Use of dual methods to infer methane bubble populations in gassy sediment: Inversion of propagation data

Abstract

The inversion of the acoustic properties of gassy sediments presents the optimum manner of determining the in situ distribution of sediment-based methane bubbles. An in situ device that measures both compressional wave attenuations and combination-frequency components in gassy sediment lying within 2 m of the seabed has been developed at the University of Southampton. This device was deployed at an inter-tidal site along the South coast of England. Compressional wave attenuations were measured from 10 to 100 kHz though the analysis of propagation signals transmitted from a variety of sources to a buried co-linear hydrophone array, with propagation distances spanning 0.5 to 2 m. Measured attenuations were inverted to infer in situ bubble size distributions using both established and new acoustic models for gassy sediment. The analysis and results of the combination-frequency component are described in a companion paper.

Acoustical methodology for determination of gas content in aquatic sediments, with application to Lake Kinneret, Israel, as a case study

“…The inclusion of gas bubbles in aquatic sediments changes their effective properties as a medium of sound propagation, which in turn affects acoustic phenomena such as the reflection, scattering, and attenuation of sound signals, even at a low (order of 1%) gas fraction within the sediment (Anderson and Hampton ; Robb et al ).…”

“…This dependence has a resonance maximum at some frequency (Minnaert ; Leighton ), depending in turn on sediment and bubble properties. As a result, if a wide‐band signal is used, then a significant peak at this resonance frequency will be observed for side‐scattered sounds. Nonlinear effects in sound signal propagation in a gassy medium (Karpov et al ; Robb et al ; Leighton et al )The following typical nonlinear effects can take place: generation of the second and higher harmonics for a single frequency incident signal, and generation of combination frequencies for two and more signals with different frequencies. The latter usually appears in the high frequency regime (ultrasound, tens of kHz) and was implemented, for instance, in Leighton et al (), where bubble distribution over size was estimated.…”

“…Nonlinear effects in sound signal propagation in a gassy medium (Karpov et al ; Robb et al ; Leighton et al )…”