Characterization of gas‐bearing sediments in the coastal environment using geophysical and geotechnical data

Abstract

Seismic investigation in marine gas-bearing sediments fails to get information below the acoustic mask created by free gas. To circumvent this problem, we combined collocated multichannel ultra-high resolution seismic imaging, marine electrical resistivity tomography (MERT) and core sampling to study the physical properties of gas-bearing sediments in the Bay of Concarneau (France). We obtained sections of compression (P-) wave velocitvalues where free gas was identified in seismic data. We tested a joint processing workflow combining the 1D inversion of the MERT data with the 2D P-wave velocity through a structural coupling between resistivity and velocity. We obtained a series of 2D resistivity models fitting the data whilst in agreement with. The resulting models showed the continuity of the geological units below the acoustic gas fronts which is associated with paleo-valley sediment infilling. We were able to demonstrate relationships between resistivity and velocity differing from superficial to deeper sediments. We established these relationships at the geophysical scale then compared the results to data from core sampliand porosity). We inferred the porosity distribution from the MERT data. At the core locations, we observed a good agreement between this geophysical scale porosity and the core data both within and outside the gas-bearing sediments. This agreement demonstrated that resistivity could be used as a proxy for porosity where no was available below gas caps. In these regions, the observed low resistivity showed a high porosity (60-70%) down to about 10-20 m in depth in contrast with the surrounding medium with porosity less than 55%. These results support the hypothesis that failures inside the paleo-valley sediment could control the gas migration

A workflow for processing mono‐channel Chirp and Boomer surveys

“…Helle et al (2003) analysed the effects of gas saturation on both velocity and anelastic absorption. Dusart et al (2022) jointly inverted seismic and electrical measurements in an area with gas‐bearing shallow sediments, getting P velocities between 1200 and 1400 m/s. Chen et al (2023) obtained similar results for gas‐saturated sands using X‐ray and acoustic tomography.…”

“…Zero‐ or short‐offset mono‐channel data cannot estimate an angle‐dependent reflectivity. Another useful parameter, that is resistivity, can be jointly inverted with seismic data (Dusart et al., 2022).…”

Coupled Inversion of Amplitudes and Traveltimes of Primaries and Multiples for Monochannel Seismic Surveys

“…For monochannel systems, the estimation of rock parameters such as P velocity, density and thickness of shallow marine sediments becomes a challenge. However, these parameters are needed by offshore engineers for pipeline and jetty design [21,22] and by marine geologists to assess recent depositional conditions [23][24][25].…”