Patrícia Represas

A case study of two and three-dimensional inversion of dipole-dipole data: the Enfermarias Zn-Pb (Ag, Sb, Au) Prospect (Moura, Portugal)

The Enfermarias Zn-Pb(Ag, Sb, Au) prospect, located in the Moura-Ficalho region (Alentejo, Portugal), was intensively explored during 1980s. A drilling programme was successful, intersecting massive and disseminated sulphide mineralizations. The geological and geophysical data set available for Enfermarias is quite vast and comprises 13 borehole geological logs, as well as gravity, magnetic and resistivity data. In this paper, the dipole-dipole resistivity surveys are re-examined and interpreted using both 2D and 3D inversion to provide an interesting comparison of the two different approaches. Results from gravity data modelling and data from borehole logs are used as independent validity tests on the quality of the results. The 2D and 3D models indicate a conductive overburden covering more resistive bedrock, which appears at a depth of 70–100 m in the western part of the profiles. Results obtained from 3D inversion, however, show a very clear separation of the most resistive domains in the two different bodies, whereas this separation does not appear in the 2D models. The 3D model also shows a higher degree of spatial continuity than the partial 2D models, which are, in general, richer in shortwavelength anomalies, and show a higher variation from profile to profile. Comparison of the resistivity results with gravity in two coincident profiles reveals an excellent agreement between the main shallow features of both 2D and 3D models. This suggests that the resistivity model does indeed satisfy the main features of the available geological and geophysical data.

Application of Time-Domain Electromagnetic Method in Investigating Saltwater Intrusion of Santiago Island (Cape Verde)

Santiago Island, the biggest and most populated island of the Cape Verde Republic, is characterised by limited surface waters and strong dependence on groundwater sources as the primary source of natural water supply for extensive agricultural activity and human use. However, as a consequence of the scarce precipitation and high evaporation as well as the intense overexploitation of the groundwater resources, the freshwater management is also in a delicate balance with saltwater at coastal areas. The time-domain electromagnetic (TDEM) method is used to locate the extent of saltwater intrusion in four important agricultural regions in Santiago Island; São Domingos, Santa Cruz, São Miguel, and Tarrafal. The application of this method in Santiago Island proves it to be a successful tool in imaging the fresh/saltwater interface location. Depths to the saline zones and extensions of saline water are mapped along eight TDEM profiles.

On the applicability of joint inversion of gravity and resistivity data to the study of a tectonic sedimentary basin in Northern Portugal

Abstract The Chaves basin is a pull-apart tectonic depression implanted on granites, schists, and graywackes, and filled with a sedimentary sequence of variable thickness. It is a rather complex structure, as it includes an intricate network of faults and hydrogeological systems. The topography of the basement of the Chaves basin still remains unclear, as no drill hole has ever intersected the bottom of the sediments, and resistivity surveys suffer from severe equivalence issues resulting from the geological setting. In this work, a joint inversion approach of 1D resistivity and gravity data designed for layered environments is used to combine the consistent spatial distribution of the gravity data with the depth sensitivity of the resistivity data. A comparison between the results from the inversion of each data set individually and the results from the joint inversion show that although the joint inversion has more difficulty adjusting to the observed data, it provides more realistic and geologically meaningful models than the ones calculated by the inversion of each data set individually. This work provides a contribution for a better understanding of the Chaves basin, while using the opportunity to study further both the advantages and difficulties comprising the application of the method of joint inversion of gravity and resistivity data.

Measurements of motinally induction voltage in the Ria de Aveiro lagoon (Portugal)

Techniques based on motional induced voltage (MIV), which is induced by the water flow across Earths magnetic field has motivated and increase of interest of oceanographers, regarding studies of large scale ocean flows. The major part of these experiences has been applied to large-scale ocean studies. This technique has been demonstrated during last decade to provide reliable information when applied to the flow monitoring in large oceanic channels. In this paper we present the preliminary results of the implementation of these techniques to smaller scales systems like the one of Ria de Aveiro lagoon (Portugal). A submarine cable crossing the channel at the entrance of the lagoon (Barra channel) allows the estimation of the water flow at this place. The preliminary analysis of the collected data shows that there is a good potential to obtain the water transport (by tidal and residual flows) through the canal by measuring the differences of electrical potential.