Acoustic and Petrophysical Properties of Sea-floor Bedrocks
Authors: Assefa S. , Sothcott J. 
Published: October 2008

There are few published velocity and attenuation data for sea floor bedrocks at appropriate confining pressures (1 MPa to 10 MPa). Velocities and attenuations measured at such low pressures are affectedmainly by open microcracks, However, as the pressure is increased and the microcracks are closed, the effect of microcracks on the velocities and attenuations will be minimal. At elevated pressures, the velocities and attenuations in reeks depend primarily on the elastic moduli of the reek forming minerals and porosity which results from the degree of alteration and diagenetic processes that the reeks underwent. The aim of this paper is to investigate the effect of microeracks on the acoustic properties at low pressures (10 MPa effective pressure) and the relationship between the acoustic, petrophysical and geological properties of sea-bed rocks at high pressure (40 MPa effective pressure) where the effect of microeracks is minimal. Laboratory measurements of acoustic velocity and attenuation of this kind give a better understanding of these properties at the appropriate pressure (depth) for reeks undergoing different geological (e.g. diagenesis) processes than those buried at greater depth.

Compressional and shear wave velocity and attenuation measurements have been carried out on a suite of sea-bed sedimentary and igneous rocks at effective pressures (Confining less pore-fluid pressure) ranging from 10 MPa to 40 MPa. The measurement frequencies range from 0.5 MHz to 1 MHz. The pulse-echo method devised by Winkler and Plona (1982) has been used to measure velocity and attenuation (quality factor). The porosity and permeability of the rocks range from O to about 20 % and O to about 5 mD respective y. X-ray diffraction analysis, metrological studies of polished thin-sections andscanning eleetron microscope (SEM) observations show that most of the sandstones have a significant clay content (e.g. kaolinite, illite and chlorite) and fractures. Most of the igneous reeks are chlontised.

It is shown that the velocities and quality factors of the rocks studied are much lower than those of similar rocks of inner continents. Microporosity formed by the alteration of feldspars, micas and mafic minerals to clays (e.g chloritisation of pyroxenes) and the corresponding reduction of the elastic moduli are the major processes resulted in lowvelocity and quality factors. Velocity and attenuation models of sea-bed reeks are critical for the interpretation of seismic data for rocks buried at greater depth. The results of this study suggest that values of velocities and quality factors used for modelling purposes should be lower than usually assumed.

Introduction

There are few published velocity and attenuation data for sea-floor bed rocks at appropriate (low) pressures. Knowledge of the acoustic (velocities and attenuations) properties of sea-floor bedrocks is importantto predict the corresponding properties at greater depth and to obtain realistic velocity and attenuation models. The relationships between the acoustic, petrophysieal and metrological properties of these rocks give a better understanding of the major geological factors affecting the acoustic properties and interpretation of the seismic measurements will also be more tangible.