GB2132357A

GB2132357A - Buried object location

Info

Publication number: GB2132357A
Application number: GB08236000A
Authority: GB
Inventors: Peter Sothcott
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1982-12-17
Filing date: 1982-12-17
Publication date: 1984-07-04

Abstract

The detection of electrically non- conductive objects, such as plastics anti-personnel mines, relies on the fact that such an object appears as a local increase in the resistivity of the soil in which it is buried. To monitor the area of ground, an arrangement based on that shown in the drawing is used. Here we have two probes (3, 4) to which a constant level AC is applied, AC to prevent polarization problems, and two more probes (6, 7) to which a voltmeter (5) is provided. The meter reading indicates the potential gradient in the soil, and hence the resistivity thereof. If a non- conductive object is located the resistivity is higher than normal. Various arrays for applying this basic method are described. <IMAGE>

Description

SPECIFICATION Buried object location This invention relates to method of, and apparatus for, the location of buried objects made of electrically insulating material, such as plastics anti-personnel mines.

According to the invention, there is provided a method of locating objects made of electrically insulating material buried in the ground, which includes inserting a first electrode set to which an alternating current source is connected into the ground to cause electrical current to flow through the soil between the electrodes of the set, inserting a second electrode set to which a measuring instrument is connected into the region of the ground in which the current from said source flows, so that the potential gradient (and hence the electrical resistivity) in the region subjected to the current is determined, the measurements being repeated at a number of locations, such that the pattern of potential gradients thus determined indicates the presence of and location of a said object or objects of insulating material.

An embodiment of the invention will now be described with reference to the accompanying drawing.

The arrangements to be described was developed for the specific purpose of locating buried plastics anti-personnel mines. Since such mines are almost wholly constructed of electrically insulating material, conventional mine detectors which rely on the same principles as metal detectors are not effective. When such a plastics mine is buried, the fact that it is electrically nonconductive means that it forms a high resistance obstacle when buried in an otherwise electrically conductive soil mass. The measurement of soil resistivity is in fact used in geophysics, and the present method is an extension of the principles used in geophysics to other appiications.

If two electrodes are planted just below the surface of the soil and are spaced apart in the horizontal plane, the application of a potential difference between them causes an electrical current to flow between them. This current is not limited to the space between the electrodes, but extends downwards below the surface, its variation with depth depending on the electrode spacing and the soil resistivity. It will also extend sideways of, and upwards from, the electrodes.

The pattern of current flow is disturbed if a mass of insulator is in the volume in which current normally flows. The disturbance can be detected at the surface if the obstacle is not too deeply buried.

A method of current measurement suitable for the detection of a disturbance due to a buried object, such as a plastics mine is the measurement of the potential difference between two probes, again inserted just below the surface.

This gives the potential gradient, in volts per unit length, which is a measure of resistivity. Any unexpected increase in resistivity then indicates the location of a non-conductive mass. The energising signal used is a low frequency constant level alternating current, so that polarizing effects are eliminated. By mapping the potential gradients at different points on the surface of the area being dealt with, and after three-dimensional calculations, the nature of the sub-surface volume can be assessed. A number of different electrode arrangements can be used, one of which is indicated schematically in the accompanying drawing.

The arrangement shown in the drawing is a simple device, included mainly for explanatory purposes. It consists a bar 1 of an insulating material, which carries four downwardlyextending probes, each of which is 2.5 inches long, the probes being spaced by 2 inch increments, as shown. A constant current source 2 which supplies a constant alternating current of 1 mA at 220 cycles is connected to the outer probes 3 and 4. An electronic millivoltmeter 5 is connected to the inner probes 6 and 7. Thus when the device is in use with its four probes projecting into the earth, the meter 5 records the potential gradient in millivolts per two inches : this is a rough measurement of the total resistance of the volume sampled by the inner probes.

To monitor an area for possible buried objects the surface is explored at a number of places spaced apart by about 2 inches in one of two coordinates and 3 inches in the other co-ordinate.

The array is arranged in the X direction. The readings of potential gradients thus obtained show definite, but fairly small, changes of potential gradient near the position of objects to be located. When the array passes over the position of an object, the traverse shows a "hump" over the position, which hump tends to reduce for more distant traverses.

The potential gradients as thus measured may include minor humps, some of which are due to the presence of stones and some to bad or uneven contact with the soil. Improvements in this respect may be obtained by suitable design of the probes, e.g. flat "chisel-like" probes, or each probe being formed by a group of closely-spaced nail-like probes. Where the soil to be monitored is too dry for good readings to be got, it would be possible to water the area under investigation.

To reduce the labour involved in monitoring an area, e.g. a suspected minefield, a larger and more complex array of probes can be used. Thus a rectangular matrix of probes, like a bed of nails, can readily be constructed so as to cover, for instance, a metre square. These probes can then be sampled sequentially so as to move a Wenner array (like that shown) over the whole square at a high rate. The instantaneous position of the test array is shown on a display device such as a cathode ray tube screen, the output of the array controlling the spot's intensity. The brightness of the spot is thus proportional, or dependent on, resistivity, i.e. to the probability of an insulating body near the sampled part of the array under investigation. Another form of electrode array which can be used is the Schlumberger configuration, in which the whole area is energised from two fixed probes, one on each side, and then explored by a pair of closely-spaced potential probes. Three orthogonally disposed pairs of potential probes are then used to get the magnitude and direction of the current immediately below the surface, from which the current flow pattern at greater depths can be determined. From this, the shape and the location of sub-surface obstacles can be obtained in three dimensions. This involves a different calculation, so a suitably programmed microprocessor is used for that calculation.

Claims

CLAIMS,

1. A method of locating objects made of electrically insulating material buried in the ground, which includes inserting a first electrode set to which an alternating current source is connected into the ground cause electrical current to flow through the soil between the electrodes of the set, inserting a second electrode set to which a measuring instrument is connected into the region of the ground in which the current from said source flows, so that the potential gradient (and hence the electrical resistivity) in the region subjected to the current is determined, the measurements being repeated at a number of locations, such that the pattern of potential gradients thus determined indicates the presence of and location of a said object or objects of insulating material.

2. A method as claimed in claim 1, and in which the electrodes are four in number, and are arranged in a straight line with the alternating current source connected to the outer pair, which form the first set, and the measuring instrument connected to the inner pair, which form the second set.

3. A method as claimed in claim 2, and in which the electrodes include a number of said sets of four, with means for scanning over the electrodes to obtain said potential measurements.

4. A method as claimed in claim 1, in which the first set of electrodes is a pair of electrodes one at each side of the area to be monitored, and in which the second set of electrodes is an array of pairs of electrodes which can be successively connected to the measuring instrument.

5. A method of locating objects made of electrically insulating material, such as plastics anti-personnel mines buried in the ground, substantially as herein described with reference to the accompanying drawing.

6. Apparatus for executing the method of any one of the preceding claims.