Sea Bed Sampling with an Expendable Acoustic Penetrometer System

Abstract

This paper describes an expendable, acoustic, dynamic penetrometer system that is used to determine the undrained shear strength of ocean bottom sediments to a depth of approximately 9 m in water depths to 6 km. Doppler frequency shifts associated with changing velocities during bottom penetration are measured relative to a highly stable sound source which is attached to a slender weighted body. The varying rates of deceleration are correlated by a topside algorithm into forces which characterize the undrained shear strength of sediment layers down to the depth of penetration. In addition to a physical description of the hardware developed for this purpose, the method for determining soil strength and correlation to unconsolidated, undrained triaxial shear and mini-vane tests on samples recovered with a Shelby tube sampler are discussed.

Though initially designed for site selection and design of embedment anchors, the geotechnical properties that are measured make it a useful tool for a much broader applications spectrum, such as large scale EEZ surveys. In addition, though expendable, it offers a cost-effective alternative to augment and, in some cases, replace coring surveys. Expansion concepts into “in situ” soil sound velocity and acoustic propagation losses are suggested.