Downhole measurements strategy

Wireline logging

Downhole logs will be acquired at the Expedition 344 sites to measure in situ physical properties, estimate compaction, and evaluate permeable horizons. Electrical resistivity and ultrasound images referred to magnetic north will be useful to determine fracture orientations and stress directions and to orient core samples. Downhole logging will also provide key data to correlate depth in the hole with traveltime in seismic sections. These logging measurements include check shot data and formation density and velocity measurements that allow for constructing synthetic seismograms.

Three wireline tool strings are planned for the Expedition 344 sites (Fig. F11). The first tool string to be deployed in each hole is the triple combination (triple combo)-Ultrasonic Borehole Imager (UBI), which measures hole diameter, natural gamma ray, bulk density, electrical resistivity, and ultrasonic images of the borehole. The images will be acquired by the UBI, a tool that measures a detailed borehole radius and reflection coefficient image with a rotating ultrasonic beam. We added the UBI to the standard triple combo tool string to attempt measuring borehole breakouts, which indicate the orientation of the in situ principal horizontal stresses. Success in imaging breakouts will depend on favorable conditions (borehole diameter ~12 inches or less; well-preserved breakouts after coring), and even covering a partial depth interval will provide valuable scientific information.

The second Formation MicroScanner (FMS)-sonic tool string will consist of a FMS resistivity imaging tool, a Dipole Sonic Imager (DSI) that measures P- and S-wave velocities, and a natural gamma ray sensor. The third tool string will be deployed in a check shot experiment where a Versatile Seismic Imager (VSI) records the arrival of acoustic pulses generated by air guns fired from the JOIDES Resolution. The FMS-sonic and VSI tool strings will be run depending on ship heave and borehole conditions. Detailed descriptions of wireline tools and operational constraints are at​TOOLS_LABS/​index.html.

Wireline logging is expected to be challenging at the Expedition 344 sites because deep holes in a convergent margin environment are likely to be unstable after coring. To maximize the chances of obtaining useful logging data, the operations plan (Table T1) calls for two stages in logging at two sites. Sites CRIS-9A and CRIS-13B will be cored in two holes: Hole A to ~500 mbsf (cored by APC/XCB) and Hole B to total depth (cored by RCB). The triple combo-UBI and FMS-sonic tool strings will be deployed first to log Hole A and then the deeper interval in Hole B. It will be important to set the drill pipe in Hole B as deep as possible during logging, ideally just above the depth of the shallower Hole A. The drill pipe allows the logging strings to pass through borehole obstructions that may make it impossible to reach the deep interval to be logged. The final drill pipe depth will be determined based on hole conditions and after consultation between the IODP Operations Superintendent, the drilling subcontractor, and the Logging Staff Scientist. The operations plan also includes a check shot survey with the VSI tool string in the deepest holes drilled at Sites CRIS-9A, U1380, and CRIS-13B.

Temperature and core orientation measurements

The scientific objective of the temperature measurement plan is to provide data to reconstruct the thermal gradient at each site. This information will be a key input to estimate heat flow, infer fluid flow, model pore water geochemistry, and constrain the sediment diagenesis history. We plan on deploying the advanced piston coring temperature tool (APCT-3) in the interval where cores will be taken by APC and the SET or sediment temperature pressure (SET-P) tool further downhole where sediments are more consolidated. Temperature measurements will be carried out every 40–50 m down to the maximum depth where the SET tool can penetrate sediment. This maximum depth depends on the formation, and we estimate it to be ~500 mbsf. We also plan to deploy the SET-P tool to measure in situ formation pressure. Taking these measurements will require a stable hole, because the probe needs to be seated in the formation for ~10 min (45 min for pressure) with no drill string rotation and no drilling fluid circulation. APC cores will be oriented with the FlexIt tool for paleomagnetic studies.