GB2135458A - A magnetic anomaly detector - Google Patents

Abstract

A detector capable of detecting magnetic field gradients that are too small for detection by conventional magnetometry techniques is described. The voltage signal induced by a magnetic field gradient across a rotated D-shaped coil (8) is converted to an optical signal in a self-powered transmitter (12) rotating with the coil and transmitted via an optical slip-ring including transmitter (13) and receiver (14) to remote processing and indicating circuits (15). In a method of using the detector for locating buried ferrous objects, the burial ground is first swept with a magnet so as to magnetise the objects, into saturation if possible, and thereby maximise the magnetic field anomalies for subsequent detection. <IMAGE>

Description

SPECIFICATION A magnetic anomaly detector This invention relates to a magnetic anomaly detector suitable for use, in particular but not exclusively, in a method for detecting buried ferrous objects which are too small for detection by conventional magnetometry methods.

Various magnetometers are known for the purpose of mineral prospecting which operate by detecting local distortions in the earth's magnetic field. In particular, rotating coil magnetic induction gradiometer systems have been proposed for use in investigating the static gradient of the field.

These are of two basic types, the first of which uses the well known principle of the earth inductor and comprises two spaced-apart identical conductive coils which are both synchronously rotated in the field about a mutual or parallel axes diametral to the coils. The coils are opposingly connected and balanced so that the voltages induced across each by rotation in the field will cancel when the field is uniform and any resultant induced voltage will be indicative only of a difference in the field strengths at the separate locations of the two coils, ie the field gradient. This type of gradiometer suffers from all the disadvantages inherent in any system requiring two identically moving components.

A second less well known type proposed in Patent No GB676404, overcomes these disadvantages by requiring one coil only. The coil is a semi-circular annulus, ie wound on a former of semi-circular cross section, and is rotated about an axis extending orthogonally from the plane of the annulus through its circular centre, ie mid-way along the diametral portion of the coil. When this coil is rotated in a magnetic field no current is induced in the semi-circular portion, which always moves along its own length, but opposing currents are induced in the two equal halves of the diametral portion, which halves always move perpendicularly to their own length in opposing directions. These two opposing currents are selfcancelling when the field is homogeneous but will provide a differential output voltage when a field gradient exists.The laminar rotation envelope of this gradiometer is particularly advantageous when it is desired to rotate the coil as close as possible to the earth's surface.

Neither of these gradiometer types have resulted in apparatus of any practical value for detecting small anomalies in the earth's magnetic field such as might induce a signal of no more than a microvolt in the gradiometer coil for example. This is no doubt due to a major extent to the difficulties associated with transferring small signal voltages from rotating circuit elements to static circuit elements via moving contact systems, any of which systems inevitably generate random voltage fluctuations that can well exceed the signals of interest.

The present invention seeks to provide a magnetic anomaly detector capable of detecting induced coil voltages down to at least microvolt level, and further to provide a method of locating small ferrous objects using the detector.

In accordance with the present invention, a magnetic anomaly detector having a housing, at least one electrically conductive coil rotatable within the housing about an axis, and a drive means for rotating the coil about the said axis, is characterised by including: a self-powered electrooptic transmitter electrically connected to the coil so as to emit optical signals representative of voltage signals induced across the coil during rotation in a magnetic field, which transmitter is disposed within the housing so as to rotate in fixed relationship with the coil and to transmit the optical signals along the said axis; a photo-electric receiver having a photo-receiving surface fixed to the housing on the said axis in optical confrontation with the transmitter, which receiver is arranged for generating voltage output signals in response to the optical signals received from the transmitter; and output signal processing and indicating means external to and remote from the housing, electrically coupled to the receiver.

The photo-electric receiver may be located either in the housing or remote from the housing with the signal processing and indicating means, in which latter case the photo-receiving surface is coupled to the receiver via fibre optic link.

Preferably the coil is of the previously described second gradiometer type, ie a semi-circular annulus rotated about an axis perpendicular to its own plane and extending through the circle centre. Conveniently a pair of such semi-circular coils may be disposed in complementary arrangement upon a disc-shaped support member so as to both rotate about a common central axis and thereby provide a balanced rotating mass.

This paired coil arrangement hereinafter referred to as the revolving disc arrangement, has the added advantage that the two coils may be electrically connected in series summation so as to provide a substantially doubled output voltage.

Alternatively the coil may comprise two opposingly connected, spaced-apart coils of the previously described first gradiometer type, arranged to rotate together either both about a common diametral axis or each about respective diametral axes disposed in parallel. In this latter arrangement a separate transmitter and receiver is required for each axis, the output signals from the two receivers then being differenced by the signal processing means.

Preferably the drive means is fluidic so as to avoid generation of any magnetic anomalies in the vicinity of the coils, as would occur with conventional electric drive systems. A preferred pneumatic arrangement comprises a vaned rotatable disc driven by compressed air. This disc may also conveniently comprise the coil support member of the revolving disc arrangement.

An embodiment of the invention will now be described by way of example, only with reference to the attached drawings of which Figure 1 is a perspective view of a revolving disc magnetic anomaly detector, Figure 2 is a base view of the same detector with the base cover removed to show the two semi-circular coils, Figure 3 is a vertical part-section of the same detector taken on line Ill-Ill of Figure 2 and Figure 4 is a block schematic diagram of the electrical arrangement of the same detector.

The magnetic anomaly detector illustrated in Figures 1 to 3 comprises a housing 1 having a base cover 2 and a handle 3, all fabricated from a lightweight non-magnetic material eg a plastics material.

Mounted within the housing 1 and the base cover 2 respectively are two opposed, frustoconical annular bearings 4 and 5 within which is journalled a shaft 6 so as to rotate about an axis A.

The bearings and shafts are fabricated from a low friction plastics material such as polytetrafluoroethylene.

Mounted upon the shaft 6 is a disc 7 of an electrically non-conductive material which supports upon one side adjacent to the base cover 2, a pair of substantially semi-circular coils 8, and on the other side a peripheral ring of turbine blades 9. Compressed air is admitted into the housing 1 via an orifice 11 adjacent to the blades 9 so as to impel the disc 7 and the shaft 6 about the axis A.

The coils are connected in voltage summation to a transmitter circuit pack 12 attached to the shaft 6 and having an output element comprised by a light emitting diode 1 3 located upon the axis A at the upper end of the shaft 6. Attached to the housing 1 on the axis A and confronting the diode 13 is a photo-diode 14 comprising the input element of a receiver circuit pack 1 5 located at the remote end of the handle 3 (see Figure 1).

The circuit arrangement will now be described with reference to Figure 4. The presence of a magnetic anomaly within the rotation envelope of the coils 8 will cause an alternatively positive and negative voltage signal at each 1 800 of revolution.

The series summed voltage signals of the two coils 8 are amplified in a first AC amplifier 20 to provide an input to a light emitting diode modulator 21. Both of these circuits 20 and 21 are located with their own battery power supply 22 within the transmitter circuit pack 12, all the components of the pack being selected to be of low ferrous mass and being located with respect to the coils 8 so as to cause minimal distortion of the magnetic field. The output of the modulator 12 is coupled to the light emitting diode 1 3 symmetrically located on the axis A.Light signals representative of the coil voltage signals are thus transmitted by the rotating diode 1 3 to the confronting stationary photo diode 14, which diode is electrically coupled to the remote receiver circuit pack 1 5 containing a further AC amplifier 23, a threshold circuit 24, an audio alarm 25 and an associated power supply 26.

In use, the air line 10 is connected to a compressed air supply and the detector is scanned over the region of interest. Any magnetic anomaly present within the rotation envelope of the coils 8 which is of sufficient size to induce a resultant signal at the threshold circuit 24 in excess of a pre-selected threshold level will cause the alarm to sound.

In a method for detecting small ferrous articles buried in the soil, the ground is first transitted by a sweep magnet, which may be a permanent magnet or an electromagnet, so as to induce magnetism in the articles for subsequent detection with a detector in accordance with the present invention. Preferably the sweep magnet is of sufficient strength to drive the articles into a condition of saturation.

The size and distance-off of a magnetised article that can be detected is of course dependent upon the parameters of the detector coils. For example, the total voltage induced across the two coils 8 of the previously described revolving disc arrangement by an object having a flux density of 1 0-4T buried 20 cm below the periphery of the rotation envelope, will be about 2 microvolts if the coils each have 5000 turns, a diametral length of 20 cm and a rotation speed of 1OHz.

It will be apparent to those skilled in the art that various other embodiments of the invention are possible using either single coil or opposed twincoil gradiometer systems.

Claims (3)

1. A magnetic anomaly detector having a housing, at least one electrically conductive coil rotatable within the housing about an axis, and a drive means for rotating the coil about the said axis, is characterised by including; a self-powered electro-optic transmitter electrically connected to coil so as to emit optical signals representative of voltage signals induced across the coil during rotation in a magnetic field, which transmitter is disposed within the housing so as to rotate in fixed relationship with the coil and to transmit the optical sigals along the said axis; a photo-electric receiver having a photo-receiving surface fixed to the housing on the said axis in optical confrontation with the transmitter, which receiver is arranged for generating voltage output signals in response to the optical signals received from the transmitter; and output processing and indicating means external to and remote from the housing, electrically coupled to the receiver.

2. A magnetic anomaly detector substantially as hereinbefore described with reference to the accompanying drawings.

3. A method of using the detector claimed in either of the preceding Claims for detecting ferrous objects buried in the ground, having an initial step of transitting the ground with a sweep magnet so as to induce magnetism in the object for subsequent detection with the detector.