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- Volume 34, Issue 8, 2016
First Break - Volume 34, Issue 8, 2016
Volume 34, Issue 8, 2016
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Application of geophysical techniques to support geological mapping projects
Authors Beatriz Benjumea, Anna Gabàs, Albert Macau, Sara Figueras and Fabian BellmuntBetter and more detailed information on the bedrock and near-surface geology is needed to assist geological risk assessments, efficient use of water resources or management of rapidly growing regions. Since 2007, the Cartographic and Geological Institute of Catalonia (ICGC) has developed a geological mapping programme, which includes information related to hydrogeology, urban geology or geological risks among others. Within this framework, geophysical techniques make an important contribution refining geological mapping by adding subsoil information to traditional geological data. In this work, we present two studies carried out by ICGC in order to map bedrock geometry or to characterize near-surface sediments in areas with scarce borehole information. The first example of this support focuses on the combination of passive and active geophysical techniques in order to image the Quaternary/Neogene boundary, the bedrock top and to locate faults or the geological map of the Girona urban area (NE of Spain). The second case study shows the potential of reprocessing vintage seismic oil datasets to increase knowledge of the near-surface geological structure in a Neogene Basin located north of Girona (Empordà Basin). The reprocessed seismic reflection image of the first km depth has been interpreted using the refraction velocity model and passive seismic information as constraints. The main objectives are to recover bedrock geometry and structure as well as fault imaging, which are critical for geological mapping projects. The common pattern of the methodologies applied to each case is the integration of different geophysical datasets and the use of both active and passive techniques.
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The gravity method: new challenges for delineation diapiric structures in north-east Spain
Authors A. Casas, P. Santolaria, L. Rivero, A. Casas-Sainz, M. Himi, V. Pinto and A. SendrósThe gravity method was the first geophysical technique that demonstrated its usefulness for oil exploration one century ago with the delimitation of the anticlinal structure of Egbell in 1916 from detailed measurements conducted by Roland von Eötvös using a torsion balance. This experience was followed by the discovery of the oil field related to the salt diapir of Nash (Texas, USA). Despite the development of other geophysical techniques, gravity prospecting continues to offer significant and sometimes essential evidence in many exploration projects. In recent decades, the emergence of a new generation of digital gravimeters, which has increased the precision of the measurements. The support of the GPS technique for accurate location and measuring of the height of the station, as well as the availability of digital terrain models, have renewed interest in this technique. This paper illustrates some recent applications of this technique in the southern Pyrenees that is an interesting evaporitic province characterized by the existence of several diapiric bodies of Triassic and Oligocene evaporites, related to a thrust-and-fold system. Gravimetry began with Galileo (1561-1642), who discovered the laws of free fall and of pendular motion. For this reason the unit in terms of which geophysicists express their gravity measurements is the ‘gal’, so named after Galileo. To Newton (1642-1727) we owe the law of universal gravitation and Huyghens (1629-1695) first used pendulums in clocks. Newton and Huyghens pointed out independently, and almost simultaneously, that the observed effect must be owing to the departure of the Earth from the spherical form. Geodesy was the first application of gravity measurement. The object was to throw light on the shape of the Earth and Bouguer (1698-1759) was probably the first to make pendulum observations for this purpose. His name is perpetuated in the Bouguer corrections and Bouguer anomalies.
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CO2 sequestration scenarios: challenges and opportunities for EM exploration techniques at Hontomín
The geological storage of CO2 is presented as a transitory solution to reduce the human emission of CO2. To be efficient and safe, it is required to obtain a very good knowledge of the subsurface and to have tools to control the evolution of the CO2 sequestrated into reservoirs. Geophysical techniques are essential for characterizing and to monitoring the storage sites. The paper presents an overview of existing electromagnetic methods which have been conducted at the Hontomín CO2 storage site (in Spain) during the past few years. These studies cover numerical simulations and geophysical campaigns at different scales and with different electromagnetic techniques: the magnetotelluric method, control source electromagnetic and electrical resistivity tomography at lab-core scale. The objective of the paper is to offer a critical review of the EM methods in meeting challenges related of achieving high resolution for deep targets and in a relatively noisy environment. We conclude with the lessons learnt from the Hontomín case study. Research for new natural resources as well as new complex pollution problems require new efforts from scientists, in particular the environmental earth sciences community, to analyse the problems and to find appropriate solutions and tools. In this way, advances and improvements in geophysical exploration techniques depend, in some part, on the challenges of these new problems. The evolution of the electromagnetic and electric methods (EM) shows how they have broadened their application fields when these demanding tasks have been faced. Recent studies (i.e. Constable, 2010; Strack, 2014; Muñoz, 2014; Streich, 2016) review the main contributions of these methods in specific fields such as geothermal and hydrocarbons exploration and monitoring and point out their potential as well as some future directionsin each field. EM methods play an important role in these fields given the resistivity dependency on temperature and/or on the presence of fluids.
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A review of geothermal exploration by magnetotelluric imaging in Tenerife
Authors P. Piña-Varas, J. Ledo, J. Campanyà, P. Queralt and A. MarcuelloThe geothermal potential of the Canary archipelago has been extensively studied since the mid-seventies, with a particular focus on Tenerife. Several agencies and institutions conducted magnetotelluric surveys in Tenerife characterizing the electrical resistivity distribution of the subsurface. The resultant 3D resistivity model shows the classical geoelectrical distribution of a high-temperature geothermal system, with a conspicuous low resistivity structure interpreted as the hydrothermal alteration zone (clay cap) that sits above a more resistive zone, which may correspond to rocks at higher temperatures. This paper is a review of the MT studies carried out in Tenerife for geothermal exploration, including the results obtained when correlating the resistivity model with other geophysical and geochemical data. Additionally, a second-derivative model of the resulting MT model has been used to define the main interfaces between the geoelectrical structures. Geothermal exploration in the Canary Islands dates back to the mid-Seventies, when the Spanish Geological Survey (IGME) started a nationwide project to study this resource in the whole Spanish territory. This archipelago was the main target of this study owing to its active volcanism. The works were focused particularly on the islands of Gran Canaria, Lanzarote, La Palma and Tenerife, including volcanological, petrological, geochemical and geophysical studies. In the particular case of Tenerife, the main activity took place in the last stage of this nationwide project. Among the studies performed during these years, two magnetotelluric (MT) surveys were conducted in 1987 and 1991, covering the centre of the island. As a result of these works, four major areas of interest were defined, and the gradient well, Tenerife-1, was drilled in the north-west of the island. The geothermal exploration in the Canary Islands stopped in 1993. In 2007 Petratherm S.L and the Technological Institute of Renewable Energy (ITER) started a new project encouraged by the geothermal potential revealed by the previous studies. The collection of vintage data, as well as new geostructural, geochemical and geophysical surveys, were the main activities conducted in this new research stage (Hidalgo, 2010). Tenerife was, once again, the main target of the geothermal investigation. During this period several MT surveys were conducted in the island, in 2009, 2012 and 2013. The resulting 3D resistivity model images the internal structure of the island help us to characterize the seal/reservoir system.
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The leading role of geophysical methods in the discovery of the La Magdalena VMS deposit in the Pyrite Belt, Huelva
Authors A. Granda, T. Granda, J.M. Pons and J.C. VideiraThe La Magdalena deposit is located in the central, northern part of the IPB (Figure 1) in an area where the three main regional lithostratigraphic units of the Belt are well represented. The stratigraphic package is complicated by deformation from the different tectonic events resulting in a degree of ‘disorder’ in the units on the geological map as a result of this tectonic complexity. As a result of the tectonic deformation the oldest rocks corresponding to the PQ Group (phyllites and quartzites) appear thrusted over the VS (Volcano-sedimentary complex) in the northern limits of the prospect, but also appear as a thin band in the western half of the study area. To the south and away from the area of La Magdalena there are extensive areas of Lower Carboniferous Culm rocks, which are the youngest stratigraphic units and comprised of phyllites and greywackes. Between the Lower Carboniferous Culm in the south and the Devonian PQ in the north, the VS Complex outcrops and is well developed in the prospect area. The VS Complex comprises of mainly volcanic rocks (lavas and pyroclastics) with the rare appearance of sediments and it is in this VS sequence where many of the sulphide deposits are hosted including Aguas Teñidas, Esperanza and La Magdalena. The local geological map around La Magdalena deposit is shown in Figure 2.
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Indirect geophysical characterization of geohazards in mantled karst environments in the Ebro Basin
More LessKarst hazards in the central Ebro Basin are related to subsidence and collapses owing to evaporitic rocks located several tens of metres below the surface. In this type of karst (mantle karst) cavities propagate upwards through a non-soluble rock, in this case Quaternary alluvial deposits. The typical geological series comprises a heterogeneous alluvial unit underlain by a soluble, mainly gypsiferous, substratum and several water tables that usually present high conductivity variations. The highest concentration of karstic evidence is identified along the fluvial flood plain of the Ebro River where clayey deposits dominate at the surface. These boundary conditions produce complex environments for the evaluation of geophysical data related to i) karst activity, ii) the presence of cavities below the water table and within the substratum and iii) the variable sedimentary architecture of fluvial deposits. During the past ten years different geophysical techniques have been used in order to characterize karst hazards in the Central Ebro Basin. These experiences have enabled the evaluation resolution, discrimination characterization and finding of karstic evidence using different approaches. The main success in the application of these geophysical techniques has been the quantification of karst hazards from the record of subsidence processes through changes in density, magnetic susceptibility, apparent conductivity and structure of the alluvial deposits. However, the identification of cavities below the alluvial series still represents a serious handicap wing to nonunivocal interpretations and low resolution or penetration of geophysical techniques.
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Empirical correlation of shear wave velocity (vs) with spt of soils in Madrid
More LessEmpirical correlations are usually used as a predictive tool in geotechnical engineering. However, equations calculated for soils very different to the ones to be characterized are frequently used, and so they are not representative of their mechanical properties. This fact, added to the increasing interest of civil engineering in knowing the shear wave velocity (Vs) of the ground, has led to the calculation of different empirical equations to predict the Vs value of the soils of Madrid. In this study this has been achieved by calculating the empirical correlations between the Vs value obtained through the ReMi (Refraction Microtremor) technique and the Standard Penetration Test (500 NSPT values). The empirical correlations proposed are applicable to the whole metropolitan area of Madrid, and have an excellent predictive capability owing to the incorporation of the measurement depth to the equations, which has an important influence in the resistance properties of soils. It is always better to have data collected in the studied site. But often there are too many difficulties to carry out some of these surveys. It may also happen that the surveys take too long or are too expensive to carry out. In these situations, empiric equations can be used to estimate this data. In geotechnical engineering, these empiric correlations are used frequently as a predictive tool, especially in the project’s design primary phases, when vast extensions of terrain are to be characterized in a short period of time, and to define if the soil fits a specific purpose. Because of this factor, there are plenty of estimations published that link between them different types of mechanical properties and geotechnical parameters.
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Non-destructive geophysical characterization of cultural heritage buildings: applications at Spanish cathedrals
Authors Mahjoub Himi, Vega Pérez-Gracia, Albert Casas, Oriol Caselles, Jaume Clapés and Lluís RiveroThe exploration, conservation, and management of cultural heritage are of paramount importance. In Spain, much effort is devoted to exploring, protecting, and managing archaeological sites, historical buildings, museums and, in the case of our research, cathedrals. Spain is home to 44 World Heritage Sites (39 of them cultural) nominated by Unesco – only Italy and China have more. Cathedrals became the most prominent buildings in European cities in the Middle Ages. Architectural styles range from pre-Romanesque to Baroque and Neoclassical. Undeniably, one of the most important styles of the architecture of cathedrals was Gothic. The slender columns, high walls, elaborate rose windows, ceiling medallions, and colourful stained glass windows are some of their typical elements. Some of these structures are of stone and some of brick. Unfortunately, cathedrals have deteriorated owing to age and weather. In addition, earthquakes or fires have caused structural damage. In most cases, cathedrals are built over important buildings of previous civilizations and some structures could remain hidden under the present construction. In other cases, Roman or Greek temples, or Arabian mosques were partially demolished or enlarged to be transformed into cathedrals.
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Volumes & issues
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Volume 42 (2024)
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Volume 41 (2023)
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Volume 40 (2022)
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Volume 39 (2021)
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Volume 38 (2020)
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Volume 37 (2019)
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Volume 36 (2018)
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Volume 35 (2017)
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Volume 34 (2016)
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Volume 33 (2015)
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Volume 32 (2014)
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Volume 31 (2013)
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Volume 30 (2012)
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Volume 29 (2011)
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Volume 28 (2010)
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Volume 27 (2009)
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Volume 26 (2008)
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Volume 25 (2007)
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Volume 24 (2006)
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Volume 23 (2005)
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Volume 22 (2004)
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Volume 21 (2003)
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Volume 20 (2002)
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Volume 19 (2001)
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Volume 18 (2000)
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Volume 17 (1999)
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Volume 16 (1998)
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Volume 15 (1997)
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Volume 14 (1996)
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Volume 13 (1995)
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Volume 12 (1994)
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Volume 11 (1993)
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Volume 10 (1992)
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Volume 9 (1991)
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Volume 8 (1990)
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Volume 7 (1989)
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Volume 6 (1988)
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Volume 5 (1987)
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Volume 4 (1986)
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Volume 3 (1985)
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Volume 2 (1984)
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Volume 1 (1983)