LOGGING/DOWNHOLE MEASUREMENTS PLAN
The current operations plan for Expedition 306 includes downhole logging at the first proposed site, currently IRD-3A (alternatively, if a different site is drilled first then that site will be logged in place of IRD-3A), and at one of the Eirik Drift (LAB) sites. Logging operations at additional sites will take place if time permits.
The two standard IODP tool string configurations will be deployed. The triple combination (triple combo) tool string logs formation resistivity, density, porosity, natural gamma radiation, and borehole diameter and will be run first, followed by the Formation MicroScanner (FMS)-sonic tool string, which provides an oriented 360° resistivity image of the borehole wall, logs of formation acoustic velocity, natural gamma radiation, and borehole diameter. The Lamont-Doherty Earth Observatory (LDEO) high-resolution Multisensor Gamma Tool (MGT) will be deployed on the top of the triple combo tool string and run on a separate pass.
In addition to the standard logging tools, the Inline Checkshot Tool (QSST) or the Well Seismic Tool (WST) might be deployed (subject to clearance and time availability) at one of the Eirik Drift sites to facilitate core-seismic-log integration. The Schlumberger QSST is a single-axis seismic checkshot tool that runs in-line with the triple combo tool string. The QSST consists of a single hydrophone and does not utilize a clamping arm. Seismic coupling is achieved by setting the tool on the hole bottom and allowing the tool to lean against the borehole wall, coupling passively to the formation. The QSST then records the vertically incident signals at the bottom of the hole that are generated by a seismic source on the JOIDES Resolution (e.g., generator injector [GI] guns) positioned just below the sea surface. The recorded signals enable a one-way traveltime to be determined from the surface to total depth, providing a check of sonic velocity versus depth for calibration of seismic profiles and correction of sonic logs. The rig time required to run this single-point checkshot survey is negligible, as the QSST may be included during the triple combo logging run.
The WST is used to produce a zero-offset vertical seismic profile and/or checkshots in the borehole. The WST consists of a single geophone that records the full waveform of acoustic waves generated by a seismic source positioned just below the sea surface. The WST is clamped against the borehole wall at regular predetermined intervals (usually 550 m) with a hydraulic arm, and the air gun is typically fired between 5 and 15 times at each station. The recorded waveforms are stacked, and a one-way traveltime is determined for each station, thus providing calibration of the time/depth relationship in the seismic survey. The WST requires a separate logging run, generally after completion of the triple combo and FMS-sonic runs. The WST would provide more detailed data than the QSST.
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