3D POROSITY BLOCK OF A FRACTURED-KARST AQUIFER: COMPARISON BETWEEN THE POROSITY MODEL ACHIEVED BY 3D SEISMIC AND ERT IMAGING IN THE EXPERIMENTAL SITE OF POITIERS (FRANCE)

Three-dimensional total porosity blocks of a Dogger Limestone fractured-karst aquifer were obtained at the Hydrogeological Experimental Site of Poitiers, France. Dogger Limestone occurs at a depth ranging between 30 and 120 m and is overlain by argillaceous limestone. Three karst intervals at depth of 35-40 m, 85-87 m and 110-115 m have been revealed from vertical flowmeter data and high-resolution imagery of borehole walls. This paper compares the total porosity blocks obtained from 3D seismic and resistivity imaging. Reflection seismic survey allowed generating a 3D seismic pseudo velocity block. The seismic interval velocities have been converted into resistivity. The empirical relationship between seismic velocity and true formation resistivity proposed by Faust (1953) has been used. Resistivity values were then converted into porosity values, by using the Archie's law (1942). The 3D seismic pseudo porosity block allowed identifying three different large porosity layers that are consistent with flowmeter and borehole imagery data. Three-dimensional resistivity imaging was obtained from full inversion of combined 2D ERT data collected using both Wenner-Schlumberger (WS) and Pole-Dipole (PD) array sequences. 3D resistivity block obtained from the hybrid (combination of WS and PD) sequence seemed to apparently identify the first two main karst intervals. It was converted into porosity by using the Archie law (1942). The total porosity blocks obtained from seismic and resistivity imaging show very different resolutions (above all at the karst layers depth), but agree in identifying the karst layers depth. The resistivity block shows an overestimation of the total porosity values: this is due to the occurrence of the shallow, thick, low resistivity layer (drift deposits and argillaceous limestone) that led to a severe underestimate of the limestone bulk resistivity values. Both the seismic and the resistivity blocks agree in suggesting that not karst carbonates would be affected by low permeability but significant storability.