Abstract. Subrosion is the subsurface leaching of soluble rocks that results in the formation of depression and collapse structures.This global phenomenon is a geohazard in urban areas. To study near-surface subrosion structures four shear-wave reflection seismic profiles with a total length of ca. 332 m were carried out around the famous leaning church tower of Bad Frankenhausen in northern Thuringia, Germany, which shows an inclination of 4.93° from the vertical. Most of the geological underground of Thuringia is characterized by soluble Permian deposits, and the Kyffhäuser-Southern-Margin Fault is assumed to be a main pathway for water to leach the evaporite. The seismic profiles were acquired with the horizontal micro-vibrator ELVIS developed at LIAG and a 72 m long landstreamer equipped with 72 horizontal geophones. The high-resolution seismic sections show subrosion-induced structures to a depth of ca. 100 m and reveal five features associated with the leaching of Permian deposits: (1) lateral and vertical varying reflection patterns caused by strongly heterogeneous strata, (2) discontinuous reflectors, small offsets and faults, which show the underground is strongly fractured, (3) formation of depression structures in the near-surface, (4) diffractions in the unmigrated seismic sections that indicate an increased scattering of the seismic waves, (5) varying seismic velocities and low-velocity zones that were presumably caused by fractures and upward-migrating cavities. A previously undiscovered southward-dipping, listric normal fault was also found, located northward of the church. It probably serves as a pathway for water to leach the Zechstein formations below the church and causes the tilting of the tower. This case study shows the potential of horizontal shear-wave reflection seismics in imaging near-surface subrosion structures in an urban environment with a horizontal resolution of less than 1m in the uppermost 10–15 m.
Near-surface fault detection using high-resolution shear wave reflection seismics at the CO2CRC Otway Project site, Australia