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
Résumé
A tomographic P-wave velocity model is inferred from a ground level-to-gallery vertical 500 m × 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 VP 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. Taking advantage of the known geology of the site, this study explores the influence of the acquisition geometry impacted by the topography and of the near-surface weathered zone onto the shallow VP tomography resolution ability. Considering the mesoscopic scale of the targeted medium, reliable imaging of hectometric geological bodies with 10% contrasts in porosities can be achieved only with the simultaneous association of (i) a high density of sources and receivers in the monitoring array geometry, and (ii) the equal consideration of surface-to-gallery and surface-to-surface first-arrival travel times, as an essential constraint to correctly image the underlying structures.