Subsurface Imaging of Water Infiltration into the Oil-saturated Sand using the Microwave-induced Thermoacoustics Imaging Techniques

The study of non-invasive imaging techniques for multiphase flow plays a vital role in the hydrology contamination researches and water remediation efforts. Although some imaging techniques have been sufficiently investigated and widely applied in this field, none of them is perfect for the real-time 3D imaging application at the scale of several centimeters. Meanwhile, microwave-induced thermoacoustics (TA) imaging method is an emerging technique which couples electromagnetic (EM) field and acoustic/seismic (AC/S) field through the thermal expansion process. Previous studies reported that the TA waves can be generated at the surface of water-saturated sand, where the signal amplitude is directly related to the water saturation level. This work will extend the research to the sub-surface imaging applications using the TA waves to monitor the water infiltration into the oil-saturated sand. In the experiment, the water-saturated sand is geometrically constrained to offer the ground truth profile, and embedded in the oil-saturated sand at a certain depth as the target. A transducer moved by a raster scanning system is used to collect the measurements. It is experimentally demonstrated that a detectable TA signal can be obtained when the target is placed 15mm underneath the sand surface, when a 4kW microwave power and 1us excitation pulse are used. During the propagation in a porous medium such as oil-saturated sand, the AC wave always suffers from frequency-dependent attenuation and velocity dispersion, which will change the profile of measurements. In order to compensate for these effects, the porous medium is calibrated to obtain the attenuation and dispersion factors. In the imaging reconstruction process, the TA imaging sensing matrix is modified accordingly to accommodate the attenuation and dispersion effects in the propagation medium. The imaging results reveal the feasibility of applying TA waves to monitor the sub-surface water infiltration process in oil-saturated sand. This work is funded by DOE (Award DE-SC0017614) and the NSF CAREER Program (Award No. 1653671).