A series of vertical electrical soundings (VES) carried out in Schlumberger array is revisited to produce 2D resistivity model for the north central part of the Saiss Basin (Morocco). This paper is an attempt to show how data from traditional single soundings in Schlumberger configuration can be rearranged using geostatistical tools to estimate apparent resistivity values with a format compatible with a Wenner-Schlumberger array. The result is a series of estimated pseudo-sections that respect the spatial structure (covariance model) inherent in the original data set (VES), which finally are inverted to produce a 2D resistivity model of the basin along geophysical profiles. The estimated apparent resistivity field gives also valid hints about the 2D geo-electrical structure of the basin. Applying this methodology to different parallel VES profiles along the basin a 3D resistivity model can be generated.
The studied area is located between the cities of Meknes and Fez (Morocco) respectively at 33°53 N-34°04 N latitude and 5°30 W-4°57 W longitude at an altitude about 400 m; it is a structural depression oriented NE-SW between Middle Atlas to the south and Rides Prerifaines to the north. This basin is filled with sedimentary units deposited on a peneplained Paleozoic fold belt from the Triassic to the Pliocene and Quaternary. This depression is named the Saiss basin; we are concerned with its north central part (Figure 1).
For hydro-geological purposes, conventional 1D resistivity sounding surveys has occurred in 1984 resulting in a series of vertical electric soundings (VES). Based on these data we propose a methodology to convert them into two dimensional resistivity model; the Res2Dinv© software is used to produce a 2D resistivity model. The application of this methodology to different parallel VES profiles along the basin serves to generate a 3D resistivity model, drastically saving time and efforts in comparison with the individual inversion and interpretation of the VES sounding data. Also, it allows the reinterpretation of historical data in countries where only low-cost methods, as VES soundings, can be used, due to technology and economic constraints.
The set of data is composed of 220 vertical soundings. Resistivity measurements were made at the nodes of a more or less regular square grid with an SW-NE orientation parallel to the structural orientation of the Saiss basin (Figure 1); the size of the mesh is approximately 1 km. VES were carried out in Schlumberger array, half the distances between current electrodes (C1C2/2 = L/2) being systematically 3, 4, 5, 7, 9, 10, 15, 30, 40, 50, 75, 100, 150, 200, 300, 400, 500, 750, 1000, 1500, 2000 and 3000 m. Current and potential electrodes were mostly SW-NE oriented parallel to the structural orientation of the stratified basin. To summarize we have got a set of 17 profiles standing in straight lines along the SW-NE direction; each profile is composed of 2 to 22 VES depending of its location. The aim of this study is to develop a method to generate pertinent 2D resistivity models parallel to these profiles.