Capability of cross-hole electrical configurations for monitoring rapid plume migration experiments

Cross-hole electrical resistivity tomography is a useful tool in geotechnical, hydrogeological or fluid/gas plume migration studies. It allows better characterization of deep subsurface structures and monitoring of the involved processes. However, due to the large amount of possible four-electrode combinations between boreholes, the choice of the most efficient ones for rapid plume migration experiments (real-time monitoring), becomes a challenge. In this work, a numerical simulation to assess the capabilities and constraints of the most common cross-hole configurations for real-time monitoring is presented. Four-electrode configurations, sensitivity, dependence on the body location and amount of data were taken into account. The analysis of anomaly detection and the symmetry of the sensitivity pattern of cross-hole configurations allowed significant reduction of the amount of data and maintaining the maximum potential resolution of each configuration for real-time monitoring. The obtained results also highlighted the benefit of using the cross-hole AB-MN configuration (with both current - or potential - electrodes located in the same borehole) combined with other configurations with complementary sensitivity pattern

Land CSEM Simulations and Experimental Test Using Metallic Casing in a Geothermal Exploration Context: Vallès Basin (NE Spain) Case Study

Controlled-source electromagnetic (CSEM) measurements are complementary data for magnetotelluric (MT) characterization although its methodology on land is not sufficiently developed and tested as in marine environments. Acquiring expertise in CSEM is crucial for surveys in places where MT cannot be performed due to high levels of cultural noise. To acquire that expertise, we perform CSEM experiments in the Vallès fault [Northeast (NE), Spain], where MT results have been satisfactory and allow us to verify the CSEM results. The Vallès basin is relevant for potential heat generation because of the presence of several geothermal anomalies and its nearby location in urban areas. In this article, we present the experimental setup for that region, a 2-D joint MT+CSEM inverse model, several 3-D CSEM simulations in the presence of metallic casing, and its comparison with real data measurements. We employ a parallel and high-order vector finite element algorithm to discretize the governing equations. By using an adapted meshing strategy, different scenarios are simulated to study the influence of the source position/direction and the conductivity model in a metallic casing presence. An excellent agreement between the simulated data and analytical/real field data demonstrates the feasibility of study metallic structures in realistic configurations. Our numerical results confirm that metallic casing strongly influences electromagnetic (EM) responses, making surface measurements more sensitive to resistivity variations near the metallic structure. It could be beneficial getting higher signal-to-noise ratios and sensitivity to deep targets. However, such a casing effect depends on the input model (e.g., conductivity contrasts, frequency, and geometry)

Joint interpretation of magnetotelluric, seismic, and well-log data in Hontomín (Spain)

Acknowledgements. This work is dedicated to the memory of Andrés Pérez-Estaún, brilliant scientist, colleague, and friend. The authors sincerely thank Ian Ferguson and an anonymous reviewer for their useful comments on the manuscript. Xènia Ogaya is currently supported in the Dublin Institute for Advanced Studies by a Science Foundation Ireland grant IRECCSEM (SFI grant 12/IP/1313). Juan Alcalde is funded by NERC grant NE/M007251/1, on interpretational uncertainty. Juanjo Ledo, Pilar Queralt and Alex Marcuello thank Ministerio de Economía y Competitividad and EU Feder Funds through grant CGL2014- 54118-C2-1-R. Funding for this Project has been partially provided by the Spanish Ministry of Industry, Tourism and Trade, through the CIUDEN-CSIC-Inst. Jaume Almera agreement (ALM-09-027: Characterization, Development and Validation of Seismic Techniques applied to CO2 Geological Storage Sites), the CIUDEN-Fundació Bosch i Gimpera agreement (ALM-09-009 Development and Adaptation of Electromagnetic techniques: Characterisation of Storage Sites) and the project PIERCO2 (Progress In Electromagnetic Research for CO2 geological reservoirs CGL2009-07604). The CIUDEN project is co-financed by the European Union through the Technological Development Plant of Compostilla OXYCFB300 Project (European Energy Programme for Recovery).Peer reviewedPublisher PD

On the detectability of Teide volcano magma chambers (Tenerife, Canary Islands) with magnetotelluric data

Tenerife has been the subject of numerous studies covering a wide range of fields. Many studies have been focused on characterising the magmatic plumbing system. Even so, a controversy still exists regarding the location and size of the current magma chambers. Several magnetotelluric (MT) surveys have been carried out in the island, but no conductivity anomalies associated with the chambers have been detected. We report the results of a set of tests conducted against the 3-D resistivity model of the island, to determine the characteristics of the detectable chambers with the MT data. The most remarkable results indicate that the MT dataset is incompatible with a large-scale mafic reservoir located at shallower depths than 8 km b.s.l. However, shallower phonolitic chambers smaller than 3 x 3 x 1 km(3) could be undetected by the existing MT sites and new data should be acquired to confirm or not their existence. This new information is essential in volcanic islands like Tenerife, since many volcanic hazards are related to the size and depth of the sources of magma. Additionally, a joint interpretation of the obtained results together with other information is summarised in a hypothetical model, allowing us to better understand the internal structure of the island

A layer stripping approach for monitoring resistivity variations using surface magnetotelluric responses

The resolution of surface-acquired magnetotelluric data is typically not sufficiently high enough in monitoring surveys to detect and quantify small resistivity variations produced within an anomalous structure at a given depth within the subsurface. To address this deficiency we present an approach, called "layer stripping", based on the analytical solution of the one-dimensional magnetotelluric problem to enhance the sensitivity of surface magnetotelluric responses to such subtle subsurface temporal variations in resistivity within e.g. reservoirs. Given a well-known geoelectrical baseline model of a reservoir site, the layer stripping approach aims to remove the effect of the upper, unchanging structures in order to simulate the time-varying magnetotelluric responses at depth. This methodology is suggested for monitoring all kinds of reservoirs, e.g. hydrocarbons, gas, geothermal, compress air storage, etc., but here we focus on CO2 geological storage. We study one-dimensional and three-dimensional resistivity variations in the reservoir layer and the feasibility of the method is appraised by evaluating the error of the approach and defining different detectability parameters. The geoelectrical baseline model of the Hontomín site (Spain) for CO2 geological storage in a deep saline aquifer is taken as our exemplar for studying the validity of the 1D assumption in a real scenario. We conclude that layer stripping could help detect resistivity variations and locate them in the space, showing potential to also sense unforeseen resistivity variations at all depths. The proposed approach constitutes an innovative contribution to take greater advantage of surface magnetotelluric data and to use the method as a cost-effective permanent monitoring technique in suitable geoelectrical scenarios

Improving the modeling of geomagnetically induced currents in Spain

Vulnerability assessments of the risk posed by geomagnetically induced currents (GICs) to power transmission grids benefit from accurate knowledge of the geomagnetic field variations at each node of the grid, the Earth's geoelectrical structures beneath them, and the topology and relative resistances of the grid elements in the precise instant of a storm. The results of previous analyses on the threat posed by GICs to the Spanish 400 kV grid are improved in this study by resorting to different strategies to progress in the three aspects identified above. First, although at midlatitude regions the source fields are rather uniform, we have investigated the effect of their spatial changes by interpolating the field from the records of several close observatories with different techniques. Second, we have performed a magnetotelluric (MT) sounding in the vicinity of one of the transformers where GICs are measured to determine the geoelectrical structure of the Earth, and we have identified the importance of estimating the MT impedance tensor when predicting GIC, especially where the effect of lateral heterogeneities is important. Finally, a sensitivity analysis to network changes has allowed us to assess the reliability of both the information about the network topology and resistances, and the assumptions made when all the details or the network status are not available. In our case, the most essential issue to improve the coincidence between model predictions and actual observations came from the use of realistic geoelectric information involving local MT measurements

El potencial espontáneo como herramienta para la caracterización de la afección por hidrocarburos

El objeto del presente documento es poner en conocimiento de la sociedad la disponibilidad de una técnica de prospección geofísica no destructiva que permite detectar y localizar la afección por hidrocarburos en el subsuelo de instalaciones industriales.La memoria recoge la descripción del fenómeno físico y los antecedentes de dicha técnica, citándose también novedades respecto a su aplicación en instalaciones industriales.El documento puede ser de utilidad para los agentes que intervienen en actividades potencialmente contaminantes del suelo, así como de cualquier entidad que pueda acometer trabajos de caracterización de suelos con técnicas que complementen la ejecución de estas tareas

Integrated interpretation of geophysical data from Zagros mountain belt (Iran)

Fluid composition and distribution, the key factors determining geoelectric structure in a seismically active region, are controlled by local and regional stresses and rheological contrasts. In the central Zagros collision zone, one of the world's most seismically active mountain belt, almost coincident magnetotelluric and seismic velocity profiles are jointly interpreted to recover more accurately structural boundaries and fluid distribution within the crust. A multi-site and multi-frequency approach was used for the strike analysis of regional structure and decomposition of distortion effects on magnetotelluric data. Distortion corrected magnetotelluric data were then used for two- dimensional inversion modeling. The results image a thick conductive overburden in the southwest of the profile, high conductivities attributed to the fault zone conductors (FZCs) and an almost concave conductor extending from middle to lower crust in the central- eastern portion of the mountain belt, beneath the High Zagros (HZ). Comparison with the already available S- velocity structure, obtained by joint inversion of P-wave receiver functions and surface wave dispersion data, shows that these main conductive features are spatially correlated with a low-velocity layer representative of the sedimentary cover overlying the Arabian platform and a velocity contrast bounded by the main Zagros thrust (MZT) fault, indicating the presence of fault zone fluids. The joint interpretation of magnetotelluric inverse modeling and seismicity data also shed light on fluid generation influencing rock deformation and seismicity in this region. It suggests that beneath the HZ, deep crustal fluids generated through metamorphism may promote aseismic deformations before high stresses are buildup and cause the north- eastern part of the Zagros Fold and Thrust Belt (ZFTB) to be seismically inactive compared to its south- western part

On the observation of magnetic events on broad-band seismometers

The objective of this contribution is to get new insights into the effects of magnetic field variations of natural and anthropogenic origin on broad-band seismic stations. Regarding natural sources of magnetic perturbations, we have investigated if the Sudden Storm Commencements (SSC) cataloged during the 24th solar cycle (2008-2019) can be systematically identified in broad-band seismic stations distributed worldwide. The results show that the 23 SSC events with a mean amplitude above 30 nT and most of those with lower energy but still clearly identified in the magnetometer detection network can be observed at broad-band stations' network using a simple low-pass filter. Although the preliminary impulse of those signals is usually stronger at stations located at high latitudes, major SSC are observed at seismic stations distributed worldwide. Regarding anthropogenic sources, we focus on the short period seismic signals recorded in urban environments which are correlated with the activity of the railway transportation system. We have analyzed collocated measurements of electric field and seismic signals within Barcelona, evidencing that significant changes in the electric field following the activity of the transportation systems can be attributed to leakage currents transmitted to the soil by trains. During space weather events, electric currents in the magnetosphere and ionosphere experience large variations inducing telluric currents near the Earth surface, which in turn generate a secondary magnetic field. In the case of underground trains, leakage currents are transmitted to the soil, which in turn can result in local variations in the magnetic field. The observed signals in modern seismometers can be related to the reaction of the suspension springs to these magnetic field variations or to the effect of the magnetic field variations on the force transducers used to keep the mass fixed

New geoelectrical characterization of a continental collision zone in the Central - Eastern Pyrenees: Constraints from 3-D joint inversion of electromagnetic data

Continent-continent collisions are responsible for the formation of large mountain ranges like the Himalayas and the Alps and play a primary role in the development of the continents. The continental collision between the Iberian and European plates during the Alpine Orogeny resulted in the formation of the Pyrenees. In this study new electromagnetic data from the Eastern Pyrenees were complemented with older data from the Central Pyrenees, constraining the physical and geological processes at the eastern end of the Pyrenean mountain range. The electrical resistivity distribution beneath the Central-Eastern Pyrenees was characterized by means of three-dimensional (3-D) joint inversion of three electromagnetic datasets: (1) the MT impedance tensor (Z), (2) the geomagnetic transfer function (T), and (3) the inter-station horizontal magnetic transfer function (H). The main finding was the non-continuity to the east of the major conductive anomaly observed previously beneath the Central and West-Central Pyrenees related to partial melting of the Iberian subducted lower crust. Lower amounts of water (related to the presence of muscovite and biotite) in the subducted lower crust beneath the Eastern Pyrenees were suggested to explain the lack of partial melting in this part of the mountain range. The electrical resistivity model also revealed higher electrical resistivity values for the lithospheric mantle beneath the Eastern Pyrenees than beneath the Central Pyrenees, thus supporting the hypothesis of an heterogeneous Iberian plate inherited from the Variscan Orogeny. A less clear signature was the lateral variation along the strike direction of the lithosphere-asthenosphere boundary beneath the Eastern Pyrenees (relatively flat, between 110 km and 140 km depth) and the Central Pyrenees (north dipping, between 80 km and 120 km depth beneath the Iberian Plate and between 110 km and 160 km depth beneath the European plate), supporting the hypothesis of a missing lithospheric root beneath the Eastern Pyrenees