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Assessing Spatio‐Temporal Variability of Free Gas in Surficial Cohesive Sediments Using Tidal Pressure Fluctuations

Abstract : From a geohazard assessment perspective, the distribution, content, and dynamics of free gas in surficial sediment were addressed by imaging and monitoring the upper 15 m beneath the shelf offshore Nice, France. Based on high-resolution seismic data covering three sites where pore pressure was recorded over three and a half years, the presence of free gas was determined in the upper 2.75–14.75 m of cohesive, silty clay. Seismic velocity changes delineate two layers with gas volume fractions ranging from 0.12% to 1.89%. By considering the tidal response recorded by eight pore pressure sensors, estimates of gas volume fractions vary from 0.26% to more than 9.4% on a spatio-temporal scale which cannot be achieved with seismic data. To depict spatio-temporal patterns three types of free gas occurrence (FGO) were distinguished. Type 3 FGO uniquely showed sawtooth fluctuations in overpressure of 27%–45% of the hydrostatic effective stress as evidence of bubble growth and rise. The other two types showed long-term overpressure trends indicative of a situation whereby bubble growth has ceased. Type 1 FGOs are distinguished from type 2 by their gas volume fraction lower than 9.4% and ratios of overpressure to hydrostatic effective stress lower than 0.3. Values higher than this threshold are considered sufficient for shear failure to initiate from the steep shelf edge (>20°). Beyond site-specific insights, the distinction of FGO from their overpressure levels yields testable implications for the dynamics of methane in sediments.
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Submitted on : Friday, November 26, 2021 - 4:35:24 PM
Last modification on : Friday, March 18, 2022 - 2:58:01 PM

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Sebastien Garziglia, Nabil Sultan, Yannick Thomas, Stephan Ker, Bruno Marsset, et al.. Assessing Spatio‐Temporal Variability of Free Gas in Surficial Cohesive Sediments Using Tidal Pressure Fluctuations. Journal of Geophysical Research: Earth Surface, American Geophysical Union/Wiley, 2021, 126 (10), pp.Article number e2021JF006131. ⟨10.1029/2021JF006131⟩. ⟨hal-03451895⟩

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