GB2382149A

GB2382149A - Method of suppressing spurious magnetic signals detected by a magnetic gradiometer

Info

Publication number: GB2382149A
Application number: GB0222498A
Authority: GB
Inventors: Alexey Vladimirovich Veryadkin
Original assignee: Gravitec Instruments Ltd
Current assignee: Gravitec Instruments Ltd
Priority date: 2001-09-27
Filing date: 2002-09-27
Publication date: 2003-05-21
Anticipated expiration: 2022-09-27
Also published as: GB0222498D0; GB0123269D0; GB2382149B

Abstract

A method of suppressing spurious magnetic signals in a magnetic gradiometer comprises providing two gradiometers S1, S2 spaced apart with their sensitive axis aligned in the same direction; applying a reference magnetic signal to the gradiometers, which is distinguishable from the said spurious signals, and analysing the signals form the gradiometers and the reference signal to derive a signal indicative of the target signal. The reference and spurious magnetic signals may be from near field sources to the gradiometers and the target signals for a far field source. The output signal form one gradiometer is subtracted from that of the other to provide a first signal containing information about the spurious signals. A second signal with information about the reference signal is provided and used in conjunction with a gradiometer signal to produce a third signal. The three signals are processed to obtain a signal indicative of the target signal. The reference magnetic signal may be provided by an alternating current flowing in a conductive loop 1 formed around the gradiometers S1, S2. The first and third signals may be low-pass filtered 4, 6 to the same bandwidth.

Description

A Method for Real-Time Suppression of Spurious Magnetic Signals Detected by Direct Magnetic Gradiometer This invention relates to mobile magnetic gradiometry, particularly, though not exclusively, to an airborne magnetic surveying technique for measuring magnetic gradients directly inside an airplane.

Currently, there are no methods allowing magnetic sensors to take measurements onboard an airplane due to the high level of in-cabin spurious magnetic signals.

Normally, they originate from built-in magnetic sources like wires carrying an electric current or magnetised moving parts of that plane. Since the distance from the centre of a plane to a source like that is quite short compared to the distance to a far target source under surveying (for example, an ore deposit), the corresponding magnetic "noise" inside the plane may overcome a desired signal by orders of magnitude.

In WOOO/68702 a method and apparatus for the measurement of two off-diagonal magnetic gradients (Bxz, Byz) is described. This is done by the use of a stretched metallic current-carrying string fixed at both ends. The string is excited at its second eigenmode in the presence of a quasi-static magnetic gradient, provided that the current pumped into the string is an alternating one and it is tuned to that eigenmode's frequency. The amplitude of the string's vibration is coupled only to that magnetic gradient.

In order to measure a magnetic gradient, the string is pumped by an additional carrier-frequency current and is inductively coupled to a resonant bridge tuned to the carrier frequency (approx. 3 MHz). Then mechanical displacements of the string give a low-frequency modulation (at the rate of the second eigenmode of the string which normally lies between 600 and 1000 Hz) of

the carrier signal. A double-lock-in scheme provides firstly amplification and detection of the carrier signal and secondly amplification and detection of the low-frequency envelope with an amplitude proportional to the magnetic gradient measured.

Therefore, the sensor described above is a direct magnetic gradiometer which gives absolute magnetic gradient readings at its output.

It is an object of the present invention to provide a method for the measurement of desired magnetic gradients from a far target source based upon direct magnetic gradiometers where an immunity to the near noise-like magnetic signals is provided.

According to the invention, there is provided a method of suppressing spurious magnetic signals arising from relatively near sources to a direct magnetic gradiometer used for detecting magnetic signals arising from a relatively far target source, comprising: providing two direct magnetic gradiometers spaced apart with their sensitivity axes aligned along the same direction; providing a relatively near reference source of magnetic signals of a characteristic distinguishable from the spurious signals and to which the two gradiometers are arranged to respond; subtracting the outputs of the gradiometers one from the other to form a first electrical signal containing information about the spurious signals and not the target signals; providing a second electrical signal containing information about only the reference signal; using the second signal to extract the response of at least one of the gradiometers to the reference signal and to produce a third electrical signal indicative thereof; and processing the first, second and third electrical signals so as to form a signal indicative of the target signals.

The basic idea is that instead of separating a desired signal (i. e. signal from a far target source) from noise, this invention separates firstly that noise

(i. e. unknown signals from near magnetic sources) from a desired signal, and then, in real time, subtracts that noise from the total output provided by a magnetic gradiometer.

An embodiment of the invention will now be described by way of example and with reference to the accompanying drawing, in which Fig. 1 is a schematic view of a direct magnetic gradiometer arrangement suitable for carrying out the invention.

In order to distil near signals from far signals two direct magnetic gradiometers Si, S2 are placed side by side and separated by a distance 2d (say 10 cm).

Their sensitivity axes are aligned along the same, say, X direction (see Fig. 1).

A complementary pair of direct magnetic gradiometers S3, S4 is placed along the Y direction. It is assumed that each of the four direct magnetic gradiometers is placed vertically and measures one of the off-diagonal components of the magnetic gradient tensor Bij (ij = x, y, z).

With respect to the local coordinate frame chosen, as depicted in Fig. 1, the outputs of the gradiometers read as follows: sensor 1 (Si) measures Bxz (0 + d) ; sensor 2 (S2) measures Bxz (0 - d) ; sensor 3 (S3) measures Byz (0 + d) ; sensor 4 (S4) measures Byz (0 - d) ; where 2d is the distance between magnetic gradiometers.

By differencing the outputs of Sl and S2 one can obtain a differential combination

output (Sl)-output (S2) =Bxz (0+d)-Bxz (0-d) =2d Bxxz (O) (1) where Bxzx (O) = Bxxz (O) is a second spatial derivative of the magnetic induction component Bx taken

at the origin of the local coordinate frame XOY.

If the distance to a far target source is, say, R > > 2d and it is more than 100 m, then the combination (1) yields 2d Bxxz (far)- (2d/R) Bxz (far) < 10-3 Bxz (far) Therefore, all far signals of less than 50 nT/m will be completely suppressed from the output (1) at a coarse sensitivity level which is assumed to be 0.1 nT/m per 1 sec measurement interval. On the other hand the "near"contribution to the combination (1) is assumed to be reasonable as it is supported by multiple modelling results.

The only problem we have now is how to establish single-valued correspondence between the output (1) and that magnetic gradient which is measured by a single magnetic gradiometer. In other words, we need to establish absolute measure for the unknown near signals.

In order to do that, an ac near reference source (a current loop, see Fig. 1) is placed in the vicinity of all the single magnetic gradiometers providing a unique measure for all possible in-cabin magnetic sources. The plane of the current loop is the coordinate XOY plane which crosses all sensors at their mid points.

As is shown in Fig. 1, all the single magnetic gradiometers are placed symmetrically inside a current loop 1, which provides the same magnitude of an external near magnetic gradient at the gradiometer's inputs. The current loop is fed by an ac current source 2, at, say, a rate of a few cycles per second (3 to 10 Hz).

Actually, this frequency has to be chosen from a"quiet window" the magnetic noise spectrum of a particular aircraft. As an additional duty, the same current loop can be used for the system calibration.

The sensors (sol... S4) must be aligned inside that loop with a certain accuracy which is to be determined

from a concrete design of the system.

The amplitude of the reference magnetic gradient signal is obtained at the output of a synchronous detector 3, more correctly, at the output of its lowpass filter 4 having the same bandwidth as that of the output of the whole system (say 1 Hz).

The outputs of the gradiometers 81 and S2 are subtracted in subtracter 5 and filtered in low-pass filter 6 having the same bandwidth as filter 4. This provides a signal containing information on the near sources but not the far source.

The sum of the outputs of 81 and S2 is filtered by low-pass filter 7 again having the same bandwidth.

An adaptive filtering function is provided in which the gain k of gain elements 8 and 9 can be controlled.

At the output of the system we have only the following combination of signals Bxz (far, O) + Bxz (near, 0)-k Bxz (ref) (2) where k is a function of the differential output (1).

The adaptive filter shown in Fig. 1 calculates k in such a way that the last two terms in Eq. 2 cancel each other.

All functional blocks in Fig. 1 can be built as either hardware or virtual electronic components within a LabView environment and can be run as a part of the system's digital signal processing software.

Claims

Claims 1. A method of suppressing spurious magnetic signals arising from relatively near sources to a direct magnetic gradiometer used for detecting magnetic signals arising from a relatively far target source, comprising: providing two direct magnetic gradiometers spaced apart with their sensitivity axes aligned along the same direction; providing a relatively near reference source of magnetic signals of a characteristic distinguishable from the spurious signals and to which the two gradiometers are arranged to respond; subtracting the outputs of the gradiometers one from the other to form a first electrical signal containing information about the spurious signals and not the target signals; providing a second electrical signal containing information about only the reference signal; using the second signal to extract the response of at least one of the gradiometers to the reference signal and to produce a third electrical signal indicative thereof; and processing the first, second and third electrical signals so as to form a signal indicative of the target signals.
2. A method as claimed in claim 1 wherein the reference magnetic signal is generated from an alternating electric current of a frequency distinct from the effects of the spurious and target signals, and the second signal is proportional to said alternating electric current.
3. A method as claimed in claim 2 wherein the alternating current is supplied to a current loop adjacent the gradiometers.
4. A method as claimed in claim 2 or 3 wherein a synchronous detector arranged to receive the second signal is used to extract the response of the at least

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one of the gradiometers to the reference signal and to produce said third signal.
5. A method as claimed in any preceding claim wherein said first and third signals are subject to low-pass filtering to essentially the same bandwidth.
6. A method of suppressing spurious magnetic signals arising from relatively near sources to a direct magnetic gradiometer used for detecting magnetic signals arising from a relatively far target source, substantially as hereinbefore described with reference to the accompanying drawing.