Second Projet de Renforcement Institutionnel du Secteur Minier de la Republique Islamique de Mauritanie (prism-ii)

5 Aeromagnetic and Gravity Data for evaluating hydrologic resources

A more detailed discussion of the relation of geophysical data to hydrology is in the hydrology section of this report (Friedel and Finn, 2012). Radiometric data (Plate 4) reflect the distribution of dune ridges and valleys. The valleys are occasionally prospective for ground water. Basins with sufficient depth are necessary to host aquifers. The primary basins in Mauritania are the Taoudeni and Coastal Basins with small parts of the Tindouf Basin in the north (Fig. 32). Depth estimates from the merged new and UN data over the Coastal and Taoudeni Basin calculated using a structural index of 0 and a window of 7 produce depth estimates that must be treated with care (Fig. 33). Depth estimates from the more widely spaced UN data seem to be ~1 km deeper than those from the newer data. The magnetic depth estimates indicate that Taoudeni Basin depth increases rapidly from ~500-1 km at the edge to greater than 2 km within 25 km of the edge. The greatest depths are >6 km in the areas not covered by dolerite sills. The minimum depth to the Precambrian basement in the center of the basin is ~2900m as determined by the Abolag-1 well (Fig. 34). Depths to the Jurassic sills are ~0.5- 1.5 km, in reasonable agreement with the seismically determined thickness of the Devonian section over which the sills lie (Fig. 34). The southern region covered by the magnetic data is covered by sedimentary rocks and sills underlain by sedimentary rocks that could source aquifers. Maximum depth estimates (euler structural index of 1) from the southeastern portion of demonstrate a pattern of shallow estimates over dolerites and deeper over basins (Fig. 35). The boundary between regions of shallow and deeper estimates corresponds to a boundary between dry and aquifer areas delineated by nuclear magnetic resonance and DC electrical resistivity data (Bernard, Lemine et al. 2004). The magnetic sources lie much deeper (>2000m, Fig. 35) than the magnetite poor clay base of the aquifer sandstones (at ~300m depth, Bernard et al., 2004), the pattern of depth estimates may indicate regions where thick sandstones with aquifer potential (Fig. 35) and can be used to target hydrogeophysical studies.