| Co-Chief: Casey Moore | Cruise Dates: 21 December, 1996-7 January, 1997 |
| Staff Scientist: Adam Klaus | Operations Superintendent: Scott McGrath |
Deformation and fluid flow in sedimentary sequences cause changes in physical properties. In situ measurement of physical properties evaluates processes (consolidation, cementation, dilation) operating during deformation, fluid flow, and faulting. Because seismic images are affected by changes in physical properties, their measurement allows for calibration of seismic data as a tool for remotely sensing processes of deformation and fluid flow. Logging-while-drilling (LWD) provides an industry-standard tool for in situ evaluation of physical processes, including transient borehole conditions. Leg 171A will drill a series of LWD holes to measure the physical properties of sediments through a deforming accretionary prism and across plate-boundary faults off Barbados. Extensive drilling and three-dimensional seismic surveys provide a rich framework for log interpretation, seismic calibration, and process evaluation. The results will assist with the interpretation of similar, but less active, systems in sedimentary basins elsewhere, thereby contributing to the analysis of groundwater, hydrocarbon migration, and earthquake processes.
Deformation of accretionary prisms changes the physical properties of sediments, thereby producing fluid, controlling fluid flow, altering rheologic properties, and affecting seismic arrival times and reflection characteristics. Consolidation and chemical diagenesis change the specific physical properties of porosity, density, and sonic velocity. These changes are both distributed (because of the loss of fluids in response to accumulating stresses) and localized along discrete structures (such as faults) in response to overpressuring, fluid migration, or fault collapse. Because consolidation and fluid overpressuring affect seismic arrival times and seismic reflections, seismic data provide direct clues to physical properties evolution and to physical properties changes coupled with deformation.
Physical properties evolution in sedimentary sequences cannot be comprehensively evaluated from recovered cores. Elastic rebound and microcracking of coherent sedimentary samples degrade shipboard physical properties measurements. Fault gouge and other incoherent lithologies are either not recovered or cannot be measured after recovery; therefore, transient properties (e.g., overpressuring) must be measured in situ.
Sediments in tectonically active areas undergo rapid changes in physical properties. Because of this rapid deformation and the shallow burial depth of the deformed features, accretionary prisms are exceptional, natural laboratories to study these changes that can therefore be drilled and imaged seismically. The information discerned at convergent margins about fault geology and overall sedimentary consolidation, in addition to seismic imaging of these processes, will be applicable to other, less active, sedimentary environments, and therefore will impact our understanding of hydrocarbons, groundwater, and aspects of earthquake systems. To better understand the interrelationships of deformation, fluid flow, seismic imaging, and changes in physical properties, an LWD transect of a setting dramatically influenced by pore fluids is proposed: the Barbados accretionary prism.
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 60-120 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 or ocean-bottom seismograph 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, 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. 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 are of little utility in this environment; the velocity-porosity relationship must be determined for each prism empirically, and the possibility that this relationship changes laterally within a prism must be investigated. 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. The series of LWD holes planned here, plus existing penetrations, will help constrain this problem. Excellent in situ porosities at all sites is anticipated. The velocity-porosity relationship will be constrained by wireline sonic logs at proposed Site NBR-5A, and from the previously logged Site 948.
An LWD transect across the Barbadian décollement can address the following questions: (1) do faults collapse and strain harden with displacement, (2) does active fluid flow retard this process, and (3) are collapsed faults inactive with respect to fluid flow? Structural, biostratigraphic, and seismic reflection criteria identify faults. Anomalies in pore-water geochemistry and thermal anomalies 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.
At Site 948 in the Barbados prism, high-quality density measurements demonstrated under-consolidation 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.
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. Negative polarity reflections have been interpreted as resulting from either (1) overthrusting of higher-impedance sediment over lower-impedance sediment in Costa Rica, or (2) the reduction of fault-zone impedance through dilation at Barbados. 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. 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. 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.
[ Contents of the Semiannual Report, No. 2, June-November 1996 |
| Program Updates | New Initiatives | Project Summaries | Laboratory Working Groups |
| Panel Recommendations | Appendixes |
| Semiannual Report, No. 1, December-May 1996 ]