COCHONAT, PIERRE, BRUNO SAVOYE, JEAN FRANCOIS BOURILLET, and AGNES BALTZER, IFREMER, Laboratoire "Environnements Sedimentaires", Brest, France; THIERRY MULDER, Laboratoire de Geomecanique, ENSG, Nancy, France; and DAVID J. W. PIPER, Atlantic Geoscience Center, Dartmouth, Nova Scotia, Canada

ABSTRACT: Combined High Resolution Side-scan Sonar Survey and In-situ Geotechnical Measurements: an Efficient Modern Approach of Subsurface Mapping

The recent development of high resolution deep-towed geophysical systems and in-situ geotechnical measurement tools allows to better investigate the subsurface and map sea-floor instability areas.

During the last four years, IFREMER carried out several marine surveys using the french side-scan sonar SAR and the IFREMER geotechnical Module in areas of specific interests in term of slope stability: the slope of the Baie des Anges (Nice, France, 20-1800 m waterdepth) and the 1929 "Grand Banks" earthquake area (East Canada, 200-4000 m water depth) for example.The "Module Geotechnique" is an autonomous preprogrammed equipped frame suspended from a cable that can operate down to 6000 m water depth, penetrate the seafloor up to 2 m, and acquire 3 parameters each 2 cm: cone resistance (1000 kPa), sleeve friction (25 kPa), and excess differential pore pressure (1000 kPa). It can also obtain one 2 m long core per launching.

It was used on transects identified from SAR or seismic imagery to make in-situ measurements on particular stratigraphic horizons or SAR facies. The measured geotechnical parameters associated with other data (seismic, sonar, submersible, cores, ...) allow assessment of the lithology and give an excellent indication of soil deposit stress history. They allow to map in detail subsurface sediment properties, determine type of failure and factors controlling failures and better understand the relationship between slides and their associated deposits.

The geotechnical quantitative approach conducted on Nice submarine slope has led to the classification and the detailed mapping of different areas and to a preliminary attempt of regional stability analysis.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.