LOGGING STRATEGYDownhole logging provides continuous in situ measurements of geophysical properties such as resistivity, porosity, natural radioactivity, or sonic velocity, which can be used to assess the physical, chemical, and structural characteristics of the formation. Logging of the equatorial Pacific sites (proposed Sites EQP-1A and EQP-2A) and Peru Margin sites (proposed Sites PRU-1A, PRU-2A, PRU-3A, and PRU-4A) will help characterize current physical properties that may set important constraints on the downhole microbial community. For example, at Sites PRU-1A and PRU-2A, downhole logs will enable an assessment of the depth extent of high porosity/flow zones for the deep brine incursion. Comparison of the logs between the sites will allow some parameterization of the horizontal flow gradient. Similarly, at Sites EQP-1A and EQP-2A, logs may allow assessment of the flow regime at the base of the sediment column. In addition, logging will allow us to assess the occurrence of hydrates at Site PRU-4A (ODP Site 685), where the zone of gas hydrate stability extends down to >600 mbsf. Intervals containing hydrate are best characterized in situ by increases in resistivity and velocity (Vp and Vs) logs. These may be the only direct indicators of the presence of hydrate at Site PRU-4A. Logs might be particularly useful to draw a complete characterization of the subseafloor environment of the microbial communities at some sites visited during ODP Leg 112, where XCB core recovery was very poor (often <5%).
The logging program will consist of at least one logging run with the triple combination (triple combo) tool string (porosity, resistivity, density, natural gamma, and temperature) at all sites in the Equatorial Pacific (EQP-1A and EQP-2A) and at the sites on the Peru margin with sufficient penetration (>200 m) (PRU-1A, PRU-2A, and PRU-4A). Hole conditions and time permitting, the Formation MicroScanner (FMS)/sonic tool string might be run at any of these sites, where high resolution electrical images from the FMS could help refine our characterization of the downhole environment.
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