Detection and localization of hydromechanical disturbances in a sandbox using the self‐potential method

Abstract

Four sandbox experiments were performed to understand the self‐potential response to hydro‐mechanical disturbances in a water‐infiltrated controlled sandbox. In the first two experiments, ∼0.5 mL of water was abruptly injected through a small capillary at a depth of 15 cm using a syringe impacted by a hammer stroke. In the second series of experiments, ∼0.5 mL of pore water was quickly pumped out of the tank, at the same depth, using a syringe. In both type of experiments, the resulting self‐potential signals were measured using 32 sintered Ag/AgCl medical electrodes. In two experiments, these electrodes were located 3 cm below the top surface of the tank. In two other experiments, they were placed along a vertical section crossing the position of the capillary. These electrodes were connected to a voltmeter with a sensitivity of 0.1 μV and an acquisition frequency of 1.024 kHz. The injected/pumped volumes of water produced hydro‐mechanical disturbances in the sandbox. In turn, these disturbances generated dipolar electrical anomalies of electrokinetic nature with an amplitude of few microvolts. The source function is the product of the dipolar Green's function by a source intensity function that depends solely on the product of the streaming potential coupling coefficient of the sand to the pore fluid overpressure with respect to the hydrostatic pressure. Numerical modeling using a finite element code was performed to solve the coupled hydro‐mechanical problem and to determine the distribution of the resulting self‐potential during the course of these experiments. We use 2D and 3D algorithms based on the cross‐correlation method and wavelet analysis of potential fields to show that the source was a vertical dipole. These methods were also used to localize the position of the source of the hydromechanical disturbance from the self‐potential signals recorded at the top surface of the tank. The position of the source agrees with the position of the inlet/outlet of the capillary showing the usefulness of these methods for application to active volcanoes.