Location of buried valley aquifers using transient electromagnetic soundings

In Denmark, 99% of the water supply is based on groundwater abstraction. Increasing problems with the groundwater quality due to urban growth and agricultural contamination in 1995 caused the Danish Minister of Environment to approve a plan for improving ground water protection. In July 1998, the Danish Parliament decided to launch an ambitious plan to significantly intensify the hydrogeological investigation of the Danish groundwater resources. This plan runs for the next 10 years and includes all areas characterized as groundwater recharge and aquifer areas, a total of 16,000 km2 or 1/3 of the country. In Denmark, a large number of aquifers are composed of Quaternary sand and gravel deposited in deep valleys eroded in Tertiary clays. The valleys are often covered by an inhomogeneous moraine cap. In order to protect the aquifers, it is necessary to delineate the buried valleys and to locate permeable sandy areas in the inhomogeneous clay cap. It is generally observed that variations in hydraulic conductivity of these formations are closely related to the variations in electrical formation resistivity. Numerous investigations during the nineties showed that the application of electrical and electromagnetic methods (Christensen N.B. and Sørensen K.I., 1998, McNeill, J. D., 1990, Meju, M. A., 1999) was successful and provided the hydrogeologists with detailed information on location of the buried valleys and the inhomogeneities of the protecting surface clays. In Denmark, the largest investigation of this kind so far is the transient electromagnetic (TEM) mapping of the municipality of Aarhus (Sørensen et al. 2003). The survey area is approximately 120 km2. In the period 1991 - 1998, more than 6000 TEM soundings were acquired with a density of 16 per km2. In 2000, the data were reinterpreted to ensure uniformity using current processing, modeling and inversion techniques. The investigation was carried out by cooperation between the Municipal Water Supply of Aarhus, the County of Aarhus and the Department of Earth Sciences at the University of Aarhus. The Department conducted the measurement, the data processing and the geophysical interpretation of the soundings, and the hydrogeological interpretation was performed by the County of Aarhus. The cost of the survey was approximately 1 million euro. In this presentation we give an overview of the gained experience and the new techniques developed through the investigation period. On a general level, it is development of standards which ensure enhanced data density and reliability, increased acquisition efficiency as well as standardization of mapping methods in a region. The latter provides the precise, detailed mapping needed to determine the extent, vulnerability and water quality of aquifers. The following geophysical developments have taken place: - Development of field methodologies for operating the TEM method in densely inhabited and culturally developed areas (Sørensen et al. 2003). - Development of new and enhanced TEM instrumentation. These are the PATEM method (Sørensen, 2000) and the HiTEM method (Danielsen et al., 2002). - Development of new data processing and interpretation software packages. This is enhanced modeling of instrument transfer functions (Effersø et al., 1999), MCI and LCI interpretation (Auken et al., 2001, Auken et al., 2000) and instrument calibration (Halkjaer et al. 2002).