L. Rivero

INTEGRATED GRAVITY AND SEISMIC INTERPRETATION OF DUPLEX STRUCTURES AND IMBRICATE THRUST SYSTEMS IN THE SOUTHEASTERN PYRENEES (NE SPAIN)

Abstract Combined surface, subsurface and gravity data were used to determine the deep structure of the thrust belt in the southeastern Pyrenees. The Ripolles and La Garrotxa areas are part of the Cardona oil exploration permit. Hydrocarbons pervade the concession with numerous oil seeps, abandoned oil wells, and gas or condensate are in these two wells associated with the overthrust belt. The source rocks, the Armancies formation (upper Cuisian-lower Lutetian), consist of alternating layers of organic-rich shales and thin limestones; this formation outcrops in the Cadi thrust sheet. In the Ripolles area, the Cadi unit overlies the Serrat unit with a hanging-wall ramp geometry, as deduced from the Serrat-1 well. The structure of the Garrotxa area is more complex. It consists of a duplex structure folded by antiformal stack structures with basement rocks in their cores. The northern boundary of these antiformal stacks represents an inversion of pre-existent extensional faults. The basement rocks involved in the structure represent shortcuts formed in the footwall of the extensional faults. A negative anomaly is shown in these antiformal stacks. These anomalies are interpreted as a combination of salt accumulation, evaporites and sedimentary rocks, under the antiformal stacks. In the Serrat-1 well, the Serrat unit is made up of a thick Middle Eocene evaporitic sequence. But the gravity and seismic data evidence the presence of non-evaporitic sedimentary rocks in the northern part of the Serrat unit. These sediments correspond to the footwall of the Cadi series, so the probable existence of the Armancies formation in the Serrat unit is a very good target for oil prospecting. The existence of the previous extensional faults only in the Garrotxa area, determines the difference in the deep structure between the Ripolles and Garrotxa areas.

Moho depth structure of the eastern part of the Pyrenean belt derived from gravity data

Abstract This paper proposes a new and updated interpretation of the gravity map of the eastern part of the Pyrenean belt based on the well-established correlation existing between gravity anomalies and crustal structure. A new set of gravity data, collected over the past few years by the University of Barcelona both for academic purposes and hydrocarbon exploration, have been merged with the data previously available. First a quantitative interpretation of the crust’s thickness has been estimated by establishing a local empirical relationship between Bouguer anomaly and the Moho depth calculated from deep seismic refraction and reflection profiles. Then, a spectral analysis of the radial wavenumber of the Bouguer anomaly values using the FFT and a 3D inversion has been performed. Finally, a 2 1 2 D density model from a SE–NW gravity profile linking the southern edge of the coastline with the northern part of the ECORS–Pyrenean profile was also established.

Teaching oriented geophysical software

Abstract Interactive teaching techniques encourage students to adopt an active role in their education and should therefore be used at different levels of the teaching sequence. In order to mitigate the lack of educational software for Applied Geophysics, a fully interactive graphic software has been developed. The program is written in Visual Basic with some subroutines in FORTRAN and is designed for IBM-PC microcomputers using a Windows environment. The program offers the majority of the processes involved in geophysical data handling, modelling, tutorials, and instrument simulators.

Río de la Plata inner estuary (Argentina): main forms, sedimentary infill and Holocene evolution

Abstract The emerged part of the Río de la Plata estuary is characterized by striking coastal geomorphic features. Aerial and satellite images, field observations of the outcrops, shallow boreholes, radiometric dating of sand and shells and Ground Penetrating Radar surveys were analysed. The main sedimentary features are: a large coastal barrier (up of 220 km long), old coastal lagoons, attached beach ridge (about 210) sets distributed along 170 km, the Ibicuy sand body variously interpreted as a reworked shoal/island or delta, and cheniers. The tidal flats and the modern Paraná Delta are located in the centre and on the right side of the inner estuary. The height of the estuarine coastal barrier represents the maximum flooding surface (MFS) of the last transgression at about 6000 aBP. Other main sedimentary forms developed as a falling stage systems tract (FSST) during the sea-level drop between the aforementioned date and the present. The sea surface was about 5 m above sea-level in the inner part of the estuary during the middle-Holocene. This MFS was subsequently followed by a continuous and slow sea-level fall that controlled the generation of the large sedimentary forms developed in the innermost part of the Río de la Plata Estuary.

An evaluation of aquifer vulnerability in two nitrate sensitive areas of Catalonia (NE Spain) based on electrical resistivity methods

The protection of groundwater has become one of the most important European environmental policies as evidenced by the Orders relating to the protection of water from contamination, Directive 2000/60/CE of the European Parliament and the European Union Council, and more concretely Directive 2006/118/118/CE, related to the protection of groundwater from pollution and degradation. Traditional methods for assessing vulnerability include soil surveys, drilling and analysis of lithology logs from wells with the objective of characterising the thickness, hydraulic properties and lateral extend of the protective layers. However, such studies can be labour-intensive and expensive. In addition, the parameters measured may have high spatial variability, which makes accurate characterization over large areas difficult. Fortunately, a numerical index of protection can be assigned from the longitudinal electrical conductance parameter derived from electrical resistivity surveys (VES, ERT or any other electrical or EM method). This can be more accurate and reliable than any other vulnerability index derived only from visual inspection or interpolated from sparse borehole data.

Quaternary over-elevated torrential channels. Characteristics and depositional significance: the Maresme model (Catalonia, NE Spain)

Channel-levee deposits that occur in some large sand-dominated fluvial systems are commonly elevated above the surrounding floodplain. However, the over-elevation processes of small and isolated fluvial channels are poorly documented. The Maresme zone located NE of Barcelona (Spain) offers many examples of small over-elevated channels. This area is characterised by weathered granodiorites forming a thick coarse-grained sand-dominated regolith, which was initially covered by dense forests. In the XVIII and XIX centuries human activity led to the destruction of the vegetal cover, resulting in subsequent erosion and in the intense remobilisation of sediments during storms. Thus, large amounts of sand were transported during historical times. The intermittent discharges were confined to short (few km) and straight channels with high and uncommon gradients from 3.2 to 3.4%. These discharges flowed at high velocities towards the Mediterranean Sea, the regional base-level. High infiltration rates contributed to the accretion of sandy sediments along the channels coevally with levee development. This sandy lithosome, which is usually elevated above the surrounding floodplain, displays a characteristic convex-up cross section.

Hydrogeological and geophysical characterization of fractured aquifer of Òdena (Barcelona, Catalunya)

Summary The results of the geophysical survey reflects the presence of heterogeneity distribution of electrical resistivity values. This allowed to delimit the geometry of the three layers, as indicated by borehole logging. At the same time, it has been possible to highlight several zones with low electrical resistivity values in the central part of some profiles. These have been considered as fractured areas. In many cases, these areas of low resistivity fractures coincide with areas where the wells have registered low values of the electrical conductivity. This fact reveals the effect as preferential flow areas that have these fractures throughout the circulation system of the contaminant.

Conceptual Model for Exploring Geothermal Resources in La Seva Basin (NE Spain) from Integrated Geophysical Data

An integrated geophysical interpretation has been conducted around La Selva basin that show strong geochemical and geologic evidences of hydrothermal with the intent of define a conceptual model for exploration of low and intermediate enthalpy geothermal resources. The geophysical data involved in this study consist of gravity, magnetic and geoelectrical, supplemented by geological and hydrogeochemical information. The interpretation of gravity anomalies has been useful for defining the main structural discontinuities, magnetic map show the existence of several anomalies related to mafic intrusions while geoelectrical prospecting let delineate more conductive areas which could correspond to an increase in thermal fluid circulation. The results obtained by the different geophysical methods are in good agreement and allows defining a reliable geostructural model of the subsurface of the studied area.

Geophysical Evaluation of the Volume of a Mine Tailing Dump (Osor,Girona,NE Spain) Using ERT

Mine tailing ponds are environmentally hazardous because of high susceptibility to leaching and erosion by water and wind. In this study we demonstrates the usefulness of electrical resistivity tomography for assessing volume and internal structure of mine tailing ponds in abandoned Osor mine as significant information for decision-making measures of environmental protection.

Urban Archaeophysics - The Case of Three Cathedrals

Cultural heritage buildings are irreplaceable vestiges of the civilizations. The cathedrals are usually a wide representation of styles and constructive techniques, being clear examples of the development and history of the societies. Geophysical surveys using different non-destructive methods (electrical resistivity tomography, ground penetration radar, frequency electromagnetic domain, passive seismics …) are carried out in many cases to provide information related to these two aspects: existence of older remains under the Cathedral and definition of structural health, detection of possible damage or even, evaluating possible causes of the damage. This paper present three case study in different Cathedrals describing geophysical methodologies used and obtained results.