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- ItemShear wave reflection seismic yields subsurface dissolution and subrosion patterns: application to the Ghor Al-Haditha sinkhole site, Dead Sea, Jordan(Göttingen : Copernicus Publ., 2018) Polom, Ulrich; Alrshdan, Hussam; Al-Halbouni, Djamil; Holohan, Eoghan P.; Dahm, Torsten; Sawarieh, Ali; Atallah, Mohamad Y.; Krawczyk, Charlotte M.Near-surface geophysical imaging of alluvial fan settings is a challenging task but crucial for understating geological processes in such settings. The alluvial fan of Ghor Al-Haditha at the southeast shore of the Dead Sea is strongly affected by localized subsidence and destructive sinkhole collapses, with a significantly increasing sinkhole formation rate since ca. 1983. A similar increase is observed also on the western shore of the Dead Sea, in correlation with an ongoing decline in the Dead Sea level. Since different structural models of the upper 50 m of the alluvial fan and varying hypothetical sinkhole processes have been suggested for the Ghor Al-Haditha area in the past, this study aimed to clarify the subsurface characteristics responsible for sinkhole development. For this purpose, high-frequency shear wave reflection vibratory seismic surveys were carried out in the Ghor Al-Haditha area along several crossing and parallel profiles with a total length of 1.8 and 2.1 km in 2013 and 2014, respectively. The sedimentary architecture of the alluvial fan at Ghor Al-Haditha is resolved down to a depth of nearly 200 m at a high resolution and is calibrated with the stratigraphic profiles of two boreholes located inside the survey area. The most surprising result of the survey is the absence of evidence of a thick (> 2–10 m) compacted salt layer formerly suggested to lie at ca. 35–40 m depth. Instead, seismic reflection amplitudes and velocities image with good continuity a complex interlocking of alluvial fan deposits and lacustrine sediments of the Dead Sea between 0 and 200 m depth. Furthermore, the underground section of areas affected by sinkholes is characterized by highly scattering wave fields and reduced seismic interval velocities. We propose that the Dead Sea mud layers, which comprise distributed inclusions or lenses of evaporitic chloride, sulfate, and carbonate minerals as well as clay silicates, become increasingly exposed to unsaturated water as the sea level declines and are consequently destabilized and mobilized by both dissolution and physical erosion in the subsurface. This new interpretation of the underlying cause of sinkhole development is supported by surface observations in nearby channel systems. Overall, this study shows that shear wave seismic reflection technique is a promising method for enhanced near-surface imaging in such challenging alluvial fan settings.
- ItemHigh-resolution shear-wave seismic reflection as a tool to image near-surface subrosion structures – a case study in Bad Frankenhausen, Germany(Göttingen : Copernicus Publ., 2016) Wadas, Sonja H.; Polom, Ulrich; Krawczyk, Charlotte M.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.
- ItemDistinct element geomechanical modelling of the formation of sinkhole clusters within large-scale karstic depressions(Göttingen : Copernicus Publ., 2019) Al-Halbouni, Djamil; Holohan, Eoghan P.; Taheri, Abbas; Watson, Robert A.; Polom, Ulrich; Schöpfer, Martin P. J.; Emam, Sacha; Dahm, TorstenThe 2-D distinct element method (DEM) code (PFC2D_V5) is used here to simulate the evolution of subsidence-related karst landforms, such as single and clustered sinkholes, and associated larger-scale depressions. Subsurface material in the DEM model is removed progressively to produce an array of cavities; this simulates a network of subsurface groundwater conduits growing by chemical/mechanical erosion. The growth of the cavity array is coupled mechanically to the gravitationally loaded surroundings, such that cavities can grow also in part by material failure at their margins, which in the limit can produce individual collapse sinkholes. Two end-member growth scenarios of the cavity array and their impact on surface subsidence were examined in the models: (1) cavity growth at the same depth level and growth rate; (2) cavity growth at progressively deepening levels with varying growth rates. These growth scenarios are characterised by differing stress patterns across the cavity array and its overburden, which are in turn an important factor for the formation of sinkholes and uvala-like depressions. For growth scenario (1), a stable compression arch is established around the entire cavity array, hindering sinkhole collapse into individual cavities and favouring block-wise, relatively even subsidence across the whole cavity array. In contrast, for growth scenario (2), the stress system is more heterogeneous, such that local stress concentrations exist around individual cavities, leading to stress interactions and local wall/overburden fractures. Consequently, sinkhole collapses occur in individual cavities, which results in uneven, differential subsidence within a larger-scale depression. Depending on material properties of the cavity-hosting material and the overburden, the larger-scale depression forms either by sinkhole coalescence or by widespread subsidence linked geometrically to the entire cavity array. The results from models with growth scenario (2) are in close agreement with surface morphological and subsurface geophysical observations from an evaporite karst area on the eastern shore of the Dead Sea.