Applications of magnetotelluric and transient electromagnetic methods in groundwater and engineering studies

The main aim of this study is to use the transient electromagnetic (TEM) and magnetotelluric (MT) methods to determine the electrical resistivity distribution of the subsurface and locate possible structural features controlling groundwater distribution in the northern and south-eastern margins of Parnaiba basin, Brazil. MT data from 24 stations along two profiles across the margins of the basin have been processed using standard tensorial techniques to obtain the interpretable response functions. The TEM data recorded at the same sites facilitated the removal of static shift and the recovery of the near-surface structure. One-dimensional (1-D) joint inversion of TEM and MT data yielded an approximate geoelectric structure for each profile. Subsequent two-dimensional (2-D) modelling revealed a more realistic resistivity distribution for each profile. The result of 2-D regularized inversion of MT data delineated the main sedimentary sequences and deep basement features. A resistive crystalline basement (≥ 200 &OHgr;m) is overlain by relatively conductive sedimentary sequences of varying resistivities and thicknesses. The existence of a major basement trough at the expected position of a concealed fault in the north-eastern margin of the basin was proved by the MT model. This anomalous zone is interpreted to have a thick development of granular sediments and may be a good site for groundwater development. The 2-D model for the south-eastern profile defined the position of a major fault, which is interpreted as Picos fault and may have implication for groundwater development. A graben-like structure is also suggested ∼10 km further east from Picos fault and considered to be a good target for groundwater development. Overall, the 2-D MT inversion results are consistent with the available geological information and offer new insights into the deep structure of the basin margins of Parnaiba basin..