Coastal Aquifer Characterization Based on Seismic and Resistivity Imaging Methods: a Case Study from Costa Brava, Spain

Coastal aquifer formations located in Northeast of Spain affected by saline intrusion has been the focus for a multi-technique geophysical study. Two aquifers are present: a shallow aquifer in the quaternary sediments and a deeper karstic aquifer. The objectives were: (1) estimate quaternary sediment thickness and characterize weathered/fractured rock (2) qualitative assessment of water degradation changes within the study area. The methodology consisted of combining and integrating seismic and resistivity imaging techniques: seismic refraction tomography (SRT), electrical resistivity tomography (ERT) and H/V microtremor technique. Three ERT and SRT coincident profiles have been acquired from sea to inland. Comparison of velocity and electrical models has allowed obtaining quaternary and weathered/fractured bedrock thickness. Electrical resistivity differences from one to another profile help mapping changes in saline intrusion. H/V technique has been suitable to obtain bedrock depth in the profile close to sea. However, strong attenuation of the microtremor signal has precluded its application in the rest of the study area. This attenuation could be related to the dissipation of seismic energy in moving fluids through a thick zone of fractured/weathered bedrock. All this information is critical in order to establish preferential water flows and groundwater degradation of the aquifers.