Combinig fiber optic DTS, cross-hole ERT and time-lapse induction logging to characterize and monitor a coastal aquifer

Abstract

The characterization of saline water intrusion (SWI) and its hydrodynamics is a key issue to understand sub-marine groundwater discharge (SGD) and manage groundwater resources in coastal areas. To test and comparedifferent methods of characterization and monitoring, a new experimental site has been constructed in a coastalalluvial aquifer north of Barcelona city (Catalonia, Spain). The site is located between 30 and 90 m from theseashore and comprises 16 shallow piezometers organized in nests of three with depths ranging between 15 and25 m and 4 solitary piezometers. The objective of this paper is to combine different recently developed mon-itoring techniques to evaluate temporal variations in the aquifer hydrodynamics of the site at different spatialscales before and after the dry season of 2015. At the site scale,fibre optic distributed temperature sensing (FO-DTS), for thefirst time applied to study SWI, and cross-hole electrical resistivity tomography (CHERT) has beenapplied. At the meter/borehole scale, electrical conductivity of the formation has been applied not only in arepeated manner (¿time lapse¿), but also for thefirst time at relatively high frequency (1 sample every 10 min).CHERT has provided a better characterization of the seawater intrusion than electrical conductivity data ob-tained from piezometers. The combination of techniques has allowed improving the understanding of the systemby: 1) characterizing the extent and shape of SWI; 2) differentiating two different dynamics in the aquifer; and 3)identifying preferentialflow paths over different time and spatial intervals. Future challenges and the appli-cation of these techniques in other areas are also discussed.