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doi:10.2204/iodp.sp.340T.2011 Logging/Downhole measurements strategyDownhole logging will be the primary operation to achieve the scientific objectives of Expedition 340T by providing continuous, in situ geophysical measurements of the drilled basement at Site U1309. The logging program will establish the current temperature profile in the borehole fluid, which will provide information on active fluid flow in previously observed faulted intervals and temperature gradient changes that may be associated with lithologic variations. In addition, sonic logging and a zero-offset VSP will extend into the deeper interval (>800 mbsf) of the borehole, where no such data were previously recorded because of weather and instrument problems encountered during Expedition 305. These new velocity data will allow for direct, high-resolution correlation of wireline measurements with core measurements made during Expeditions 304 and 305. The operations schedule for Expedition 340T is limited to 3 days on site (Table T1), based on ancillary project letter (APL) guidelines. As a result of the time limitation, the logging program is designed to be flexible and to include as many as four different tool strings:
Following two phases of drilling during Expeditions 304 and 305, Hole U1309D penetrated to a total depth of 1415.5 mbsf, with 13⅜ inch casing to 20.5 mbsf to provide stable reentry. The hole was left in good condition at the end of Expedition 305, so we anticipate reasonably good hole conditions for logging operations. We will reenter the hole with a logging bit and maintain the pipe within the casing, if possible. The first logging run will be the triple combo tool string with the MTT in order to recover an equilibrium temperature profile of the hole, which has been undisturbed since Expedition 305 logging operations concluded on 26 February 2005. The caliper on the triple combo will be used to assess hole size and condition and to identify favorable intervals for anchoring the VSI during the VSP. The logging bit will allow for rotation, if a need for minor clean-up of the hole is indicated by the caliper log. The order of the other tool strings will be adjusted to (1) ensure that the VSP is acquired during daylight operations to accommodate marine mammal observations while using the seismic source and (2) optimize time on site to collect the full suite of logging data within the 3 days available for operations. Characteristics of the tools are described briefly below. For more information on specific logging tools, please refer to iodp.ldeo.columbia.edu/TOOLS_LABS/. Triple combo tool stringThe triple combo consists of five main tools:
Sonic tool stringThe main component of the sonic tool string is the Dipole Shear Sonic Imager (DSI), which records a full set of acoustic waveforms to measure the compressional (VP) and shear velocity (VS) of the formation. In addition to VP and VS, the DSI will be run with different modes to allow the estimation of properties such as anisotropy and permeability. VP can also be combined with the density log from the triple combo to generate synthetic seismograms and provide high-resolution seismic/well integration. A gamma ray tool will be run in the sonic tool string in order to depth match the different logging runs, as well as potentially the MTT, to record temperature rebound in the borehole. Versatile Seismic Imager tool stringThe VSI will be used to acquire a zero-offset vertical seismic profile (VSP) for high-resolution integration of borehole and seismic data. The tool string will be lowered into the hole and anchored at specified intervals against the borehole wall to record the waves emitted by the seismic source. The survey is planned with 25 m spacing of stations over the entire open hole interval (~20 mbsf to total depth of ~1415 mbsf). Spacing may be adjusted based on hole condition/size determined from the caliper log. If a full period of daylight is available, under optimal conditions the entire open hole VSP may be acquired during a single run. However, given the uncertainty of arrival time on site, VSP operations may require two daylight phases of logging. Phase 1 would capture the deeper interval (~800–1415 mbsf) where no VSP has been acquired, and Phase 2 would investigate the shallow interval (~20–800 mbsf) with closer station spacing than obtained during the Expedition 305 VSP. If time is limited, the deeper interval below ~800 mbsf is the higher priority. The seismic source for the VSP will be a parallel cluster of two 250 in3 Sercel G guns (Table T2), positioned 2–7 m below sea level and offset by ~50 m from the side of the ship. VSP operations are subject to the IODP marine mammal policy and may be postponed or cancelled if policy conditions are not met. Magnetic Susceptibility Sonde tool stringThe MSS measures magnetic susceptibility, which would provide an assessment of formation alteration (serpentinization) and could be correlated with magnetic susceptibility measurements from Expedition 304/305 cores. LDEO is currently building a replacement for the MSS that was deployed and lost during IODP Expedition 320. Current production and testing timelines indicate that the MSS will be available for Expedition 340T. The MSS will be run in a modified triple combo (MSS replaces HRLA) or in a condensed tool string (MSS with HNGS and/or HLDS). Temperature considerations may prevent or limit the deployment of the MSS (rated up to 80°C); measurements at the end of Expedition 305 indicate a temperature of at least 119°C at ~1400 mbsf. Magnetic susceptibility data are lower priority than temperature and velocity data from the other tool strings; therefore, temperature and time constraints will be used to determine whether the MSS tool string is run. Even a partial run of the MSS tool string in the shallower, presumably cooler interval (e.g., ~20–800 mbsf) would likely provide scientifically valuable data. |
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