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Basic Theory of the Method
Seismoelectric effects are initiated by seismic waves, usually p-waves, passing through a porous rock and inducing relative motion between the rock matrix and the fluid within the rock pores. Motion of ionic fluid through capillaries in the rock occurs with cations preferentially adhering to the capillary walls, so that the applied pressure and resulting fluid flow relative to the rock matrix separates the cations and anions thus producing an electric dipole. This is called the Seismoelectric effect.
This is illustrated in Figure 1. A seismic source produces a seismic compression wave, which then propagates into the ground at a speed depending on the rocks through which it passes. Generally this speed varies from about 5000 ft/sec to over 10,000 ft/sec in sedimentary rocks, but can be faster in igneous and metamorphic rocks. The wave spreads out to form a hemisphere as illustrated in Figure 1. When the initial pressure pulse reaches the water table, or a rock saturated with water, electrical charges are separated as described above, and the electrical signal is transmitted back to the ground surface at approximately the speed of light. Conversely, when the wavefront emerges from the saturated zone (aquifer) at depth into a layer with little water, the signal decays to zero. The signal also usually decays to zero if the water in the aquifer becomes saline. Generally, the amplitude of the signal will also decay slowly with depth, as the spreading seismic wave loses its high frequency components and its amplitude decreases due to spherical divergence along with other factors. The fundamental relationships between the spreading seismic wave, the resulting electrical dipoles (charge separations) and the voltage at the ground surface are complex.
Figure 1. Schematic drawing illustrating the basic principles of the Electroseismic method at the top of an aquifer. This diagram should be rotated about its axis (seismic source) by 180° to image the hemispherical nature of the seismic wave.
The circular area (in plan view) encompassed by the leading edge of the pulse when the negative part the pulse just intersects the interface is called the first Fresnel zone. As can be seen in Figure 2, the curvature of the wavefront and the Fresnel geometry ensures that the signal is focused back to the shot point.
Figure 2. Schematic showing the focusing of the electrical signals back to the shot point.
Measuring the Seismoelectric Effect.
The geometry of the seismic source and electrode array used to measure the Seismoelectric effect are illustrated in Figure 3. The electrodes in the array are spaced symmetrically about the seismic source at distances from the source of about 2.5 and 8 feet. The seismic wave is created and the instrument measures the resulting electrical signal.
Figure 3. The geometry of the seismic source and electrodes used to measure the Seismoelectric effect.
One of the instruments used to measure the Seismoelectric effect is called the AquaLocate GF6. This instrument incorporates the floating electrode system described earlier in this text.
Interpretation
Water can move within the pores of the rock easier in good aquifers (high permeability and porosity) than in poor ones and this provides the basis for assessing aquifer quality. If the water moves easily then it will move rapidly when under the influence of the seismic pulse. If the rock has a low permeability or hydraulic conductivity, then the water will move slowly. This causes the shape of the Seismoelectric signal to be different in these two cases. A good aquifer will produce a more rapid rise in the signal amplitude than a poor one, all else being equal. A steeper rise time implies that the signal contains higher frequencies than a slow rising signal and the signal is said to have a greater bandwidth. Water yield estimates can be obtained from the signal bandwidth and the calculations to do this are programmed into the GF2500 instrument. The depth to the top of the aquifer is found from the time taken for the seismic signal to travel to the aquifer, which can be found from the time to the first arrival of the Seismoelectric signal. Likewise, the depth to the bottom of the aquifer can be estimated from the time when the ESI signal decays to zero. In other words, the aquifer thickness can be found from the length of the Seismoelectric signal. The velocity of seismic waves in different rock types is generally well known from seismic surveys, although there can be significant variations in the velocity of rocks, depending on several factors.
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