Integrating Hydrogeological and Geophysical Methods for the Characterization of a Deltaic Aquifer System

Groundwater management needs detailed aquifer characterization, especially in semiarid costal aquifer systems that are under hydrological pressure. Our study area is in the Tordera delta, northeastern coast of Spain, where a detrital fluvio-deltaic aquifer system has been developed above granitic basement. The main purpose of this study is to characterize the complex lithological structure and the seawater intrusion state by combining hydrological information, audiomagnetotelluric (AMT) and seismic reflection and refraction models. This allowed us to provide spatially continuous information about aquifer properties and processes. Thus, we have determined the thickness and continuity of the aquifer units, as well as the morphology and depth to the basement. The models revealed that the main seawater intrusion main path is found in the western deltaic area that coincides with an existing buried paleochannel. This new result explains the anomalously high chlorine concentrations observed in the deep semiconfined aquifer more than 1,500 m inland.     

Feasibility of Monitoring the Hontomín (Burgos, Spain) CO2 Storage Site Using a Deep EM Source

Geophysical methods have been used experimentally during the last decade, a period of strong development, being adopted as complementary techniques for characterizing and monitoring hydrocarbon and gas reservoirs. In this study, we evaluated the ability of the controlled source electromagnetic (CSEM) method to monitor the storage of CO₂ at the Research Laboratory on Geological Storage of CO₂ at Hontomín (Burgos, Spain). Two aspects of the CSEM monitoring were examined considering the geoelectrical structure at the site, the technological constraints and the noise conditions of the Hontomín area. Borehole-to-surface simulations were performed to evaluate the detectability of the resistivity changes in the reservoir and the capacity to determine the location of the CO₂ plume. The synthetic time-lapse study explores the possibilities of CSEM monitoring with a deep electric source. Three depths of the source are analyzed: above the plume, inside the plume, and beneath the stored CO₂. In terms of the Hontomín storage site, the study confirmed that a deep electric source located beneath the injection depth can provide valuable information on the behavior of the stored CO_2.     

The North Maladeta Fault (Spanish Central Pyrenees) as the Vielha 1923 earthquake seismic source: Recent activity revealed by geomorphological and geophysical research

The Spanish Central Pyrenees have been the scenario of at least two damaging earthquakes in the last 800 years. Analysis of macroseismic data of the most recent one, the Vielha earthquake (19 November 1923), has led to the identification of the North Maladeta Fault (NMF) as the seismic source of the event. This E–W trending fault defines the northern boundary of the Maladeta Batholith and corresponds to a segment of the Alpine Gavarnie thrust fault. Our study shows that the NMF offsets a reference Neogene peneplain. The maximum observed vertical displacement is  730 m, with the northern downthrown sector slightly tilting towards the South. This offset provides evidence of normal faulting and together with the presence of tectonic faceted spurs allowed us to geomorphically identify a fault trace of 17.5 km. This length suggests that a maximum earthquake of M_w = 6.5 ± 0.66 could occur in the area. The geomorphological study was improved with a resistivity model obtained at Prüedo, where a unique detritic Late Miocene sequence crops out adjacent to the NMF. The section is made up of 13 audiomagnetotelluric soundings along a 1.5 km transect perpendicular to the fault trace at Prüedo and reveals the structure in depth, allowing us to interpret the Late Miocene deposits as tectonically trapped basin deposits associated with normal faulting of the NMF. The indirect age of these deposits has been constrained between 11.1 and 8.7 Ma, which represents a minimum age for the elevated Pyrenean peneplain in this part of the Pyrenees. Therefore, we propose the maximum vertical dip-slip rate for the NMF to be between 0.06 and 0.08 mm/a. Normal faulting in this area is attributed to the vertical lithospheric stress associated with the thickened Pyrenean crust.