Physical properties of 110 MA oceanic crust at Site OSN-1: Implications for emplacement of a borehole seismometer

An extensive suite of downhole geophysical logs was acquired through a 70 m thick section of basalt drilled by the Ocean Drilling Program (ODP) into 110 Ma oceanic crust 225 km SW of Hawaii. These data include standard sonic, electrical, and nuclear logs, as well as electrical and acoustic images of the wellbore, and enable a vertically continuous characterization of the physical properties of shallow, old oceanic crust. Fracture porosity and alteration increase with depth in this hole, and consid- ered together with results obtained elsewhere in both young and old (5 to 110 Ma) crust, suggest that physical proper- ties gradients in the uppermost few hundred meters of oceanic basement are controlled by crustal morphology and alteration and are ubiquitously variable. As this hole was drilled primarily for pilot experiments at the first site of the Ocean Seismic Network, evaluation of the log data demon- strates the importance of careful in-situ data collection in order to emplace a broadband monitoring device below the seafloor for long-term seismic measurements.