WO2002067015A1

WO2002067015A1 - An apparatus and method for detecting an object in a medium

Info

Publication number: WO2002067015A1
Application number: PCT/AU2002/000183
Authority: WO
Inventors: James Macnae; Ying Ping Yang
Original assignee: Macquarie Research Ltd
Priority date: 2001-02-21
Filing date: 2002-02-21
Publication date: 2002-08-29
Also published as: AUPR323001A0

Abstract

An apparatus (1) for detecting an object in a medium includes means for inducing an electrical field in the medium suitably in the form of an inductive transmitter loop (7); at least one pair of detectors (2), suitably an array of capacitive electrodes (2), the at least one pair of detectors (2) being located at spatially distinct locations from one another and from the means (7) for inducing an electrical field in the medium, the detectors (2) being capable of detecting charge selected from the group consisting of charge induced in an object and charge induced on an object in the medium and outputting at least one measurement signal.

Description

AN APPARATUS AND METHOD FOR DETECTING AN OBJECT IN

A MEDIUM

Technical Field

The present invention relates to an apparatus and method for detecting an object in a medium.

Background of the Invention

The detection of objects below the surface is becoming increasingly necessary both for land development and for maintenance including clearing of unexploded ordnance and detection of subsurface pipelines, tunnels and road cracks. Clearing of unexploded ordnance on military bases using currently available methods (using a combination of magnetics and electromagnetics) is estimated to cost of the order of $180 billion US dollars over the next decade on US military bases alone and the methods are far from reliable. Such methods suffer from the disadvantage that some unexploded ordnance, for example land mines, are encased in plastic, are electrically resistive and cannot be detected by the conventional use of magnetic methods (which are sensitive to ferrous or nickelferrous conductive materials) and electromagnetic methods (which are sensitive to conductive objects).

Aside from the use of magnetic and electromagnetic methods, other methods currently in use are either too costly, have low resolution or are directed to specific applications. For example, the use of radio waves (radar) and the use of nuclear magnetic resonance (mapping hydrogen in water, prospecting and medical uses). None of the previously described methods have the ability to detect both subsurface resistive and conductive objects.

One method that is known and has the ability to locate both resistive and conductive objects involves the use of grounded electric field sources and receiver electrodes. This method suffers from the disadvantage however that the resolution is typically low, the electric field sources must be grounded and the method is slow in operation. A capacitive non-contacting electrode method using high frequency in the hundreds of kiloHertz range is also known however this method is limited in its potential applications.

Object of the Invention

It is the object of the present invention to overcome or substantially ameliorate at least one of the above disadvantages or at least provide a suitable alternative. • Summary of the Invention

According to a first aspect of the invention there is provided an apparatus for detecting whether an object is present in a medium, said apparatus including means for inducing an electrical field in the medium; at least one pair of detectors located at spatially distinct locations from one another and from said means for inducing an electrical field in the medium, said detectors being capable of detecting charge selected from the group consisting of charge induced in an object and charge induced on an object in the medium and outputting at least one measurement signal. Typically the apparatus according to first aspect further includes an analyser cooperating with the detectors to determine from the measurement signal whether an object is present in the medium.

According to a second aspect of the invention there is provided a method for detecting whether an object is present in a medium including: inducing an electrical field in the medium; detecting any charge selected from the group consisting of charge induced in object and charge induced on an object present in the medium by means of at least one pair of detectors located at spatially distinct locations from one another; and outputting at least one measurement signal. Typically the method according to the second aspect further includes determining from the measurement signal whether an object is present in the medium.

There is disclosed an apparatus for detecting whether an object is present in a medium including means for inducing electrical signals in the medium; at least one pair of detectors located at spatially distinct locations from one another and from said means for inducing electrical signals in the medium, said detectors detecting any signals relating to charge induced in/on an object in the medium; and analysis means for interpreting from the detected signals whether an object is present in the medium. There is also disclosed a method for detecting whether an object is present in a medium including: inducing electrical signals in the medium; detecting any signals related to charge induced in/on an object present in the medium by means of at least one pair of detectors located at spatially distinct locations from one another; and determining from said detected signals whether an object is present in the medium. There is also disclosed an apparatus for detecting whether an object is present in a medium including means for inducing electrical signals in the medium, said means in the form of an array of sequentially operable transmitters; an array of simultaneously operable detectors, pairs of detectors being located at spatially distinct locations from one another and from said means for inducing electric signals in the medium, said array of simultaneously operable detectors for detecting any signals relating to charge induced in/on the surface of an object in the medium; and analysis means for interpreting from the detected signals whether an object is present in the medium, There is also disclosed a method for detecting whether an object is present in a medium including: sequentially inducing electrical signals in the medium with an array of transmitters; simultaneously detecting any signals related to charge induced in/on the surface of an object present in the medium by means of an array of detectors, pairs of detectors being located at spatially distinct locations from one another; and determining from said detected signals whether an object is present in the medium. By use of the apparatus and method of the invention it is possible to detect the contact between either a resistive or conductive object and its surrounding medium and in preferred embodiments one or more of: location, size or shape of the object. Charge can be induced on the object and, if present, at one or more boundaries within the object.

The means for inducing an electrical field in the medium is typically provided by at least one transmitter, preferably an array of transmitters. Suitable transmitters include inductive and/or grounded source transmitters. Preferably an ungrounded inductive loop is used through which low frequency (sub 10kHz) current of a few milliAmps to a few Amps is passed. By use of an inductive loop a horizontal circulating current and electric field is created in the medium, which in use is largely independent of any horizontal layering in the medium and thereby responds primarily to lateral changes. For example if no object is present in a layered medium, representation of the medium is transparent and independent of medium conductivity at suitable frequencies. Preferably a number of inductive loops of various sizes are used to enable penetration of electrical signals to various depths in the medium and across various directions of excitation (smaller loops penetrating the medium at lesser depths). Typically the observable field of each transmitter loop penetrates the medium to a depth of about three times the maximum width of the loop. Sequential energisation of each transmitter loop creates a sequence of different electromagnetic fields enabling differing depth penetrations and subsurface orientations to be analysed.

The detectors for detecting charge selected from the group consisting of charge induced in and charge induced on any object present in the medium are preferably provided by at least one pair of electric field sensors, preferably an array of electric field sensors which are preferably operated at low frequency of the order of 100 Hz to 10 KHz, more preferably 100 Hz to lKHz (the higher the frequency used, the better the signal to noise ratio except that if too high a frequency is used, the electric field created becomes a function of the conductivity of the medium). By an array is meant that the sensors are not constrained along a single line but are spread in two dimensions. Typically the electric field sensors are in the form of capacitive electrodes, which capacitively couple to any charges induced near the surface of the medium although grounded electrodes such as porous pot electrodes or a combination of capacitive electrodes and grounded electrodes can be used. Suitable grounded electrodes include copper sulphate electrodes (CuCuSO₄) or stainless steel rods. If used, the capacitive sensors are preferably provided with a guard cover to minimise electrical interactions with an environment other than the medium, for example atmospheric and ionospheric electric fields such as wind blown charge on dust particles present in the atmosphere. Such guard covers are suitably conductive and connected to the output of a suitable preamplifier. With an average vertical gradient of lOON/m (within the range -100 to +300N/m), electrode movements whether wind or otherwise caused can create a signal of the order of lOOmN/mm of movement. With signal levels of interest in the low mV range, vertical movements of 0.01mm may be critical to the performance of the electrodes. In dry conditions wind-blown or dust carried charge (static) may build up on electrodes and with the extreme impedance of the measurement system this charge is not dissipated, guarding ensures that wind-blown charge and ionospheric coupling noise affect the potential on the cover rather than directly on the sensor electrode. '

The pair of detectors can be spaced apart at any suitable distance, for example from say 5cm up to say 50m apart. The detectors can be used for downhole measurements for example in dry/plastic encased borehole environments or for surface measurements suitably for use in arid environments. Capacitive electrode sensor dipoles couple well to sites carrying current and it is desirable when using such sensors to separate current and potential dipole wiring. It is therefore not possible to obtain measurements when a transmitter pole is located at the bottom of the same borehole as the sensor electrodes.

In preferred embodiments for surface measurement, the sensor array is in the form of a moveable mat. The number of sensors used in the array and their arrangement is dictated by the object to be detected and its depth. Suitably from 2 to 2048 or more sensors are present, most preferably 100 to 200 sensors, for example 200 sensors in a 10 by 20 rectangular arrangement or 126 sensors in a 9 by 14 rectangular arrangement, the mat suitably having an area of about 2 square meters. When the sensor array is in the form of a mat, the mat is suitably larger than the object to be detected with sensors spaced at intervals less than the size of the object to be detected. Typically the maximum depth of investigation is less than the overall mat dimensions, objects detectable at depths less than their diameter up to 2 to 3 times their diameter.

In preferred embodiments the transmitter array is located within or adjacent to the sensor array, although it is possible to locate various inductive loops outside of the sensor array. By use of a receiver array it is possible to undertake simultaneous data acquisition which is typically more stable than sequential measurement. Typically a factor of ten noise reduction can be achieved by making simultaneous electric field measurements.

Where a low signal to noise ratio is present, a high impedence preamplifier is preferably connected to each electrode suitably by means of coaxial cables so as to amplify any measurement signal. Like the capacitive electrodes, the preamplifier and connecting coaxial cables are also preferably guarded, all gaps in the guarding being preferably minimised for successful noise minimisation. Preferably at least two perpendicular reference electrodes may also be present to reduce noise through referencing. By use of two perpendicular electrodes, prediction of noise at a local site can be made. It is desirable where there is likely to be temperature fluctuations to calibrate and/or compensate the preamplifier for any such variations or alternatively regulate the temperature environment around the preamplifier for example by providing suitable thermal insulation.

The analyser is preferably provided by means of computer, which may be connected to at least one A/D multichannel digital converter. For a 126 sensor array, the A/D converter typically comprises 128 channels. Typically the computer is a laptop type computer. For personal computer based digital acquisition systems with a limited number of bits (12 or 16), an additional DC bucking circuit is desirable to increase recording sensitivity.

In preferred embodiments the data can be analysed by obtaining the relative voltage difference between pairs of detectors based on a signal transmitted from each transmitter and processing the measurement signals to determine locations of any lateral interfaces of conductivity in the medium (preferably in three dimensions). In preferred embodiments, based on estimated resistivity contrasts, the size and shape of the object and its nature (resistive or conductive) can also be determined. Signals can be processed by using a suitable algorithm to calculate and/or calibrate expected signals induced by each transmitter at each detector pair. Observed readings are then compared with calculated or calibrated ones to determine lateral location and orientation of any subsurface inhomogenieties (to determine whether an object is present in the medium) and further determining those signals relating to near-surface origin from those relating to deeper origin. Numerical models can then be used to determine the location of the object in the medium, to determine whether the object is conductive or resistive and ascertain its likely source from the geometrical and conductivity structure so determined.

In preferred embodiments the apparatus and method of the invention is used over bare ground and if ungrounded detectors are used, these are regularly grounded when not in use to dissipate any accumulated charge. Prior to use each transmitter and each pair of electrodes are preferably calibrated in air or over a uniform medium with no embedded objects.

By use of the invention it is possible to detect plastic land mines, unexploded ordnance, road cracks, buried gas lines, concrete sewers and plastic piping.

Brief Description of the Drawings

A preferred form of the present invention will now be described by way of example with reference to the accompanying drawings wherein:

Figure 1 is a schematic representation of one suitable apparatus for detecting an object in a medium; Figure 2 is a schematic detailed diagram of a detector array suitable for use in an apparatus of the invention;

Figure 3 is a schematic diagram of a guarded preamplifier circuit suitable for use in an apparatus of the invention; Figure 4 is a schematic diagram of a circuit design of a capacitive electrode suitable for use with the invention;

Figure 5 is a schematic diagram of a suitable field setup;

Figure 6 shows apparent resistivity data of a capacitive electrode as used in the present invention compared with a porous pot electrode;

Figures 7a, 7b, 7c, 8a, 8b, 8c, 9a, 9b, 9c, 9d and 10a, 10b, 10c show measured data using capacitive electrodes in PVC cased boreholes compared with data obtained using conventional porous pot electrodes.

Detailed Description of the Preferred Embodiments Referring to Figure 1 an apparatus 1 suitable for detecting an object in a medium is shown. The apparatus comprises an array of capacitive electrodes 2 mounted on a moveable mat 3. Each capacitive electrode 2 is connected to a multichannel A D converter 4 by means of coaxial cables 5 and thereby to a computer 6 which is connected to a power or battery source (not shown). At least one inductive transmitter loop 7 is also connected to the computer 6 by means of coaxial cables 8. As shown in Figure 2, when more than one inductive loop 7 is used, such loops can be located inside or outside the array of electrodes 2 and may be of varying size and offsets with respect to the array and lie in the plane of the array, or above or below it.

Where there are low signal to noise ratios, a preamplifier is desirably connected to each capacitive electrode. A suitable guarded preamplifier circuit 9 for each half of a complete dipole set up is shown in Figure 3 typically having an input resistance of 100GΩ.

Figure 4 shows a suitable circuit design and construction detail for a capacitive electrode suitable for use in the present invention. In this embodiment the electrode is in the form of an electrode plate 10 of a nominal size (10 x 10 cm). The electrode plate is electrically insulated on the bottom surface however this is not necessary. The top surface of the electrode plate 10 is surrounded by a guard plate 11 which is insulated 12 at both ends. The electrode plate 10 is connected by means of a cable 13 to a preamplifier box 14. Both the preamplifier box 14 and cable 13 are also guarded. The signal output of the preamplifier box 14 is connected to a receiver input terminal (not shown).

Figure 5 shows wiring details suitable for use in the invention using a pair of capacitive electrodes 2 and preamplifiers 14 connected to a receiver input terminal 15. Many pairs of such electrodes form the array shown in Figure 1. Example 1

A field test using the capacitive electrodes as shown in Figures 4 and 5 was carried out near Parkes New South Wales. A survey line was laid out to cover a known sub- economic skarn sulphide deposit. A 15kW Geotronics transmitter and a Huntec Mk4 receiver were used for the test. The dipole-dipole geometry with a = 50m (the distance between the capacitive electrode pair) and n = 1,6 (transmitter-receiver separations of na or 50m to 300m) was chosen for the test. At each station, the capacitive electrodes were laid out immediately adjacent to a porous pot electrode for comparison. The results are shown in Figure 6. It was clear from the field test that capacitive electrodes are more than adequate for accurate resistivity measurements using induced polarisation equipment. One complete cycle (8 seconds) was adequate for either measurement of the primary voltage V_p and the resulting reading was virtually identical between the capacitive and porous pot electrodes. Figure 6 shows effectively identical pseudosection plots of the apparent resistivity data.

Example 2

Application to borehole measurements in PVC cased holes

Experiments using a capacitive electrode for PNC cased bore hole induced polarisation and resistivity measurements were conducted North West of Sydney. Two 3 meter long PNC tubes, one 10 cm diameter and the other 9 cm diameter were buried about 10 cm under relatively wet sandy ground to simulate PNC cased 'boreholes'. A small preamplifier with input resistance of 100 GΩ, specially built for downhole measurement, was used as the down hole electrode amplifier. A metal cylinder made from a steel plate and a metal pipe were used as a downhole capacitive electrode. A porous pot electrode located on the ground was used as reference electrode. A Scintrex transmitter was used for transmitting signal to the ground.

The measurement results and processed (deconvolved) results are shown in Figures 7 to 10.

Figure 7a shows the raw measured data using a metal cylinder capacitive electrode of small diameter (~ 7 cm) and of 54 cm length inside the small (9 cm) 'borehole'. In this case, the borehole had a 9.05 cm external and 8.8 cm internal diameter. The measured signal was distorted due to low transfer function parameter τ value caused by low capacitance to ground. Figure 7b shows the recovered signal. An estimated capacitance of 16 pF for the capacitance of the electrode to the ground was obtained by deconvolution of the raw data using a transfer function parameter τ of 1.6 sec and using the input resistance Ri of 100 GΩ. Figure 7c shows the signal measured with a conventional porous pot electrode pair located on the surface. Figure 8a shows the raw measured data using a similar metal cylinder capacitive electrode as that used in Figure 8a but of enlarged diameter (~ 10 cm) inside the larger (10 cm) PNC cased 'borehole' (having an external diameter of 11.1 cm and an internal diameter of 10.4 cm). A better signal was obtained due to a substantial increase of the capacitance between the electrode and the ground. A capacitance of 110 pF was estimated from the deconvolution of the raw data using a transfer function parameter τ of 11.0 sec and an input resistance of 100 GΩ (Figure 8b). Figure 8c shows the signal measured using conventional porous pot electrodes.

Figures 9 and 10 show data using 6.07 cm diameter metal pipes and 80 cm length as capacitive electrodes respectively inside the large and small 'boreholes' described in Figures 7 and 8 above. Figure 9a shows the raw measured signal using the capacitive electrode in the large borehole, Figure 9b the raw data deconvolved using a transfer function parameter τ of 1.6 sec, Figure 9c the raw data deconvolved using a transfer function parameter τ of 1.1 sec and Figure 9d the signal measured with conventional porous pot electrodes. Figure 10a shows the raw signal measured using the metal capacitive electrode in the small PNC cased 'borehole', Figure 10b deconvolution of the raw data using a transfer function parameter of 3.2 sec and Figure 10c the signal measured with conventional electrodes. In Figures 9 and 10 the electrode lay asymmetrically in the shallowly dipping PVC tube. Due to an unknown reason, the 'ideal' waveform could not be obtained from the two measurement data as shown in Figures 9b, 9c and 10b. This causes a problem in estimating the value of the capacitances, but respective capacitances of 14 and 32 pF are still estimated for the pipe inside the big and small 'boreholes'.

Capacitance between two hollow coaxial cylinders one inside the other can be calculated from

where D, and D_e are internal and external diameters of the PNC case, ε_r the relative dielectric permittivity of the PNC, D_c and L the diameter and length of the cylindrical electrode and ε₀ the dielectric permittivity of free air. The calculated capacitance and estimated (measured) capacitance for the electrodes used in the experiment are listed in Table 1. The calculation assumes that the electrode and the PNC case are coaxial and the relative electric peπnittivity of PVC is 1. From Table 1 it can be seen that the calculated capacitance is 3 to 7 times larger than the measured capacitance. The first reason for this may be that the input resistance of the amplifier was lower than the nominal resistance of the input resistor, 100 GΩ, due to the conduction of the circuit board or due to moisture in the air thus causing the estimated capacitance to be too low. The second reason is that the soil may not be a perfect conductor for the capacitance calculation and the assumption of the soil as a perfect conductor may cause the calculated capacitance to be too large.

Table 1 Com arison of estimated and calculated capacitances

From the above it can be concluded that:

1. Capacitive electrodes can be used for PNC cased or dry-hole down hole IP and resistivity measurement.

2. To obtain the maximum capacitance between electrode and ground, it is better to keep the electrode diameter as close to the bore hole diameter as possible.

3. The calculated capacitance between electrode and ground using equation 1 can be 3 to 7 times larger than the true value. This needs to be kept in mind when designing an electrode.

4. To measure an IP signal without distortion, it is desirable to keep the transfer function parameter τ, the product of capacitance of the electrode to the ground and the input resistance of the preamplifier, larger than 10 times the on-time period T, τ > 10 T. This requires very long electrodes. For example for 100, 80, 50, 40 and 30 mm holes, with electrodes 5 mm in diameter, and with the preamplifier of 100 GΩ nominal input resistance used, the length of the electrode will respectively need to be 930, 1200, 1900, 2400 and 3300 mm (with calculated τ over 100 sec) in order to measure a 2 second on time signal without significant distortion. A 3m long electrode is not unreasonable however. 5. For the data measured with low τ value, the true signal can potentially be obtained by deconvolving the raw data with a transfer function. As the true signal can be recovered from data measured with τ values as small as 0.05T, the above electrode lengths can be 200 times shorter and still be expected to obtain data from which the true signal can be recovered.

6. The implications of this experiment to surface measurements are that 10cm x 10cm electrodes would be suitable for operation at frequencies in the 100Hz to lOKHz range.

Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.

Claims

CLAIMS:

I. An apparatus for detecting whether an object is present in a medium, said apparatus including means for inducing an electrical field in the medium; at least one pair of detectors located at spatially distinct locations from one another and from said means for inducing an electrical field in the medium, said detectors being capable of detecting charge selected from the group consisting of charge induced in an object and charge induced on an object in the medium and outputting at least one measurement signal.

2. An apparatus according to claim 1 further including an analyser cooperating with the detectors to determine from the measurement signal whether an object is present in the medium.

3. The apparatus according to claim 1 wherein the means for inducing an electrical field in the medium is provided by at least one transmitter.

4. The apparatus according to claim 3 wherein the means for inducing an electrical field in the medium is provided by an inductive source transmitter, a grounded source transmitter or a combination thereof.

5. The apparatus according to claim 1 wherein the means for inducing an electrical field in the medium is provided by one or more ungrounded inductive loops.

6. The apparatus according to claim 5 wherein ungrounded inductive loops of varying sizes are provided.

7. The apparatus according to claim 1 wherein the detectors for detecting charge selected from the group consisting of charge induced in and charge induced on an object present in the medium is provided by at least one pair of electric field sensors.

8. The apparatus according to claim 7 wherein the electric field sensors are in the form of capacitive electrodes, grounded electrodes or a combination thereof.

9. The apparatus according to claim 2 wherein the analyser is provided by a computer optionally connected to at least one A/D multichannel digital converter.

10. The apparatus according to claim 1 further including at least two perpendicular reference electrodes.

II. An apparatus according to claim 1 wherein the means for inducing an electrical field in the medium is in the form of an array of sequentially operable¹ transmitters.

12. An apparatus according to claim 1 or 11 including an array of simultaneously operable detectors, pairs of detectors being located at spatially distinct locations from one another and from said means for inducing an electrical field in the medium, said array of simultaneously operable detectors being capable of detecting charge selected from the group consisting of charge induced in and charge induced on an object in the medium.

13. The apparatus according to claim 12 wherein the array of simultaneously operable detectors is in the form of a moveable mat.

14. The apparatus according to claim 12 wherein the transmitter array is located within or adjacent to the sensor array.

15. A method for detecting whether an object is present in a medium including: inducing an electrical field in the medium; detecting any charge selected from the group consisting of charge induced in an object and charge induced on an object present in the medium by means of at least one pair of detectors located at spatially distinct locations from one another; and- outputting at least one measurement signal.

16. The method according to claim 15 further including determining from the measurement signal whether an object is present in the medium.

17. The method according to claim 15 wherein the electrical field is induced by one or more ungrounded inductive loops through which low frequency (sub 10kHz) current of a few milliAmps to a few Amps is passed.

18. The method according to claim 17 wherein a number of inductive loops of various sizes are used to enable penetration of the electrical field to various depths in the medium and across various directions of excitation.

19. The method according to claim 18 wherein the observable field of each loop penetrates the medium to a depth of about three times the maximum width of the loop.

20. The method according to claim 18 wherein each loop is energised sequentially to create an electromagnetic field enabling differing depth penetrations and subsurface orientations to be analysed.

21. The method according to claim 15 wherein the charge induced is detected by one or more pairs of electric field sensors, which are operated at low frequency of the order of 100 Hz to 10 KHz.

22. The method according to claim 16 wherein determining whether an object is present includes processing detected signals to determine locations of any lateral interfaces of conductivity in the medium.

23. The method according to claim 22 including determining the size, shape, nature (resistive or conductive) of the object or a combination thereof.