Logging/Downhole measurements strategy

Logging will play an important role in achieving the scientific objectives of Expedition 351. Previous drilling in similar environments, particularly during ODP Legs 125 and 126, has achieved only partial recovery in some intervals. Downhole data will complement core measurements by determining the characteristics of lithologic units in any intervals where core recovery is poor. In addition, wireline logging data can be compared to analyses of discrete core samples. The gamma ray tool should clearly distinguish lithologic units, including those comprising pyroclastic debris, and aid with the reconstruction of paleo-oceanographic conditions in the late Mesozoic, including possible ocean anoxic events. The magnetic susceptibility sonde (MSS) will further help identify and delineate volcaniclastic units. The Formation MicroScanner (FMS) electrical images will help to characterize any structural deformation as well as the deposition sequences, particularly to delineate sedimentary packages, their thickness and direction, and provide evidence of how the upper plate responded to subduction initiation. Check shots with the Versatile Seismic Imager (VSI) will be acquired, and synthetic seismograms can be generated using density and velocity logs. The results from which will allow the drilled boreholes to be tied into the site survey data.

Three primary logging strings will be deployed (see for more details regarding logging tools). First, the traditional triple combination (“triple combo”) tool string (gamma ray, porosity, density, and resistivity) will be run with the MSS to provide a full characterization of the formation and of hole conditions. The FMS-sonic tool string will record compressional and shear velocity and capture high-resolution electrical images of the borehole. Finally, the VSI tool will be used to acquire the check shot survey. It will be anchored to the side of the borehole at fixed intervals (20–100 m) to record the waveforms generated by a seismic source (a Sercel G-gun 250 inch × 250 inch × 250 inch parallel cluster) held ~7–11 m below the sea surface. The order of deployment of the last two strings will depend on the requirement to limit operation of the seismic source to daylight in order to be able to monitor the presence of protected species and interrupt operations if necessary. There is a possibility that the Ultrasonic Borehole Imager (UBI) will be run where borehole conditions allow (i.e., the hole is “in gauge”; ≤11 inch diameter). The UBI provides high-resolution acoustic amplitude images with 100% borehole wall coverage. The high-resolution FMS and UBI images will allow detection of small-scale fractures and lithologic variations, enable the tilt of units to be evaluated, and highlight borehole breakouts for regional stress analysis. The General Purpose Inclinometry Tool will be deployed with both image tools to collect accelerometer and magnetometer data, which will allow orientation of the images and provide information about borehole geometry. Additionally, pending funds, tool availability, and available operations time, a borehole magnetometer (either the third-party Göttingen Borehole Magnetometer [GBM] or Lamont Multisensor Magnetometer Module [MMM]) may be deployed as a separate run to obtain three-component magnetic field measurements in order to investigate rotational history.

The main logging operations are currently planned to take place after full completion of coring in the RCB pilot hole (Hole C) and in the reentry hole (Hole D following coring and casing operations). However, depending on the drilling scenario (see “Drilling and coring strategy” and “Risks and contingency”) and based on the progress of coring and on drilling conditions, it may be necessary to adapt this plan.