Articles | Volume 7, issue 5
Solid Earth, 7, 1491–1508, 2016
Solid Earth, 7, 1491–1508, 2016

Research article 28 Oct 2016

Research article | 28 Oct 2016

High-resolution shear-wave seismic reflection as a tool to image near-surface subrosion structures – a case study in Bad Frankenhausen, Germany

Sonja H. Wadas1, Ulrich Polom1, and Charlotte M. Krawczyk1,2,a Sonja H. Wadas et al.
  • 1Leibniz Institute for Applied Geophysics, Stilleweg 2, 30655 Hanover, Germany
  • 2Institute for Applied Geosciences, TU Berlin, Ernst-Reuter-Platz 1, 10587 Berlin, Germany
  • anow at: GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany

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 seismic reflection 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 Leibniz Institute for Applied Geophysics (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 heavily fractured, (3) formation of depression structures in the near-surface, (4) diffractions in the unmigrated seismic sections that indicate increased scattering of the seismic waves, and (5) varying seismic velocities and low-velocity zones that are presumably caused by fractures and upward-migrating cavities. A previously undiscovered southward-dipping listric normal fault was also found, to the north of the church. It probably serves as a pathway for water to leach the Permian formations below the church and causes the tilting of the church tower. This case study shows the potential of horizontal shear-wave seismic reflection to image near-surface subrosion structures in an urban environment with a horizontal resolution of less than 1 m in the uppermost 10–15 m.

Short summary
Subrosion is the subsurface leaching of soluble rocks. It is a global phenomenon and a geohazard in urban areas because it causes depressions and sinkholes. This is the case in the study area, the town of Bad Frankenhausen, in northern Thuringia, Germany. Using shear-wave seismic reflection we are able to image these structures at high resolution to a depth of ca. 100 m. We observe that the underground is strongly fractured and there are indications of cavities.