WO1998037439A1

WO1998037439A1 - Water prospecting method and apparatus

Info

Publication number: WO1998037439A1
Application number: PCT/GB1998/000383
Authority: WO
Inventors: Charles John Watkinson; Gregory Eric Blight
Original assignee: Water Prospecting International Limited
Priority date: 1997-02-20
Filing date: 1998-02-19
Publication date: 1998-08-27
Also published as: AU6106498A; ZA981427B; GB9703573D0

Abstract

A method of prospecting for underground water (12) at an over ground test site comprises the following steps. Arranging at least three electrical probes (13, 14, 17) in the ground at spaced positions along a straight line. A first set of three probes is selected comprising a first probe (13), a second probe (14) and a third probe (17) between them. An electrical potential (15) is applied across the first and second probe (13, 14) to cause an electrical current (16) to travel through the ground between the first probe and the second probe. The potential difference between the first probe (13) and third probe (17) and between the third probe (17) and second probe (14) is measured. The measurement is repeated for a further set of three probes displaced along the straight line from the position of the first set of probes. Comparison of the relative magnitude of the same measurements taken for the first and second set determines the presence of underground water.

Description

WATER PROSPECTING METHOD AND APPARATUS

This invention is concerned generally with prospecting for underground water at a particular ground test site i.e. detecting and locating the presence of underground water, and using electrical resistivity measurements of the ground at the test site.

Many different send / receive techniques have been used to monitor underground conditions. For example, sonic wave transmissions can be used to monitor the presence / thickness of underground strata during geological surveys e.g. during exploration for underground oil and / or gas reservoirs. It is also known to use electrical monitoring techniques in order to determine the presence and depth of buried pipelines, and also in some cases to monitor the integrity of any protecting coatings on buried pipelines.

The present invention utilises electrical resistivity measurements of test volumes of underground samples at a test site, and deriving therefrom information which is indicative of the presence / absence of underground water at the test site.

According to one aspect of the invention there is provided a method of prospecting for underground water at an over ground test site comprising the steps of: arranging at least three electrical probes in the ground at spaced positions along a straight line; selecting a first set of three probes comprising a first probe and a second probe with a third probe between them; applying an electrical potential across the first probe and the second probe so as to cause an electrical current to travel through the ground between the first probe and the second probe; measuring the potential difference between the first probe and the third probe and between the third probe and the second probe; repeating the measurement for a further set of three probes located at a position displaced along the straight line from the position of the first set of probes; and comparing the relative magnitude of the same measurements taken for the first and further set of probes so as to determine the presence of underground water.

The method therefore allows the presence of underground water to be detected along a line by scanning along a line of probes and applying a voltage to a pair of probes so as to measure the voltages between the probes of a pair and an intermediate probe. The changes in the magnitude of the intra pair unbalanced voltages indicates the presence of underground water as the water causes the resistance underground to be lower at certain positions which results in the un-balanced voltages.

The method may include the further step of taking measurements for multiple sets of three probes having positions displaced along the straight line, in which the separation between the first and second probes of each set is the same for each measurement so as to determine the presence of underground water at a particular depth.

As the separation of the pair of probes determines the path taken by the electrical current, it also determines the test depth. Hence by maintaining the inter pair separation constant but sequentially moving the probe pair along a line, a scan at a certain depth along a certain direction can be carried out to locate the presence of water at that test depth.

The method may include the step of taking measurements for multiple sets of three probes, in which the separation between the first and second probes of each set varies for each measurement so as to determine the presence of underground water at different depths.

The form of the water bearing geological structure may be investigated by changing the separation of the pair of probes so as to scan vertically through the geological feature. This can help to determine the vertical centre of a water zone.

The method may have all the probes equally spaced along the straight line. This can help in automating the method so that scans at fixed steps of position and depth can easily be carried out.

The method may include the step of automatically switching the electrical connection between the probes and an interface device so as to select which of the probes act as the first, second and third probes of a set of probes. By automating the process a full set of results can be obtained providing a two dimensional scan through a geological formation so as to make identification of a water bearing formation easier.

The method may include the step of repeating the method with the straight line displaced relative to the straight line used initially. By carrying out a further survey the overall shape of the geological feature can more accurately be mapped. The straight line may be rotationally displaced. The position of the displaced straight line may cross the position of the initial straight line. By re-scanning across an initial survey a three dimensional picture of the geological feature can be built up and the centre of any water bearing feature may be identified so as to indicate the optimum above ground position for locating a bore hole.

According to a second aspect of the invention there is provided a method of prospecting for underground water by measuring underground soil resistivities at an over ground test site, comprising the steps of: arranging at least four electrical probes in the ground at spaced positions along a straight line; applying an electrical potential across a first outer pair of probes so as to cause an electrical current to flow through the ground between the first outer pair of probes at a test depth determined by the separation of the outer pair of probes ; measuring the potential difference across the first inner pair of probes; and deriving from the measured potential difference across the inner . pair of probes a value which is indicative of the presence of water at the test depth.

The method may including the step of measuring the strength of the electrical current and combining the measured current strength with the measured potential difference in deriving a value indicative of the presence of water at the test depth. By combining current strength and soil resistivity measurements a more reliable indication of the likelihood of water can be obtained from the data. This is especially useful when clay forms part of the geological feature of interest.

The method may include providing a plurality of pairs of outer electrical probes centred on the inner pair and at different distances from the inner pair and including the step of taking measurements of the potential difference across the inner pair with an electrical potential across different pairs of outer probes so as to determine the presence of water at different depths. In this way a vertical scan through a geological feature can be carried out by measuring the soil resistivities at different depths .

The method may include using a further set of four electrical probes and the step of applying an electrical potential across the further outer pair of probes and measuring the potential across an inner pair of probes displaced along the straight line from the first inner pair of probes so as to determine the presence of water at different positions along the straight line. In this way soil resistivity data at different positions along the straight line and at different, or the same, depths may be taken.

The method may derive an indication of the presence of underground water from measurements taken according to the first aspect of the invention in combination with measurements taken according to the second aspect of the invention. In this way soil resistivity and current strength measurements may be combined with un-balanced voltage measurements to provide sufficient data to allow verification of the analysis when complicated geological structures are being surveyed.

According to a third aspect of the invention there is provided underground water prospecting apparatus for use above ground comprising: a plurality of electrical probes for inserting in the ground in a straight line; electrical cabling connected to the electrical probes for communicating electrical signals with the probes; an interface unit to which the cables are connected and including switching means; a voltage power supply connected to the switching means; and voltage measuring means, in which the switching means selectively applies an electrical potential across a pair of electrical probes, on either side of a third probe, and also connects the voltage measuring means between a first probe of the pair and the third probe and the third probe and a second probe of the pair to provide an indication of the presence of underground water.

According to a fourth aspect of the invention there is provided underground water prospecting apparatus for use above ground for measuring soil resistivities comprising: a plurality of electrode probes for inserting in the ground in a straight line; electrical cabling connected to the electrical probes for communicating electrical signals with the probes; an interface unit to which the cables are connected and including switching means; a voltage power supply connected to the switching means; and voltage measuring means, in which the switching means selectively applies an electrical potential across an outer pair of electrode probes, either side of an inner pair of electrode probes, and also connects the voltage measuring means across the inner pair of probes to provide an indication of the presence of underground water at a depth determined by the separation of the outer pair of probes.

Preferred examples of water prospecting methods and apparatus according to the different aspects of the invention will now be described in more detail below, with reference to the accompanying drawings, in which:

Figures 1 and 2 are schematic illustrations of a basic water prospecting system for use in carrying out the method according to the first aspect the invention, and showing diagrammatically how electrical resistivity measurements can be made, to determine the presence of a water bearing geological structure at a test site;

Figure 3 is a schematic view, partly in plan, of the water prospecting system according to the second aspect of the invention;

Figure 4 is an illustrative graph showing typical results for measurements taken by the system shown in Figures 1 to 3.

Figure 5 is an illustrative graph showing typical results for a set of scans carried out at different positions along a scan line and at different test depths and showing the optimal position for a bore hole based on an analysis of the measurements;

Figure 6 is a schematic diagram of apparatus to be used with the water prospecting system to allow finer scale measurements to be made at a particular depth and location;

Figure 7 is a set of graphs of typical data measured across a site by the system at different depths illustrating the use of soil resistivity data to interpret the results of the system;

Figure 8 is a set of graphs showing measurements made by the system across a site at different depths and illustrating location of a water bearing geological feature in the form of an evenly saturated clay; and Figure 9 is a schematic diagram showing a basic water prospecting system for measuring soil resistivities and current strengths in accordance with the second and fourth aspects of the invention.

The same items in different Figures share common reference numerals. It should be understood that the particular measured values will vary from site to site, and that the particular measured operating parameters which will be described e.g. the voltages, the electric current, the electric resistivity, and the spacing apart of the probes are illustrative examples only.

Water Prospecting System

Firstly, with reference to Figures 1 and 2 the basic principles of the water prospecting system and method will be described. The water prospecting system, designated generally by reference numeral 10, works by inducing an electrical current flow between two pins located in the surface of the ground of a test site. The majority of the electrical current induced will follow a parabolic path 11 underground at a particular depth. If there is a zone of moisture 12 on one part of the parabolic path then the electrical resistance of that part of the path will be less as the water increases the electrical conductivity of that part of the path. This will result in an imbalance of voltage drops across different parts of the parabolic path. By controlled relocation of the position of the parabolic current path the position of zones of relatively higher moisture 12 can be mapped out. Analysis of the voltage drops resulting from zones of higher moisture allows the position of the moisture zone within the current path to be determined. Interpretation of the results of the system utilising the method of the invention then allows area with a high likelihood of water bearing formations to be identified.

The basic water prospecting system 10 has a pair of electrode pins 13, 14 placed in the ground and connected in series to a direct electrical current source 15. One electrode pin has positive polarity and one electrode pin has negative polarity. The electrical current induced to flow between the electrodes by the DC source follows a parabolic path underground and the depth of the parabolic path can be calculated approximately by

(x*1.5)³

((x*1.5) - (x²) )

where y is the parabolic path depth in metres and x is the distance in metres from a central position equidistant between the pins.

For example to test a depth of 10m requires a spacing between the pins of approximately 30m. An electrolyte is required at each pin to ensure good electrical conductivity between the electrode pin and the ground and to ensure conductivity in the ground immediately surrounding each pin.

The basic system includes a DC power supply connected at a central point to the two outer electrode pins with an ammeter 16 capable of reading milliamps connected across the current shunt and a switch (not shown) connected in series with the two pins. A third electrode pin 17 is placed centrally between the two outer pins; i.e. approximately equidistant between the outer pins and on a straight line between the outer pins. The central pin is connected to a first outer pin by a first voltmeter 18 and to the second outer pin by a second voltmeter 19. This arrangement allows the voltage drop between each outer pin and the inner pin to be measured, enabling half potentials across the parabolic current path to be measured. The central pin does not need to be precisely positioned as it is indications of imbalance in each half of the parabolic current path that are all that are required to reposition the equipment for more precise measurement.

This water prospecting system can now carry out a test on relative changes in soil moisture/conductivity and current behaviour as governed by bodies of water or moisture in the ground at the desired test depth dependent on the electrode pin spacings .

The test sequence is as follows. The current is switched on by closing the switch and readings of the current strength and voltages are quickly taken so as to avoid changes due to polarisation of the electrode pins. The two voltage readings, i.e. the voltage between the first outer pin 13 and the central pin VP1 and the voltage between the second outer pin 14 and the cental pin VP2, can be described as balance potentials. In the absence of moisture and assuming a generally uniform distribution of underground matter, the resistivity will be approximately uniform along the path and so the resistance of the first half of the path and the resistance along the second half of the path will be approximately equal and so the voltage drops between each outer pin and central pin will also be approximately equal .

However, as illustrated in Figure 1 the resistance of the first part of the parabolic current path would be lower due to the presence of water and so the electrical current flows more easily and so there is a lower voltage drop between the first outer pin 13 and the central pin 17 than there is between the central pin 17 and the second outer pin 14. Hence the reading on the first voltmeter 18 would be substantially lower than that on the second voltmeter 19. Example values of the results are VP1=32V, VP2=68V and a current strength of 345mA.

The overall system can be considered to be the balance of two halves of the parabolic path PI and P2, and the sum of the voltage drops across the two halves should equal the DC voltage applied by the power source. This can condition can be used as a validity check for the correct operation of the system. The existence of current flow is also an indication that the system is functioning correctly and when little or no current is flowing, this indicates a fault or discontinuity in somewhere in the system.

It follows that if the pins are moved so that the central pin lies to the left of the water zone 12 as shown in Figure 2, then the values of VP1 and VP2 will be reversed. It is possible therefore to test from one side of a water mass through to the other side, in the case of confined aquifers, or, in the case of unconfined aquifers, to determine differentials in conductivity which would result from zones of even and uneven saturation.

In order to obtain a full picture of the structure of an underground water mass, several surveys need to be carried out in a straight line to determine where along the line the voltages VP1 and VP2 balance indicating the centre of the water mass. A full survey also includes a series of surveys at different depths along a straight line by utilising electrode pins with different separations as the test depth is determined by the outer pin-central pin separation. In order to facilitate the process without moving the equipment, a string of electrode pin probes, which constitute multiples of the basic system shown in Figures 1 and 2, are used with all the resultant pairs using a common central electrode pin probe.

With reference to Figure 3, a water prospecting system according to the third aspect of the invention, designated generally by reference numeral 40, includes a first arm 41 and a second arm 42 of electrode pin probes 43. Each arm has fifteen probes in the form of electrode pins which are in communication with multi-core cables 47 to connect all the probes. The arms are connected to a central interface unit 44 which is connected to a DC power supply 45. The Interface unit is connected to a first voltmeter 18 and a second voltmeter 19 which are used to measure the voltage drops between pairs of outer probes and a central probe. The interface unit is also connected, to an ammeter 16 for measuring the current strength in the system.

The voltmeters 18, 19 and ammeter are connected to a computerised data analysis and control system 46. The computerised control system is also connected the interface unit and controls the interface unit to selectively transmit electrical signals to different pairs of probes in the arms so as to scan surveys along the arms and select pairs of probes with different separations to provide different test depths. The computerised control system also automatically logs measurement data taken from the voltmeters and ammeter.

The example system shown in Figure 3 has fifteen probes on each arm separated by 15m so that the total arm span is 450m together with a common central probe. This allows a test depth of up to 120m to be used. The water prospecting system described is essentially the basic system repeated 30 times.

The DC power supply is in the form of a 100V DC battery power supply of eight rechargeable 12V batteries connected in series giving a total charged voltage of 105.6V as each battery has a nominal voltage of 13.2V.

The central interface unit controls switching to allow any one of the 31 separate probes to be switched in via the multi-core cables 47 which make up the arms of the system. By appropriate switching control the system can access any two probe combination. The required multi core cables 47 to act as current carrying conductors are as follows:

2x90m lengths of 18 core (0.55mm²) cable

2x90m lengths of 18 core (0.05mm²) cable

2x90m lengths of 12 core (0.55mm²) cable 2x9Om lengths of 12 core (0.05mm²) cable

2x45m lengths of 12 core (0.55mm²) cable

2x45m lengths of 12 core (0.05mm²) cable

2x15m lengths of 12 core (0.55mm²) cable

2x15m lengths of 12 core (0.05mm²) cable

The probes are attached to the multi-core cables at attachment points located along the cables. At each point there is a current and voltage sensing conductor.

The electrode pin probes are in the form of 33x316 stainless steel pins (12mm diameter x 200mm long) with a short 200mm lead (0.75mm²) hard soldered into a 4mm diameter hole drilled in the side of the pin 15mm from one end. Above this connection is a "blob" of weld to prevent the attached wire being struck off by a hammer blow. The opposite end is turned to a point to aid ground penetration. The wire end is covered by a Raychem MWTM 25/8 EZ20271-04 sealant loaded heatshrink.

Both the voltmeters 18,19 should be capable of measuring 0-lOOV DC and preferably have an input impedance of at least 10 egaohms. Also a DC meter capable of measuring from 0-300mV is used to measure the voltage drop across the current shunt. The DC current is, for example, in the range 0-1800mA. A single trace oscilloscope is also used to help identify superimposed noise signals from, e.g., communication equipment and power lines etc.

A laptop computer is used for data recordal together with a printer for printing results. The data acquisition equipment may be in the form of 2 x Hewlett Packard 34970A data acquisition switch units with 4 x Hewlett Packard 34903 switch modules and 1 X Hewlett Packard 34901 20-channel multiplexer. Hewlett Packard interface cards and cabling connect the data acquisition articles to the purpose built interface unit which communicates with the rest of the water prospecting system. Power is supplied by 2x 12V DC to 240V AC inverters. Other subsidiary equipment is also used including: 15 litres of towns water, 2 x motorised cable reels, 2 x standard hand wind cable reels, 2 x small sledge hammers and a tool kit for maintenance.

The above specified system provides a survey span of 450m but shorter survey spans, e.g. 200m, also work adequately. It has been found that the 15m separation of probes has significant advantages and is the preferred separation of probes. Smaller spacing results in overlapping of results and larger spacing results in gaps in the data both of which make the measurements lower in quality.

With the above configuration having probes at 15m intervals, connection between two electrodes gives a test depth of approximately 10m; i.e. the test covers 0 to 10m in depth. By switching between probes it is possible to carry out a survey testing at 10m depth along the whole of the cable span. By using probes separated by greater distances and switching to use sets of probes along the cable span, surveys at greater test depths and extending along the whole length of the span can also be carried out. The maximum test depth available with a 450m span system being approximately 120m. By appropriate switching between the probes it is possible to determine the conductivity in the parabolic path at approximately 10m deep intervals throughout the whole length of the parabolic path

Figure 4 shows a typical set of results from a test span. As can be seen from the results, the centre of the water mass is at 49m from the middle point of the arms, where the two lines on the graph cross.

In order to verify the position a further survey can be conducted oriented at 90° to the original in order to build up a more accurate picture of the moisture within the ground at that position. It is also possible to more accurately asses the situation at a particular depth by utilising lm electrode steps at a particular point in the survey.

An example of a full set of results for a survey are shown in the graphs of Figure 5. As can be seen, by switching the position and separation of the probes used sets of voltages at 10m depth intervals and across the span of the arms have been taken. Line 60 indicates the optimal position for a proposed bore hole based on an analysis of the graphical results.

Two different types of graphical curves can be used in the analysis of the data: single curves as shown in Figure 4 and single curves combined together on two charts only to determine any trends such as fracture zones.

The information obtained by the water prospecting system can be enhanced by utilising a soil resistivity check and combining the data with that obtained by measuring parabolic half potentials as per the above discussion. The soil resistivity analysis system is described in detail later.

Although operating the system is fairly straight forward, interpretation and analysis of the results gathered, and the gathering of appropriate results itself, is an important factor in the overall use of the system. The use of the system and interpretation of data will now be discussed.

System Use And Data Analysis

The survey data is based upon the particular geological feature being surveyed; e.g. the procedure for surveying a confined aquifer is different to that for mapping out an unconfined aquifer.

Confined Aquifer Procedure

(i) Firstly the geology of the area within which a survey is to be carried out must be identified. The geological strike, i.e. the horizontal direction which is at right angles to the dip of a rock, must be identified, e.g. the centre line of a ridge, and the survey cables must be set out in a line perpendicular to the strike. The geological strike could be identified on a geological survey chart or by local on-site interpretation.

(ii) If an area with unfamiliar or complicated geology is being surveyed then it is preferable to survey an existing bore hole to obtain a known base line. Each geological structure or formation has its own signature or VP1/VP2 curve shapes and it is good practice to obtain a local baseline.

(iii) If there is choice of different potential water bearing formations, then the formation most likely to provide the best supply should be initially surveyed followed by the other formations in turn.

(iv) The centre of a zone of saturation is shown by a definite cross of the VP1 and VP2 curves, i.e. VP1 and VP2 curves must stay crossed over the span in known dry areas. For geology where there are multiple saturated zones then a secondary cross will represent another saturated zone.

(v) If the VP1 and VP2 curves cross and meet again or come close, e.g. 8V or less, this signifies that the first crossing of the VP1 and VP2 curves is only an indication of moisture. The location indicated by a crossing in the curves is only slightly higher in water saturation than the moisture zone indicated by the narrowing of the curves.

An important consideration is the zones of influence; i.e. the underground zones through which the electrical current travels. The majority of the electrical signal follows the general path of a parabolic arc. Therefore when a survey is conducted by carrying out scans along a survey span, the electrical paths have a degree of overlap which increases as the depth of testing increases. Approximately 80% of the area of the first parabolic arc of the first scan on a survey span is covered by the next scan's parabolic arc. This overlap continues right across the survey span.

This results in scans at several depths being affected by apparently higher conductivity caused by earlier scans within the survey span which have substantial common zones of influence. The first five survey span depths (i.e. O-lOm, 11- 20m, 21-30m, 31-40 & 41-50m) do not have large zones of influence. At a depth of 30m there is an overlap of three parabolic scan paths which are all actually detecting a mass of water within the scan path of the first scan. Once past the 30m depth, the zone of influence equates to the distance between the pair of survey probes in use divided by two. Hence, deeper tests have much larger zones of influence. The zones of influence assist in determining the effective water saturation at the cross over points of the PI and P2 curves as indicated above. The deeper depths encompass almost the whole survey span; e.g. 91-100m deep test survey has a single zone of influence of approximately 200m wide, where as the 0-lOm deep test survey has a zone of influence approximately 30m wide.

The zones of influence can be used effectively to determine the ratio of water saturation to rock. A single cross of the VP1 and VP2 curves, with the curves remaining relatively flat for the remainder of the 200m test span, indicates a suitably saturated zone dependent on the geology.

(vi) Geological structures become more apparent when survey spans at different depths are combined as shown in Figure 5. In this example, line 60 represents a water bearing fracture zone within a ridge. If there is an interference zone between formation types which has moisture/water present then these can easily be identified. Other features such as the angle of dip, faults etc., can be identified using this system. (vii) Once a water bearing structure has been identified, the survey system can be rotated in line with the structure; e.g. a ridge would be surveyed parallel to the strike. This is done to help identify the extent of the structure and also and better zones of saturation which would be better for a water bore. A suitable structure of a bore should be at least 50m long.

(viii) If a better location is found then another transverse survey should be carried out to verify the better location.

(ix) It is now appropriate to survey at lm intervals across an appropriate part of the water bearing structure. This allows a better determination of the size or width of the structure, i.e. its vertical height or thickness, and helps provide a more accurate position for the detailed test depth.

(x) This finer scale surveying is done by using a system as shown in Figure 6 which includes a smaller second set of cables 71 which are 15m long and bear closely spaced probes and which can be connected to the ends of the arms and positioned on the surface to only test the 10m depth band of interest.

For example, at a particular site there is a water bearing structure 70 that passes through the 71 to 80m deep survey at +27m along from the centre of the cable span. The short cables are then positioned so that the probes start at the 70m depth probe positions on the main cable plus 27m. Therefore the finer scale probe cable will start 127m from the central interface unit and finish 142m from it. The probes are spaced 1.5m apart allowing testing to be carried out at depth intervals of lm across the zone of interest, i.e. 71m, 72m, 73m, 74m, 75m, 76m, 77m, 78m & 79m. A similar procedure is carried out on the opposite side of the central interface unit, but instead subtracting 27m from the 70m depth probe position of 100m from the interface unit so that the cable is finer spaced probe cable is placed 73m from the central unit. Therefore a fine lm scale survey can be carried out across a depth of 70m to 80m but displaced by 27m from the central position so as to test across the water bearing zone of interest.

A typical set of results follows:

The above results indicate water at 72m and 75m to 77m. This is confirmed by the low difference between VP1 and VP2 and by the rise in current. The difference should be as small as possible. In this case, and many others like it, the water bearing structure exits on an angle and the survey is scanning across it in a straight line. Therefore the results at the 75m depth level arise from the centre of the parabolic arc being to the left of the water bearing fracture zone, the 76m depth level arising from the parabolic arc centred over the water mass and the 77m depth level from the arc to the right.

This fine scale procedure is useful in verifying the earlier less detailed survey and is good for accurate mapping of the structure. An accurate position of the depth of the centre of the water mass can typically be given to within ±3m. (xi) Clays and shales are difficult formations to survey in, as they contain a very high saturation of water but do not release it as clays have a sponge-like characteristic. Shales and some alluvials contain very fine particles which restrict water flow, even though the resource is large.

Clays

Clays are sponge-like in character and the level of saturation is even throughout the formation. Surveying in this type of structure will produce a set of VP1 and VP2 curves that will start wide apart, then gradually taper towards each other cross and gradually separate in the same fashion. There will be no separation immediately prior to the VP1/VP2 curves crossing and when using the fine scale step detailed survey, there are not usually increases in current flow at a single step. However, this last step must be treated with caution as there might be a thin sand layer which will cause a rise in current flow due to the higher saturation of water.

Soil resistivity measurements along with system current flow requirements are also recorded as clays have low resistance, i.e. high conductance. The soil resistivity measuring system is described later in detail.

If formations containing clay are being surveyed then the conductivity or more simply the current flow in the system increases and current flow data is plotted on graphs together with VP1 and VP2 curves and soil resistivity data as shown in Figure 7. The extra soil resistivity and current flow curves assist in determining whether or not the survey is being conducted within clay. For example in Figure 7, the 10m depth data indicates that to the left of the figure the survey is passing through clay (high current flow) and then to the right the formation changes to a chalk structure, i.e. low current flow. Unconfined Aquifer Procedure

The procedure starts as per steps (i) and (ii) of the confined aquifer procedure.

(iii) An evenly saturated zone in an unconfined aquifer is required for a suitable position for a bore. The size of the even zone of saturation is dependent upon the water flow required. A typical zone of saturation would have a radius of 30m. A typical survey result indicating a saturated zone is shown in Figure 8.

(iv) Zones of influence have the same effect as in the confined aquifer process and are equally important.

(v) Geological structures become apparent when spans form different depths are combined on the same graph as in Figure 5. If there is an interference zone between formation types which has water/moisture present then these can be more easily identified. Other features, such as the angle of dip, faults, etc. can easily be identified.

(vi) Once a water bearing structure has been identified, then the surveying equipment is rotated through 90° and another survey across the centre of the zone of even saturation identified by the first survey is carried out. This helps to identify the extent of the structure and to ensure even saturation on all four points of the compass. If the zone of saturation is poor in the second direction, which tends to be unlikely, then another location may be used.

(vii) Again the discussion on clays and the use of soil resistivity and system current flow measurements in the confined aquifer process apply equally to the unconfined aquifer process.

Soil Resistivity Measurement System There now follows a description of a soil resistivity measurement system according to the second and fourth aspects of the invention. It may be used in combination with the water prospecting system to assist in locating water in conductive clays and. other formations.

The soil resistivity measurement system is used in conjunction with the water prospecting system to help identify the presence of ground water at a particular test site. A typical test site may be a 5m diameter "test column" extending down into the ground to a desired test depth. This is an operational system which tests to 150m (500 ft) and is suitable for most requirements. There is no reason why this cannot be expanded to test to 300m (1000 ft).

Basis of Operation

With reference to Figure 9, the basic system, designated generally by reference numeral 100, uses 4 x 304 stainless steel pins 101,102,103,104 (can be mild steel or other conducting material) (12mm diameter x 150mm long) placed in the ground at equal distances d apart and in a straight line. The distance between the pins governs the depth at which testing is carried out. The "test column" is at the midpoint of the two inner pins 102,103. Water is poured onto each pin to ensure the best possible electrical conductivity with the ground and to ensure that all pins behave in a similar manner. From the central "test column" point two cables are run to each pin (0.55mm² multi-strand single core for the current carrying conductor and 0.5mm² multi strand single core for the potential measurement) .

At the central point, cables to the two outer pins 101,104 are connected in series with a minimum 100V DC power supply 105, a meter 106 capable of measuring milliamps across a current shunt with peak hold function and a switch (not shown) is connected in series with the two outer pins. The two inner pins 102,103 are connected to a very high input impedance voltmeter 107 and are used to measure the potential drop across the two inner pins when current is caused to pass between the two outer pins. The input impedance desirably should be. greater than or equal to 10 egaohms.

The system is now ready to carry out the first half of a test on average soil resistivity from ground level to the desired test depth and current behaviour as governed by bodies of water of moisture in the ground to the desired test depth.

The test sequence is as follows. Switch on the system and record the current flow and potential drop across the inner pins.

The first test depth is normally 10m. This is followed by measuring the average soil resistivity to 20m deep and in 10 metre steps thereafter until 150m is reached. The 10m steps can be reduced to smaller steps for shallower testing and for greater test detail. The test depth is determined by the spacing of the pins

To test, the power is switched on for a maximum of 5 seconds and the current flow is displayed on the ammeter. The relative change in potential is displayed on the voltmeter, and these readings are recorded.

Basically, electrical current is passed between the selected pair of outer two pins whilst measuring the resultant potential drop across the central two pins. Current flows through the soil in a semi-circular fashion as illustrated in Figure 9 and the soil resistivity is calculated.

When the system is briefly switched on readings must be taken very quickly to avoid polarisation of the outer pins. Therefore, the voltage reading before current is applied and after current has been applied is recorded. The ammeter temporarily stores the peak current flow automatically. This is recorded and the ammeter reset for the next set of readings.

Normally,- four sets of data are recorded to ensure stable readings which indicate repeatability. Sometimes wind blowing across the cables causes static charges to build on the cables thereby causing the voltage readings to violently fluctuate. Watering of the pins goes a long way to reducing this effect and screened earthed cable can eliminate the problem.

An oscilloscope is used to determine the usable signal strength going to ground, the degree of polarisation on the pins and whether there is a faulty connection or very high resistance to ground connection. The oscilloscope records a capacitance type discharge curve when current is applied and the shape of this curve has proved to be highly beneficial in confirming correct operation of the system.

From four sets of readings, the average is calculated for both types of data (i.e. potential drop and current flow). A t ical set of recorded values are as follows:

This is a typical set of results from which the averages are calculated:

The potential drop (or delta mV) is volts #2-volts #1=4V Current flow = 9.2mA

This is repeated for all test levels surveyed which then produces a set of raw data shown below which can be analysed.

Analysis of Data

Soil resistivity gives an indication of ground water but cannot be relied upon in its entirety. Therefore, the data needs to be analysed using three main methods detailed below. The results of all the methods are shown graphically on Figure 7.

Soil Resistivity

If there is a relatively high concentration of ground water then the soil resistivity will drop significantly at that level, even though the measurement is an average from the surface to that test depth. Therefore, the best way to interpret this information is to assess the change between test depths in soil resistivity. It has been found that large decreases in soil resistivity are a strong indicator of water. Also slight positive difference in resistivities and small decreases in resistivity can indicate water but must be supported by other results,

The soil resistivity is calculated using the following formula:

R = 2 x π x D x V / I where R = Soil Resistivity (ohm-metres) D = Test Depth and pin spacing (m) V = Relative change in voltage measurement when current is applied (mV) I = Current Flow (mA) The example set of data below shows soil resistivity in ohm-metres :

Changes in Current

A positive change in current flow is a strong indicator. If used as the only indicator then the change must be dramatic (i.e. greater than 100mA). If the change is between 50 and 100mA then all factors must be considered. Any changes less than 50mA are usually only an indication of soak. Method A

This third step is where several factors are combined together to produce a summarised result. The changes in current flow have to be adjusted depending upon the relative changes in soil resistivity. This is done by averaging all the current measurements and dividing the result into each current reading per level. This gives a percentage change in current flow based on local site conditions. This result is then biased by standard deviation and the average of all current measurements. As only relative changes between levels are looked at, this step emphasises any major differences.

The bias is further enhanced by the relative change in soil resistivity. The greater difference between levels results in a higher bias toward the appropriate direction. A positive bias can also be a result of a slight increase in soil resistivity between two levels. The limiting difference is 100 ohm-metres and a relatively sharp rise in current flow.

Calculations are as follows:

1. Average of all milliamp values for all levels = Avg(l_!..I„)

2. Standard Deviation of all milliamp values for all levels = Std(I_x..I_n)

3. The difference between the current and previous soil resistivities is calculated. If the difference is greater than-100 then the value has a sign change to positive. The deciding factor of (-100) is conservative and can be increased or made more negative after further development.

4. These results are then used in the following formula and the results are shown in the table below:

1 x (Std(I_x..I_n) x Avg(I₁..I_n)) - (DR x -1)

Avg(I₁..I_n) n

Where I = Current (mA)

DR = Delta soil resistivities n = Number of test levels in survey

Method B

As current flow is the most critical factor after resistivity and is assessed by biasing each levels current flow with the degree of change in current flow between levels. Soil resistivity is used in the calculation as it allows for variances between sites. Thereby, amplifying the survey results within the restrictions of the local test site.

Calculations are as follows;

1. Average of all milliamp values for all levels = Avg(Ii..I_n)

2. The difference between the current and previous soil resistivities is calculated. If the difference is less than 15mA then the value has a sign and value change to - 1. The deciding factor of (15mA) is conservative and can increase or made more negative after further development. This has the effect of making all changes in current flow less than 15mA be shown as a negative value, therefore excluding it from the final graphical result.

3. The standard Deviation is calculated for all current measurements to obtain the degree of variance which controls the magnitude of the results.

4. The average soil resistivity is calculated in Avg (R_^.R-) and the percentage is derived for the test layer. The percentages are not allowed to exceed 100% as this over amplifies the result. This reduces the following formula's result to suit local conditions.

DR

I x Std( I₁... J_n) x Diff I x -1

Avg( DR_ ... DR_n

Res =

Avg( I_ ...J

Where I = Current (mA)

DR = Delta soil resistivities n = Number of test levels in survey

Average of Biased Resistivities and Current Flows

Methods A and B are averaged for each level and shown on the attached graph as an area shaded curve. This is very useful when estimating where the stream starts between levels or that it starts at an earlier level. This also applies to where streams finish.

Relative Changes in Current Flow

The results of current flows at each level and the differences between each level are plotted. These curves are quite often useful in tipping the scales in borderline situations as the degree of change is more easily recognised in a graphical form. This is very important as it is the relative change between levels which must be considered.

Safety Factors

There are three safety factors built into the system and these are as follows:

1. The standard deviation of the current flows (mA) of all levels provides a quick easy way to check the degree of change between levels. Therefore, on a test site where the standard deviation is less than 15mA the site is below the lower limits of the system. Any recommendations are an indication only. A result from 15 to 20mA is a site which has to be carefully assessed and caution must be observed when advising. If the soil conductivity result is good and the potential drop recorded at that level is good, then there is a 95% possibility that there is water at a specified level, but the water will only be of small volume.

2. The average is calculated for the milliamp results on all levels. The average has to be high (greater than 30mA) . This indicates the pins are functioning correctly and high current flows indicate a very strong site, where there is no doubt water exists .

3. All the resistivities are averaged and if that average exceeds 500 ohm-metres caution must be observed. Verification Method

When locating small streams or when making recommendations at the lower limits of the system then the site must be verified. This is done by running a second set of cables 90° to the original set and only testing for the level where water is considered to be present. If water is present then the new survey results will be similar to the first set.

It has been found that if the survey results are higher on the second set then the semi-circular test pattern is more in line with direction of a water stream.

Homing Method

When attempting to locate small streams the test column can fall on the edge of a stream giving a slight indication of water at a certain test level. Without going to the extent of shifting the main cables and pins a second set is used as in the verification method. The difference is the second set of cables is run in the same line as the original set except the configuration is set to only test at one depth once it is set up.

When set up at the desired depth, the survey column is shifted laterally along the cables because of the new pin locations and configuration. The first test column is 15m along the side of the positive current pins and is then moved in 5m steps until 15m past the original test column (i.e. testing is radial along the original cable line in 5m steps up to a maximum of 15m from the interface unit of the original test column).

The same type of data is recorded as before (i.e. potential drops and current flows) and if there is a stream in this line of testing then there will be a peak in the values recorded.

The position can then be more accurately located by rotating the second set cables and pins 90° to the second survey and another lateral survey completed. The central position of this third survey must be over the location where the peak readings were observed on the second survey. This will result in another set of peak values which then indicates the ideal location to drill for water.

Hardware

The system described in the foregoing for the initial survey is a basic soil resistivity measurement system. For ease of operation and speed, the electrodes are multiplied 16 times to have one system and a single set-up to test to a depth of 150m (approx 500 ft). It will be appreciated that the soil measurement system is essentially similar to the water prospecting system and the same system may be used for both purposes provided appropriate switching between pins can be achieved so that the appropriate voltage measurement for each type of system can be obtained.

1. 100V DC battery powered supply which is a series of high capacity 12V batteries (rechargeable).

2. A central control and interface unit which allows switching between the appropriate set of survey pins along the multi core cables.

3. A high input impedance voltmeter capable of measuring from 0 to lOOOmV. Input impedance must be switchable between 1 giga-ohm and 10 mega-ohms.

4. A meter capable of measuring current across a shunt and temporarily storing the peak result.

5. Multiple core cables as follows:

2 x 90m lengths of 32 core cables with pin connectors. 2 x 90m lengths of 18 core cables with pin connectors. 2 x 45m lengths of 6 core cables with pin connectors. The sensor attachment points are at predetermined locations along these cables.

6. 64 x 304 stainless steel pins (12mm dia x 150mm long), can be other good conductor materials.

7. 1 x single trace oscilloscope.

8. 1 x laptop computer and printer.

9. 15 Lt of towns water. 10. 4 x motorised cable reels.

11. 2 x standard hand wind cable reels.

12. 2 x small hammers.

13. Tool kit for repairs

Claims

CLAIMS :

1. A method of prospecting for underground water at a over ground test site comprising the steps of: arranging at least three electrical probes in the ground at spaced positions along a straight line; selecting a first set of three probes comprising a first probe and a second probe with a third probe between them; applying an electrical potential across the first probe and the second probe so as to cause an electrical current to travel through the ground between the first probe and the second probe; measuring the potential difference between the first probe and the third probe and between the third probe and the second probe; repeating the measurement for a further set of three probes located at a position displaced along the straight line from the position of the first set of probes; and comparing the relative magnitude of the same measurements taken for the first and further set of probes so as to determine the presence of underground water.

2. A method as claimed in claim 1, including the further step of taking measurements for multiple sets of three probes having positions displaced along the straight line, in which the separation between the first and second probes of each set is the same for each measurement so as to determine the presence of underground water at a particular depth.

3. A method as claimed in claim 1 or claim 2, including the step of taking measurements for multiple sets of three probes, in which the separation between the first and second probes of each set varies for each measurement so as to determine the presence of underground water at different depths .

4. A method as claimed in any preceding claim, in which all the probes are equally spaced along the straight line.

5. A method as claimed in any preceding claim, and including Γûá the step of automatically switching the electrical connection between the probes and an interface device so as to select which of the probes act as the first, second and third probes of a set of probes.

6. A method as claimed in claim 1 and including the step of repeating the method of claim 1 with the straight line displaced relative to the straight line used initially.

7. A method as claimed in claim 6, in which the straight line is rotationally displaced.

8. A method as claimed in claim 6 or claim 7, in which the position of the displaced straight line crosses the position of the initial straight line.

9. A method of prospecting for underground water by measuring underground soil resistivities at an over ground test site, comprising the steps of: arranging at least four electrical probes in the ground at spaced positions along a straight line; applying an electrical potential across a first outer pair of probes so as to cause an electrical current to flow through the ground between the first outer pair of probes at a test depth determined by the separation of the outer pair of probes ; measuring the potential difference across the first inner pair of probes; and deriving from the measured potential difference across the inner pair of probes a value which is indicative of the presence of water at the test depth.

10. A method as claimed in claim 9, and including the step of measuring the strength of the electrical current and combining the measured current strength with the measured potential difference in deriving a value indicative of the presence of water at the test depth.

11. A method as claimed in claim 9 or 10, in which a plurality of pairs of outer electrical probes are provided centred on the inner pair and at different distances from the inner pair and including the step of taking measurements of the potential difference across the inner pair with an electrical potential across different pairs of outer probes so as to determine the presence of water at different depths.

12. A method as claimed in claim 9, claim 10 or claim 11 in which a further set of four electrical probes are used and including the step of applying an electrical potential across the further outer pair of probes and measuring the potential across an inner pair of probes displaced along the straight line from the first inner pair of probes so as to determine the presence of water at different positions along the straight line.

13. A method of prospecting for underground water comprising deriving an indication of the presence of underground water from measurements taken according to the methods of claim 1 and claim 9.

14. Underground water prospecting apparatus for use above ground comprising: a plurality of electrical probes for inserting in the ground in a straight line; electrical cabling connected to the electrical probes for communicating electrical signals with the probes; an interface unit to which the cables are connected and including switching means; a voltage power supply connected to the switching means; and voltage measuring means, in which the switching means selectively applies an electrical potential across a pair of electrical probes, on either side of a third probe, and also connects the voltage measuring means between a first probe of the pair and the third probe and the third probe and a second probe of the pair to provide an indication of the presence of underground water.

15. Underground water prospecting apparatus for use above ground for measuring soil resistivities comprising: a plurality of electrode probes for inserting in the ground in a straight line; electrical cabling connected to the electrical probes for communicating electrical signals with the probes; an interface unit to which the cables are connected and including switching means; a voltage power supply connected to the switching means; and voltage measuring means, in which the switching means selectively applies an electrical potential across an outer pair of electrode probes, either side of an inner pair of electrode probes, and also connects the voltage measuring means across the inner pair of probes to provide an indication of the presence of underground water at a depth determined by the separation of the outer pair of probes.