WO2004097459A1

WO2004097459A1 - Seismic-electric water-table detection system and method

Info

Publication number: WO2004097459A1
Application number: PCT/GB2004/001817
Authority: WO
Inventors: Ken O'hara-Dhand
Original assignee: Ken O'hara-Dhand
Priority date: 2003-04-29
Filing date: 2004-04-29
Publication date: 2004-11-11
Also published as: WO2004097459B1

Abstract

A water-table detection system (1) has a seismic wave generation unit (2) and a remote detector unit (3). Sledgehammer (4) strikes hammer plate (5) to generate a trigger pulse P which is then output by transmitter (8) and tranmitted to the detector unit (3) for triggering of its operation. Detector unit (3) detects pulse P from unit (2), and electro-magnetic signals produced when the seismic waves from unit (2) interact with a soil-water interface (i.e. the water-table) within the ground for storage and processing.

Description

WATER-TABLE DETECTION SYSTEM AND METHOD

The present invention relates to a system and method for water-table detection.

There have been proposals for detecting electro-magnetic waves generated at soil-water interfaces from seismic waves by using embedded stainless steel electrodes which are inserted into ground, typically to depths of 0.15 metres.

There are numerous and considerable disadvantages to such arrangements, including the lack of effectiveness of the electrode's electrical contact with the ground under differing conditions of moisture content, lack of consistency of depth to which they are inserted in the ground, and the immovability of the detection system. Also such systems suffers from low sensitivity and problems with noise, both seismic and electrical.

The present invention provides a water-table detection system comprising means to generate a seismic wave; electric field dipole and/or magnetic field loop aerial means to receive electro-magnetic radiation produced by travel of the seismic wave through at least one moisture-containing region in the ground and interacting at the soil water interface; clock means to measure the time delay between generation of the seismic wave and reception of the electro-magnetic radiation.

The system may include any one or more of the following preferred features:

• Means to store a plurality of said time delays;

• Storage means for a plurality of said time delays for a plurality of different detection locations; • Storage means for a plurality of said time delays for a plurality of different detection locations at different distances from the location of generation of the seismic waves;

• Means to measure the amplitude of the received electro-magnetic radiation;

• Means to measure the amplitude of the received electro-magnetic radiation at a plurality of locations at different distances from the location of generation of the seismic waves;

• Means to determine the position of the source of the electro-magnetic radiation derived from the time delay and the electric field strength at a given location from seismic source;

• Means to pass a signal representing the time of generation of the seismic wave to a processor unit to determine the time delay;

• The signal passage means comprises a module to send an electro- magnetic signal representing the time of generation to a processor unit;

• The dipole/loop aerial means is operable to receive a signal representing the time of generation of the electromagnetic wave by means of the seismic wave;

• Means to determine the depth of the source of the electro-magnetic radiation derived from the time delay;

• Means to determine the position/depth of the water-table using seismic data on soil structure and the time delay data;

• Means to determine the position/depth of the water-table using data on the change of amplitude of the electro-magnetic field dipole radiation from different detection positions;

• Means to determine any one or more of the following features from the delay results :-

• Depth and/or location of sub-surface fractures;

• Ground porosity; • Estimation of ground permeability; • Effects of ground compaction;

• Soil stiffness;

• Ground pollutants.

The present invention also provides a method of detecting a water-table comprising: generating a seismic wave; operating electric field dipole and/or magnetic field loop aerial means to receive electro-magnetic radiation produced by travel of the seismic wave through a moisture-containing region in the ground; measuring the time delay between generation of the seismic wave and reception of the electro-magnetic radiation.

The method may include the features of any one or more of dependent Claims 16 to 28.

According to the present invention, there is also provided a computer program product directly loadable into the internal memory of a digital computer, comprising software code portions for performing the method of the present invention when said product is run on a computer.

According to the present invention, there is also provided a computer program directly loadable into the internal memory of a digital computer, comprising software code portions for performing the method of the present invention when said program is run on a computer.

According to the present invention, there is also provided a carrier, which may comprise electronic signals, for a computer program embodying the present invention. According to the present invention, there is also provided electronic distribution of a computer program product, or a computer program, or a carrier of the present invention.

Such a system and method may readily provide data enabling the determination of the position and depth of the water-table. Moreover, the results may be further processed to obtain additional information and data, including determining the depth and location of sub-surface fractures, estimating ground porosity, estimating ground permeability, measuring ground compaction determining soil stiffness and determining ground pollutant characteristics.

In one preferred form, the system provides a seismic wave generation unit and a separate, readily moveable detector/processor unit, the two units being in signal communication either by a screened cable to transmit information to trigger the recording system at the moment of impact of the seismic hammer with the plate or by any appropriate wireless form e.g. using radio transmission, WiFi, Bluetooth or other protocols.

The present invention also provides a generator unit for use in a water-table detection system, the unit comprising means to produce a signal representing the time of generation of the seismic wave and means to send the signal to a detector unit.

The present invention also provides a detector unit for use in an water-table detection system, the unit comprising means to receive a signal representing the time of generation of a seismic wave, electric dipole and/or magnetic loop aerial means to receive electro-magnetic radiation produced by travel of the seismic wave through at least one moisture-containing region in the ground, and clock means to measure the time delay between generation of the seismic wave and reception of the electro-magnetic radiation.

Preferably the units are operable to communicate by any appropriate wireless form, e.g. using radio transmission, WiFi, Bluetooth or other protocols.

In order that the present invention may more readily be understood, a description is now given, by way of example only, reference being made to the accompanying drawings in which:-

Figure 1 is a block schematic diagram of a system embodying the present invention;

Figure 2 shows possible detection positions of the system of Figure i; Figure 3 is a graph of the data from one detection position of the system;

Figures 4 to 6 are graphs of the data from three further detection positions of the system;

Figure 7 is a circuit diagram of part of the system of Figure 1.

Figure 1 shows a water-table detection system 1 comprising a seismic wave generation unit 2 and a remote detector unit 3 which is spaced at a distance 'n' metres from the unit 2, where the distance 'n' is decided at time of survey.

The seismic wave generation unit 2 has a 4.5 Kg sledgehammer 4 to strike an Ultra High Molecular Weight (UHMW) polyethylene hammer plate 5 of thickness 38 x 10^" metres and diameter 2.5 x 10^" metre, the material being chosen in order to ensure that no electro-magnetic signals are generated when a strike occurs. Plate 5 is embedded within the ground, having a top surface exposed and protruding above the ground. Each time sledge hammer 4 hits plate 5, an impact sensor 6 in trigger unit 7 and formed of two relay contacts generates a pulse P which is then output by transmitter 8 and to the detector unit 3 for triggering of its operation.

Detector unit 3 is positioned to receive a trigger pulse P from unit 2, and aerial 10 to also receive electro-magnetic signals produced when the seismic waves from unit 2 interact with a soil-water interface (i.e. the water-table) within the ground.

Aerial 10 is a fully-insulated dipole of 2 metres length of PVC-covered copper tube of diameter 1.5 x 10^" metres. In an alternative form, the aerial may be formed by 16 strands of 2 x 10^"4 metre diameter wire coated with PVC and sealed in a 1.2 x 10^" metre diameter plastic tube for rigidity and support.

Aerial 10 also has a coil loop to detect the magnetic component of the electro-magnetic wave with 7 strands of 2 x 10^"4 metres PVC-coated wire with a loop diameter of 2 metres and 160 turns, tuned for maximum sensitivity at 100 Hz with a parallel capacitor.

In variants, the overall length of dipole aerial 10 is increased by multiples of 2 metres or more to increase detection sensitivity, the detection sensitivity being proportional to length. In another variant, a down-hole water-proof version of the dipole is provided for borehole investigation, especially for ground permeability investigations.

The output signal from aerial 10 is coupled, via a high-quality low-noise microphone cable, to a very-low-noise 16 channel preamplifier 11 with a bandwidth 0 to 3 KHz and input noise less than 1 microvolt. The output from pre-amplifier 11 is fed to a precision 16-bit, 16-channel analogue-to- digital (A/D) converter 12 with a maximum sampling rate of 1000 kHz; therefore for 16 channels, the individual sample rate for each channel is 100 kHz/ 16 = 6.25 kHz, giving a Nyquist frequency in excess of 3.0 kHz per channel. The effective dynamic range of the amplifier and A/D converter is 21 bits or 126 db. The minimum sample rate is set to be 0.25, 0.50, 1.00, 1.25, 2.5, 5.00 and 10 ms, all controllable via software.

In the case of the seismic or acoustic signals, there is a time shift with distance away from the strike point due to the speed of propagation of sound waves in various media, for example sand with a typical velocity of 800 m/s or 0.8 metres a millisecond. However, an electro-magnetic wave has a group velocity of 2.99 x 10⁸ m/s and therefore at the various offsets of distance away from the strike point there is no observable move out. Reception of the electro-magnetic signal is effectively instantaneous. In fact this is proof that the signal is electro-kinetic in origin and is produced by the water-table.

The recorded data is stored in buffer 13 for subsequent processing in microprocessor unit 14. Typically, five readings are made for each position of aerial 10, and aerial 10 is placed at each of a plurality of positions being at 3-metre intervals in orthogonal directions in a grid arrangement from the location L of seismic wave generation unit 2 as shown in Figure 2.

Figure 3 shows the signals recorded in a buffer 13 when remote monitor unit 3 is positioned at 3 metres from unit 2 at location Ll. The signals comprise:

1. A negative pulse Al of amplitude 105 microvolts at 19.25 milliseconds, representing the arrival of the seismic wave at the capillary zone above the water table. 2. A positive pulse A2 of amplitude 100 microvolts at 22.00 milliseconds representing the electro-magnetic wave generated by the seimic wave intercepting the water table.

3. A negative pulse A3 of amplitude 25.0 microvolts at 22.75 milliseconds representing a lightly fractured zone.

4. A positive pulse A4 of amplitude 75.00 microvolts at 24.50 milliseconds representing a more heavily fractured zone.

Figures 4 to 6 shows the signals recorded in buffer 13 when unit 3 is positioned at locations L2, L3 and L4 namely at distances of 6, 9 and 12 metres from unit 2 at L, see Table 1. Note that the timings of the pulses (e.g. Bl, B2, B3 and B4 in Figure 4) are identical to Al to A4 of Figure 3, showing that the signals received are the electro-magnetic signals generated by the water-table while the comparative amplitudes between Ll, L2, L3 and L4 are different, in accordance with the distance of the dipole aerial from the location at which hammer strikes plate 5.

TABLE 1

In a first method of determining the depth of the water-table from the recorded values as shown in Figures 3 and 4, in addition to recording the electro-magnetic signal, a shallow seismic survey is conducted in order to ascertain the seismic velocity in the various layers down to the water-table. Knowing the seismic velocities in the various layers, one can calculate the time it takes for the seismic wave to traverse each layer and hence the depth of the water-table.

Another method of determining the water-table depth, which eliminates the necessity to carry out a seismic survey, is noting the strength of the electric dipole field at various distances away from L and from this calculating the point of origin of the dipole electric field, i.e. the water-table interface.

The amplitude of a dipole field falls off as 1/r² where r is the distance from the source. The formula for the electric field Ε' is: E =p(l + 3cos²θ)°^"5/4πε₀r³ (1) where p is a vector p = Qxs Q being the electric charge of the two point charges forming the dipole and s is pointing from -Q to +Q.

Knowing the distance from the source and the strength or magnitude of the electric field 'E' at this distance, one calculates the distance to the origin using formula 1. Microprocessor unit 14 performs any one or more of the following processing operations:-

• Determining the depth of the water-table from the time delay of receipt of the electro-magnetic wave together with the soil strata information provided from records and/or seismic readings or geophone calculations;

• Determining the depth of the water-table from the time delay as above, and based on a calculation based on amplitudes of the received electro-magnetic wave at different detector distances; • Determining the depth and location of sub-surface fractures;

• Estimating ground porosity by analysis of the electro-magnetic response over the capillary zone above the fully water-saturated ground;

• Estimating ground permeability by use of electro-magnetic transport equations to determine (w);

• Measuring extent of ground compaction by noting the electo- magnetic response before and after compaction;

• Determining ground pollutant characteristics by comparison of the electro-magnetic signal from a known unpolluted site with a similar geological profile and that from the polluted site, (there being a change in electrical conductivity which will reflect in the difference of the electro-magnetic response);

• Determining soil stiffness.

The electro-magnetic transport equations include:-

J = σ(τσ)-E + L(tπ)-(-Vp + ω²p_fu(τσ) +f) (2)

-iτσw(rø) = L(τσ E + (k(τπ)/η)-(-Vp + ω² p_f U(TO) +f) (3)

Where tσ = 2πf, being the frequency of the seismic disturbance. -i σw(τσ), the filtration velocity, represents the volume of the fluid traversing unit area in unit time J is the current density k is the fluid flow permeability L is the Electrokinetic coupling coefficient tensor

E is the Electric field σ = electrical conductivity p = fluid pressure η = fluid shear viscosity / = f_f fluid force i = the complex operator

Trigger unit 7 operates either from a seismic hammer contact closure or trigger pulse of minimum amplitude + 0.20 volts. For calibration or noise monitoring purposes, it is internally triggered on a repetitive basis via the software. An analogue 50.60 Hz notch filter was not included as it can lead to signal distortion and phase shift of the received signal, a very important consideration when detecting very low level signals. Recording on one channel, and extending the recording time e.g. for a sample rate of 0.125 ms and record length of 8196 samples and using one channel results in a total recording time of 8196 x 16 x 0.000125 = 16.392 seconds. The seismic data files can be saved for later data analysis on a high density 3.5 inch floppy disc.

Pre-amplifier 11 is a precision low power instrumentation amplifier with minimum voltage resolution of the A-D converter 10 is 1.25/2¹⁶ volts, which is 19.07 μ volts. To provide a higher resolution, the gain of preamplifier was designed to be adjustable to 8, 16 or 32 to give resolutions of 2.38, 1.19 and 0.57 μ volts. At these gain levels, the frequency response of the pre-amplifier is flat to 12 kHz. The input noise figure for pre- amplifier 11 is 0.28 nanovolt at a gain of 1000 over a frequency range of 0.1 to 1 kHz, the source impedance R_s being zero.

The circuit diagram of one of the pre-amplifier channels is shown in Figure 7 with the trigger circuit shown as well. The total current consumption is typically 6 milli-amperes, giving a total operating time in the field of 33 days based on 9 hours use per day.

Detector unit 3 also has inputs from a GPS unit 16 to provide location data relevant to the measurements recorded, a mobile phone and modem interface 17 for data transmission for detailed later analysis, and back-up power supply 18.

Soil stiffness can be determined by use of the shear wave velocity which has been observed during experiments with the magnetic field loop aerial, the relationship being

G = (Vs) (Vs).p Mpa (4)

Where G is the bulk modulus and p the bulk density.

Further, using the relationship

G=(E/(2(1 +v) (5)

Where v is Poisson's ratio and E is Young's modulus.

E can easily be found from the equation 5.

Claims

1. A water-table detection system comprising: means to generate a seismic wave; electric field dipole and/or magnetic field loop aerial means to receive electro-magnetic radiation produced by travel of the seismic wave through at least one moisture-containing region in the ground ; clock means to measure the time delay between generation of the seismic wave and reception of the electro-magnetic radiation.

2. A system according to Claim 1 comprising means to store a plurality of said time delays.

3. A system according to Claim 1 comprising storage means for a plurality of said time delays for a plurality of different detection locations.

4. A system according to Claim 1 comprising storage means for a plurality of said time delays for a plurality of different detection locations at different distances from the location of generation of the seismic waves.

5. A system according to any preceding claim comprising means to measure the amplitude of the received electro-magnetic radiation.

6. A system according to any preceding claim comprising means to measure the amplitude of the received electro-magnetic radiation at a plurality of locations at different distances from the location of generation of the seismic waves.

7. A system according to any preceding claim comprising means to determine the position of the source of the electro-magnetic radiation derived from the time delay.

8. A system according to any preceding claim comprising means to pass a signal representing the time of generation of the seismic wave to a processor unit to determine the time delay.

9. A system according to Claim 8 wherein the signal passage means comprises a transmitter to send an electro-magnetic signal representing the time of generation to a processor unit.

10. A system according to Claim 8 or 9 wherein the aerial means is operable to receive a signal representing the time of generation of the seismic wave.

11. A system according to any preceding claim comprising means to determine the depth of the source of the electro-magnetic radiation derived from the time delay.

12. A system according to any preceding claim comprising means to determine the position/depth of the water-table using data on the soil structure and the time delay data.

13. A system according to any preceding claim comprising means to determine the position/depth of the water-table using data on the change of amplitude of the electro-magnetic field dipole radiation from different detection positions.

14. A system according to any preceding claim comprising means to determine any one or more of the following features from the time delay results :-

• Depth and/or location of sub-surface fractures; • Ground porosity;

• Estimation of ground permeability;

• Effects of ground compaction;

• Soil stiffness;

• Ground pollutants.

15. A method of detecting a water-table comprising: generating a seismic wave; operating electric field dipole and/or magnetic field loop aerial means to receive electro-magnetic radiation produced by travel of the seismic wave through at least one moisture-containing region in the ground and interacting at the soil water interface; measuring the time delay between generation of the seismic wave and reception of the electromagnetic radiation.

16. A method according to Claim 15 comprising storing a plurality of said time delays.

17. A method according to Claim 15 comprising storing a plurality of said time delays for a plurality of different detection locations.

18. A method according to Claim 15 comprising storing a plurality of said time delays for a plurality of detection locations at different distances from the location of generation of the seismic waves.

19. A method according to any of Claims 15 to 18 comprising measuring the amplitude of the received electro-magnetic radiation.

20. A method according to any of Claims 15 to 19 comprising measuring the amplitude of the received electro-magnetic radiation at a plurality of locations at different distances from the location of generation of the seismic waves.

21. A method according to any of Claims 15 to 20 comprising determining the position of the source of the electro-magnetic radiation derived from the time delay and the electric field strength at that point.

22. A method according to any of Claims 15 to 21 comprising passing a signal representing the time of generation of the seismic wave to a processor unit to determine the time delay.

23. A method according to Claim 22 comprising transmitting an electromagnetic signal representing the time of generation to a processor unit.

24. A method according to Claim 22 or 23 wherein the aerial means is operable to receive a signal representing the time of generating of the seismic wave.

25. A method according to any of Claims 15 to 24 comprising determining the depth of the source of the electro-magnetic radiation derived from the time delay and the electric field strength at that point.

26. A method according to any of Claims 15 to 25 comprising determining the position/depth of the water-table using data on the soil structure and the time delay data.

27. A method according to any of Claims 15 to 26 comprising determining the position/depth of the water-table using data on the amplitude of the electro-magnetic radiation from different detection positions.

28. A method according to any of Claims 15 to 27 comprising determining any one or more of the following features from the time delay results: • Depth and/or location of sub-surface fractures;

• Ground porosity;

• Estimation of ground permeability;

• Effects of ground compaction;

• Soil stiffness; • Ground pollutants.

29. A computer program product directly loadable into the internal memory of a digital computer, comprising software code portions for performing the method of any one or more of Claims 15 to 28 when said product is run on a computer.

30. A computer program directly loadable into the internal memory of a digital computer, comprising software code portions for performing the method of any one of more of Claims 15 to 28 when said program is run on a computer.

31. A carrier, which may comprise electronic signals, for a computer program of Claim 30.

32. Electronic distribution of a computer program product of Claim 29 or a computer program of Claim 30 or a carrier of Claim 31.