Acoustic and Petrophysical Properties of Seafloor Bedrocks

Abstract

Summary There are few published velocity and attenuation data for seafloor bedrocks at appropriate confining pressures (1 MPa to 10 MPa). Velocities and attenuations measured at low pressures are affected mainly 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 rocks depend primarily on the elastic moduli of the rock-forming minerals and porosity, which results from the degree of alteration and diagenetic processes that acted on the rocks. The aim of this paper is to investigate the effect of microcracks on the acoustic properties at low pressures (10 MPa effective pressure) and the relationship between the acoustic, petrophysical, and geological properties of seabed rocks at high pressure (40 MPa effective pressure), where the effect of microcracks 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 rocks undergoing different geological (e.g., diagenesis) processes than those buried at greater depth. We measured compressional- and shear-wave velocity and attenuation on a suite of seabed sedimentary and igneous rocks of the northern U.K. continental shelf 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. We used Winkler and Plona's 1 pulse-echo method to measure velocity and attenuation (quality factor). The porosity and permeability of the rocks range from 0% to about 20% and 0 to about 5 md, respectively. X-ray diffraction (XRD) analysis, petrological studies of polished thin-sections, and scanning electron 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 rocks are chloritized. We show that the velocities and quality factors of the rocks studied are much lower than those of similar continental rocks. Microporosity formed by the alteration of feldspars, micas, and mafic minerals to clays (e.g., chloritization of pyroxenes) and the corresponding reduction of the elastic moduli are the major processes that result in low velocity and quality factors. Velocity and attenuation models of seabed rocks 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 modeling purposes should be lower than assumed normally.