Velocity and porosity of sediments from the CRP-1 Drillhole, Ross Sea, Antarctica

The relationship between whole-core compressional wave velocities and gamma-ray attenuation porosities of sediments cored at CRP-1 is examined and compared with results from core-plug samples and global models. Both core-plug and whole-core velocities show a strong dependence on porosity; this relationship appears to be independent of lithology. In the range from 0.1 to 0.4 of fractional porosity (Miocene strata), plug velocities are generally 0.2 0.5 km s higher than whole-core velocities. Possible reasons include decreased rigidity in the whole core and diagenetic changes in the plugs. Possibly both velocity measurements are correct but neither is fully representative for in situ conditions. It appears that the core-plug results are more compatible with data from other regions than the whole-core data. After removing first-order compaction control from the whole-core porosity record, a second-order control by clay content can be quantified as a simple positive linear regression (R=0.6). In contrast, after correction for first-order control, porosity and velocity are not significantly influenced by lonestone abundance except for rare, very large lonestones. approaches zero, and therefore internal friction, attenuation, and frequency dependence can be neglected, compressional velocity can be specified largely in terms of dynamic elastic moduli (Gassmann, 1951a, 1951b). However, these moduli are difficult to predict for high-porosity sediments such as those cored at CRP-1. An alternative approach to sediment velocities is based on empirical, rather than theoretical, relationships. For example, Wyllie et al. (1956) recognised that porosity dominates most variables controlling velocity, and he therefore introduced an empirical relationship between porosity and compressional velocity. His time average equation uses grain and fluid velocities, rather than bulk moduli, to describe the velocity/porosity relationship. This equation most closely approximates the behaviour of lithified sedimentary rocks. Its validity is usually restricted to sediments with porosities <0.3. Later investigators extended this empirical approach to include variations in sand/shale content (Castagna et al., 1985; Han et al., 1986) and higher porosities (Raymer et al., 1980). Erickson & Jarrard (in press) proposed “global” empirical relationships for predicting velocity based on porosity, sand/shale content, and consolidation history. Physical properties of CRP-1 core, including porosity and P-wave velocity, were determined in two ways: by measuring gamma-ray attenuation and P-wave travel time on the whole core prior to core cutting at the drill site (Cape Roberts Science Team ,1998), and by using core-plug samples (Brink & Jarrard, this volume). Down-core variations in the whole-core data are plotted and their implications are discussed by Niessen et al. (this volume). The aim of this study is to examine the relationship INTRODUCTION The Cape Roberts Project (CRP) is investigating the Cenozoic and Cretaceous climate history of the Antarctic by coring scientific drillholes offshore Cape Roberts, Ross Sea. The first drillhole (CRP-1) penetrated 148 metres of Quaternary and Miocene sediments. P-wave velocities of these sediment cores provide a bridge between core depths and regional seismic profiles (Cape Roberts Science Team, 1998). Relationships of P-wave velocity and porosity can be diagnostic tools for the interpretation of a complex imprint on the petrophysics of sediments and sedimentary rocks. For example, porosity-velocity relationships by Wood (1941) and Wyllie et al. (1956) were long used to describe the petrophysical characteristics of high porosity sediments and low porosity sedimentary rocks, respectively. The Wood equation simplified the theoretical Hookean elastic equations (e.g., Gassmann, 1951a, 1951b), by assuming a sediment suspension in which shear modulus and frame bulk modulus are zero. Consequently, the relationship is, at best, appropriate only at very high porosities of >0.7. In fact, a multitude of variables affects the elastic moduli (and, therefore, the compressional-wave velocities) of siliciclastic, sand-shale sediments and sedimentary rocks. In a classic series of papers (e.g., Biot, 1962), Biot developed a general theoretical description for the viscoelastic responses of fluid and mineral framework of a porous medium. Biot theory, though rigorous and powerful for sensitivity studies, requires specification of 13 parameters, a formidable hurdle for most geophysical studies of sound propagation. For the special case in which frequency West Antarctic Rift System WAIS