Magnetotelluric (MT) and differential geomagnetic sounding (DGS) surveys were made in the Senegal basin, West Africa, to investigate the conductivity structure of the crust and upper mantle. Magnetic and electric variations were recorded at 11 sites along a east-west profile 220 km in length. The period range of the measurements was 10--10,000 s. For the western stations, the MT response displays low skew with moderate anisotropy. The DGS data indicate the presence of an anomalous geomagnetic variation field in the deep basin and on the eastern margin of our stuidy area.

A telluric current concentration in the thick, conductive sediments of the deep basin might cause this anomaly. Both MT and DGS data are then compared with two-dimensional model results to explain the observed features along the profile. Results reveal a strong anomalous crustal structure in the central part of the profile involving a conductive zone (2 ohm m) at depths in the range 4--23 km. Within the deep basin the results of the two-dimensional modeling are not conclusive; acceptable fits between observed and calculated data have been obtained with alternate models. A continuous conductive zone (0.7-2 ohm m) may exist beneath the uppermost layers at a depth of approximatively 1 km, but the observed data can be explained equally well by poorly conducting material intruded into the highly conductive sedimentary cover. This ambiguity is inherent in the two-dimensional models themselves. Both models show that the sediments increase westward to reach a thickness greater than 4000 m when approaching the coast. It is suggested that the flow of current observed in this study is largely controlled by strong lateral variation in the resistivity of the basin and/or by variation of the sedimentary thickness. The models also indicate the existence of two decreased resistivity layers in the eastern part of the profile, one at the base of the crust and the second at 80 km with a resistivity of 20 ohm m. No such layers were detected at these depths below the deep basin. At some depth greater than 150 km there is a general trend toward lower resistivities from 2000 to about 2 ohm m across the entire profile. This may indicate the presence of a small degree of partial melt.