Microwave Induced Thermoacoustic Imaging With Multi-Pulse in Geological Application

The accurate and real-time monitoring of fluid flow in porous media can boost the prediction of mass transport and chemical reactions, which profoundly impacts the subsurface exploration and hydrocarbon extractions. Our preliminary effort has shown the efficacy of employing a thermoacoustic (TA) technology for imaging an immobile rock sample. The results support the applicability of making this methodology to move forward for imaging a dynamical process. But the real-time monitoring of fluid flow requires the target under test to excite TA signals with a higher signal-to-noise ratio (SNR), which will promise a sufficient image resolution with fewer necessary measurements or less averaged times, and then lead to a faster scan. It is proved that the excitation pulse is directly proportional to the microwave absorption rate, and thus determines the observability of the corresponding TA signals. Unfortunately, due to the thermal and stress confinements, a microsecond-width pulse envelope is greatly limited and is not sufficient for achieving a high SNR. Although a recently proposed Frequency Modulation Continuous Wave (FMCW) showed an improvement on SNR, it signifies a deficiency of the long-time irradiation and additional electronic disturbance especially at a high peak power. To address this issue, we propose a new excitation envelope with multi-pulses, to favor the coherent frequency-domain signaling method for optimizing the image reconstruction while shortening the total envelope duration than that of the FMCW. In the present paper, the TA sensing of a dry sandstone sample is presented, which efficiently enhances the SNR of TA signals and the image resolution, thus validating the appropriateness of the proposed multi-pulse envelope. The current study also promises a future possibility towards its application for dynamically exploring the underground flow transport.