Geophysical monitoring of subsurface contamination in two-phase porous media

Abstract

We have explored a new technology based on using low-frequency strain attenuation data to monitor the infiltration of contaminants into two-fluid phase porous materials. The attenuation mechanism is related to the loss of energy due to the hysteresis of resistance to meniscus movement (changes in surface tension and wettability) when a pore containing two fluids is stressed at very low frequencies. This phenomenon was verified in our laboratory experiments and applied to a field study near Maricopa, Arizona. In the field study we conducted controlled experiments with the aim of detecting and monitoring the infiltration of a contaminant – a biosurfactant – into groundwater. Three sets of geophysical instruments, each consisting of a 3-component seismometer and a tiltmeter, were installed near an irrigation site. The experiment lasted about 3years with controlled irrigations of initially water only and then water with contaminant into the vadose zone. We used naturally occurring signals of microseisms for our seismic sources and solid Earth tides as sources for the tilt signals.To process the tilt data we extracted the tidal signal from the raw data, and then compared the measured tidal signals with site-specific theoretical data. Changes in tidal signals indicate a strong anomaly associated with the irrigation when a 150ppm concentration of a biosurfactant was present. No such anomaly was detected when only water was used. Further data processing with an automated procedure shows that the anomalies still persisted. Furthermore, the microseismic results from the seismic data suggest that there may also be a detectable anomaly.