Sara Jorreto

Automated monitoring of coastal aquifers with electrical resistivity tomography

An Automated time-Lapse Electrical Resistivity Tomography (ALERT) system has been developed for the long-term monitoring of coastal aquifers. This ALERT system has been permanently installed in the River Andarax, Almeria, Spain to monitor and manage the impact of climatic change and land-use practice on the underlying Quaternary aquifer. An electrode array, nearly 1.6 km long, has been buried below the normally dry riverbed with electrode take-outs at regular intervals of 10 m. The maximum depth of investigation is about 160 m below ground level. An unmanned, permanent control station, in a secure location, allows the aquifer to be interrogated remotely from the BGS office in the UK. Volumetric geoelectric images of the subsurface can be obtained ‘on demand’ or at regular intervals; thereby eliminating the need for expensive repeat surveys. The entire process from data capture to image on the office PC is fully automated and seamless. The ALERT technology can provide early warning of potential threats to vulnerable water systems such as over-exploitation, rising sea levels, anthropogenic pollutants and seawater intrusion. The electrical images obtained (in space and time) are interpreted in terms of the hydrogeologic features including the seawater-freshwater interface. The timely detection and imaging of groundwater changes can help to regulate pumping and irrigation schemes.

Characterization of seawater intrusion using 2D electrical imaging

We have investigated the potential of 2D electrical imaging for the characterization of seawater intrusion using field data from a site in Almeria, SE Spain. Numerical simulations have been run for several scenarios, with a hydrogeological model reflecting the local site conditions. The simulations showed that only the lower salt concentrations of the seawater-freshwater transition zone could be recovered, due to the loss of resolution with depth. We quantified this capability in terms of the cumulative sensitivity associated with the measurement setup and showed that the mismatch between the targeted and imaged parameter values occurs from a certain sensitivity threshold. Similarly, heterogeneity may only be determined accurately if located in an adequately sensitive area. At the field site, we identified seawater intrusion at the scale of a few kilometres down to a hundred metres. Borehole logs show a remarkable correlation with the image obtained from surface data but indicate that the electrically derived mass fraction of pure seawater could not be recovered due to the discrepancy between the in-situ and laboratory-derived petrophysical relationships. Surface-to-hole inversion results suggest that the laterally varying resolution pattern associated with such a setup dominates the image characteristics compared to the laterally more homogeneous resolution pattern of surface only inversion results and hence, surface-to-hole images are not easily interpretable in terms of larger-scale features. Our results indicate that electrical imaging can be used to constrain seawater intrusion models if image appraisal tools are appropriately used to quantify the spatial variation of sensitivity and resolution. The most crucial limitation is probably the apparent non-stationarity of the petrophysical relationship during the imaging process.

Constraining Geostatistical Simulations of Delta Hydrofacies by Using Machine Correlation

In some hydrogeology applications, the only subsurface geological information available comes from a small number of boreholes from which hydrofacies have been intersected and identified. Geostatistical simulation is a widely used stochastic technique for generating a set of possible hydrofacies images that cover the range of the complexity and heterogeneity of the structures. However, the uncertainty due to the very sparse data may be significant to the extent that the simulated images cover an unrealistically large range of possibilities for the hydrofacies characteristics. In such cases it may be desirable to constrain the simulations so as to provide a more realistic, or plausible, set of simulations. In the absence of wireline logging, outcrops, geophysics, production data or any other types of hard data, we propose the use of machine numerical correlation between hydrofacies at the boreholes as a means of constraining the range of plausible simulations. The procedure is used to simulate delta hydrofacies in a coastal aquifer in Almeria (Southern Spain) where the variability of the hydrofacies is critical for managing problems related to seawater intrusion.

Hydrological modeling of a semi-arid Andarax river basin in Southern Spain

The 2,265 km2 Andarax river basin is located in Southern Spain. The basin is strongly affected by tectonic activity and coincides with one of the existing depressions in the Betic Cordilleras. The lithology is mostly Triassic schists and limestone in upland areas and Neogene-Quaternary sandstones, conglomerates and loams of marine origin in medium and lower-lying areas. The terrain changes from sea level at the coast to more than 2,500 m in the Sierra Nevada Mountains. It is one of the most arid regions in Europe with a precipitation of 200-350 mm/year, which mainly falls (70%) in autumn and winter. In the mountainous areas, where the unit Triassic schists dominates nearly all precipitation is converted into overland flow which runs directly to the river. When the river reaches the medium and lower-lying areas most of the water infiltrates into the highly permeable Detritic aquifer. Only at rare occasions and for high rainfall events the river discharges into the Mediterranean Sea and consequently nearly all excess rainfall generated from the upstream catchments areas is discharged as groundwater flow which thus determines the water availability in the delta region. The rather unique hydrological behavior of the Andarax river basin is simulated by the MIKE SHE code, which is a physically based, distributed and integrated hydrological model. Particular emphasis is given to the simulation of the seasonal and spatial distribution of recharge within the river catchment. As the availability of traditional hydrological data for the Andarax river basin data is rather sparse remote sensing data will be used to improve the estimation of the spatial and temporal distribution of the evapotranspiration. For this purpose an energy based model included as an option in the MIKE SHE code will be used based on remote sensing data for leaf area index (LAI), global radiation, albedo and air temperature.

Constraining a 2D/3D density dependent saltwater intrusion model using time-lapse electrical imaging data

In groundwater model development, calibration is one of the critical aspects that determine its reliability and applicability in terms of e.g. system (hydrogeological) understanding, groundwater quality predictions, and general use in water resources context. The result of a groundwater model calibration is determined by different factors, where both data quantity and quality is of crucial importance. A density dependent saltwater intrusion model has been established for a coastal aquifer in Almeria, SE Spain, where hydraulic and solute transport parameters had to be calibrated in order to simulate the intrusion dynamics realistically. Furthermore, the sources of saltwater in the coastal aquifer are not unambiguously defined yet but current state of knowledge seems to suggest the possibility of multiple sources e.g. present seawater intrusion, solution of evaporites, fossil waters and anthropogenic influences. Typically the availability of conventional monitoring installations (e.g. depth specific monitoring wells for Chloride measurements) and the cost of constructing new ones limit the amount of obtainable data. These factors can seriously hinder a successful monitoring strategy and consequently limit the reliability of the calibrated model. An effective alternative to conventional measurements is the use of geophysical methods to monitor the saltwater intrusion front. If applied in a time-lapse manner, electrical images give both spatial and temporal information on the salinity distribution compared to conventional methods, which only give (few) point information. As a result a more comprehensive understanding of the hydrogeological system is obtained and a greater confidence in the calibrated model is achieved. In this (case) study from Almeria, we demonstrate how the use of time-lapse electrical imaging data improve the calibration of a density dependent intrusion model compared to using data from conventional (existing) measurements installations. This was achieved by calibrating the model with the different data sets and subsequently evaluate the performance of the model. The results illustrate the benefit of using electrical images for calibration purposes but also its contribution to system understanding and how it, together with modeling, can assist in the identification of saltwater sources in the area.