Energy amplification of repeated radiating ultrasonic pulse signals transmitted through gas-bearing sands: potential for gas bubble manipulation

SUMMARY Gas-bearing sediments in shallow-water environments have recently attracted attention from the perspectives of energy resources, potential geohazards and climate change. Our laboratory measurements demonstrate the potential for gas-bubble manipulation in gas-bearing sands via radiating repeated pulse signals. By monitoring the temporal changes in the waveform of repeated ultrasonic pulse irradiation with a dominant frequency of 100 kHz transmitted through gas-bearing sand, we observe energy amplification and a frequency shift towards the dominant frequency in the recorded waveforms over 880 hr. These results imply that gas bubbles may deform or move autonomously to propagate irradiated ultrasonic waves most efficiently and thereby minimize the energy loss of the transmitted waves. Gas bubbles larger than sand grains detected using 3-D X-ray computed tomography cannot explain the phenomena of energy amplification and frequency shifts, indicating the need for a higher spatial resolution to capture the behaviour of smaller gas bubbles.