Seismic study of the low-permeability volume in southern France karst systems

Abstract

Locating groundwater in deep-seated karst aquifers is inherently difficult. With seismic methods, we studied the upper epikarst and the underneath low-permeability volume (LPV) of several karst systems located in the southern Quercy and Larzac regions of France and found that refraction tomography was effective only in the epikarst and not in the LPV. We evaluated a 3D case study using a combination of surface records and downhole receivers to overcome this limitation. This 3D approach unveiled a set of elongated furrows at the base of the epikarst and identified heterogeneities deep inside the LPV that may represent high-permeability preferred pathways for water inside the karst. To achieve the same result when no borehole was available, we studied seismic amplitudes of the wavefield, recognizing that wave-induced fluid flow in low-permeability carbonates is a driving mechanism of seismic attenuation. We developed a workflow describing the heterogeneity of the LPV with spectral attributes derived from surface-consistent decomposition principles, and we validated its effectiveness at benchmark locations. We applied this workflow to the 3D study and found a low-amplitude signal area at depth; we interpreted this anomaly as a water-saturated body perched above the aquifer.

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

“…The use of this method is widespread since it is easily carried out with a one‐dimensional (1D) to three‐dimensional (3D) coverage, quick to implement and relatively inexpensive (Galibert et al . ). However, if measurements of V P are performed quite efficiently for many years, retrieving V S remains complex since it requires the use of horizontal component geo‐phones difficult to set up horizontally (Sambuelli et al .…”

“…For these shallow-target studies, and are generally retrieved with seismic refraction tomography using both P and SH (shear-horizontal) waves (Turesson 2007;Grelle and Guadagno 2009;Fabien-Ouellet and Fortier 2014;Pasquet et al 2015). The use of this method is widespread since it is easily carried out with a one-dimensional (1D) to three-dimensional (3D) coverage, quick to implement and relatively inexpensive (Galibert et al 2014). However, if measurements of are performed quite efficiently for many years, retrieving remains complex since it requires the use of horizontal component geophones difficult to set up horizontally (Sambuelli et al 2001) and specific sources strenuous to use (Sheriff and Geldart 1995;Jongmans and Demanet 1993;Xia et al 2002;Haines 2007).…”

Application of combined time‐lapse seismic refraction and electrical resistivity tomography to the analysis of infiltration and dissolution processes in the epikarst of the Causse du Larzac (France)

“…), which 3D seismic surveys have shown to correspond to a weathered corridor (Valois ; Galibert et al . ).…”

“…; Galibert et al . ), particularly for the identification of caves and their connections with the surface.…”

Quantitative estimation of water storage and residence time in the epikarst with time‐lapse refraction seismic

“…It is beyond the scope of this paper to report on the saturation‐related seismic attenuation effects; this topic has been discussed for the data used in this work in (Galibert et al . ). We focus here on the velocity dispersion effect because I plan to invert the velocities for the water content.…”