 

The accurate and real-time monitoring measurements of fluid flow can boost the prediction of mass transport and chemical reactions critically impact the efficiency of subsurface exploration and hydrocarbon extractions. Our preliminary effort has shown the efficacy to employ thermoacoustic (TA) imaging for detecting geophysical samples immersed in the oil-bath and has a profound implication of making this methodology applicable for underground water-flow imaging. To achieve the real-time monitoring of fluid flow in porous media at a cm/dm-scale, it entails a fast scan covering the large domain. The reduction of necessary intensive sampling of TA measurement has been conducted by introducing the compressive sensing theory. Another essential problem that impedes the applicability of real-time TA imaging has found to be the low signal-to-noise ratio (SNR) of generated TA signals and the long scan time.

The theoretical analysis is conducted in this work to demonstrate the dominant role of excitation pulse envelop of a frequency-modulation in TA imaging. It is proved that the excitation pulse can greatly affect the time derivative of the absorbed RF energy (SAR) and the generated acoustic pressure waves, which agrees well with the experimental validation. Unfortunately, neither short nor the recently proposed long pulse, known as Frequency Modulation Continuous Wave (FMCW), can perform well to generate a high SNR and meanwhile achieve the short-time scanning.  As seen from the reconstructed TA images for a geological media, the TA signal excited by a short pulse (1us) with a peak power constrain is shown to be incomplete imaging, which is unable to reveal both front and back interfaces of the square sample under the test. Applying a longer pulse can store more thermal energy in target and induce a better SNR of TA signal, but it results in a narrower bandwidth in the frequency-domain and a larger imaging voxel size, causing a lower image resolution. Although the proposed FMCW excitation has been proved to be capable to enhance SNR as well as wide bandwidth in frequency-domain, the extra-long time (pulse width of 30 us) spent at each spot significantly increase the total scan time.

To resolve this dilemma, we exploit the frequency-modulation mechanism and propose a new excitation envelop with multi-pulse, aiming at enhancing the SNR of TA signals, decreasing the irradiation time and remaining the high imaging resolution. In this work, we theoretically analyze the frequency-domain TA method and discuss the principles to optimize the multi-pulse envelop. And we experimentally examine the different excitation methods (single short pulse, multi-pulse and FMCW) in both time and frequency-domain. Compared to oil, water, as the fluid to be injected in the dry porous media, signifies a prominently high contrast. Therefore, we assess the performance of different excitation pulse envelops on the capacity of TA imaging of water-saturated sandstone. Preliminary results demonstrate the prominent enhancement of SNR and TA imaging performance by using multi-pulse and FMCW; the multi-pulse is promised a much faster scanning due to effectively shortening the scan period. In the future work, the enhanced frequency-domain thermoacoustic imaging with the multi-pulse excitation envelop will be in-depth applied for the real-time monitoring of fluid flow in porous media.

 

 


Microwave Induced Thermoacoustic Imaging With Multi-Pulse in Geological Application

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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