Deep-Water Seabed Characterization Using Geostatistical Analysis of High Density/High Resolution Velocity Field

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.

This geostatistical analysis of HDHR velocity field calibrated on in situ measurement pro­vides a relevant methodology for regional deep water seabed characterisation.