Travel time inversion from ground level to gallery: protocol for the characterization of P-wave seismic signature in a fractured-porous Urgonian platform at hectometric scale

摘要

A tomographic P-wave velocity model is inferred from a ground level-to-gallery vertical 500 m x 800 m seismic experiment conducted at the inter-Disciplinary Underground Science and Technology Laboratory (LSBB, France). No initial knowledge of the velocity structure of the surrounding fractured-porous carbonates was previously available. Ninety-four shots at the surface were recorded by a line of 189 seismometers on the steep slope of the topographic surface and by a line of 150 geophones in an 800 m-long, 250-500 m-depth gallery. The P-wave velocities inferred from first-arrival travel time inversion display a relatively large set of values ranging from 4000 to 6000 m/s. Such V-P variations correlate well with the 5 to 20% porosity variations between the main geological units that consist of two sedimentary facies affected by a complex cemented fault zone.