A New Seismo-Acoustic Probe for Seabed Measurements

Abstract

ABSTRACT A new seismo-acoustic probe has been developed to measure the propagation velocity and attenuation of compressional and shear waves in seafloor sediments. These data provide information important to the study and analysis of sound propagation in shallow ocean areas and for inferring the geotechnical parameters of offshore sediments. The probe employs a vibrator-driven compaction penetration technique to overcome the problems of ship-based open hole drilling and wire line probe entry into boreholes in unstable sediments. INTRODUCTION The propagation of underwater sound in shallow ocean environments is influenced significantly by the acoustical characteristics of the water-sediment bottom interface and, in particular, by the compressional wave and shear wave parameters of the sediment materials. Substantial progress has been made in predicting and better understanding of long-range sound propagation in shallow ocean areas through computational model simulation studies(l-4) as well as from full-scale experiments conducted at sea(5-8) Extended progress and refinements of these studies now require more detailed information on the acoustical parameters of the bottom sediment materials, including vertical profiles of these parameters to depths comparable with the acoustic wavelengths of interest. Accurate determination of the desired acoustical and physical properties of seafloor sediments is difficult and often impractical using bottom sampling techniques because of their disturbance of the material specimens. Measurements restricted only to the bottom interface are convenient but are incomplete because of their limited depth of penetration and inadequate depth profile information. Therefore, sub-bottom in situ measurements are necessary in order to fully evaluate the acoustical characteristics of the sediment materials, particularly in those cases where vertical variations in homogeneity are important. Since compressional waves and shear waves are to be measured as a function of depth in the seafloor, the measurement requirement is equivalent to that of full-waveform sonic logging; a technique well developed for use in oil and gas formation evaluation in open boreholes. Recordings of compressional and shear waveforms measured over two known path lengths can be analyzed to yield the propagation velocity and the attenuation coefficient of each wave type as averaged over the differential wave propagation path length common to the two measurements. These experimental data on the velocity of compressional andshear waves together with knowledge of the density of the medium can be further analyzed to yield values of Young's modulus, shear modulus, and Poisson's ratio averaged over the path length common to the two measurements. Additionally, by taking into account the acoustic attenuation effects and frequency dependence effects in the analysis of the elastic module, estimates of the inelastic parameters of kinematics viscosity and particle internal friction can be derived. These parameters of the sound propagation medium, derived from acoustical measurements, derme the dynamical behavior of the medium. Empirical interpretations of these parameters may also become practical in the future whereby the static module may be inferred from the dynamic values to yield estimates of the in situ geotechnical and engineering characteristics of the medium as averaged over thepath length common to the two measurements.