Geoelectric Complex Resistivity Measurements of Soil Liquefaction Features in Quaternary Sediments of the Alpine Foreland, Germany

Soil or rock liquefaction is a well-known process initiated by seismic shaking during strong earthquakes and exemplarily revealed by, e.g., the 1811/1812 devastating New Madrid, Missouri, earthquake series. In the Bavarian Alpine Foreland, typical soil liquefaction surface features have been investigated by complex 4-frequency resistivity measurements in the form of 2D electrical imaging. We used a pole-dipole configuration and a 1 m station spacing resulting in apparent resistivity and apparent induced polarization (IP) pseudosections. The profiles crossed a recently collapsed 1 m-diameter sink hole (a so-called thunderhole) and a 4.5 m-diameter active soil subsidence. Both resistivity and IP (selected phase shift at 8,33 Hz) pseudosections reveal that sink hole collapse and active depression are only small-scale snapshots in time of a much larger geologic scenario going off in the subsurface. Over at least 20 m in the first case and 40 m in the latter, the underground structures, normally well bedded Quaternary fluvio-glacial sands and gravels, loamy moraine material and loess deposits, show a drastically disturbed resistivity pattern. Conspicuously, the IP pattern is significantly more affected by small-scale structures and obviously much more sensitive to even minor changes of rock facies. Thus, in the investigation under discussion the IP pattern points to distinct multiple intrusion and extrusion features typical of soil liquefaction, while in this respect the resistivity expression is comparatively poor. A deep excavation of the thunderhole on a larger scale following the geophysical measurements confirmed the prediction of the complex resistivity survey in very detail. Moreover, it gave insight into a geological underground liquefaction process that must have released enormous energies leading to the assumption that the liquefaction, because of absent earthquake activity in the region, has been induced by the recently proposed Holocene so-called Chiemgau meteorite impact event. Since the liquefaction features including the widespread sink hole activity, known in the region within living memory, have engineering-geology implications we suggest the application of complex resistivity measurements as an important tool for the investigation of subsoil properties in construction works. Because of the frequently thick loamy and clayey low-resistivity cap rocks in the liquefaction-affected region, complex resistivity surveys may be superior to ground penetration radar (GPR) measurements otherwise useful in subgrade geophysics.