Characterization of a fractured granitic pluton: P- and S-waves' seismic tomography and uncertainty analysis

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A detailed characterization of the physical properties of a granitic pluton, including a three dimensional reconstruction of the fractures network and its surrounding are obtained by high resolution seismic tomography of P and S-waves. The input P- and S-wave travel times were picked from three-component offset and azimuth variable seismic profile (OVSP) acquired in a 500 m deep vertical borehole. The travel time inversion resulted in two three-dimensional P- and S-wave velocity models, and the uncertainty, robustness and sensitivity are estimated by performing a Monte Carlo-type analysis and checkerboard test. This study provides an indicator of the uniqueness of the determined model. The 500 m deep borehole was continuously cored, and extensively surveyed by geophysical techniques. Thus, the detailed surface geological mapping, fracture index logs, borehole televiewer images and P- and S-wave velocities derived from sonic logs, provide solid constraints for the interpretation of the tomographic images. The three-dimensional tomographic images reveal fracture zones mapped at surface that intersect the borehole at different depths and they can be identified in the core descriptions and in the borehole televiewer images. The fractures identified on the cores feature relatively low values for the sonic P- and S-wave velocities, and high values for Poisson's ratio ( σ). For single type lithologies, the distribution of Poisson's ratio differentiates fractured and altered granite from fresh unfractured domains within the rock. Poisson's ratios close to 0.3 correlate with fractured volumes consistent with fracture index and sonic logs, otherwise Poisson's ratio values close to 0.25 are related to unaltered and unfractured rock.