GB2255832A - Locating buried object having an associated electromagnetic field - Google Patents

Abstract

The buried object is detected by a detector having an array (preferably triangular) of detector coils 1, 2, 3 whose extended axes define a multi-sided area on the ground, which is passed over the buried object, the change of phase in the coils being detected by a detecting circuit 10-49. The circuit is located outside the area defined by the axes of the coils, and is preferably electromagnetically screened from the coils, thus substantially eliminating interference problems. As described, the detector comprises three similar circuits 10, 20, 30 operating on three separate frequency bands ie 50-60Hz, 5KHz-5MHz and 15-100KHz. Each circuit comprises a high gain amplifier 14, 24, 34, two monostable circuits 15, 17, 25, 27, 35, 37, a 3 input gate 16, 26, 36 and an associated LED 19, 29, 39. When an object is detected, the output of one of the coils will be out of phase with the other and the LED of the relevant frequency band circuit will light, indicating the nature of the buried service. When no object is detected, equal signals are supplied to all inputs of gate 40 and LED 49 is lit. <IMAGE>

Description

IMPROVED DEVICE AND METHOD FOR LOCATING BURIED OBJECTS This invention relates to an improved device for locating buried objects, and a method for using the device in locating buried objects. Location generally involves detection of an electromagnetic field associated with an object.

GB1509380 discloses a device of this general type, intended for locating a buried conductor. The device has three vertically-extending coils whose axes define an equilateral triangular area on the ground, and a receiver and processor for comparing the phases of signals produced in the coils and indicating whether they are in or out of phase. The redeiver/processor is mounted centrally within the triangle, slightly above the coils.

The assembly of three receiving coils has the property that its component coils will provide an in-phase output signal in response to any source of electromagnetic energy having the form of a conductor much longer than the distance between itself and the receiving coils, wherein the source lies outside the area bounded by the extended axes of the coils. But where this energy source lies within the area covered by the extended axes, then one coil output will contain an out-of-phase component with respect to the others.

In practice, all space near the surface of the Earth is filled with electromagnetic energy of great diversity of amplitude and frequency, and often the amplitude of the energy source which is to be detected will be very small compared with the ambient energy level.

Accordingly, it is to be expected that when this small quantity of energy is summed with the large ambient energy it will only shift the output waveform of the coil whose extended axis which is opposite (with respect to the source) those of the other two, by a small amount.

In consequence, a practical instrument must be capable of detecting reliably very small time discrepancies between the rising (or trailing) wave fronts of the summed voltage signals provided by the three coils. The quantification of this summed voltage is a complex matter, but in practice a signal of strength 0.5W and a frequency of 100kHz at a distance of 7m from an assembly of three coils whose axes define the vertices of an equilateral triangle of side 200mm will be shifted from a typical summed radio frequency ambient field at the outof-phase coil by a time period of the order of 40ns. For reliable detection of typical re-radiated radio energy from buried services, a sensitivity to wavefront shifts of the order of 2-5ns may be necessary.However, such a sensitivity is not available in the existing devices for locating buried services such as pipes or cables.

A device of the desirable sensitivities will have to work at very high switching speeds. Therefore, its electronic circuitry will certainly generate interfering fields even though the currents may be very small.

Electromagnetic screening of the circuitry would marginally improve resistance to such interference, but at the sensitivities desirable for practical use in locating buried services, the device should adopt a layout conforming to the three coil antenna's property of inherent insensitivity to in-phase signal inputs. I have found that the apparatus as disclosed in GB1509380 is not satisfactory.

According to one aspect of the present invention, there is provided a device for locating buried objects having associated electromagnetic fields, the device comprising an array of aerial coils whose extended axes define a multi-sided area on the ground, and a detecting circuit for detecting phases of the signals produced in the coils and indicating whether such signals are in or out of phase; wherein the detecting circuit is located outside the multi-sided area defined by the axes of the coils. Preferably there are three coils and the area defined by their axes is an equilateral triangle.

Preferably the detecting circuit is electromagnetically screened from the array of aerial coils.

According to another aspect of the present invention, there is provided a method for locating buried objects by using the device of the present invention, comprising: keeping the extended axes of aerial coils of the device at a first direction relative to vertical and locating the device at a first position relative to a buried object such that out-of-phase signals are produced in the coils; keeping the extended axes of aerial coils of the device at a second direction relative to vertical and displacing the device to find a second position such that out-of-phase signals are produced and; determining the location of a buried object and the depth thereof below the device from the angle between the first and the second directions and the distance between the first and second device positions.

The object to be detected may be an elongate conductor that carries a current, giving rise to an electromagnetic field. (The current may be present anyway, or applied specially for purposes of detection.) In such a case, as the device passes horizontally over the conductor, a phase change is produced in the array of coils.

The object to be detected may be a non-conductor (or not actually be conducting). A sonde or probe may then be positioned in relation to the object for detection (e.g. inside a non-conductive pipe). A sonde may have a relatively small coil for radiating a field, typically with a horizontally-extending axis. Passing the detector over such a sonde gives an in-phase response when the sonde is vertically beneath the cent-re of an array of three receiver coils (or close thereto).

The foregoing will have given some indication of the very great directional acuity and potential sensitivity of the 3 coil antenna arrangement according to the present invention. It must be accepted that the device of the present invention will be equally sensitive to electromagnetic fields generated by its own circuitry, and that the antenna (the array of coils) must be protected from such feedback by the same means that enable it to distinguish small local fields from large distant ones: it is specified that all alternatingcurrent circuitry must be situated outside the extended three axes of the antenna. It is highly preferable that all alternating-current circuitry is also electromagnetically screened from the antenna, especially when the circuitry is located very close to the antenna to make the device compact.With this elimination of an inherent disposition to feedback the practical sensitivity of the proposed device would be limited only by the switching speeds achievable.

An embodiment of the present invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a block diagram of an embodiment of the present invention; Fig. 2 shows waveforms that may be produced by coils of the embodiment; Fig. 3 shows a plan view of the layout of the embodiment of the present invention; and Fig. 4 illustrates an embodiment of the method according to the present invention.

Fig. 1 shows the connection of the different components in an embodiment of the present invention.

Three coils 1, 2, 3 form an antenna array for detecting the existence of an electromagnetic energy source.

Three parallel detecting circuits 10,20,30 are each arranged to receive signals from the coils 1,2 and 3 as inputs. Each circuit has three branches, connected to respective coils 1,2,3. The three circuits 10,20,30 work on three separate frequency bands, e.g. 50 to 60Hz. 5KHz to 5MHz, and 15 to 100 KHz. The first band corresponds to the frequency of the electrical power lines, the second one to an RF source, and the third one to the frequency of a signal generator. The structures of the circuits 10,20,30 are very similar to one another. Each has in each branch a high gain amplifier 14,24,34. Those of different circuits work at different frequency bands.

Each amplifier is coupled to a monostable circuit 15,25,35 which acts to produce a pulse output of a period of about 1-5ns when the outputs of the amplifiers 14,24,34 reach a predetermined threshold voltage. The three branches of each circuit converge at a 3-input gate 16,26,36 which receive output pulses from the monostable circuits 15,25,35 respectively. The waveforms of these detecting circuits are shown in Fig. 2. Each circuit includes a further monostable circuit 17,27,37 which receives the output of the gates 16,26,36 and in turn provides signals to an LED 19,29,39 according to the output of the gates 16,26,36. The lines to the LED's are coupled to another 3-input gate 40, which provides a signal to a fourth LED 49 according to the signals to the other three LEDs 19,29, 39.When an energy source is detected within the area defined by the axes of the three coils 1,2,3, one of the coils will provide a signal outof-phase with the signals of the other two coils. In this situation, the detecting circuit of the corresponding frequency band will provide an output signal different from the other two circuits and its LED will provide a read-out accordingly. An operator can tell from the indication of three LEDs what kind of buried service has been detected by the antenna array.

Only when there is no energy source located under the antenna array will the output signals of the three coils all be inphase, and the three LEDs 19, 29 and 39 will then receive the same signal. In that case the fourth LED 49 will provide a signal to show that it is clear of any energy source, i.e. that there are no buried conductors in the area covered by the extended axes of the coils of the antenna array.

Fig. 2 shows the waveforms of the signals processed by the detecting circuits shown in Fig. 1: Fig. 2A is the summed ambient RF output from each of the three coils 1, 2 and 3; Fig. 2B is the output of the amplifier 14, 24 or 34, which has been driven into saturation by high amplification (80 - 120 dB voltage gain.); Fig. 2C shows threshold voltage inputs to monostables 15, 25 or 35 (which are desirably adjustable, via an integrator as shown here, or simple voltage output control of the steep wavefronts of the RF amplifier outputs), in order that the short-pulse outputs of the monostables may be finely adjusted for synchronisation in the absence of an energy source within the extended coil axes; and Figs. 2D, 2E, 2F show synchronised short pulse outputs from the three monostable circuits 15, 25 or 35, causing the 3-input gate 16, 26, or 36 to give an output to the final monostable circuit 17, 27 or 37. Fig. 2E shows (dotted) an out-of-step pulse which would shut down the output gate 16, 26 or 36 and cause (via an inverter) its LED to illuminate.

Fig. 3 shows a preferred layout of a device embodying the present invention. It is compact, with a single enclosure, with screening of the circuitry and display, from the antenna array. The antenna array consists of three coils 1, 2, 3 extending in parallel at the corners of an equilateral triangle, which is slightly spaced from a screened enclosure, which houses the circuitry and display, and thus avoids any interference between the antenna array 1, 2 and 3 and the circuitry of the device. The screened enclosure 4 includes a display board 5, with LED's 19, 29, 39 and 49.

Preferably a further internal screening board would be placed between the circuit boards and the display LEDs.

Conveniently the device includes a battery housing 8, and has a carrying handle 7.

It should be noted that in the compact structure shown in Fig. 3 the circuitry part should be very well screened by the enclosure 4 to ensure the sensitivity of the device and avoid any electromagnetic feedback by the circuitry to the antenna array 1, 2 and 3. However, if the circuitry portion 4 is situated far away from the antenna array, for example more than 30cm away, electromagnetic screening will be less important or even not necessary.

Fig. 4 shows a method for locating and determining the depth of a buried conductor by using the device according to the present invention. Firstly, the buried conductor C is located by keeping the axes of the antenna array vertical and a position A is determined in which the device is over the conductor, according to the signals shown by LEDs 19, 29, 39 and 49. Then the antenna array is held at 45 degrees to the vertical and moved laterally until once again the detection signals show that the axis passes through the conductor C. The new position of the device is indicated by letter B. The depth of the buried conductor C can be easily determined by simple geometry from the distance AB. The device can be kept at 45 degrees by arranging the handle so that when the device is held by the rear corner, it will be kept at 45 degrees.Certainly this can also be done by any supporting means to keep the device at a desired angle. According to the same general idea shown in Fig.

4, the depth of a buried conductor can be easily determined even if the surface of the ground is very uneven or sloping, or is obstructed by structures, or there is more than one conductor buried at different depth under the same location. The angle of 45" makes the calculation particularly simple, but other angles may be used.

Some modification of the operation of the illustrated device is desirable for use in detecting the position of a sonde having a relatively small, horizontally extending, signal-generating coil. With a detector having an array of three detector coils, there is an in-phase response at the three receiver coils where it is vertically below or nearby the centre of the array.

Accordingly, the "Signal" receiving circuitry in this mode must be arranged such that the output "CLEAR" LED illuminates with one of the inputs slightly out-ofphase, and the three input pulses to the gate closely but not exactly synchronous. The AND gate will be off in these circumstances, the first transistor will be "on" and the second (inverting) transistor "off". The "Signal" LED in this mode should be driven from the collector of the first transistor. Then, in the absence of the sonde the "CLEAR" LED will light. The presence of the sonde will force the three outputs into phase, extinguishing the "CLEAR" light and turning on "SIGNAL".

Usually the sonde signal will be pulsing on and off to simplify its identification: since the method for estimating depth is "trigonometrical", the pulsed signal is as simple to determine as a steady signal would be.

Referring to Fig. 2,E: the dotted and solid lines would be reversed with respect to D and F to show the sonde absent. Omitting the dotted lines of E would show the sonde present.

Claims (8)

1. A device for locating buried objects having associated electromagnetic fields, the device comprising an array of aerial coils whose extended axes define a multi-sided area on the ground, and a detecting circuit for detecting phases of the signals produced in the coils and indicating whether such signals are in or out of phase; wherein the detecting circuit is located outside the multi-sided area defined by the axes of the coils.

2. A device according to claim 1 wherein there are three coils and the area defined by their axes is an equilateral triangle

3. A device according to claim 1 or 2 wherein the detecting circuit is electromagnetically screened from the array of aerial coils.

4. A device for locating buried objects substantially as any herein described with reference to and as illustrated in the accompanying drawings.

5. A method for locating buried objects by using a device according to any preceding claim comprising: keeping the extended axes of aerial coils of the device at a first direction relative to vertical and locating the device at a first position relative to a buried object such that out-of-phase signals are produced in the coils; keeping the extended axes of aerial coils of the device at a second direction relative to vertical and displacing the device to find a second position such that out-of-phase signals are produced and; determining the location of a buried object and the depth thereof below the device from the angle between the first and the second directions and the distance between the first and second device positions.

6. A method according to claim 5 wherein the buried object is an elongate conductor.

7. A method according to claim 5 wherein a signalproducing sonde is located in or adjacent the object for detection.

8. A method for locating buried objects substantially as any herein described with reference to and as illustrated in the accompanying drawings.