Frequency dependence in seismoacoustic imaging of shallow free gas due to gas bubble resonance

Abstract

Shallow free gas is investigated in seismoacoustic data in 10 frequency bands covering a frequency range between 0.2 and 43 kHz. At the edge of a gassy patch in the Bornholm Basin (Baltic Sea), compressional wave attenuation caused by free gas is estimated from reflection amplitudes beneath the gassy sediment layer. Imaging of shallow free gas is considerably influenced by gas bubble resonance, because in the resonance frequency range attenuation is significantly increased. At the resonance frequency of the largest bubbles between 3 and 5 kHz, high scattering causes complete acoustic blanking beneath the top of the gassy sediment layer. In the wider resonance frequency range between 3 and 15 kHz, the effect of smaller bubbles becomes dominant and the attenuation slightly decreases. This allows acoustic waves to be transmitted and reflections can be observed beneath the gassy sediment layer for higher frequencies. Above resonance beginning at ∼19 kHz, attenuation is low and the presence of free gas can be inferred from the decreased reflection amplitudes beneath the gassy layer. Below the resonance frequency range (<1 kHz), attenuation is generally very low and not dependent on frequency. Using the geoacoustic model of Anderson and Hampton, the observed frequency boundaries suggest gas bubble sizes between 1 and 4–6 mm, and gas volume fractions up to 0.02% in a ∼2 m thick sediment layer, whose upper boundary is the gas front. With the multifrequency acoustic approach and the Anderson and Hampton model, quantification of free gas in shallow marine environments is possible if the measurement frequency range allows the identification of the resonance frequency peak. The method presented is limited to places with only moderate attenuation, where the amplitudes of a reflection can be analyzed beneath the gassy sediment layer.

Annual variability and regulation of methane and sulfate fluxes in Baltic Sea estuarine sediments

“…A possible explanation could be that changes in upward transport of methane are due to variability in hydrostatic pressure and the associated diffusive and advective upward transport of methane from depth. The free gas depth of methane is thought to follow changes in hydrostatic pressure and temperature (Mogollón et al, 2011;Tóth et al, 2015). An estimated 10 % of the fine-grained sediments in the Stockholm archipelago area are underlain by pockets of free methane (Persson and Jonsson, 2000) and these free gas pockets are preferentially located in areas with the thickest postglacial mud accumulation, generally in the center of the subbasins and along fault lineaments (Söderberg and Flodén, 1992).…”

State‐of‐the‐art remote characterization of shallow marine sediments: the road to a fully integrated solution

“…Bubble resonance, for example, can lead to a significant underestimate of the gas saturation due to a reduction in the apparent velocity change of the gassy sediments (e.g., Toth et al . ). However, if geophysical measurements can be made across a broad‐enough bandwidth, this frequency‐dependent response can be highly diagnostic (Marin‐Moreno, Sahoo and Best ).…”

“…) and interval velocities (e.g., Leighton and Robb ; Toth et al . , ; Vardy et al . ), P‐wave attenuation measurements (e.g., Morgan et al .…”

“…; Cevatoglu et al . ; Toth, Spiess and Kiell ; Vardy et al . ), that are of broader interest to geologists and geotechnical engineers.…”

“…Ultimately, we present a number of ideas for future research activities in this field. published literature demonstrably derives more advanced properties, such as gas saturation (e.g., Morgan et al 2012;Morgan, Vanneste and Vardy 2014;Toth et al 2014;Cevatoglu et al 2015;Toth, Spiess and Kiell 2015;Vardy et al 2015), that are of broader interest to geologists and geotechnical engineers.This limited quantitative characterization and disparate terminology often leads to problems integrating geophysical data with the results from directly sampled geological and geotechnical data. Purely qualitative integration based on seismo-and lithostratigraphic correlation is often difficult because, while the geological/geotechnical data are recorded in depth below seafloor (SF), the geophysical data are acquired in two-way travel 19693-NSG17 August BOOK.indb 387 12/07/17 09:56 19693-NSG17 August BOOK.indb 388 12/07/17 09:56 Remote characterization of shallow marine sediments 389…”