171A Scientific Prospectus


1. Overall Prism Consolidation.

Porosity is the foundation for a variety of studies about the large-scale, long-term fluid budget of accretionary prisms. Logs can be used to determine a continuous record of density and porosity as a function of depth, as was done during Leg 156. Between-site variation in the porosity-depth relationship provides an estimate of the amount of fluid expulsion (and therefore volumetric strain). Unfortunately, measurements of volume change are usually impossible with standard logs, as they frequently fail because of bad hole conditions in this setting. Even under ideal conditions wireline logs do not obtain data from the top 60 to 120 m because the drill pipe must extend below the seafloor during logging, nor do they often sense the bottom 60-120 m of the hole because of fill. The shallowest 100 m, where porosity reduction is the greatest, is of particular interest in this study. Only LWD can obtain reliable porosity logs from the entire depth range, including the critical top 100 m.

Profiles of porosity vs. depth provide a tantalizing but incomplete view of the fluid expulsion pattern of an accretionary prism. Velocity data, either from multichannel seismic data (Bray and Karig, 1985; Bangs et al., 1990; Cochrane et al., 1994) or ocean-bottom seismograph (OBS) studies, are powerful tools for studying prism porosity structure. The fundamental limitation in determining porosity from velocity is the conversion between these two parameters. This relationship is well known for normally consolidated, low-porosity sediments (e.g., Gardner et al., 1974), but it is much less certain for high-porosity sediments, where changes in terms of fluid production and volumetric strain are more important. Furthermore, our analysis of logs from the Cascadia accretionary prism indicates that prism deformation dramatically changes the porosity-velocity relationship (Jarrard et al., 1995). In contrast to pelagic sediments, accretionary prism sediments of the same porosity can exhibit a wide range of elastic moduli and, therefore, velocities; this complexity results from variability in cementation, compression-induced modification of intergrain contacts, and fracturing. Theoretical relationships of porosity to velocity (e.g., Gassman, 1951) are of little utility in this environment; we must determine the velocity-porosity relationship for each prism empirically, and we must investigate the possibility that this relationship changes laterally within a prism. In situ velocity and porosity logs that sample the section completely are the only means of reaching this objective.

The overall fluid budget of the Barbados prism requires analysis to evaluate the fluid loss and geochemical budgets (e.g., Bekins et al., 1995). The series of LWD holes planned here, plus existing penetrations, will help constrain this problem. We anticipate obtaining excellent in situ porosities at all sites. The velocity-porosity relationship will be constrained by wireline sonic logs at proposed Site NBR-5A, and from the previously logged Site 948.

2. Correlation of Physical Properties of Faults with Displacement and Fluid Flow.

An LWD transect across the Barbadian décollement can address the following questions: (1) do faults collapse and strain harden with displacement (e.g., Karig, 1986), and (2) does active fluid flow retard this process, and are collapsed faults inactive with respect to fluid flow (e.g., Brown et al., 1994)? Structural, biostratigraphic, and seismic reflection criteria identify faults. Anomalies in pore-water geochemistry (e.g., Kastner et al., 1991) and thermal anomalies (Fisher and Hounslow, 1990) indicate fluid flow. With the positive identification of faults, LWD can measure their physical properties. These properties then can be correlated to variations in displacement and fluid activity.

3. Consolidation State of Sediments in and Around Faults.

At Site 948 in the Barbados prism, high-quality density measurements demonstrated underconsolidation around faults, indicating that the faults had recently loaded subjacent sediments. The consolidation state can also be interpreted in terms of effective stress and fluid pressure. Clearly, consolidation varies around faults and should be defined to develop any tectonic-hydrologic model of the fluid expulsion system.

4. Polarity and Shape of the Seismic Waveform from Fault Zones.

Seismic reflections are created by changes in physical properties that can in turn be measured in boreholes. In principle, the seismic data provide a proxy for these larger-scale changes in physical properties. The polarity and shape of the seismic waveform were mapped and various models formulated for the waveform across décollement zones beneath accretionary prisms (Bangs and Westbrook, 1991; Moore and Shipley, 1993). Negative polarity reflections have been interpreted as resulting from either (1) overthrusting of higher-impedance sediment over lower-impedance sediment in Costa Rica (Shipley et al., 1990), or (2) the reduction of fault-zone impedance through dilation at Barbados (Bangs and Westbrook, 1991; Shipley et al., 1994; Bangs et al., 1996). The modeling, however, is incomplete without ground truthing by the in situ measurement of physical properties across fault zones in areas with high-quality, three-dimensional seismic data.

Logging data have only been acquired at one décollement locality (Shipboard Scientific Party, 1995). These LWD data from Barbados are in an area of positive reflection polarity, and show impedance increases that reproduce the positive polarity in synthetic seismograms (Shipboard Scientific Party, 1995). The LWD results also suggest thin (0.5-1.5 m) hydrofractures within the interval of positive polarity in the décollement zone. The hydrofractures apparently are too thin to be resolved seismically. A major question is whether negative polarities elsewhere in the Barbados décollement consist of thicker zones of hydrofractures.

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