Electrical resistivity tomography revealing the internal structure of monogenetic volcanoes

Eruptive activity of individual monogenetic volcanoes usually lasts a few days or weeks. However, their short lifetime does not always mean that their dynamics and structure are simple. Monogenetic cones construction is rarely witnessed from the beginning to the end, and conditions for observing their internal structure are hardly reached. We provide high-resolution electrical resistivity sections (10m electrode spacing) of three monogenetic cones from northeastern Spain, comparing our results to geological observations to interpret their underground continuation. The 100m maximum depth of exploration provides information on almost the entire edifices, highlighting the relationships between Strombolian and hydromagmatic deposits in two multiphase edifices. A main observation is a column of distinct resistivity centered on the Puig d"Adri volcano, which we interpret as the eruptive conduit. This method can provide valuable information on the past volcanic dynamics of monogenetic volcanic fields, which has real implications for the forecast of future activity

ERT monitoring of two Managed Aquifer Recharge ponds in the alluvial aquifer of the Llobregat River (Barcelona, Spain)

Over the past twenty years, there has been growing interest in the use of the subsurface for water storage using shallow ponds, where water is infiltrated to the subsurface and later groundwater is recovered from pumping wells. This scheme is designed as surface Managed Aquifer Recharge (MAR). Llobregat artificial recharge ponds are MAR systems located in alluvial aquifers near Barcelona with strong significance for water supply to the city. The recharge ponds showed low infiltration rates since the beginning (Ca n'Albareda) and a significant decrease after some months (Sant Vicenç). Consequently, different methodologies were designed for monitoring the systems and evaluating the effectiveness of the selected areas and maintenance procedures. For this purpose, we combined the use of Electrical Resistivity Tomography (ERT) with standard hydrogeological methodologies, including water table monitoring from piezometers and infiltration tests. The combination of direct and indirect methods have allowed us to improve the diagnosis of the subsurface involved in the managed recharge system. The ERT technique has shown to be a cost‐effective and high‐resolution tool, flexible and well adaptable for surveying at different scales without disturbing the recharge process. As a consequence, we demonstrate the usefulness of ERT imaging to unveil hydrogeological heterogeneities and monitoring infiltration, clogging effect and clean‐up processes in surface MAR projects

Geophysical Characterization of Hydraulic Properties around a Managed Aquifer Recharge System over the Llobregat River Alluvial Aquifer (Barcelona Metropolitan Area)

Managed aquifer recharge using surface or regenerated water plays an important role in the Barcelona Metropolitan Area in increasing storage volume to help operators cope with the runoff variability and unexpected changes in surface water quality that are aggravated by climate change. The specific aim of the research was to develop a non-invasive methodology to improve the planning and design of surface-type artificial recharge infrastructures. To this end, we propose an approach combining direct and indirect exploration techniques such as electrical resistivity tomography (ERT), frequency domain electromagnetics and data from double-ring infiltration tests, trial pits, research boreholes and piezometers. The ERT method has provided much more complete and representative information in a zone where the recharge project works below design infiltration rates. The geometry of the hydrogeological units and the aquifer-aquiclude contact are accurately defined through the models derived from the interpretation of ERT cross-sections in the alluvial aquifer setting. Consequently, prior to the construction of recharge basins, it is highly recommended to conduct the proposed approach in order to identify the highest permeability areas, which are, therefore, the most suitable for aquifer artificial recharg

Resiliencia en sistemas dunares litorales altamente antropizados: la restauración del sistema dunar de la playa del Remolar (Viladecans, Barcelona)

Con el fin de recuperar y restaurar el sistema playa-duna del Remolar (700 m de longitud y más de 100 m de ancho, de dirección 070N), se propusieron en 2004 una serie de medidas de restauración blanda. Para valorar dichas medidas, transcurridos 10 años, se efectuó un estudio de la morfología, sedimentología y de la vegetación del ecosistema. Se realizó el levantamiento de perfiles topográficos y de GPR, un análisis granulométrico y un análisis de las comunidades vegetales encontradas a lo largo de los perfiles topográficos. Los datos obtenidos han sido comparados con datos de un estudio previo, realizado en el 2004. Los resultados muestran que la morfología dunar se ha recuperado, originándose una nueva duna primaria. El sistema presenta actualmente un proceso mayor de agradación que de progradación. La vegetación ha recuperado una composición global de sistema dunar, con una comunidad típica de duna embrionaria y otras de duna primaria, en bandas paralelas a la costa. A pesar de esta mejora, la componente de tipo oportunista y ruderal de la vegetación de duna primaria denota una fuerte herencia antrópica en el sistema

Assessment of the Restoration of the Remolar Dune System (Viladecans, Barcelona): The Resilience of a Coastal Dune System

The Remolar beach-dune system (700 m long and more than 100 m wide, 070N direction) borders a campground that was closed (2003), due to the Barcelona airport expansion. In order to recover and restore the dune ecosystem, a series of soft measures were performed. After 10 years, a study of the morphology, sedimentology, and vegetation of the ecosystem was carried out to evaluate the results of these measures. For this purpose, a series of topographic and groundpenetrating radar (GPR) profiles, grain-size analysis, and an analysis of plant communities found along the profiles were carried out. The data obtained were compared with data from a former 2004 study. The results show that the morphology of the dunes recovered, and a new primary dune has arisen. The system now has a greater process of aggradation than of progradation. The vegetation has recovered the global composition of dune systems, with a typical community of embryo dunes and others of primary dunes that are set in strips parallel to the coast. Despite this improvement, the opportunistic and ruderal component in the primary dune vegetation evidences a strong anthropic inheritance in the system

Enhanced Groundwater Protection and Management Using Gravity and Geoelectrical Data (Valls Basin, Spain)

The basis for the protection and prevention of groundwater pollution lies in the accurate assessment of vulnerability in terms of the exposure of groundwater bodies to contaminants before they are potentially discharged into the environment. The vulnerability assessment consists of calculating the ease with which pollutants can reach the aquifer from the surface through the vadose zone, which effectively reduces the pollutant load when the transit time is long. Index methods are mostly used, as they are based on input data that are readily available, easy to implement and interpret, and which are simple and practical. However, there are also limitations, as some methods are somewhat subjective and provide only a qualitative approximation. This case study aims to develop a methodology that can quantitively estimate the hydrogeological parameters of the aquifer formations of the Valls basin using geophysical methods and the Dar Zarrouk parameters. The specific treatment carried out on data from gravity stations and vertical electric soundings, supported by the available well data, allows for the delineation of the most favourable areas for the exploitation of groundwater resources (higher hydraulic transmissivity) and the areas most susceptible to pollution (with a shorter transit time) on a regional scale. Geophysical methods have proved useful, sustainably providing valuable information without the need to drill new boreholes that could act as preferential pathways for pollutants into the aquifer

Relación entre las resistividades y la mineralogía ¿litofacies de explotaciones activas de yeso

El sulfato de calcio, y en particular su fase hidratada, el yeso, es uno de los minerales evaporíticos más frecuentes en la naturaleza y el más abundante en afloramiento, debido principalmente a su menor solubilidad relativa en comparación con el resto de minerales evaporíticos

Estudio comparativo de diferentes técnicas geofísicas para el control de la intrusión salina en acuíferos costeros: Aplicación al acuífero de Oued Laou (Marruecos)

Groundwater is a fundamental and strategic resource for water supply of coastal areas, especially in the Mediterranean basin where the irregularity of precipitation, both seasonally and year-onyear, put at risk the availability of surface water in the rivers, which go from being dry for long periods to causing torrential discharges. In particular, if the exploitation of coastal aquifers is not properly managed and the quality of groundwater is not monitored regularly, important sectors of the aquifer may be affected by saltwater intrusion, and cause the abandonment of the wells. Geophysical methods are a very effective complement to provide indirect information on groundwater salinity in areas of the aquifer where wells or piezometers are not available for sampling. This study presents the results obtained from the application of different geophysical methods in the coastal aquifer of Oued Laou (Morocco) as a complement to hydrochemical data. Four geophysical methods, two electrical: vertical electrical sounding and electrical resistivity tomography, and two electromagnetic: frequency domain EM and time domain EM sounding. The comparative analysis of the results obtained by each method has made it possible to assess the coherence and complementarity of each method, as well as their corresponding advantages and limitations in terms of resolution and cost-effectiveness

Characterization of a Shallow Coastal Aquifer in the Framework of a Subsurface Storage and Soil Aquifer Treatment Project using Electrical Resistivity Tomography (Port de la Selva, Spain)

Water percolation through infiltration ponds is creating significant synergies for the broad adoption of water reuse as an additional non-conventional water supply. Despite the apparent simplicity of the soil aquifer treatment (SAT) approaches, the complexity of site-specific hydro-geological conditions and the processes occurring at various scales require an exhaustive under-standing of the system's response. The non-saturated zone and underlying aquifers cannot be considered as a black box, nor accept its characterization from few boreholes not well distribut-ed over the area to be investigated. The electrical resistivity tomography (ERT) is a non-invasive technology, highly responsive to geological heterogeneities that has demonstrated useful to provide the detailed subsurface information required for groundwater modeling. The relation-ships between the electrical resistivity of the alluvial sediments and the bedrock and the differ-ence in salinity of groundwater, highlight the potential of geophysical methods over other more costly subsurface exploration techniques. The results of our research show that ERT coupled with implicit modeling tools provides information that can significantly help to identify aquifer geometry and characterize the saltwater intrusion of shallow alluvial aquifers. The proposed approaches could improve the reliability of groundwater models and the commitment of stakeholders to the benefits of SAT procedures

Application of Resistivity and Seismic Refraction Tomography for Landslide Stability Assessment in Vallcebre, Spanish Pyrenees

Geophysical surveys are a noninvasive reliable tool to improve geological models without requiring extensive in situ borehole campaigns. The usage of seismic refraction tomography (SRT), electrical resistivity tomography (ERT) and borehole data for calibrating is very appropriate to define landslide body geometries; however, it is still only used occasionally. We present here the case of a Spanish Pyrenees slow-moving landslide, where ERT, SRT and lithological log data were integrated to obtain a geological three-dimensional model. The high contrasts of P-wave velocity and electrical resistivity values of the upper materials (colluvial debris and clayey siltstone) provided accurate information on the geometry of the materials involved in the landslide body, as well as the sliding surface. Geophysical prospecting allowed us to identify the critical sliding surface over a large area and at a reduced cost and, therefore, gives the geophysical method an advantage over borehole data. The three-dimensional model was used to carry out stability analyses of a landslide in 2D and 3D, which, coherently with previous studies, reveal that the lower part is more unstable than the upper units