Cauquil, Eric1, Vincent Curinier1, Stéphanie Legeron-Cherif2, Florent Piriac2, Alain Leron2, Nabil Sultan3 (1) Total, Paris La Défense, France (2) ERMS (3) IFREMER
Deepwater geohazards present geophysical signatures including interval velocity anomalies, with high velocity for gas hydrates / hardground and low velocity for gas bearing sediments. This paper describes a methodology for regional geohazard assessments using geostatistical analysis of high density / high resolution (HDHR) velocity field.
A 3D HDHR velocity field is obtained using Total’s internal velocity picking software (DeltaStack3D) that picks velocity automatically at every CDP gather. Time sampling, depending on seismic frequency content, is driven by the definition of constraints along a seed line which are propagated in 3D in order to take into account lateral geological variations. The interval velocity cube is then computed from this RMS velocity field.
From this interval velocity field, ERMS applied a standard Spatial Quality Assessment (SQA) procedure using geostatistics, with spatial analysis of velocity data, estimation (factorial kriging) of “possible artefacts” and “geological” spatial components, and derivation of Spatial Quality Index (SQI„¥, patented by ERM.S). Final products are interval velocity charts, where filtered residuals could detect small scale geological features.
Velocity anomaly charts have been compared to in-situ geotechnical measurements conducted by IFREMER with PENFELD (deep water CPT). A total of 21 CPT and sonic measurements have been acquired over the study area, allowing detection of gas, gas hydrates layers and carbonate concretions. The comparison between HDHR velocity field and in situ measurements showed a very good correlation.