Induced Polarization

Sudden collapse of the Quaternary soil to form sinkholes on the order of meters and tens of meters has been a geologic phenomenon within living memory in a localized area north of Lake Chiemsee in Southeast Germany. Failing a satisfying explanation, a relation with an undefined glaciation process has always been proposed. Excavations and geophysical measurements at three newly affected sites show underground features such as prominent sandy-gravelly intrusions and extrusions typical of rock liquefaction processes well known to occur during strong earthquakes. Since strong earthquakes can reasonably be excluded to have affected the area under discussion, it has been suggested that the observed widespread liquefaction is related with the recently proposed Holocene Chiemgau meteorite impact event. Except for one earlier proposed but unassertive relation between impact and liquefaction, the obviously direct association of both processes in the Chiemgau area emphasizes that observed paleoliquefaction features need not necessarily have originated solely from paleoseismicity but can provide a recognizable regional impact signature.

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.

Nanoparticles have grown in importance over the last decade with significant consumer and industrial applications. Yet, the behavior (fate and transport) of nanoparticles in the environment is virtually unknown. Research is needed to identify, characterize, and monitor nanomaterials in the subsurface. Here, we investigate the spectral induced polarization (SIP) response of nanometallic powders (nZVI, nAg, nTiO2, nZnO, and nCeO2) in porous geologic media. Our main objective is to determine the sensitivity of the SIP response (0.1–10,000 Hz) to the presence of nanoparticles (metals and metal oxides) in porous media. The SIP response was tested under various conditions: increasing particle concentration under constant solution chemistry; varying solution molarity (0.0 M–1.0 M), and varying solution valence (+ 1, + 2, + 3 valence) under constant particle volume. We examine the results in terms of phase shift and resistance magnitude. Our data suggest that the oxide nanoparticles do not show SIP responses to increasing particle concentration, solution valence, and molarity, while the metallic particles show a clear response to increasing particle concentration, and frequency. Silver was the only material to show any significant response to increasing solution molarity, valence, and frequency. Because of the high propensity of the nanoparticles to form aggregates, they essentially behave as colloidal and clay particles allowing us to apply conventional SIP theory to our interpretation. We suggest that the oxidation state of the metals diminishes their SIP response consistent with more recent studies that have documented that polarization decreases with oxidation of metallic particles. We infer from our results that nanoparticle crystalline composition and aggregation effects control the SIP response of nanoparticles in porous media.