In Situ Measurement of Marine Sediment Acoustical Properties during Coring in Deep Water

In order to obtain in situ measurements, to several meters depth in a marine sediment, of compressional wave speed and attenuation, a device has been built for attachment to ocean bottom sediment corers. This profilometer consists of transducers on the lower end of the coring tool which are connected by armored cables to an electronics package on the upper end of the tool, thus allowing acquisition of acoustical data with a minimum of modification to a standard corer. Early measurements with a prototype profilometer, which required electrical cables to the surface, established the accuracy and usefulness of the technique. The new deep water version of the profilometer is self contained including provisions for recording the data in the electronics package on the corer. Design operating water depth for the new system is 5 km. Test results for the new profilometer are presented including several high resolution in situ sound speed profiles to 12 m depth in deep water (4.5 km) sediments. Detailed layering is described including a high strength, high water content layer, several thin high speed sand layers, and several low speed layers. The data indicate that the near surface low speed zone in these sediments is composed of several thin low speed layers imbedded in a higher speed matrix.