated in Fig. 3. Each ring-core device can measure the magnetic field in two orthogonal directions in the the plane of the coil, so that with suitable drive currents a full "five axis" gradiometer can be constructed with just five ring-core devices 1 to 5.

Another type of sensor is the superconducting quantum interference device (SQUID) magnetometer which is extremely sensitive. A single-axis gradiometer consists of a pair of coils which are sensitive to the field perpendicular to the plane of the coils. A possible implementation is shown in Fig. 4. Such an array of eight pairs of coils 6-13 would significantly reduce the volume, which is important since the entire sensor head is held at liquid helium temperature.

Other possible magnetic field sensors would be Hall effect devices and magnetoresistors, although the minimum detectable gradients would be much higher.

In our abovementioned co-pending Application No. 8504730 (P. Extance--R.E. Jones 17-17) planar, and horizontal, coils of single mode optical fibre in the arms of an all fibre Mach-Zehnder interferometer are used as the magnetic field sensing elements. The fibre of the coils is magnetically sensitised by being coated with a magnetostrictive material and optical path length fluctuations in the coils induced by means of a varying magnetic excitation field, generated by means of a solenoid to which an ac signal is applied. The effect of a dc magnetic field on the magnetically sensitised fibre is to increase correspondingly the amplitude of the first harmonic component of an optical signal applied to the fibre.A magnetic gradient detector described in the abovementioned co-pending Application and able to detect two magnetic gradients at right angles to one another is schematically shown in Fig. 5. Two such magnetically sensitised portions B and C (planar horizontal fibre coils) of one arm 32 of the interferometer each have a respective electrical winding 47 and 48 to which a respective bias field frequency w, and w2 are applied. The other arm 30 of the interferometer has one sensitised portion A with which are associated two electrical windings 45 and 46, each of which is connected in series with a respective one of windings 47 and 48 whereby both bias field frequencies are applied to portion A.The signals at the detectors 39, 40 at frequency w, will relate to A and B and thus dBK/dy, whereas the signals at frequency w2 will relate to A and C and thus to dBK/dx. In Fig. 5 reference numeral 38 indicates a stabilised laser source, 34 and 35 indicated 3dB couplers, 33 indicates a PZT phase modulator, 41 indicates a differential amplifier and 42 indicates an integrator. The outputs of the detectors are also used to control the interferometer via the amplifier 41, integrator 42 and phase modulator 33.Whilst such planar fibres coils could be used in the planar detector arrangements described above with reference to Fig. 2 the construction would be complicated by the numbers of elements required A magnetically sensitised single mode optical fibre can alternatively be formed into a spherical shell shape by winding it in random directions on a support sphere of non-magnetic material. An omnidirectional response is thus achieved, that is a magnetic field in any direction will cause a length change of the optical fibre that can be detected interferometrically. Such a spherically wound fibre sensor is the subject of our co-pending Application No. 8504731 (P. Extance-R.E. Jones 20-20) which also describes a magnetic gradient sensor of the arrangement I type.With a spherically wound fibre sensor specific directionality is obtained by the application of a d.c. bias field in a desired direction. All three perpendicular components of a magnetic field can be measured by applying three perpendicular a.c. bias magnetic fields, each at a different frequency, to the spherically wound fibre. Each ac field may be provided by disposing the spherically wound fibre between a respective pair of Helmholtz coils or in a respective solenoid, for example. For a basic gradient detector two such spherically wound fibres separated by a predetermined baseline spacing are required for one component of the gradient, and the differential response of the two spherically wound fibres is measured.

Arrangement II with the use of "golf-ball" sensors is shown in Fig. 6. It is equivalent to Fig. 2 but only three spaced-apart golf-balls are required to produce the effect of eight field sensors.

Bias fields are required in the x, y and z directions. Fig. 6 shows a "central" "golf-ball" 64 at the origin of the x, y and z axes and two golf-balls 65 and 66 spaced apart from the origin in the z and y directions, respectively, by baseline spacings dx and dy respectively. In order to obtain the dB,/dx and dB,/dx terms of arrangement I, "golf-balls" 64 and 65 each require the application of the same bias fields in the x and z directions, that is bias fields fi, and v For the dB,/dy, dBv/dy and dB,/dy terms, "golf-balls" 64 and 66 each require the application of the same bias fields in the x, y and z directions, that is bias fields ,ii',, Se and ',. Thus five bias fields, or rather fields at five different frequencies, are required. At golf-ball 64 the bias fields in the x and z directions will each have two components at different frequencies so that separate bias field generating coils may be necessary, although suitable means for using only one coil can be designed. The three "golf-balls" of Fig. 6 can simply be inserted in an interferometer similar to Fig. 5 in place of the planar sensing coils, although more bias field coils would be necessary and more detectors, or alternatively a spectrum analyser, would be required so that the five frequencies of the bias fields can all be detected. Thus the five independent components of a field gradient, which are required to determine the remaining four components of the field gradient, can be measured and the full field gradient tensor can be determined.

Claims

1. A magnetic field gradient detector comprising a plurality of magnetic field sensors disposed in a planar arrangement such as to permit measurement of the five-axis magnetic field gradients from which the full nine axis magnetic field gradient tensor can be determined without requiring measurements to be taken at a position spaced apart from the planar arrangement in a direction orthogonal thereto.

2. A detector as claimed in claim 1, wherein the magnetic sensors are such as to measure the magnetic fields at three directions at right angles at a first position in the planar arrangement, are such as to measure the magnetic fields in the three directions at right angles at a second position in the planar arrangement and are such as to measure the magnetic fields in two of the three directions at right angles at a third position in the planar arrangement, which first position is disposed at the origin of a pair of orthogonal axes, which second position is spaced apart from the first position and lies on one of the pair of orthogonal axes and which third position is spaced apart from the first position and lies on the other of the pair of orthogonal axes.

3. A detector as claimed in claim 2 and comprising a respective three magnetic field sensors disposed at each of the first and second positions and two magnetic field sensors disposed at the third position.

4. A detector as claimed in claim 3 wherein the magnetic field sensors are comprised by superconducting quantum interference device magnetometers.

5. A detector as claimed in claim 2 and comprising a respective pair of orthogonal ring-core fluxgate magnetometers disposed at each of the first and second positions and a further ringcore fluxgate magnetometer disposed at the third position.

6. A detector as claimed in claim 2 and including at each of the first second and third positions a respective "golf-ball" comprising a length of magnetically sensitised single mode optical fibre wound in substantially random directions into a spherical shell shape together with means for producing a respective bias magnetic field thereat in each of the directions for which measurements are required, which bias fields have respective different frequencies.

7. A detector as claimed in claim 6 wherein the "golf-ball" at the first position is coupled in one arm of an all-fibre Mach-Zehnder interferometer and wherein the "golf-balls" at the second and third positions are coupled in series in the other arm of the interferometer.

8. A detector as claimed in claim 1 or claim 2, wherein the magnetic field sensors are comprised by Hall effect devices.

9. A detector as claimed in claim 1 or claim 2, wherein the magnetic field sensors are comprised by magnetoresistors.

10. A detector as claimed in claim 1 or claim 2, wherein the magnetic field sensors are comprised by magnetically sensitised optical fibre.

11. A detector as claimed in claim 10, wherein the optical fibre is formed into coils.

12. A magnetic field gradient detector substantially as herein described with reference to Fig.

2 with or without reference to Fig. 3, Fig. 4 or Figs. 5 and 6 of the accompanying drawings.

13. A method of determining the full nine axis magnetic field gradient tensor by measuring five independent magnetic field gradient tensor terms with magnetic field sensors disposed in a planar arrangement and calculating the remaining tensor terms therefrom.