HR: 14:10h
AN: H23G-03 INVITED     [Abstracts]
TI: The Contribution of Hydrogeophysics to Hydrogeological Modeling
AU: * Christensen, N B
EM: nbc@geo.au.dk
AF: Department of Earth Sciences University of Aarhus, Finlandsgade 8, Aarhus, 8200 N Denmark
AU: Auken, E
EM: esben.auken@geo.au.dk
AF: Department of Earth Sciences University of Aarhus, Finlandsgade 8, Aarhus, 8200 N Denmark
AU: Sorensen, K
EM: kurt.sorensen@geo.au.dk
AF: Department of Earth Sciences University of Aarhus, Finlandsgade 8, Aarhus, 8200 N Denmark
AB: Electrical and electromagnetic (E&EM) methods are some of the most commonly used geophysical techniques for hydrogeophysical investigations. In this presentation, the use of E&EM methods for watershed-scale hydrogeological investigations are reviewed. Over the past two decades a tremendous development has taken place with regard to E&EM instrumentation, field procedures and interpretation algorithms; a process that to a large extent has been focussed on hydrogeological investigations. The primary parameter mapped by E&EM methods is the electrical resistivity (or the inverse: conductivity). High and low values of the resistivity of geological materials enable the discernment between sand and clay, unsaturated and saturated, fresh and salt water, unaffected and polluted, bedrock and sediment, respectively - all fundamental to hydrogeological modeling. Time-consuming, single-site, individual electrical sounding acquisition geometries have now been replaced by multi-electrode, profile oriented measurements that have the capability to image the variation in resistivity with both depth and along profiles to a depth of 70-100m and a productivity of 1-1.5 km/day/field person. Pulled-array methods, which acquire measurements using multiple electrode configurations while moving, can traverse 10-15 km per day with a depth penetration of approximately 20 m. Transient electromagnetic soundings are carried out as both single-site and pulled-array methods, and recently by helicopter. Very cost-efficient transient methods are now commercially available. E&EM data are complicated, nonlinear functions of the resistivity distribution and the full potential of the data can only be realized by inverting the data to obtain a physical model describing the subsurface resistivity distribution. Model calibration and inverse hydraulic modeling is most often carried out based on very sparse data sets and geological information from a few boreholes. Geophysical models covering an extended area support interpolation between the sparse data and can often be decisive in building a hydrogeological model. E&EM models contribute mainly within three areas: defining the geometrical extent of aquifers by locating impermeable boundaries (clay and bedrock), estimating the vulnerability of aquifers to infiltration of unwanted substances from the surface, and in defining the internal structure (permeability and saturation) of an aquifer. We present several different examples of the use of E&EM methods for assisting in hydrogeological investigations at the regional scale in Denmark. These investigations have primarily been used to define the boundaries between permeable (sand) and impermeable (clay), thus pointing to the presence of possible aquifers and reducing the volume of flow modeling. Important aquifers must be protected by public authorities and geophysical models with good surface resolution can be used to support the necessary physical planning by pointing to areas where aquifers are vulnerable, i.e. areas with little or no capping clay. The use of geophysical models to constrain the internal structure of aquifers is the most complicated of the three and is the subject of recent efforts. Even though there is no general functional relationship between hydraulic conductivity and electrical resistivity, there is sometimes a locally valid correlation that can be utilized in a variety of statistical techniques that will correlate higher resistivities with higher hydraulic conductivities, often in the formulation of an inverse hydraulic modeling. Our efforts suggest that E&EM methods have great potential to assist in watershed characterization studies.
DE: 1879 Watershed
SC: Hydrology [H]
MN: Fall Meeting 2005