Our aim was to qualitatively infer fracture density of oceanic crystalline basement from full waveform acoustic logging data from Site 1253, Leg 205, and from that to validate the usefulness of DSI logs for assessing hydrological properties of oceanic basement rocks.
For this purpose, we processed the Stoneley wave for velocity and energy content by deploying a straightforward semblance algorithm. We observed a correlation between borehole geometry and Stoneley wave energy and velocity, as expected. Our further interpretations therefore concentrated on five smooth borehole sections in order to minimize the geometry effect. Combination with fracture observations performed downhole and on recovered core material linked the local low-energy locations to local maxima in fracture abundance. We identified five zones at depths of 468, 492, 500, 508, and 518 mbsf as probably being of higher permeability.
A quantitative permeability estimate is not possible with our processing, and the qualitative investigation reported here also requires, at the current state, comparison with other fracture observations such as core or FMS measurements.
An improvement of our algorithm would be possible by comparing it with Stoneley wave energy distribution calculated by the Geoframe software. We also suggest modeling the influence of borehole geometry on Stoneley velocity and energy and implementing such a model in Matlab. More precise diameter information would be helpful, as well as an extended and undisturbed borehole section for measurements and comparison.
Nevertheless, we have shown that the Stoneley wave energy distribution delivers additional information to the standard data curation process for crystalline rocks. This is especially interesting in cases where no cores could be recovered. The possibility of calculating Stoneley wave energy distribution is included in the Geoframe software, and it would be worth, for future research, to make use of this possibility, especially because recording Stoneley waves is already a standard and does not require additional time.