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- ItemStructural analysis of S-wave seismics around an urban sinkhole: evidence of enhanced dissolution in a strike-slip fault zone(Katlenburg-Lindau : European Geophysical Society, 2017-12-19) Wadas, Sonja H.; Tanner, David C.; Polom, Ulrich; Krawczyk, Charlotte M.In November 2010, a large sinkhole opened up in the urban area of Schmalkalden, Germany. To determine the key factors which benefited the development of this collapse structure and therefore the dissolution, we carried out several shear-wave reflection-seismic profiles around the sinkhole. In the seismic sections we see evidence of the Mesozoic tectonic movement in the form of a NW–SE striking, dextral strike-slip fault, known as the Heßleser Fault, which faulted and fractured the subsurface below the town. The strike-slip faulting created a zone of small blocks ( < 100 m in size), around which steep-dipping normal faults, reverse faults and a dense fracture network serve as fluid pathways for the artesian-confined groundwater. The faults also acted as barriers for horizontal groundwater flow perpendicular to the fault planes. Instead groundwater flows along the faults which serve as conduits and forms cavities in the Permian deposits below ca. 60 m depth. Mass movements and the resulting cavities lead to the formation of sinkholes and dissolution-induced depressions. Since the processes are still ongoing, the occurrence of a new sinkhole cannot be ruled out. This case study demonstrates how S-wave seismics can characterize a sinkhole and, together with geological information, can be used to study the processes that result in sinkhole formation, such as a near-surface fault zone located in soluble rocks. The more complex the fault geometry and interaction between faults, the more prone an area is to sinkhole occurrence.
- ItemZero-Offset VSP Monitoring of CO2Storage: Impedance Inversion and Wedge Modelling at the Ketzin Pilot Site(New York, NY : Hindawi, 2014) Götz, Julia; Lüth, Stefan; Krawczyk, Charlotte M.; Cosma, CalinAt the CO2 storage pilot site near the town of Ketzin (35 km west of Berlin, Germany) the sandstone reservoir at 630 m–650 m depth is thin and heterogeneous. The time-lapse analysis of zero-offset VSP measurements shows that CO2-induced amplitude changes can be observed on near-well corridor stacks. Further, we investigate whether CO2-induced amplitude changes in the monitoring data can be used to derive geometrical and petrophysical parameters governing the migration of CO2 within a brine saturated sandstone aquifer. 2D seismic-elastic modelling is done to test the processing workflow and to perform a wedge modelling study for estimation of the vertical expansion of the CO2 plume. When using the NRMS error as a measure for the similarity between the modelled and recorded repeat traces, the best match is achieved for a plume thickness of 6-7 m within the reservoir sandstone of 8 m thickness. With band limited impedance inversion a velocity reduction at the top of the reservoir of 30%, influenced by casing reverberations as well as CO2 injection, is found. The relation of seismic amplitude to CO2 saturated layer thickness and CO2-induced changes in P-wave velocities are important parameters for the quantification of the injected CO2 volume.
- ItemNear-surface fault detection using high-resolution shear wave reflection seismics at the CO2CRC Otway Project site, Australia(Hoboken, NJ : Wiley, 2016) Beilecke, Thies; Krawczyk, Charlotte M.; Ziesch, Jennifer; Tanner, David C.High-resolution, near-surface, shear wave reflection seismic measurements were carried out in November 2013 at the CO2CRC Otway Project site, Victoria, Australia, with the aim to determine whether and, if so, where deeper faults reach the near subsurface. From a previous P wave 3-D reflection seismic data set that was concentrated on a reservoir at 2 km depth, we can only interpret faults up to 400 m below sea level. For the future monitoring in the overburden of the CO2 reservoir it is important to know whether and how the faults continue in the subsurface. We prove that two regional fault zones do in fact reach the surface instead of dying out at depth. Individual first-break signatures in the shot gathers along the profiles support this interpretation. However, this finding does not imply perforce communication between the reservoir and the surface in the framework of CO2 injection. The shear wave seismic sections are complementary to existing P wave volumes. They image with high resolution (better than 3 m vertically) different tectonic structures. Similar structures also outcrop on the southern coast of the Otway Basin. Both the seismic and the outcrops evidence the complex youngest structural history of the area.
- 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.
- ItemPore-scale tomography and imaging: applications, techniques and recommended practice(Göttingen : Copernicus Publ., 2016) Halisch, Matthias; Steeb, Holger; Henkel, Steven; Krawczyk, Charlotte M.[No abstract available]
- 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.