2D characterization of near‐surface : surface‐wave dispersion inversion versus refraction tomography

Abstract

International audienceThe joint study of pressure (P-) and shear (S-) wave velocities (Vp and Vs ), as well as their ratio (Vp /Vs), has been used for many years at large scales but remains marginal in near-surface applications. For these applications, and are generally retrieved with seismic refraction tomography combining P and SH (shear-horizontal) waves, thus requiring two separate acquisitions. Surface-wave prospecting methods are proposed here as an alternative to SH-wave tomography in order to retrieve pseudo-2D Vs sections from typical P-wave shot gathers and assess the applicability of combined P-wave refraction tomography and surface-wave dispersion analysis to estimate Vp/Vs ratio. We carried out a simultaneous P- and surface-wave survey on a well-characterized granite-micaschists contact at Ploemeur hydrological observatory (France), supplemented with an SH-wave acquisition along the same line in order to compare Vs results obtained from SH-wave refraction tomography and surface-wave profiling. Travel-time tomography was performed with P- and SH- wave first arrivals observed along the line to retrieve Vtomo p and Vtomo s models. Windowing and stacking techniques were then used to extract evenly spaced dispersion data from P-wave shot gathers along the line. Successive 1D Monte Carlo inversions of these dispersion data were performed using fixed Vp values extracted from Vtomo p the model and no lateral constraints between two adjacent 1D inversions. The resulting 1D Vsw s models were then assembled to create a pseudo-2D Vsw s section, which appears to be correctly matching the general features observed on the section. If the pseudo-section is characterized by strong velocity incertainties in the deepest layers, it provides a more detailed description of the lateral variations in the shallow layers. Theoretical dispersion curves were also computed along the line with both and models. While the dispersion curves computed from models provide results consistent with the coherent maxima observed on dispersion images, dispersion curves computed from models are generally not fitting the observed propagation modes at low frequency. Surface-wave analysis could therefore improve models both in terms of reliability and ability to describe lateral variations. Finally, we were able to compute / sections from both and models. The two sections present similar features, but the section obtained from shows a higher lateral resolution and is consistent with the features observed on electrical resistivity tomography, thus validating our approach for retrieving Vp/Vs ratio from combined P-wave tomography and surface-wave profiling

Estimating picking errors in near‐surface seismic data to enable their time‐lapse interpretation of hydrosystems

“…; Pasquet et al . ). Such an observatory and associated hydrogeological heterogeneities offers a suitable context for the methodological design and implementation of time‐lapse seismic acquisitions.…”

“…This lithological heterogeneity is clearly illustrated by Pasquet et al . (2015a) in electromagnetic induction (Fig. a) and electrical resistivity tomography (ERT) (Fig.…”

“…Pasquet et al . (2015a) combine the use of P‐wave velocity ( V P ) and shear (S) wave velocity ( V S ) which behave differently in the presence of water (Biot ,b) to estimate the depth of the saturated zone. On a methodological point of view, V P is retrieved by a tomographic inversion of first arrival times (Schuster and Quintus‐Bosz ; Bauer et al .…”

Time-lapse monitoring of fluid-induced geophysical property variations within an unstable earthwork using P-wave refraction

“…The embankment section covered by surfacewave data (for which V S models are available) is wider than the area surveyed with P-wave refraction (Figure 1). Significant discrepancies between the temporal behavior of V P and V S are present, and they can be ascribed to the different spatial resolution and sensitivity of the surface-wave and P-wave refraction methods (e.g., Pasquet et al, 2014Pasquet et al, , 2015b or to inversion artifacts. Another possible explanation is the occurrence of conditions of local soil saturation, which, at the investigation scale of P-wave refraction surveys, would result in increasing P-wave velocities that are not quite as high as those which would normally be expected for fully saturated media .…”

“…In near-surface geophysics, the potential use of seismic techniques for tracking time-variant processes has been assessed (e.g., Jefferson et al, 1998;West and Menke, 2000), although at reduced spatial scales and spanning shorter time intervals (from hours to some days), when compared with those involved in this paper. Recently, the use of seismic techniques has been proposed for monitoring water table levels in shallow aquifers (Pasquet, 2014;Pasquet et al, 2015aPasquet et al, , 2015b.…”