Structural mapping of large aquifer structures

Abstract

In this presentation we show results from a large hydrogeophysical survey carried out in Vendsyssel in the northernmost part of Jutland, Denmark. In the search for groundwater resources the survey clearly demonstrates the effectiveness of using cost-efficient transient electromagnetic (TEM) methods in combination with geological evaluations of existing borehole data and other geological information. Transient EM (TEM) is commonly used in hydrogeophysical investigations as a fast and relatively cheap screening method of the subsurface. The method yields a high resolution of layers with low resistivity making it suitable for delineation of high resistivity aquifers bounded by low resistivity clays, or for mapping fresh water - salt water interfaces. Recently a new helicopter-borne TEM system, SkyTEM, have been developed utilizing a fast and accurate mapping of large survey areas. The methodology used in the survey comprises five phases: Phase 1: Phase 1 involves an extensive evaluation of existing geological information. This information originates mainly from existing boreholes in the form of geological samples and geophysical logs. Phase 2: Areas of interest are chosen for extensive mapping with electromagnetic geophysical methods. Phase 3: Cross sections and geophysical thematic maps are produced based on the results from Phase 2. The information is entered into a geological/hydro- geological model for the area. Phase 4: The geological/hydrogeological model forms the basis for pointing out sites for supplementary investigation drillings or geophysical measurements. Phase 5: A final model is produced enabling hydrological modelling of the groundwater flow and assessment of the extent and vulnerability of the groundwater resources. The described methodology is cost-efficient as the number of the generally expensive drillings is cut down to a minimum. Also, the methodology is general in all aspects and directly applicable to sedimentary areas worldwide.