LOGGING PLANLogging and downhole measurements will be critically important to the scientific objectives of Leg 193, particularly because previous coring experiences in Middle Valley and TAG have been characterized by poor core recovery. The main objectives of the logging program will be to assess the changes in physical properties resulting from hydrothermal alteration and to determine how these variations relate to existing hydrological models. In addition to defining structural and lithologic boundaries as a function of depth, the downhole program will also attempt to establish hole-to-hole correlations to determine lateral stratigraphic variations in active hydrothermal systems and produce direct correlations with discrete laboratory data. Altogether, downhole measurements will be used to assess compositional variations throughout massive sulfide deposits and the underlying altered volcanic flows and to determine fracture densities that may serve as conduits for vigorous focused fluid flow. Finally, downhole measurements will complement core measurements by filling gaps in downhole stratigraphy and determining the thickness of lithological units in intervals where poor core recovery is prevalent.
Overall, if temperature and borehole conditions are favorable (T < 175°C), wireline logging operations will consist of two to three tool strings plus a fluid sampling probe. The strings will consist of the triple combo with the HNGS, the accelerator porosity sonde (APS), the HLDS, the dual induction tool (DIT), a caliper tool, and cable head temperature measurements. If electrical resistivities in the volcanic section exceed the upper limit (~200 ohm-m) of the DIT, the use of a dual laterolog (DLL) may be necessary, pending final approval from the Lamont-Doherty Earth Observatory Borehole Research Group (LDEO/BRG). Following the deployment of the triple combo, the Formation MicroScanner (FMS)/dipole sonic imager (DSI) combination will be lowered into the borehole. Temperature probes will be used to determine the presence of active fluid-flow conduits that will be potential targets for subsequent deployment of a fluid sampling tool. The temperature probes that will be available are the LDEO/BRG wireline Hi-T probe and the University of Miami GRC Ultra Hi-T Memory Tool. The LDEO/BRG Hi-T tool will deployed in cases where temperatures do not exceed an upper limit of 235°C, whereas the GRC tool will be used in cases where the temperatures exceed the upper limit of the wireline capabilities.
The triple combo with caliper measurements and cable head temperature sensors will be used to determine concentrations of K, U, and Th, obtain formation density, electrical resistivity and porosity values, and assess borehole conditions. These measurements will be utilized for characterization of stratigraphic sequences and determination of possible variations in alteration. Mapping the potassium distribution will help to 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 stratigraphic 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 may be used to determine preferred mineral and/or fracture orientations and densities, paleostress directions, and permeability estimates, all required to accurately model the hydrological characteristics of the hydrothermal system.
Projected Wireline Logging Plan
The breakdown of wireline logging operations for each hole are as follows:
|Time* with no SES
|Hole PCM-1A||Triple Combo, FMS/DSI
Temperature**, Fluid Sampler
|Hole PCM-2A||Triple Combo, FMS/DSI,
Temperature, Fluid Sampler
|Hole PCM-3A||Triple Combo, FMS/DSI,
Temperature, Fluid Sampler
|Hole PCM-4A||Triple Combo, FMS/DSI,
Temperature, Fluid Sampler
Logging While Drilling (LWD)
There are two potential plans for LWD operations during Leg 193. At the present time, the Compensated Dual Resistivity (CDR) tool is scheduled to be on board for the duration of the cruise for augmenting cruise results, especially if unstable hole conditions and poor core recovery restrict the scientific results of the leg. Our intent will be to drill three holes throughout the leg to an approximate depth of 100 mbsf. These holes will be drilled to characterize the upper intervals that are commonly not recovered and not logged with conventional wireline tools. The CDR will provide gamma-ray and borehole compensated deep and shallow resistivity measurements that will allow direct correlation with core and wireline results in nearby holes and will permit bed boundary definition.
If logistics can be arranged, a resistivity-at-the-bit (RAB) tool will be used in lieu of the CDR. The RAB tool will be brought on board at the end of the cruise and three 100-m holes will be drilled near existent conventional holes during a 6-day period. There are several scientific advantages to replacing the CDR with the RAB. The RAB is a laterolog tool that has a larger range of resistivity measurements (0.2-2000 ohm-m) than the CDR (0.2-200 ohm-m). This capability could become crucial in identifying volcanic flows that may have resistivity values much greater than 200 ohm m. The RAB also provides complete azimuthal coverage of the borehole, providing high-resistivity images comparable to those obtained with the FMS. These data will provide visual identification of massive sulfide and volcanic layers as well as identification of fracture patterns, structural orientations, and formation thickness. The availability of resistivity images will also allow better characterization of shallow deposits that are usually not accounted for because of drilling operations and lack of wireline logs. Finally, performing the RAB measurements at the end of the leg will provide the flexibility to plan hole locations in places where stable hole conditions, favorable temperatures, and penetration depths were established by previous conventional drilling.
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