GEOPHYSICAL MAPPING OF ALPINE KARST GROUNDWATER CONDUITS, HIRSCHEGG, AUSTRIA

The Gottesacker Plateau in the Austro-German Alps is a large exposure of carbonates that capture precipitation and snowmelt, transporting them in a strata-bound karst aquifer to the Schwarzwasser River in the Kleinwalsertal below. This is a well-studied karst aquifer system, with the (presumed conduit) connections between infiltration points in the mountains and spring discharges along the river well established in some detail. Modelling of spring responses to precipitation have also allowed precise estimation of the effective diameters of some conduits. What is not yet know is the geometry or paths of these conduits. This study represents an attempt to detect two conduits with a known inlet, outlet, and effective diameter, but unknown surficial trace. Four profiles were recorded using microgravity (to detect the expected mass deficiency of the conduits), seismic refraction (to correct the microgravity for the effect of bedrock relief), electrical resistivity tomography (to detect highly resistive air-filled conduits, or conductive water-filled conduits), and spontaneous potential (SP - to detect minute voltages related to the electrokinetic effect of flowing water). A fifth profile was recorded using seismic refraction, microgravity, and SP. The refraction data showed uniformly smooth (glacially polished?) bedrock surfaces requiring no corrections to the gravity data. The known conduit coming down from the plateau to feed Aubach Spring appeared distinctly on microgravity and resistivity, and could easily be detected and identified in a “blind” test. The SP signature is subtle but recognizable when the conduit location is known from gravity and resistivity. The second conduit running from an estavelle (Europe’s largest) on the banks of the Schwarzwasser to the Aubach conduit appears on resistivity profiles to actually be three small conduits, and is thus less distinguishable on microgravity (also due to depth). However, it produces a dramatic SP anomaly on some profiles – presumably due to the greater surface area provided by the multiple small conduits (as opposed to a single larger one) which enhances the presumed Helmholtz double-layer affect underlying the SP anomalies. [TB1]Missing r in Schwarzwasser