Hydraulic Tomography Using Oscillatory Pressure Waves

Hydraulic conductivity (K) is a basic physical parameter needed to understand an aquifer flow system and to design any needed control measure. This research uses sinusoidal pressure waves that are generated at a source well and allowed to propagate to a receiver well. By measuring the resulting amplitude decay and phase shift of the received signal, estimates of the hydraulic conductivity distribution between the source and receiver wells may be made. Theory for a homogeneous aquifer predicts that the exponential amplitude decay and phase shift should increase linearly with radial distance and inversely with the square root of the hydraulic conductivity. Extending this simple theory to heterogeneous aquifer systems predicts the hydraulic conductivity should be replaced with the spatially averaged value. The spatially averaged K value may be estimated by measuring the change in phase and amplitude of a sinusoidal signal as it moves from a source well to a receiver well. Our initial work has used horizontal ray paths to calculate horizontally averaged K values between wells and the results of several surveys will be presented. These averaged K value between the source and receiver wells can be compared to high-resolution slug test data at the source and receiver wells in order to estimate how the K distribution varies between the wells. In addition, we have looked at repeatability and reciprocity (reversing the source and receiver wells). In both cases, for a given well pair, the results seem to be reproducible within experimental accuracy, even in the presence of background noise. Additional work has been done involving diagonal ray paths. It is expected that this extension will provide greater definition of the K distribution by allowing tomographic reconstruction. Initial work indicates that diagonal ray paths originating from a source well do indicate areas of differing hydraulic conductivity as recorded by the measured phase and amplitude changes. This work funded by the Strategic Environmental Research and Development Program (SERDP) as project ER-1367.