As part of a program to evaluate the utility of integrated geoelectrical methods for subsurface structural mapping and groundwater resource investigation in the semiarid eastern margin of the Parnaiba basin in Brazil, several vertical electrical soundings (VES) and transient electromagnetic (TEM) and tensorial audio-magnetotelluric (AMT) measurements were carried out along a 250-km-long east-west transect (passing through major towns and cities) and a 127-km-long north-south profile (passing through small farm settlements). The various data sets have been jointly processed using a novel integration scheme and a constrained 1-D inversion technique to yield the resistivity structure underneath each observation station. Regularized 2-D inversion of static-shift-corrected, dual-mode AMT data provided additional deep structural information and, together with the joint 1-D results, enabled an assessment of the distribution of aquifers and major structural controls in the region. The east-west regional geoelectrical model evokes a picture of a gently dipping succession of conductive and resistive units in good agreement with the alternating shaly and sandy formations evinced from preexisting borehole data and previous geological studies. The geoelectric models also show the presence of the large-scale Transbrazilian lineament and other graben-like structures, previously inferred from aeromagnetic data, which may have some control on groundwater distribution. The agreement with geology and aeromagnetic interpretation emphasizes the importance of integrated geoelectrical surveying as a complementary or independent means of obtaining useful stratigraphic and structural information for hydrogeological studies in this region.

The applicability of the Transient Electromagnetic (TEM) geophysical method for stratigraphic mapping and hydrogeological delineation has been tested in terrestrial part of the Campos Basin in Brazil. The study area is located within the fluvial plain of Paraíba do Sul River, at the W edge of the continental portion of the Basin, geologically consisting of Tertiary and Quaternary sediments and a gneissic basement. Around 32 central loop TEM soundings were made with 10 or20 m-sided transmitter loops, to map the conductive and/or resistive zones in the subsurface, which were correlated with lithology from a drilled borehole in the test area, whose location was indicated by the present study. In general, the one-dimensional (1D) models constructed for the TEM field data suggest a 3-layer resistivity structure characterized by a resistor-conductor-resistor pattern. The shallow section of 100-500 ohm.m and 50 m average thickness is interpreted as correlating with clay and shaly sand formations. The intermediate layer, with thickness around 20 m and resistivity less than 10 ohm.m, corresponds to clay. The basal resistor of 10-100 ohm.m at depths of more than 60 m at most sites is interpreted as the combined signature of the basal conglomerates, shaly sands and crystalline basement. Comparison with borehole data shows that the zone corresponding to the conductive geoelectrical units contains many thin layers of clay and sand that cannot be individually resolved by the TEM data and that there are no thick aquifers in the investigated area. In addition to that the shaly sand sediments have iron rich clays, which pollute groundwater, deeper resistive layers formed by conglomerates are considered the main aquifers of this area.