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doi:10.2204/iodp.sp.301.2004
The main objectives of the downhole measurements program are to document crustal physical and hydrogeologic properties and to evaluate the nature of hydrothermal alteration. In addition to defining structural and lithologic boundaries as a function of depth, wireline logging data can be compared to results of laboratory analyses of discrete samples and should help delineate fracture densities and orientations. These measurements will complement core measurements by determining the thickness of lithologic units in intervals where core recovery is poor. Downhole measurements will also help determine the thermal state of sediments and upper basement rocks.
A series of four tool string deployments are planned for the basement section of Hole SR-1A. These tool strings include the triple combo, FMS/DSI, UBI, and WST-3. The time estimate for all four deployments is 39.4 h. Detailed descriptions of all wireline tools and applications are provided at iodp.ldeo.columbia.edu/TOOLS_LABS/index.html.
The triple combo, with caliper measurements and cable head temperature sensors, will be used to assess initial postdrilling borehole conditions such as hole size and fluid temperatures. This tool string will measure K, U, and Th concentrations; formation density; photoelectric effect; electrical resistivity; and porosity as a function of depth. These measurements will be used for characterization of stratigraphic sequences and to assess variations in alteration. Mapping the potassium distribution could help delineate acid-sulfate (K depletion) and higher-temperature phyllic (K addition) styles of alteration, particularly if core recovery is poor. The FMS will provide high-resolution borehole images of lithostratigraphic sequences and boundaries, oriented fracture patterns, and information regarding hole stability. The DSI will produce a full set of compressional and shear waveforms, cross-dipole shear wave velocities and amplitudes measured at different azimuths, and Stoneley waveforms. These types of measurements can be used to determine preferred mineral and/or fracture orientations and fracture densities and paleostress directions. The UBI will provide full coverage of the borehole walls while depicting structural and lithostratigraphic features related to paleo- and current stress environments. A VSP will be obtained using the WST-3. This tool will provide the shallow basement velocity gradient information that will be helpful for core-log-seismic correlations and for assessment of crustal layering.
The Ultra High Temperature MultiSensor Memory (UHT-MSM) tool could be used for obtaining a downhole temperature profile several days postdrilling and prior to the Cork-II installation in the SR-1A deep hole. The UHT-MSM is deployed on the coring line, rather than the logging line, and was successfully deployed during ODP Legs 169 and 193. The estimated time required for the deployment of this tool is 7–8 h.
The DVTP and the APCT tool will be used to determine sediment temperatures and heat flow at one or more locations. This work is most likely to occur where sediment cores are collected. One or both tools could also be deployed in open hole to determine whether the formation is producing fluid. Similarly, the Water Sampling Temperature Probe (WSTP) may be deployed in an open hole to collect fluid samples if the formation produces crustal fluids.
The drill string packer is used to isolate part of an open or cased hole so that hydrogeologic properties near the borehole can be assessed (Becker, 1986). Packer experiments are planned for Holes SR-1A and SR-1B. In Hole SR-1A the packer may be run in straddle mode, with inflatable elements set at the top and bottom of the interval of interest, but it will be used to isolate the entire open borehole interval in Hole SR-1B.