Comparison of high-resolution normal-incidence 3.5 kHz and 12 kHz reflections with geotechnical properties of giant piston cores

Abstract

A deep operating self-contained high-frequency echo sounder known as Deep Probe was recently developed at W.H.O.I. under ERDA and Sea Grant contracts for the purpose of resolving fine details of bottom and sub-bottom sediment layering in the deep ocean. In August 1975 this system was mounted on R.V. KNORR for a coring expedition (KNORR #51) to the Rockall Trough area east of the United Kingdom under the direction of Dr. Charles Hollister. The purpose of this exercise was to determine the correlation between the high frequency acoustic sounding records and the stratification of several Giant Piston Cores (GPC) taken in the same area. Three of these cores, GPC-13, 17 and 19, have been analyzed and compared graphically with the acoustic survey of each core location, using Deep Probe, near-bottom, both as a 12 kHz echo-sounder, and as a deep receiver for detecting returns from a 3.5 kHz pinger mounted on the surface vessel. The acoustic traces for both frequencies were then compared for detail and depth of penetration with respect to the cores. Good to excellent correlation with core stratigraphy is indicated at 12 kHz. Resolution ranged from fair to poor at 3.5 kHz although transmission losses through the core were lower at this frequency. The general conclusion is that deep echo-sounding at 12 kHz is an excellent technique for delineation of shallow bottom and sub-bottom sediments in high detail, and that deep sounders having characteristics similar to Deep Probe could therefore prove valuable for detailed surveys of small areas preliminary to coring, or in deep-towed fish form, for general seismic profiling of shallow sediments over longer tracts in the deep ocean.