Abstract
Different surface seismic surveys have been recorded on an experimental hydrogeological site that has been developed for several years near Poitiers (France). The paper shows how 3D seismic imaging can be used to describe the near-surface heterogeneous aquifer. The acquisition spread is designed to perform both 3D refraction and reflection seismic surveying. Refraction survey enables us to obtain a 3D image in depth of a low velocity superficial zone contrasting with the underlying water – bearing carbonates. Refraction survey shows the main orientations (N90 and N50) of fracture corridors. These two directions have been selected as the drilling azimuths of two deviated wells C3 and C4. Reflection survey enables us to generate a 3D seismic pseudo velocity block in depth. The 3D seismic pseudo velocity block shows the large heterogeneity of the aquifer reservoir in the horizontal and vertical planes, and confirms the main structural orientations (N90 and N50) identified by the refraction survey. The low velocity areas correspond to high hydraulic conductivity. In order to quantify the porosity of the different productive layers of the aquifer, the interval seismic velocities have been converted in resistivity. For that purpose, the empirical relationship between seismic velocity and true formation resistivity proposed by Faust (1953) [Geophysics 18, 271-288] has been used. The 3D resistivity block is converted in porosity, by using the Archie law (1942) [Petrol. Technol. 146, 54-62]. The 3D seismic pseudo porosity block allows us to identify three different water productive layers: an upper layer at 35-40 m depth, an intermediate layer at 85-87 m depth and a lower layer at 110-115 m. The intermediate layer is composed of bodies having a porosity larger than 30%. These bodies represent the most karstic part of the reservoir. The very high resolution seismic surveying has led to obtain a 3D porosity seismic block which represents a deterministic high resolution reservoir model. After further calibration, that reservoir model could be used for flow simulation.
Highlights
Many underground aquifers were developed as experimental sites during the past decade
This paper has shown how 3D seismic imaging can be used to describe the near-surface heterogeneous aquifer
Refraction survey enabled us to obtain a 3D image in depth of a low velocity superficial zone contrasting with the underlying water – bearing carbonates
Summary
Many underground aquifers were developed as experimental sites during the past decade. Different surface seismic surveys were attempted with different acquisition schemes These preliminary studies (Mari and Porel, 2007) lead us to select the following spreads for 3D imaging. A direct shot and a reverse shot have been recorded per receiver line. 3 shot points in the cross line direction have been fired at distances of 40, 50 and 60 m from the receiver line under consideration. Bottom left: example of 2D shot point for refraction survey. Reflection seismic is based on cross-line shots for 3D acquisition purpose
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