Multi-Frequency Oscillating Flow Tests—Making the Best Use of a Few Boreholes

Characterization of the deep subsurface for flow properties represents an enormous challenge, for several reasons. Surface-based geophysical methods represent a promising approach to aquifer characterization that have been applied at many near-surface aquifers, but such methods (e.g., reflection GPR, and ERT) are not able to obtain good resolution at significant depths. Similarly, the cost of installing deep boreholes and the inability to utilize direct push equipment means that direct access to the subsurface materials may be limited to a few boreholes.

In this work, I discuss Multi-frequency Oscillatory Hydraulic Testing (M-OHT), in which pumping tests of various frequencies are performed, and the response of the aquifer to these oscillations is monitored. The ability to stress an aquifer at multiple frequencies is extremely useful, since the period of oscillation represents an additional testing parameter that can be changed in order to collect different information about the subsurface. In contrast, traditional testing strategies such as (constant rate) pumping tests and slug tests have only one parameter that can be changed—i.e., the rate of pumping, or the height of the slug test—and test response is often insensitive to changes in these testing parameters.

I discuss the theory behind M-OHT and first show some example numerical experiments demonstrating the added information content from multiple-frequency aquifer stimulations. I then present laboratory and field examples of this testing (along with analysis strategies), and discuss the practical implementation issues for carrying out such tests to characterize deep aquifers.