WO1991006854A1

WO1991006854A1 - Method for use in assessing the physical state of ground materials and apparatus for use therein

Info

Publication number: WO1991006854A1
Application number: PCT/GB1990/001656
Authority: WO
Inventors: Peter Douglas Jackson; Michael Brozel; Jon Reginald Hallam; Philip Meldrum
Original assignee: Natural Environment Research Council
Priority date: 1989-11-04
Filing date: 1990-10-30
Publication date: 1991-05-16
Also published as: GB8924934D0; GB9209643D0; GB2253912A; GB2253912B

Abstract

Apparatus for assessing the physical state of a ground site has electrode assemblies (10) positioned in the site to be assessed, each comprising an electrode (12) and selector means (14) and at least one multiway cable (16) connecting a plurality of electrode assemblies (10) to a central processing unit (18). Lines of the cables (16) are connected to the unit (18) to provide control signal to the selector means (14) and further lines of the cable are connected to a current source (28) and voltage detection means (30, 32, 26). Each selector means (14) is actuated by a control signal from the unit (18) to connect the associated electrode (12) with a selected one of lines (C1, C2, P1, P2) to a current source (28) or voltage detector (30).

Description

METHOD FOR USE IN ASSESSING THE PHYSICAL STATE OF GROUND MATERIALS AND APPARATUS FOR USE THEREIN

Technical Field

This invention relates to a method for use in assessing the physical state of ground materials and apparatus for use therein and is especially concerned with the continuous monitoring of the physical state of ground materials.

Background Art

Known techniques for assessing the physical state of ground materials involve the use of electrical resistivity measurements. However, the known methods have required the use of a number of dedicated electrodes each connected to an appropriate part of a central unit. As each electrode, because of its physical connection to the central unit, is dedicated for a particular use, such a method is cumbersome and restricted physically by the number of available ports on the central unit. Thus, in these known methods, it is impossible to run many electrodes from a single central processor unit.

Such a method is of little benefit in monitoring the physical state of ground materials, especially over a fairly wide region because of the physical constraints of the known systems mentioned above.

It is desirable to provide a method for use in assessing the physical state of ground materials for example to establish the basic ground structure (e.g. where proposing to build a reservoir for example) or in a continuous monitoring situation for ground hazards, for example pollution e.g. by oil or the like, degradation or instability of the geological substructure, ingress of water into the region under investigation and other purposes.

One of the various objects of the present invention is to provide an improved method for use in assessing the physical state of ground materials.

Another object of the present invention is to provide improved apparatus suitable for use in assessing the physical state of ground materials.

Disclosure of the Invention

The invention provides in one of its various aspects apparatus suitable for use in assessing the physical state of ground materials comprising a plurality of electrode assemblies adapted to be positioned within and/or around a region of ground to be assessed, each of the assemblies comprising an electrode adapted to be placed in electrical contact with the ground material and selector means, and at least one multiway cable connecting a plurality of the electrode assemblies to a central processing unit, a plurality of lines of the cable being adapted to be connected to the central processor unit whereby to provide control signals to the selector means and further lines of the cable being adapted to be connected to a current source and to a voltage detection unit, each selector means being adapted to be actuated by a control signal from the central processor unit to connect the associated electrode with a selected one of said further lines of the cable. Preferably the central processing unit of apparatus in accordance with the invention is adapted to signal the electrode assemblies in a selected sequence; conveniently this sequence may be changed by the user if desired.

A preferred apparatus in accordance with the invention is adapted to pass a constant electrical current between a first selected pair of said electrodes and to measure the potential difference between a second selected pair of said electrodes. The apparatus is preferably arranged so that any one of the electrodes can, under the control of the central processing unit, be selected as one of either of said first or said second pair of electrodes.

Where measurements are required in a vertical direction through ground material, apparatus in accordance with the invention may comprise a probe having at least two electrode assemblies carried thereby. Suitably these are adapted to provide data permitting determination of the resistivity of the ground. Conveniently where a probe is provided, the probe may comprise a multiplicity of electrode assemblies of which the electrodes are equally spaced apart along the probe.

Such probes may be used in conjunction with electrode assemblies adapted to be positioned on the surface of the region of ground under assessment.

Preferably apparatus in accordance with the invention comprises a constant current source connected to two lines of said multiway cable. Preferably apparatus in accordance with the invention comprises measuring apparatus adapted to measure the potential difference between two lines of said multiway cable connected to said measuring apparatus.

In a preferred apparatus in accordance with the invention, two lines of said multiway cable are dedicated for connection to a constant current course and two lines of the multiway cable dedicated for connection to a measuring apparatus.

Apparatus in accordance with the invention may, if desired, comprise a plurality of multiway cables each connected to a plurality of electrode assemblies.

In another aspect the invention may be considered to provide a method for assessing the physical properties of ground materials comprising positioning a plurality of electrode assemblies at selected sites in and/or around a region of ground to be assessed, the assemblies being connected to a central processor unit by a multiway cable, and causing the assemblies to be addressed in a selected sequence to connect selected ones of the electrodes to a selected line of the multiway cable.

Preferably in carrying out a method in accordance with the invention a first pair of electrodes is selected by the central processing unit to be connected to lines of the cable connected to a constant current source to establish a constant current flow between said first pair of electrodes, and a second pair of electrodes is connected to lines of the cable connected to measuring apparatus to measure the potential difference between said second pair of electrodes for assessment by the central processing unit. In one preferred method in accordance with the invention referred to as Total Field Resistance measurement, the central processing unit selects a first second pair of electrodes, one of which is a "fixed" reference electrode, and determines the potential difference between them: this gives an indication of the potential field.

By using the same first pair of electrodes, passing the same constant current but selecting different second pairs of electrodes one of each pair always being the reference electrode, the potential field of the whole region to be surveyed can be mapped. It will be appreciated that any one of the electrodes may be selected as reference electrodes. Having measured the potential field at various electrode positions the measurements permit the resistance to be calculated between any two electrode positions. Thus all possible pairs of electrode positions can be used to reconstruct the resistance that would have been measured if they had been connected for a 4 terminal resistance measurement. Thus for N total field resistance measurements N factorial values of apparent resistivity could be calculated.

The use of the scalar potential field as the fundamental measure (in terms of total field resistance which simply normalises for current) allows the reconstruction and addition processing referred to, which is not possible using conventional methods where a reference electrode is not used.

Further measurements may be made using one second pair of electrodes (one of which is the reference electrode) to determine the potential field but using a plurality of first pairs of electrodes between each pair of which the same constant current may be passed; additional measurements may use the same set of first pairs of electrodes but a different constant current.

The data from different current source locations can be added together to replicate the case when they are all operative at the same time, enabling very complex electric fields to be built up by adding together the potential field at one set of potential electrodes for a very large number of current electrodes. The versatility of the apparatus and system (known, by the Applicants, as Rescan), permits a wide variety of measurements to be readily made. When interpreted by an expert these permit an improved assessment of the characteristics of the region of ground under investigation. For example apparent resistivity may be calculated from the potential fields determined and the associated constant current; conveniently the microcomputer is programmed to make such calculations.

Brief Description of the Drawings

There now follows a detailed description, to be read with reference to the accompanying drawings, of apparatus and methods embodying the invention. It will be realised that the apparatus and methods have been selected for description to illustrate the invention by way of example.

In the accompanying drawings:-

Figure 1 is a diagrammatic view showing the electrical layout of apparatus embodying the invention;

Figure 2 is a diagrammatic plan view showing disposition of electrode assemblies in a method embodying the invention; and Figure 3 is a diagrammatic view of a probe for use in apparatus embodying the invention.

Best Mode for Carrying Out the Invention

In Figure 1 is shown in block diagram form the electrical layout of apparatus suitable for use in assessing the physical state of ground materials. The apparatus comprises a plurality of electrode assemblies which are adapted to be positioned within and/or around a region of ground to be assessed. Each of the assemblies 10 comprises an electrode 12 adapted to be placed in electrical contact with the ground material and selector means 14. The electrodes 12 are made of any suitable material, preferably a metal which tends to oxidise in the ambient conditions for example copper, brass or steel; stainless steel which does not oxidise readily in normal conditions is not preferred. The electrode assemblies 10 are connected to one of a plurality of multiway cables 16. A plurality of electrode assemblies 10 may be connected' to each multiway cable 16. For example a twelve way cable may carry two hundred electrode assemblies.

Each of the multiway cables 16 connects the associated electrode assemblies 10 to a central processing unit 18 through an interface 20. A plurality of lines of each cable 16 are adapted to be connected to lines 22 of the central processing unit themselves being connected to a selection control unit 24 under the control of a microcomputer 26. A suitable microcomputer is an NEC Powermate portable microcomputer with hard disc, compatible with an IBM PC-AT. The selection control unit 24 converts TTL signals from the microcomputer 26 to a form suitable for serial transmission down a multicore cable. Two further ^'lines of the multiway cable 16 are adapted to be connected through the interface 20 to lines Cl, C2 and thence to a source 28 adapted to provide a constant current output. Two further lines of the multiway cable 16 are adapted to be connected through the interface 20 to lines PI, P2 and thence to a voltage amplifier 30 the output of which is connected to an analogue to digital converter 32 of which the output is applied to the microcomputer 26.

Each of the selector means 14 is adapted to be actuated by a control signal from the selection control unit 24 of the central processing unit 18 whereby to connect the electrode 12 associated with that selector means 14 with a selected one of the lines Cl, C2, PI, P2. The selector means 14 includes circuitry allowing, in a manner known to one skilled in the art, decoding of an address sent serially by the control unit 24: once addressed, further circuitry is activated to respond to data which governs which of lines Cl, C2, PI, P2 is connected to the electrode.

The source 28 has an associated current decode 29 which allows selection of a particular constant current by the same circuitry as used for electrode selection. The voltage amplifier 30 has an associated gain decode 31 which permits selection of a particular resistor to provide a desired gain to the amplifier 30, again selected by the circuitry used for electrode selection. The central processing unit 18 is adapted to signal the electrode assemblies in a preselected sequence and is a type of multiplexing system.

The cables 16 may be flexible, with the electrode assemblies 10 disposed at intervals along the cables, preferably at equally spaced intervals. Such flexible multiway cables 16 are preferably used to position electrode assemblies 10 at various preselected positions on the surface of the ground at the site to be assessed. Alternatively, a multiway cable 16 may lead to a probe 34 (see Figure 3). The probe 34 is rigid and comprises a plurality of electrodes 12 positioned at equally spaced intervals along the probe 34. Selector means 14 associated with each electrode 12 is positioned within the probe. The probe 34, which may be several metres long, may be driven into the ground for example pushed or drilled in depending on the hardness of the ground, or lowered into bore holes with a plastic well screen. Use of a probe 34 allows measurements to be taken at a variety of depths and spacings of electrode assemblies, under the control of the central processing unit 18.

In use, in carrying out a method embodying the invention for assessing the physical properties of ground materials, a central processing unit 18 and associated interface 20 are positioned at a suitable point in the site G to be investigated. An appropriate number of cables 16 are connected to the interface 20, each of the cables 16 having an appropriate number of electrode assemblies 10 connected thereto. In the case of one of the cables 16, a probe 34 carrying a plurality of electrode assemblies is used, in this example.

The electrode assemblies 10, and probe 34 are positioned at preselected points in the region to be investigated. For example, if it is wished to monitor the physical state of the ground around a lake R, a fuel tank F and a slurry pit S the electrodes are positioned around the various sites to be checked: for example, to determine leakage of water from the lake R, possible contamination from fuel leaks from the fuel storage tank F and leakage of pollutants from the slurry pit S. Because each of the electrode assemblies 10 is uniquely addressable by the selection control 24 of the illustrative apparatus, it is possible to use as many or as few electrode assemblies 10 as is needed, up to the capacity of the central processing unit 18. If only a few of the electrode assemblies 10 are to be utilised, then only these electrode assemblies will be addressed in scanning the assemblies 10 by the central processing unit 18.

In the operation of the illustrative apparatus in carrying out a method embodying the invention, the electrode assemblies 10 are scanned in a selected sequence. For each reading a first pair of electrodes are selected by the selection unit 24 under the control of the microcomputer 26 to be connected to the lines Cl, C2 and thence to the constant current source 28 thus to establish a constant current flowing between said first pair of electrodes 12. Likewise a second pair of the electrodes 12 are connected to the lines PI, P2 and thence to the voltage amplifier 30, to the A to D converter 32 and to the microcomputer 26 thus to measure the potential difference between said second pair of electrodes. Electrode assemblies 10 addressed for each reading may be on the same or different multiway cables 16, including the probe 34.

As the individual electrode assemblies desired are addressed by the selection control 24 for each reading to be taken, it is possible that for one reading a particular electrode 12 may be connected to one of the lines Cl, C2 while being connected, for a different reading, to one of the voltage detection lines PI, P2 - the electrode selection system is completely flexible. The potential difference between said second pair of electrodes 12 is stored by the microcomputer 26 and manipulated as necessary to generate the desired information.

In one method in accordance with the invention, the central processing unit 18 may be adapted to select a first second pair of the electrodes, one of which is a "reference" electrode, for a potential reading and thereafter to select further second pairs of electrodes (each pair including the "reference" electrode), for a reading of the potential difference between the other second pairs of electrodes, whilst the same constant current is flowing between the same first pair of electrodes in all such readings: the readings are stored by the microcomputer 26 and, as they are all related to the same reference electrode, permit determination of the potential field for the whole region under investigation, enabling calculation of the potential field at positions where no actual measurements have been made. Various other electrode and current combinations may usually be selected, as outlined previously.

In Figure 3 is shown an example where all of the selected electrodes are carried by a probe 34, the two electrodes 12 connected to lines PI, P2 being disposed between the electrodes 12 connected to the lines Cl, C2. In Figure 3 the probe 34 is shown inserted into a ground strata L which is porous, disposed between two impervious strata I.

By monitoring the electrode assemblies 10 over a period of time, any changes in the physical state of the ground materials under investigation can be determined. Thus it is possible to detect leakage from reservoirs and spread of fuel or other contaminants. It is also possible to monitor ground conditions of a site under investigation for building purposes.

Should it prove necessary to move the apparatus to a different site, requiring more or fewer electrode assemblies 10, it is simple to add or remove electrode assemblies and these can be accommodated merely by assigning a suitable address so that they can be addressed by the central processing unit 18. It is also simple to select different combinations of electrodes 12 for taking different sets of readings at the same sites should this be deemed necessary by the researcher.

The illustrative apparatus and method are more flexible than any previous monitoring system and are able to readily accommodate a greater number of electrode assemblies conveniently.

Claims

1. Apparatus suitable for use in assessing the physical state of ground materials characterised by a plurality of electrode assemblies (10) adapted to be positioned within and/or around a region of ground to be assessed, each of the assemblies (10) comprising an electrode (12) adapted to be placed in electrical contact with the ground material and selector means (14), and at least one multiway cable (16) connecting a plurality of the electrode assemblies (10) to a central processing unit (18), a plurality of lines of the cable (16) being adapted to be connected to the central processing unit (18) whereby to provide control signals to the selector means (14) and further lines of the cable being adapted to be connected to a current source (28) and to a voltage detection unit (30,32,26), each selector means (14) being adapted to be actuated by a control signal from the central processing unit (18) to connect the associated electrode (12) with a selected one of said further lines of the cable.

2. Apparatus according to Claim 1 wherein the central processing unit (18) is adapted to signal the electrode assemblies (10) in a selected sequence.

3. Apparatus according to either one of the preceding claims adapted to pass a constant electrical current between a first selected pair of said electrodes and to measure the potential difference between a second selected pair of said electrodes.

4. Apparatus according to any one of the preceding claims comprising a probe (34) having at least two electrode assemblies (10).

5. Apparatus according to Claim 4 wherein each probe (34) comprises a plurality of electrode assemblies (10) of which the electrodes (12) are equally spaced along the probe.

6. Apparatus according to any one of the preceding claims comprising a constant current source connected to two lines (C1,C2) of said multiway cable (16).

7. Apparatus accor ing to any one of the preceding claims comprising measuring apparatus adapted to measure the potential difference between two lines (Pl,P2)of said multiway cable (16) connected to said measuring apparatus .

8. Apparatus according to any one of the preceding claims comprising a plurality of multiway cables (16) each connected to a plurality of electrode assemblies (10).

9. A method for assessing the physical properties of ground materials characterised by positioning a plurality of electrode assemblies (10)at selected sites, in and/or around a region (R,S,F) of ground to be assessed, the assemblies (10) being connected to a central processor unit (18) by a multiway cable (16), and causing the assemblies (10) to be addressed in a selected sequence to connect selected ones of the electrodes (12) to selected lines (Cl,C2,P1,P2) of the multiway cable (16).

10. A method according to Claim 9 wherein a first pair of electrodes (12) are selected by the central processing unit to be connected to lines (C1,C2) of the cable (16) connected to a constant current source (28) to establish a constant current flowing between said first pair of electrodes and a second pair of electrodes (12) are connected to lines (P1,P2) of the cable (16) connected to measuring apparatus (30,32,26) which measures the potential difference between said second pair of electrodes (12) and supplies this information for assessment.

11. A method according to Claim 10 wherein the central processing unit (18) selects a first second pair of electrodes, one of which is designated as "reference" electrode, and determines the potential difference between them and thereafter selects further second pairs of electrodes, one of each pair being the same reference electrode, and determines the potential differences between the other second pairs of electrodes whilst the same constant current is flowing between the same first pair of electrodes, the readings enabling assessment of the potential field in the region under investigation.

12. A method according to either one of Claims 10 and 11 wherein readings are taken successively using a plurality of first pairs of electrodes with the same constant current passing between each pair.

13. A method according to any one of Claims 10 to 12 wherein readings are taken successively with different constant currents passing between a first pair of electrodes.