Characterization of a coastal aquifer using seismic and geoelectric borehole methods
H. Maurer, S. Friedel and D. Jaeggi
Journal name: Near Surface Geophysics
Issue: Vol 7, No 5-6, October 2009 pp. 353 - 366
Special topic: Hydrogeophysics - Methods and Processes
Info: Article, PDF ( 11.6Mb )
Price: € 30
To characterize a coastal aquifer in southern Mallorca (Spain), we have acquired geoelectric and seismic data. The area of investigation lies within a Miocene reefal carbonate platform consisting of lagoon sediments, reef cores and talus sediments. Since the groundwater table was at about 40 m depth (~1 m a.s.l.), the delineation of small-scale structural features (1–2 m) within the aquifer required the use of borehole methods. Geoelectric single-hole tomography was performed in six ~100 m deep boreholes. This fast and inexpensive technique provided horizontal resolution well beyond the limits of traditional logging techniques. Furthermore, the tomographic inversion results remained virtually unaffected by the insertion of slotted PVC casing tubes and comparisons with electromagnetic induction logs showed an excellent match of resistivities in regions near to the boreholes. In addition to the single-hole measurements, geoelectric and seismic cross-hole data were acquired. Combined analyses of all data sets showed the reef core to be particularly susceptible to karstification and recrystallization. These two processes have caused substantial variations in the porosity. Comparison of fluid logs and electrical resistivity versus depth and seismic velocity versus depth functions along all boreholes revealed a surprisingly high correspondence. This unexpected correlation of electrical resistivity (governed by the fluid salinity) and seismic velocities can be explained by geochemical processes occurring in the saltwater-freshwater mixing zone and/or near the groundwater table. Such processes need to be considered for establishing reliable groundwater flow models.