Integrated use of geo-electrical methods for subsurface salinity distribution mapping

Geophysical methods are established in groundwater investigations (Meju et al., 1999, Meju, 2000); their importance is underpinned by the fact that (1) groundwater distribution is controlled by mapable geological factors, (2) groundwater quality is controlled by geochemical factors, and (3) rock resistivity is inherently related to porosity, fluid content and chemistry. The electrical conductivity of the subsurface is highly influenced by dissolved solids in groundwater making electrical and electromagnetic (including GPR) methods indispensable in groundwater quality studies. Because of this, the use of electric and electromagnetic methods in hydrological research in coastal areas is well established for a long time (van Dam, 1976; Boekelman, 1991). These methods were used to obtain information on the vertical changes in the subsurface resistivity at a limited number of points on the surface (vertical soundings), or on the lateral changes at a fixed apparent depth (profiling). Both are in fact one-dimensional methods. Modern electric earth resistivity meters allow us to make true two- and three-dimensional images of the subsurface resistivity distribution. Modern computers and software allow the construction of these 2D and 3D models within a reasonable amount of computation time compared with acquisition time. Depending on the size of the research area, the use of these modern methods is feasible if used in an integrated way. We have investigated this approach to map the subsurface salinity distribution in de Nieuwe Keverdijkse Polder, NKP, in the Netherlands. As the area of investigation is approximately 15 km2, a first quick survey is carried out with a fixed frequency, fixed coil-spacing induction method (EM34), then at several locations in-situ water conductivities are measured, based on these combined results a number of detailed multi-electrode electric resistivity line surveys. To calibrate the results of these surveys, the in-situ water conductivities are translated to bulk resistivity values using the formation factor of the soil. The main purpose in this study was to investigate the added value of this integrated approach in the detailed mapping, at a scale of several meters, of the subsurface fresh and brackish water distribution. These recent developments enable the hydrologists to reconstruction of the fresh and brackish water distributions to a level of detail that is unprecedented (Post et al., 2002).