Using airborne electromagnetics surveys to investigate the hydrogeology of an area near Nyborg, Denmark
A time-domain airborne electromagnetic survey was flown in the County of Fyn, Denmark, immediately west of the city of Nyborg as a quick and efficient means of providing resistivity information for the development of a hydrogeological model. The bedrock geology is typified by a thick limestone overlain by a sequence of marine marl and a Quaternary moraine of variable thickness and composition. The limestone is the largest aquifer within the area and is used to supply water for Nyborg. The upper 100 m or so of the limestone generally contain fresh water and below that the groundwater is saline. The boundary between the resistive fresh water and the conductive saline water was the target of the airborne electromagnetic survey. Man-made structures can strongly impact electromagnetic measurements; however, they can beminimized using two tactics: (1) masking the results around large power lines, and (2) non-linear filtering of any perturbations in the interpretation that are the consequence of these or other structures. The latter strategy is possible because of the dense areal coverage provided by the airborne survey, and is particularly useful when a regional overview is required. Resistivity sections derived from the airborne electromagnetic data indicate a resistive zone of till, diluvial sand and freshwater limestone of variable thickness above a deeper conductive zone of saline water. A conductive layer within the moraine, assumed to be clay or marl, is also occasionally indicated. We used a two-layer model to estimate the depth to the freshwater/saline-water interface. Where ground soundings have been made, the depth estimates from the ground and airborne data agree. The depth estimates from the airborne data can be used to infer the thickness of the freshwater aquifer, either locally or regionally.