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. Issue 6 (1st January 2014)

• Record Type: Journal Article
• Authors: Maufroy, Emeline; Gaffet, Stéphane; Operto, Stéphane; Guglielmi, Yves; Boyer, Daniel
• Is Part Of: Near surface geophysics. Volume 12:Issue 6(2014)
• Page Start: 697
• Page End: 708
• Publication Date: 2014-01-01
• Subjects: Earth (Planet) -- Surface -- Periodicals; Geophysics -- Technique -- Periodicals; Engineering geology -- Periodicals; Geophysics -- Periodicals; Planets -- Surfaces; Engineering geology; Geophysics -- Technique; Geophysics; Earth (Planet); Periodicals; 550
• DOI: 10.3997/1873-0604.2014025
• Languages: English
• ISSNs: 1569-4445

Abstract

Abstract : 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 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. 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 V p 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.