Methane occurrence and quantification in a very shallow water environment: a multidisciplinary approach

In the last decades, geological degassing in shallow marine environments has been recognized as a significant contributor to atmospheric methane, hence to the global-warming potential. Especially in shallow water environments, a proper assessment of the distribution, quantification and migration pathways of methane within the sediments is fundamental to help forecasting the amount that could leak and eventually reach the atmosphere. Traditionally, velocity anomalies from seismic data are the ones used to assess the occurrence of gas and its concentration. However, in shallow-waters (<30 m), the post-critical conditions make the near-surface velocity estimation from P-wave reflections extremely challenging, requiring an integrated approach. Here, we propose an original joint analysis of seismic data and geophysical logs, together with information from drilling reports, with the aim of characterizing and quantifying the gas along two crossing multichannel seismic profiles in the Northern Adriatic Sea, a very shallow marine basin where methane occurrence within the sedimentary succession is widespread. We estimated the gas distribution from resistivity anomalies, which are correlated with the seismic response associated with the presence of gas through the signal frequency content. Our results show a different concentration pattern in the two seismic profiles, revealing that gas is both diffuse ad concentrated in local accumulations, in agreement with the gas-related features already identified on the seismic data. Gas concentration appears to be locally associated to the tectonic features identified in the area, indicating that faults act as preferential conduits for gas migration, locally reaching the seafloor and seeping in the water column.