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doi:10.2204/iodp.sp.350.2013

Downhole measurements strategy

Logging will play an important role in achieving the scientific objectives of Expedition 350. Previous drilling in similar environments, in particular during ODP Legs 125 and 126, has achieved only partial core recovery in some intervals, and logs will allow us to fill gaps in recovery. The gamma ray tool should clearly distinguish sediments derived from the proximal volcanic front (low K, U, Th, and Th/U) from the rear arc (opposite). The magnetic susceptibility sonde (MSS) will help identify and delineate volcaniclastic units. The Formation MicroScanner (FMS) electrical images will help characterize any structural deformation as well as the deposition sequences, in particular to delineate turbidites and their thickness and direction. A vertical seismic profile (VSP) will be acquired. Combined with synthetic seismograms generated from the density and velocity logs, the VSP will tie our results to the site survey data and calibrate the existing seismic stratigraphy models.

Three logging strings will be deployed. First, the traditional triple combination (triple combo) tool string (gamma ray, porosity, density, and resistivity) will be run with the MSS to provide full characterization of the formation and hole conditions. The FMS-sonic tool string will record compressional and shear velocity and capture high-resolution electrical images of the borehole. Finally, the Versatile Seismic Imager (VSI) will used to acquire the VSP. The VSI will be anchored to the side of the borehole at fixed intervals (20–50 m) to record the waveforms generated by a seismic source (a Sercel G-gun 250 in³ parallel cluster) held ~7–11 m below sea surface. The order of deployment of the last two strings will depend on the requirement to limit the operation of the seismic source to daylight, in order to be able to monitor the presence of protected species and interrupt operations if necessary.

Main logging operations will take place after full completion of coring. Depending on drilling scenario (see “Drilling and coring strategy”) and based on the progress of coring and on the drilling conditions, it may be necessary to log the shallower units at an earlier stage of drilling, prior to deploying casing, or using a dedicated logging hole.

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