Spatially mutually constrained inversion of hydrogeophysical surveys for water resource interest zone in eastern Denmark

Denmark is currently undertaking extensive groundwater mapping surveys in designated water interest areas. The mapping objectives include vulnerability assessments, delineation of existing aquifers, and localization of new deeper lying aquifers. Airborne electromagnetics are employed for regional mapping to depths of hundreds of meters. Resistivity profiling and pulled-array geo-electrics are utilized to map local scale near surface hydrogeological conditions. Geo-modeling platforms have been developed in Denmark with the explicit purpose of carrying out detailed geological models through integrated analyses of geological and geophysical data visualized in the same data model space. These geological models form the basis for hydrostratigraphic models used for subsequent groundwater models for each of the water resource zones. The groundwater mapping program is scheduled to be completed in a few years so therefore there is a high priority to collect a dense amount of data and develop methods for interpreting these large data quantities so they can be more readily utilized in geo-modeling. This work presents results of hydrogeophysical surveys and Spatially Mutually Constrained Inversion (SMCI) of the results from the 570 km2 large Suså Ringsted water resource interest zone located in eastern Denmark. The site geology is relatively complex characterized by a succession of Quaternary sediments of meltwater sands and moraine tills, faulting systems, and buried valley systems. Hydrogeophysical surveys carried out in this zone include 2576 line-km of airborne electromagnetics, 250 line-km of Electrical Resistivity Tomography and 38 line-km of P-wave and S-wave reflection seismic. Additional geophysical data includes 39 geophysical logs. Within the zone there are over 1600 boreholes with lithological information. The geophysical and geological data have been inverted applying the SMCI method. The SMCI involves mutually constraining different types of geophysical sounding data (electromagnetic, geo-electrical) in 3D space using borehole lithology and geophysical logs as the basis for starting models. The purpose of the SMCI method is to enhance the reliability of the inversion results and thereby optimizing time and quality of subsequent geological interpretation where significant geophysical data sets are present. The SMCI result has been used as the main basis for the detailed hydrostratigraphic model made of the area.