Pilar Queralt

WALDIM: A code for the dimensionality analysis of magnetotelluric data using the rotational invariants of the magnetotelluric tensor

In the magnetotelluric (MT) method, the analysis of geoelectric dimensionality has acquired special importance in the last years, because numerical codes have made it possible to model and invert data using either one-dimensional (1D), two-dimensional (2D) or three-dimensional (3D) approaches. We present a FORTRAN code termed WALDIM to perform the dimensionality analysis of a set of MT data, according to the WAL rotational invariants criteria. These criteria are based on the possible annulment of the invariants of the MT tensor, which allow retrieval of as much information as possible from this tensor, without taking any a priori dimensionality assumption. When determining the dimensionality of real and therefore noisy data, two problems arise. The first is due to the data errors, which propagate into the invariants values, and therefore, to the determination of the dimensionality. The second is the fact that the invariants are rarely precisely zero, and the definition of a threshold is necessary. To solve these problems, WALDIM takes into account the data errors. Additionally, the dimensionality results can be grouped into frequency bands. Thus, we provide a software utility that allows providing a robust description of the dimensionality, and the parameters necessary for data correction prior to modeling. Given its completeness at analyzing the MT tensor for both individual and bands of frequencies, this code is meant to be a practical tool for MT data analysis.

New geophysical constraints on the deep structure of the Pyrenees

A 2D modeling of geoid/quasigeoid data across the Pyrenees is consistent with electromagnetic images of the Pyrenean lithospheric structure, showing a subducted Iberian lower crust. Using new magnetotelluric data acquired through the orogen, a 3D electrical conductivity model of the Pyrenean lithospheric structure is obtained. The most outstanding feature is the presence of a high conductivity zone at lower crustal depths along the orogen in the contact zone between the Iberian and European plates. The integration of various geophysical parameters suggests that the simplest and most reasonable mechanism to explain the observed data is partial melting of the subducted Iberian lower crust.

A multidisciplinary geophysical study in the Betic chain (southern Iberia Peninsula)

Abstract The combined analysis of magnetotelluric measurements, tomographic velocity models and deep seismic reflection images confirms that the Betics orogen consists of the juxtaposition of two crustal domains characterized by distinctive physical properties. At depth these data sets show evidence for a non-coincidence of the petrological and the seismic Moho beneath the Betics chain. The data sets reveal the geophysical properties of the Alboran domain (Internal Betics) and the Iberian Massif (External Betics). According to this, the Iberian crust features a relatively high seismic velocity, is seismically transparent in the seismic reflection images and is electrically resistive. The Alboran domain crust is characterized by a low average velocity, displays high reflectivity in the seismic reflection images and is electrically conductive. The outcrops of the metamorphic complexes (Alpujarride and Nevado Filabride), showing relatively high velocities coupled with low V p /V s values (1.67) derived from the Wadati slopes, suggest the existence of rocks rich in silica beneath the Alboran domain crust. An interpreted detachment at 12 km depth imaged by deep seismic reflection suggests that these rocks could be related to the Iberian upper crust. Partial melts and fluids are proposed to explain the high conductivity observed at deep crustal levels. These would account also for the reflectivity and the low V p /V s ratios mapped beneath the Alboran domain.

Electromagnetic imaging of a transpressional tectonics in SW Iberia

Forty-one magnetotelluric soundings were carried out along a 200 km-long profile (approximately NNE-SSW) across the three major geotectonic units in SW Iberia. A model obtained from two-dimensional inversion of the magnetotelluric data set reveals high conductivity zones in the middle-lower crust (10–30 km). Two of these zones correspond to the transition between the main geotectonic units: one between the South Portuguese Zone and the Ossa Morena Zone, interpreted as having been caused by metasediments, and the other between the Ossa Morena Zone and the Central Iberia Zone associated with a shear zone and metasediments. Another high conductivity anomaly related to black shales with major graphite impregnation was detected within the Ossa Morena Zone. The resistive features, located preferentially in the upper crust (1–10 km), coincide with gabbroic and granitic complexes.

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 (

Integrated two-dimensional lithospheric conductivity modelling in the pyrenees using field-scale and laboratory measurements

Abstract Recent magnetotelluric (MT) studies have shown that the lower crust in the Pyrenees contains a high conductivity zone consistent with a subducting continental slab, whose conductivity is 0.33 S/m. Partial melting has been interpreted to be the most plausible explanation for this high conductivity. Here we report a two-dimensional conductivity model of the lithosphere by integrating field-scale and laboratory determinations of the conductivity of continental crustal and mantle rocks. The laboratory data provide empirical formulas which allow us to determine the fluid saturated rock and melt conductivity when temperature, pressure and lithology are known. Consequently, we have also calculated the density, lithostatic pressure, and several alternative temperature profiles for use in the model from gravity, seismic and thermal field data. These can be used with a prescribed melt fraction to predict the electrical conductivity at depth, which can be compared with the MT conductivity data. Alternatively, the laboratory data can be combined with the MT conductivity data to predict the melt fraction at depth. The primary outputs of the modelling are conductivity and melt fraction prediction profiles for six mixing models; (i) Waff’s model/Hashin–Shtrikman (HS) upper bound, (ii) HS lower bound, (iii) parallel layers, (iv) perpendicular layers, (v) random melt areas, and (vi) a modified Archie’s law that takes account of the presence of two conducting phases. The modelling results indicate that a good match to the MT data can be obtained along the whole profile by the influence of pressure, temperature and the fluid phase with the only exception being the subducted slab, where a minimum of 4.7% melt fraction is necessary to explain the data.

A high electrical conductive zone at lower crustal depth beneath the Betic Chain (Spain)

Abstract Magnetotelluric data with periods of up to 2000 s were collected and the conductivity structure of the crust and upper mantle was imaged in the Central Betic chain. Interpretation of the impedance tensor components and the geomagnetic transfer functions was made in terms of a two-dimensional resistivity model along a NW–SE profile from the Guadalquivir foreland basin and across the boundary between the External and the Internal Betics. Elongated high conductivity zones in the upper and middle crust related to fluid circulation along both sediments and faults were detected. The conductors in the upper crust coincide with the areal distribution of the Cenozoic and Mesozoic sedimentary rocks from the Guadalquivir basin and the External Betics and the conductors in the middle crust with the major Miocene extensional detachments affecting the basement rocks of the Internal Betic zone. A high conductivity zone at lower crustal levels beneath the outer Internal Betics was also detected next to an area where earlier seismic reflection data had imaged a duplication of the reflective Moho. This conductor was interpreted as partial melting of a southeast dipping subducted Iberian lower crust. The geodynamic significance of this subduction depends mainly on whether the overthrusting lower crust belongs to the Alboran domain or to the Iberian plate. In the former case it would correspond to a major plate boundary and in the latter to an intraplate feature.

2-D resistivity modeling; an approach to arrays parallel to the strike direction

An algorithm for two-dimensional electrical resistivity modeling using the finite-element method with mixed boundary conditions can calculate the electrical potential along any arbitrary direction. In the particular case of the direction parallel to the strike of the structure, a numerical singularity occurs. We resolved it by calculating the potential near the singularity and improving the involved interpolation of the transformed potential. Investigating the shape of potential in the Fourier transformed space, we conclude that this interpolation may combine logarithmic and exponential functions.

Monitoring freshwater-seawater interface dynamics with audiomagnetotelluric data

Saltwater intrusion is one of the main environmental concerns within coastal aquifers. In this study we test the audiomagnetotelluric (AMT) method as a technique that can detect changes in electrical resistivity as a result of seasonal groundwater salinity changes. AMT is a frequency domain electromagnetic induction technique ideally suited for hydrogeophysical investigations at the basin scale, specifically in low resistivity environments such as saltwater encroachments areas. We present numerical seawater intrusion models to explore the effects of saline content variability on the model resolution. Survey data were also acquired during a long-term AMT monitoring experiment in a natural condition aquifer system and these results were compared to the numerical modelling results. The aquifer system is located in the deltaic zone of the Tordera River (north-eastern of Iberian Peninsula), where a main paleochannel that works as a seawater intrusion path was already identified in previous studies. Every four months, between 2004 and 2006, seven AMT soundings were recorded along a 1700 m long profile over the paleochannel. The final models reveal dynamic changes in the seawater-freshwater interface that correspond directly with the hydrologic state of the aquifer system.

Feasibility of Monitoring the Hontomín (Burgos, Spain) CO2 Storage Site Using a Deep EM Source

Geophysical methods have been used experimentally during the last decade, a period of strong development, being adopted as complementary techniques for characterizing and monitoring hydrocarbon and gas reservoirs. In this study, we evaluated the ability of the controlled source electromagnetic (CSEM) method to monitor the storage of CO2 at the Research Laboratory on Geological Storage of CO2 at Hontomín (Burgos, Spain). Two aspects of the CSEM monitoring were examined considering the geoelectrical structure at the site, the technological constraints and the noise conditions of the Hontomín area. Borehole-to-surface simulations were performed to evaluate the detectability of the resistivity changes in the reservoir and the capacity to determine the location of the CO2 plume. The synthetic time-lapse study explores the possibilities of CSEM monitoring with a deep electric source. Three depths of the source are analyzed: above the plume, inside the plume, and beneath the stored CO2. In terms of the Hontomín storage site, the study confirmed that a deep electric source located beneath the injection depth can provide valuable information on the behavior of the stored CO2.