A New Shear Wave Velocity Measurement Technique in Ocean Bottom Soil Samples

Abstract

ABSTRACT The shear wave velocity together with the longitudinal wave velocity and the density of the soil completely describe the first order dynamic elastic properties of the soil. This paper deals with an improved and convenient method of obtaining the shear wave velocity by using inexpensive, portable, and durable equipment. Necessary theory for proper interpretation of data (the experimental technique and data obtained using the system are presented. INTRODUCTION The recovery of the essential engineering data necessary for the efficient utilization of the sea floor is tedious and expensive. A low cost, effective, surface-borne acoustic soil assessment system is attractive concept; however, it is not yet fully developed. Preliminary site information that is often necessary for underwater engineering projects is usually obtained by physical tests performed on sediment core samples. Recently, the value of acoustic testing of soil samples has been recognized. Acoustic tests on core samples or reconstituted ocean soils serve two basic purposes. First of all, they provide the dynamical soil constants thus making tedious laboratory measurements unnecessary. Secondly, they provide compressional and shear wave velocity and attenuation information which in turn is necessary data for remote acoustic surveys. Also, any dynamical modeling of soils requires sound velocity and attenuation information. Historically the most common acoustic measurement on soils has been the determination of longitudinal sound velocity. The longitudinal wave speed through soil samples is relatively easy to obtain and a great deal of information is available in the literature. Determinations of longitudinal sound speed have been made in situ (1, 2, 3, 4) as well as on laboratory sedimented (5), laboratory disturbed (6) and on cored (7) marine soils samples. However, comparatively little information is available on the transverse wave speed, although this parameter is just as important as the longitudinal wave speed. In situ measurements of the Stonely wave velocity (from which the shear wave velocity can be computed) have been made for the top layers of marine soil by Davis (8), Bucher, et al (9) and Hamilton et al (10). The dynamic rigidity of artificially sedimented clays has been determined in the laboratory by using a torsional-oscillator-resonance technique by Hutchins (11), Cohen (12) and Hardin (13) and Humphries (14). Also, a two-hole method has been used to obtain transverse wavespeed in marine (15) and terrestial soils (16). The purpose of this paper is to describe the theory and the operation of a system that is capable of directly measuring shear wave velocity and attenuation. Here, a shear wave velocimeter for laboratory use is described which is low in cost, portable (hand held) and easy and fast to use.