GB2292859A

GB2292859A - Buried object detection systems

Info

Publication number: GB2292859A
Application number: GB9417332A
Authority: GB
Inventors: Christopher Keith Richardson
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 1994-08-27
Filing date: 1994-08-27
Publication date: 1996-03-06
Anticipated expiration: 2014-08-27
Also published as: GB9417332D0; GB2292859B

Abstract

A system for the detection of objects buried in the ground comprising a transmitter having a whip antenna which transmits vertically polarized RF signals towards the ground, and a receiver spaced from the transmitter and tuned to receive the RF signal, the receiver having a loop antenna which is supported so as normally to lie parallel with the ground, the receiver including a detector for providing an output signal indicative of the presence of a buried object, in dependence upon the amplitude of received RF signals. Homogeneous ground will give rise to no detected signal, whilst an inhomogeneity (eg. a buried object) gives rise to a signal. The loop may be tilted to detect the angular position of the inhomogeneity. The transmitted signal frequency may be varied. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO BURIED OBJECT DETECTION SYSTEMS This invention relates to systems for the detection of underground objects such as buried pipes, wires, drains, cavities or even damp/dry patches indicative of recent ground disturbance.

Presently known systems of this kind which include metal detectors and ground pulsing radar, tend to have specific uses and are not therefore suitable for general purpose applications.

Moreover, known systems tend to be short range and therefore ineffective for the detection of objects buried at a depth of more than a few inches.

It is an object of the present invention to provide a system which is generally effective to detect the presence of buried objects, even when they are non-metallic and buried at a depth in excess of several inches.

According to the present invention, a system for the detection of objects buried in the ground comprises a transmitter having an antenna suitable for transmitting RF signals through the ether which are polarised vertically with respect to the ground, a receiver arranged in spaced-apart relationship with the transmitter and tuned to receive the RF signal, the receiver having a loop antenna which is supported so as normally to lie parallel with the ground, the receiver including a detector for providing an output signal indicative of the presence of a buried object, in dependence upon the amplitude of RF signals originating from the transmitter which have tray yelled through the ground and which are influenced by the buried object.

As will hereinafter be explained if the receiver loop aerial is normally maintained substantiall) parallel with the ground, signals transmitted directly through the ether from the transmitter antenna tend to null, as do signals reflected from homogeneous regions of the ground below the receiver antenna loop. If however, non-homogeneous signals are reflected from the ground, as may be produced by a buried object, then these can be detected at the receiver.

The transmitter aerial may simpl) comprise a vertical whip aerial which will generate a vertically polarised signal.

The loop antenna may be arranged so that in use it can be tilted away from its normal position whereat it lies parallel with the ground, whereby the position of a buried object is identifiable in dependence upon the angle and/or direction of tilt necessary to produce a predetermined output signal change.

The predetermined output signal change mar be a signal decrease to a minimum signal level obtainable b' means of tilting.

The size of objects detectable aid the depth at which detection can be effected are related to the frequent of the transmitted signal. Accordingly, the transmitter and receiver may effectively be tuned selectively to an RF frequency in the range 150 MHz.

The transmitter and the receiver may' be arranged to sweep through the frequency' range in synchronisation, the detector having operatively' associated with it the means for selecting detected signals which originate from an RF signal at a predetermined frequency.

The transmitter and receiver which are mutually synchronised so that they are set to the same frequency, may be arranged to sweep through the frequency range in steps.

The frequency sweep may be controlled in dependence upon a clock signal.

The clock signal may be generated by a crystal oscillator in the transmitter.

Alternatively, the transmitter and the receiver may each comprise similar crystal clock oscillators which are synchronised and in dependence upon which the transmitter and the receiver tuning is controlled so that they switch synchronously through the said frequency range and remain substantially in step at all times.

One embodiment of the invention will now be described by way of example only, with reference to the accompanying drawings, in which, FIGURE 1 is a somewhat schematic diagram illustrating the behaviour of transmitted signals through the ether and through the ground; FIGURE 2 is a schematic diagram illustrating the behaviour of signals reflected from ground, and; FIGURE 3 is a somewhat schematic circuit block diagram showing in more detail a transmitter and receiver as shown in Figure 2 wherein corresponding parts bear the same numerical designations.

A vertically polarised signal transmitted from a transmitter antenna spaced some distance from a receiver, tends to produce a horizontally polarised signal within the ground between the transmitter and the receiver. This is because the propagation delay within the ground is much greater than in air, and therefore, the wavefront is defracted dosllwardly as shown in Figure 1. This horizontally' polarised ground signal is then reflected upwards from the ground. The signal from the transmitter, which travels through the ether, is thus vertically polarised above the ground and the signals reflected from the ground due to the ground wave are horizontall > ! polarised.

Referring now to Figure 2, an RF transmitter 1 is positioned some distance away from an RF receiver 2, the transmitter and receiver being tuned to the same frequency. The transmitter 1 has operatively associated with it a whip antenna 3 which produces vertically polarised radiation when energised with an RF signal from the transmitter 1. The receix er 2 has operatively associated with it a loop antenna 4 which is normally positioned so as to lie substantially parallel sith the ground. Signals received by the antenna 4 pass through the receiver 2 to a detector section 5 and produce an output signal on a line 6. The horizontal loop antenna 4 serves to null the vertically polarised signal which travels through the ether from the transmitter antenna 3 and also serves to null signals as indicated by arrows 7 which are due to a ground wave and which are reflected from homogeneous parts of the ground. The signal from the transmitter which has passed through the ether is nulled because the response of the loop antenna 4 is cross polarised with respect to the transmitted signal.

A horizontally polarised signal reflected from the ground, but from one direction only, would produce a maximum output signal induced in the loop. However, the signal reflected from homogeneous ground is equally distributed all around the loop 4 and thus produces equal and opposite currents in all directions which tend to null.

However, if the ground is not homogeneous, but contains a cavity or anomaly or some other object to one side of the loop, reflected signals around the loop will not be equal and therefore the loop will produce an output signal which is fed to the receiver 2, to be detected by the detector 5 which will result in an output signal on the line 6.

If an output signal is produced with the loop positioned substantially horizontally, signals can be nulled again by tilting the loop, the direction of tilt indicating the direction of the buried object which caused the output signal to be produced.

The transmitter and receiver would normally be arranged to operate in a frequency range between 1 and 50 MHz. At a frequency of 1 MHz, buried objects are detectable at depths of about three or four metres depending on ground conditions, whereas at 50 MHz, the depth of penetration of the signals is probably only about four or five centimetres. The sensitivity of the system and the effect of the frequency of operation will depend on the character of the ground, and for example, wet boggy areas will produce results which are greatly different from dry sandy areas.

Referring now to Figure , in order to provide for detection across the whole range betcveen 1 and 50 MHz, the tuning of the transmitter 1 and the receix er 2 may be swept synchronously throughout the range. Preferably, the sweeping may be effected in steps, the receiver 2 and the transmitter 1 each being arranged to carry a simple clock, which include crystals 8 and 9 respectively as shown in Figure 3.The clocks in the transmitter 1 and the receiver 2 are initially synchronised so that their associated oscillators (not shown) operate on the same frequency, synchronisation and frequency' selection in the transmitter 1 and receiver 2 being effected by appropriate circuitry in blocks 10 and 11 respectively, whereby a selected frequency is chosen from which the output signal is produced on the line 6 as shown in Figure 2. For example, five frequencies spaced through the range 1 - 50 MHz may be used, a stitch (not shown) being provided so that signals produced in ally one of the ranges can be selected by an operator as effective to provide the output signal on the line 6.

Thus, having detected a buried object, an operator may then, by means of the switch, determine the response at different frequencies whereby the depth, for example, and/or size of the buried object may be estimated in accordance with the skill and experience of the operator.

Claims

1. A system for the detection of objects buried in the ground comprising a transmitter having an antenna suitable for transmitting RF signals through the ether which are polarised vertically with respect to the ground, a receiver arranged in spaced-apart relationship with the transmitter and tuned to receive the RF signal, the receiver having a loop antenna which is supported so as normally to lie parallel with the ground, the receiver including a detector for providing an output signal indicative of the presence of a buried object, in dependence upon the amplitude of RF signals originating from the transmitter which have travelled through the ground and which are influenced by the buried object.

2. A system as claimed in Claim 1, wherein the transmitter antenna comprises a vertical whip aerial.

3. A system as claimed in Claim 1 or Claim 2, wherein the loop antenna is arranged so that in use it can be tilted away from its normal position whereat it lies parallel with the ground, whereby the position of a buried object is identifiable in dependence upon the angle and/or direction of tilt necessary to produce a predetermined output signal change.

4. A system as claimed in Claim 3, wherein the predetermined output signal change is a signal decrease to a minimum signal level obtainable by means of tilting.

5. A system as claimed in any preceding claim, wherein the transmitter and receiver are arranged effectively to be tuned selectively to an RF frequency in the range 1-50 MHz.

6. A system as claimed in any preceding claim, wherein the transmitter and the receiver are arranged to sweep through a predetermined frequency range in synchronisation, the detector having operatively associated with it means for selecting detected signals which originate from an RF signal at a predetermined frequency.

7. A system as claimed in Claim 6, wherein the transmitter and the receiver are arranged to sweep through the said frequency range in steps.

8. A system as claimed in Claim 6 or Claim 7, wherein the frequency sweep is controlled in dependence upon a clock signal.

9. A system as claimed in Claim S, wherein the clock signal is generated by a crystal oscillator in the transmitter.

10. A system as claimed in Claim 8, wherein the transmitter and the receiver each comprise similar crystal clock oscillators which are synchronised and in dependence upon which the transmitter and the receiver tuning is controlled so that they switch substantially synchronously through the said frequency range.

11. A buried object detection system as claimed in Claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.