171A Scientific Prospectus


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; Bray and Karig, 1985; Bangs et al., 1990) and localized along discrete structures (such as faults) in response to overpressuring, fluid migration, or fault collapse (Shipley et al., 1994; Tobin et al., 1994). 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 (Fisher, Zwart, et al., 1996).

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, we propose a logging-while-drilling (LWD) transect of a setting dramatically influenced by pore fluids: the Barbados accretionary prism.

Logging-while-drilling is the most effective tool for measurement of physical properties in poorly consolidated sediments. LWD acquires data from sensors integrated into the drill string immediately above the drill bit, and records data minutes after cutting the hole when it most closely approximates in situ conditions. It is an "off the shelf" industry technology already used by the Ocean Drilling Program (ODP) during Leg 156.

This technology provides high-quality logging information in environments where standard wireline systems previously acquired either no data or poor-quality data because of the typically difficult hole conditions. Specifically, LWD provides excellent-quality results in the shallowest sediment sections and in holes with marginally stable conditions that preclude wireline log runs. Wireline tools are more sophisticated than LWD tools, and, in principle, should yield more accurate measurement of physical properties. However, the difficult hole conditions encountered by drilling, especially at active margins, destroy the inherent advantage of wireline tools. The LWD tools to be used during Leg 171A provide neutron porosity, resistivity, density, and gamma-ray data, but not sonic velocity data. If time permits, sonic log velocity data will be obtained by focused wireline measurements.

The absence or failure of wireline logging operations in convergent margins means that numerous, previously drilled, Deep Sea Drilling Project (DSDP) and ODP holes provide scientifically exciting locales for LWD. Barbados is especially attractive for focused LWD investigation because:

Drilling at Barbados has occurred with high-quality structural, pore-water chemistry, heat flow, and shipboard physical properties studies (DSDP Leg 78A, ODP Legs 110 and 156). Such information provides independent determinations of locations of faults, of fluid flow activity, and of correlative physical properties such as grain density. The scientific results from this information provide a rich framework for log interpretation.

Previous studies of Barbados show that physical properties are dramatically influenced by fluids. We anticipate observation of significant fluid-related effects from physical properties in the LWD logs.

The décollement zone occurs at easily drillable depths at Barbados, and many previously drilled holes penetrate the décollement there. In contrast, thick turbidite-dominated sequences at many other convergent margins include unstable sand layers that hinder drilling and logging operations.

Barbados is one of only two convergent margins with a state-of-the-art, three-dimensional seismic reflection survey. This extraordinary data set vastly expands the opportunity for core-log-seismic integration and three-dimensional extrapolation to problems of deformation and fluid flow in accretionary prisms (Figs. 1and 2).

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