Scientific objectives

Two observations made during Expedition 340T inform young ocean lithosphere studies, in general. Each required minimum possible disturbance approaching and reentering the hole:

  1. Visual observation of whether the well was “producing” (flow out of the hole) or not: addresses fluid flow within the crust and chemical exchange with seawater in maturing lithosphere.

  2. Measurement of borehole fluid temperature: assesses conditions that may be encountered by future ultradeep drilling/logging of an intrusive oceanic section; also tests for possible fluid flow (temperature deviations) within fault zones of Atlantis Massif’s footwall.

Focusing on our main objectives, obtaining new caliper measurements throughout the hole documents hole condition and guided selection of VSP station depths that might be optimum for instrument coupling. The zero-offset VSP data provide information on velocities in the vicinity of the hole at intermediate length scales and could increase our knowledge of local reflectivity for near-vertical waves, thereby improving core-log integration with surface seismic data. Information on the condition of the borehole is crucial for determining whether a future single-ship wireline reentry experiment is viable or whether the drillship and a second vessel would be needed to complete the VSP experiment by conducting a walk-away component. Ultimately, this full data set would enable core-log-survey integration at as high a level as possible with current geophysical data.

Sonic logs obtained during Expedition 340T will allow analysis of the relationship between lithology and velocity in the section deeper than 800 mbsf, where the least altered rock was recovered from Hole U1309D. In addition to compressional velocity (VP) and shear velocity (VS) correlations with either primary or alteration lithologies, complete hole Stoneley wave data will enable assessment of permeability/fracturing and any contrast thereof between documented fault zones and surrounding rock.

Magnetic susceptibility logs target downhole variations in magnetite that are a product of serpentinization, potentially providing constraints on extent/style of alteration that may have been missed with the finite (although very good) core recovery (nonwhite portion of Fig. F3A).