Seabottom Soil Properties And Acoustic Characteristics
Authors: Mahmood Arshud , Hough John C. 
Published: October 2008

Extensive work has been reported on attempts relating acoustic response and nature of sediments. The available acoustics technology is being used successfully in engineering studies of the ocean floor. Five examples comparing acoustic records and soil boring data are presented. The examples show that broad, loosely structured correlations can be established for extrapolation of physical property data with the help of acoustics. It is concluded that such correlations are valuable in several types of studies for offshore development.

INTRODUCTION

A considerable amount of work exploring relationships between acoustic characteristics and physical properties of ocean bottom sediments has been reported in recent literature. The various investigators have concerned themselves with different aspects of the problem including the determination of sound or compressional wave velocity and attenuation in sediments (Hamilton, 1965, 1970, 1972 and 1975; Hamilton and Others, 1970), study of reflection coefficients (Hamilton, 1970; Watters and Others, 1977) determination of sediment type (Porter and Bell, 1975; Tyce, 1977), prediction of sediment elastic properties (Hamilton 1971a, 1971b), and detection of gas-charged sediments (Wheelan and Others, 1976; Westneat and Porter, 1976).

Simultaneously with the attempts at furthering the understanding of marine acoustic reflections high-resolution geophysics is being used as an increasingly useful tool in the study of the ocean floor for purposes of engineering development and construction. In its usage as applied to these engineering studies, acoustics are still a remote-sensing tool depending on ground truth for site-specific applications. Through continuing experience more and more information is being successfully related to the acoustic survey records, thus enlarging the data base that serves as a catalog of site-verified, though remotely sensed, observations, This paper is an attempt at enlarging this base by presenting examples from some of the continental shelves around the United States.

GEOPHYSICAL RECORDS AND SOIL BORING DATA

Geophysical and geotechnical information from the Gulf of Alaska, Southern California and the Gulf of Mexico is described in this section. The information consists of five soil boring logs and Acoustipulse (a boomer-type tool) profiles across the borings. The geophysical data are unprocessed, analog recordings obtained on a proprietary basis. The geotechnical data were from tests on samples obtained by drilling soil borings and sampling by wireline techniques. The geotechnical testing consisted of determining undrained shear strength and textural properties such as gradation and plasticity.

Gulf of Alaska, Site I. The site conditions are summarized in Fig. 1. The right hand side of the figure is a portion of an Acoustipulse record. The horizontal lines are timing lines representing a two way travel time of 10 milliseconds between adjacent lines. The vertical lines are ship travel marking lines and represent a distance of approximately 1000 ft between adjacent lines.

The left hand side of Fig. 1 presents a simplified summary of soil conditions as found in the soil boring. The layer thicknesses have been marked by assuming a sound wave velocity of 5000 ft/sec. A summary of soil strength and other properties is presented in Table 1.