Fernando A. Monteiro Santos

Are some of the deep crustal conductive features found in SW Iberia caused by graphite

Abstract Recent results obtained from two-dimensional inversion of magnetotelluric (MT) data acquired in SW Iberia reveal high-conductive features at the middle–lower levels of the crust. The top of these anomalous structures correlates very well with the depth (10–13 km) of an important seismic interface that has been interpreted as a regional detachment horizon. Very shallow and relatively narrow conductors in the Ossa Morena Zone appear to correspond to small-scale fluid-deposited graphite systems in the preorogenic metasedimentary sequences. Some of the midcrustal conductors can be ascribed to graphite-bearing thrust zones, the formation of graphite with variable crystallinity being a consequence of Variscan shearing processes. Deep-seated conductors are tentatively interpreted as a result of relatively continuous, highly crystalline grain-boundary graphite films presumably preserved in basement, granulite(?) rocks. Assuming that graphite occurs as interconnected films, calculations indicate that a fraction of 0.006–0.02% of this accessory mineral is enough to explain the range of the electrical resistivity estimated on the basis of MT models. The role of graphite on the thermal behaviour of the crust is also discussed. The results show that low contents of graphite do not significantly change the thermal behaviour of earth materials.

Magnetotelluric survey of the electrical conductivity of the crust across the Ossa Morena Zone and South Portuguese Zone suture

Abstract The transition between the South Portuguese Zone (SPZ) and the Ossa Morena Zone (OMZ) is made up of a major geosuture, which is indicated by the Beja-Acebuches ophiolite. Several geological data suggest that this suture is the result of an oblique collision, with a northward propagation, between the SPZ and OMZ. From the structural point of view the OMZ is much more complex than the adjacent Central Iberian Zone and the SPZ. The frequently complicated internal structure is the result of superposition of various structural elements developed during different tectonic events. With the aim of providing new constraints to this complex deep structure a magnetotelluric (MT) survey was designed by the universities of Lisbon and Barcelona in the scope of the bilateral cooperation and the Europrobe programs. A first NNE–SSW profile across the SPZ and OMZ was carried out in September 1997. The profile is 40 km long and consists of nine deep MT soundings with periods ranging from 0.0039 to 4000 s. The time series were processed using a robust algorithm, after visual inspection. The study of the dimensionality and directionality of the regional electrical structures is based on the analysis of the induction arrows and the Groom–Bailey decomposition. A strike of N125E was determined, in accordance with regional geological features. 2-D inversion was undertaken on both TE and TM modes using the RRI method. The main feature of the model is a large conductive body (

Inversion of self-potential of idealized bodies' anomalies using particle swarm optimization

The particle swarm optimization (PSO) method was applied to the inversion of single and multiple self-potential (SP) data anomalies caused by buried bodies with simple geometry like spheres, cylinders and inclined sheets. The inversion parameters are the depth of the body, the location of the anomaly, the electric current dipole moment, the polarization angle or the inclination angle and the half-width in the sheet-like body case. The algorithm was tested on synthetic data with single and multiple SP anomalies and applied to two experimental data from Surda area (India) and Vilarelho da Raia (Portugal). The inversion results showed good agreement with the previous results obtained using other techniques. The great advantage of the PSO method is that it is fast and does not require assumptions about the shape of the source of the SP anomaly.

Magnetotelluric impedances and parametric sensitivities for 1-D anisotropic layered media

Anisotropy of the electrical conductivity within the Earth has recently provided a significant link between geoelectrical models and the underlying tectonic setting. Interpretation of magnetotelluric data for anisotropic conductivities suffers from inherent ambiguity, and even for theoretically distinguishable parameters the resolution pattern may be rather complex as compared to isotropic models. As a tool for the resolution and sensitivity studies, we present an algorithm for jointly evaluating the magnetotelluric impedances and their partial derivatives with respect to the parameters of a 1-D generally anisotropic layered medium. The algorithm is based on impedance propagation formulas through a stack of anisotropic layers and their direct differentiation with respect to the model parameters. By virtue of an equivalency principle, the situation of a generally anisotropic layered medium can be always reduced in magnetotellurics to a simpler model with horizontally anisotropic layers. By rotating the impedances into the local anisotropy strike, the two wave modes in an anisotropic layer can be quasi-separated, and elimination of the positive exponential wave factors in the impedance formulas is possible, which stabilizes both the impedance and sensitivity calculations. The performance of the algorithm is demonstrated on a simple qualitative sensitivity study for a four-layer model with depth-variable anisotropy parameters.

A spatially constrained 1D inversion algorithm for quasi-3D conductivity imaging: Application to DUALEM-421 data collected in a riverine plain

The efficient use of water in irrigated agricultural systems is of increasing importance given the changes in climatic patterns currently being experienced in the irrigated areas of the Murray-Darling Basin (MDB) in Australia. In previous research, electromagnetic (EM) induction instruments have been used to map the distribution of the clay content in those areas. However, describing their vertical extent and connectivity with groundwater tables or stratigraphic features such as paleochannels has not been studied adequately. One of the reasons for the paucity of research is the lack of suitable instrumentation or software to invert apparent conductivity (σa) data. The aim of this research is to demonstrate how DUALEM-421 equipment, which operates using electromagnetic induction theory, can be used to map not only the areal distribution of a prior stream channel but its vertical extent by inputting the data into a 1D spatially constrained algorithm for quasi-3D conductivity imaging. We discovered how the i...

Salt tectonics in the western Gulf of Cadiz, southwest Iberia

This study presents the results from the interpretation of an extensive and recent regional two-dimensional seismic survey focused on the understanding of the salt tectonics in the western Gulf of Cadiz (GoC). Two different salt units were identified: an autochthonous salt unit of the Late Triassic or the Early Jurassic (Hettangian) and an allochthonous unit that originated from the Hettangian salt. Interpretation of the pattern of distribution of the salt in the basin allowed subdivision of the area of study into three distinct salt domains: the eastern domain characterized by the presence of a conspicuous allochthonous salt nappe (Esperanca Salt), the central domain dominated by salt diapirs with mild deformation of Miocene strata and wide salt-withdrawal minibasins, and the southwestern domain where present-day tectonics induces impressive salt deformation affecting the sea floor. This complex pattern is mainly the result of the interaction of inherited basement structure, complex tectonic history, and stress regime of the basin. The intense halokinesis observed has created several salt-related trap geometries and fluid migration pathways. As the focus of worldwide exploration along passive margins is gradually shifting to deep-water regions, the western GoC has the potential to become a deep-water petroleum province in the near future.

Inversion of Conductivity Profiles from EM Using Full Solution and a 1-D Laterally Constrained Algorithm

In highly conductive environments the apparent electrical conductivity (σa) data generated from electromagnetic (EM) instruments are known to be non-linear. This is particularly the case when high conductivity bodies are present in the subsurface. However, little attention has been given to this issue in the research literature of the environmental and hydrological sciences. In this paper we describe the development of an inversion algorithm, which consists of a 1-D inversion with 2-D smoothness constraints between adjacent 1-D models, whereby the forward response is calculated using the full solution of the induction phenomena. The robustness of the algorithm is evaluated using σa data acquired from two study areas. In the first case study, σa data is acquired with a DUALEM-21 across a golf green in Guelph, Ontario Canada. In the second case study, a DUALEM-421 is used to collect σa across an irrigated field located on a clay alluvial plain of the Lower Gwydir Valley (Australia). The general patterns of ...

2D joint inversion of dc and scalar audio-magnetotelluric data in the evaluation of low enthalpy geothermal fields

Audio-magnetotelluric (AMT) and resistivity (dc) surveys are often used in environmental, hydrological and geothermal evaluation. The separate interpretation of those geophysical data sets assuming two-dimensional models frequently produces ambiguous results. The joint inversion of AMT and dc data is advocated by several authors as an efficient method for reducing the ambiguity inherent to each of those methods. This paper presents results obtained from the two-dimensional joint inversion of dipole?dipole and scalar AMT data acquired in a low enthalpy geothermal field situated in a graben. The joint inverted models show a better definition of shallow and deep structures. The results show that the extension of the benefits using joint inversion depends on the number and spacing of the AMT sites. The models obtained from experimental data display a low resistivity zone (<20 ? m) in the central part of the graben that was correlated with the geothermal reservoir. The resistivity distribution models were used to estimate the distribution of the porosity in the geothermal reservoir applying two different approaches and considering the clay minerals effect. The results suggest that the maximum porosity of the reservoir is not uniform and might be in the range of 12% to 24%.

A hydrogeological investigation using EM34 and SP surveys

EM34 and SP surveys were used to delineate shallow structures associated with the mineral water springs in the Vilarelho da Raia area located NE Portugal. This spring is part of a set of CO2-rich mineral (hot and cold) waters connected to the main Hercynian NNE-SSW fault systems. The EM34 survey was interpreted using a quasi-three-dimensional inversion approach based on a smooth-regularisation algorithm. The model put into evidence the conductive overburden as well as the fractured granitic formation. A preliminary estimation of the aquifer porosity is made based on the EM34 model. The SP anomalies were interpreted considering as having their sources on the aquifer interfaces. The modelling of two SP profiles allow the characterisation of a fault connected to the spring.

Study of the Chaves geothermal field using 3D resistivity modeling

Abstract The present paper deals with the 3D modeling of a resistivity data set, carried out over the Chaves graben where the hot springs, with a temperature reaching 78°C, have a great economic importance. The main objective of the modeling was to incorporate the partial knowledge obtained from previous ID and 2D interpretations into a three dimensional model. The 3D models of three rectangles and two dipole-dipole surveys, which were performed to detect conductive zones associated with the hydrothermal circulation, depict the general form of the graben and the spatial configuration of the low resistivity zones (10–15 μ m). The achieved models exhibit several characteristics similar to those of previous 1D and 2D interpretations and are consistent with geological information and new findings related to the bed-rock depth (1200 to 1500 m), the northern and southern borders of the conductive zones and its connection with the fault systems.