Porosity estimation of a geothermal carbonate reservoir in the German Molasse Basin based on seismic amplitude inversion

The Molasse Basin is one of the most promising areas for deep geothermal exploitation in Germany and the target horizon is the aquifer in the Upper Jurassic carbonates. Carbonate deposits can be very heterogeneous even over a small area due to diagenetic processes and varying depositional environments. The preferential targets for geothermal exploitation in carbonate deposits are fault zones, reef facies and karstified areas, since they are expected to act as hydraulically permeable zones due to high porosity and high permeability. Therefore, identifying these structures and characterizing, e.g., their internal porosity distribution are of high importance. This can be accomplished using 3D reflection seismic data. Besides structural information, 3D seismic surveys provide important reservoir properties, such as acoustic impedance, from which a porosity model can be derived. In our study area in Munich we carried out a seismic amplitude inversion to get an acoustic impedance model of the Upper Jurassic carbonate reservoir using a 3D seismic data set, a corresponding structural geological model, and logging data from six wells at the ‘Schäftlarnstraße’ geothermal site. The impedance model and porosity logs were than used to calculate a porosity model. The model shows a wide porosity range from 0 to 20% for the entire reservoir zone and the lithology along the wells reveals that dolomitic limestone has the highest porosities and calcareous dolomite has the lowest porosities. The study area is cut by a large W–E striking fault, the Munich Fault, and the footwall north of it shows higher porosities and more intense karstification than the hanging wall to the south. Considering the entire study area, an increase in porosity from east to west is observed. Furthermore, we identified a complex porosity distribution in reef buildups and pinnacle reefs. The reef cores have mostly low porosities of, e.g., < 3% and the highest porosities of up to 7 to 14% are observed at the reef caps and on the reef slopes. The reef slopes show a characteristic interfingering of the reef facies with the surrounding bedded facies, which indicates a syn-sedimentary reef development with slightly varying build up growth rates. We also assessed the reservoir quality with regard to porosity distribution and determined areas with moderate to good quality for geothermal exploitation by defining porosity evaluation levels. The porosity evaluation maps show that the carbonate rocks of Berriasian to Malm ζ1 are preferential targets for exploitation, especially in the footwall of the Munich Fault and to the west of the hanging wall, because these areas are characterized by high porosities due to intense karstification of bedded and massive facies, although the latter is mainly restricted to reef caps and reef slopes

Structural analysis of S-wave seismics around an urban sinkhole: evidence of enhanced dissolution in a strike-slip fault zone

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

Geophysical analysis of an area affected by subsurface dissolution - case study of an inland salt marsh in northern Thuringia, Germany

The subsurface dissolution of soluble rocks can affect areas over a long period of time and pose a severe hazard. We show the benefits of a combined approach using P-wave and SH-wave reflection seismics, electrical resistivity tomography, transient electromagnetics, and gravimetry for a better understanding of the dissolution process. The study area, "Esperstedter Ried"in northern Thuringia, Germany, located south of the Kyffhäuser hills, is a large inland salt marsh that developed due to dissolution of soluble rocks at approximately 300 m depth. We were able to locate buried dissolution structures and zones, faults and fractures, and potential fluid pathways, aquifers, and aquitards based on seismic and electromagnetic surveys. Further improvement of the model was accomplished by analyzing gravimetry data that indicates dissolution-induced mass movement, as shown by local minima of the Bouguer anomaly for the Esperstedter Ried. Forward modeling of the gravimetry data, in combination with the seismic results, delivered a cross section through the inland salt marsh from north to south. We conclude that tectonic movements during the Tertiary, which led to the uplift of the Kyffhäuser hills and the formation of faults parallel and perpendicular to the low mountain range, were the initial trigger for subsurface dissolution. The faults and the fractured Triassic and lower Tertiary deposits serve as fluid pathways for groundwater to leach the deep Permian Zechstein deposits, since dissolution and erosional processes are more intense near faults. The artesian-confined saltwater rises towards the surface along the faults and fracture networks, and it formed the inland salt marsh over time. In the past, dissolution of the Zechstein formations formed several, now buried, sagging and collapse structures, and, since the entire region is affected by recent sinkhole development, dissolution is still ongoing. From the results of this study, we suggest that the combined geophysical investigation of areas prone to subsurface dissolution can improve the knowledge of control factors, hazardous areas, and thus local dissolution processes

Initial results of coring at Prees, Cheshire Basin, UK (ICDP JET project): Towards an integrated stratigraphy, timescale, and Earth system understanding for the Early Jurassic

Drilling for the International Continental Scientific Drilling Program (ICDP) Early Jurassic Earth System and Timescale project (JET) was undertaken between October 2020 and January 2021. The drill site is situated in a small-scale synformal basin of the latest Triassic to Early Jurassic age that formed above the major Permian-Triassic half-graben system of the Cheshire Basin. The borehole is located to recover an expanded and complete succession to complement the legacy core from the Llanbedr (Mochras Farm) borehole drilled through 1967-1969 on the edge of the Cardigan Bay Basin, North Wales. The overall aim of the project is to construct an astronomically calibrated integrated timescale for the Early Jurassic and to provide insights into the operation of the Early Jurassic Earth system. Core of Quaternary age cover and Early Jurassic mudstone was obtained from two shallow partially cored geotechnical holes (Prees 2A to 32.2g¯m below surface (mg¯b.s.) and Prees 2B to 37.0g¯mg¯b.s.) together with Early Jurassic and Late Triassic mudstone from the principal hole, Prees 2C, which was cored from 32.92 to 651.32g¯m (corrected core depth scale). Core recovery was 99.7g¯% for Prees 2C. The ages of the recovered stratigraphy range from the Late Triassic (probably Rhaetian) to the Early Jurassic, Early Pliensbachian (Ibex Ammonoid Chronozone). All ammonoid chronozones have been identified for the drilled Early Jurassic strata. The full lithological succession comprises the Branscombe Mudstone and Blue Anchor formations of the Mercia Mudstone Group, the Westbury and Lilstock formations of the Penarth Group, and the Redcar Mudstone Formation of the Lias Group. A distinct interval of siltstone is recognized within the Late Sinemurian of the Redcar Mudstone Formation, and the name "Prees Siltstone Member"is proposed. Depositional environments range from playa lake in the Late Triassic to distal offshore marine in the Early Jurassic. Initial datasets compiled from the core include radiography, natural gamma ray, density, magnetic susceptibility, and X-ray fluorescence (XRF). A full suite of downhole logs was also run. Intervals of organic carbon enrichment occur in the Rhaetian (Late Triassic) Westbury Formation and in the earliest Hettangian and earliest Pliensbachian strata of the Redcar Mudstone Formation, where up to 4g¯% total organic carbon (TOC) is recorded. Other parts of the succession are generally organic-lean, containing less than 1g¯% TOC. Carbon-isotope values from bulk organic matter have also been determined, initially at a resolution of g1/4g¯1g¯m, and these provide the basis for detailed correlation between the Prees 2 succession and adjacent boreholes and Global Stratotype Section and Point (GSSP) outcrops. Multiple complementary studies are currently underway and preliminary results promise an astronomically calibrated biostratigraphy, magnetostratigraphy, and chemostratigraphy for the combined Prees and Mochras successions as well as insights into the dynamics of background processes and major palaeo-environmental changes

Hochauflösende Scherwellenreflexionsseismik und Bohrlochseismik als Werkzeuge zur Abbildung und Charakterisierung von oberflächennahen Subrosionsstrukturen

Subrosion and the resulting structures, such as sinkholes and depressions, pose a great hazard risk. They can cause damage to buildings and infrastructure, and lead to life-threatening situations. But compared to other natural hazards, like earthquakes and tsunamis, very little is known about the generation processes or the possibilities of forecasting. Germany suffers from a widespread sinkhole problem, because soluble deposits, such as gypsum and anhydrite, are close to the surface in many areas. One such area is the federal state of Thuringia, where the two investigation areas of this thesis, Bad Frankenhausen and Schmalkalden, are located. This thesis is concerned with high-resolution imaging and characterization of near-surface subrosion structures using shear wave reflection- and borehole seismics. The possibilities and limitations of the seismic methods are outlined, and an interdisciplinary geophysical approach for the investigation of subrosion areas is presented. Furthermore, I investigate the impact of faults on sinkhole occurrence, and I identify unstable zones using geotechnical parameters derived from seismic. I investigate the main structural characteristics of depressions and sinkholes, and the development of a multiple collapse event, using SH-wave reflection seismic imaging. A strongly heterogeneous, fractured strata with small-scale intraformational faults is identified as the subrosion horizon. The displaced reflectors at the fractured sinkhole margins dip towards the focal point of collapse structures. Consecutive collapse events are triggered more easily at the margins and they follow the direction of the local drainage system. I investigate the role of faults in sinkhole development using S-waves. I show that if an area is tectonically divided into fault blocks this enables groundwater flow and therefore leaching of soluble rocks. Steep-dipping faults can act as a barrier for horizontal groundwater flow, but can allow water to flow parallel to the fault strike. As a result, areas affected by tectonic deformation are prone to enhanced dissolution and sinkhole formation. Electrical resistivity tomography, transient electromagnetics and gravimetry were used to support the reflection seismic results. The former two methods help to correlate buried and near-surface subrosion structures, and faults and fractures that were observed in P-wave and S-wave reflection seismic sections with low resistivity zones, and therefore show the vertical and horizontal water flow. Especially at faults, water can migrate downward to dissolve soluble rocks, and artesian groundwater can migrate upward to form an inland salt marsh. The mass-movement generates local minima of the Bouguer anomaly that correlate with subrosion structures. An analysis of elastic parameters and seismic attributes derived from VSP and 2D SH-wave reflection seismic delivers information about subrosion-induced unstable zones that are important for engineering and construction. Low shear wave velocities and a reduced shear strength are identified for the subrosion horizon and the disturbed overlying deposits. Low shear strength values indicate unstable zones, which show in part high Poisson’s ratios. Shear modulus and Poisson’s ratio anomalies correlate with zones of low electrical resistivities, which indicates their high conductivity due to fluid flow. Conversion of S-to-P-wave within the subrosion horizon is probably caused by dipping layers and fractures. Seismic attribute analysis reveals strong attenuation of high frequencies and low similarity of adjacent seismic traces, which correlates with the degree of subrosion-induced disturbance of the underground. In summary, this work demonstrates the suitability of 2D SH-wave reflection seismic and VSP to investigate subrosion areas and to identify unstable zones.Subrosionsbedingte Erdfälle und Senken, die zu lebensgefährlichen Situationen und zu Schäden an Infrastruktur und Gebäuden führen können, stellen ein großes Georisiko dar. Verglichen mit anderen Naturgefahren, z.B. Erdbeben und Tsunamis, sind die Entstehungsprozesse und die Vorhersagemöglichkeiten noch unzureichend erforscht. In weiten Teilen Deutschland stellen Erdfälle ein großes Problem dar, weil sich vielerorts lösliche Gesteine wie Gips, Anhydrit und Steinsalz nahe der Erdoberfläche befinden. Ein besonders stark betroffenes Gebiet ist das Bundesland Thüringen, in dem sich die zwei Untersuchungsgebiete dieser Dissertation, die Orte Bad Frankenhausen und Schmalkalden, befinden. Diese Dissertation beschäftigt sich mit der hochauflösenden Abbildung und Charakterisierung oberflächennaher Subrosionsstrukturen unter Verwendung von 2D-Scherwellenreflexionsseismik und Bohrlochseismik. Es werden die Möglichkeiten und Grenzen der seismischen Methoden dargelegt und ein interdisziplinärer geophysikalischer Ansatz zur Untersuchung von Subrosionsgebieten vorgestellt. Außerdem wird der Einfluss tektonischer Störungen auf die Erdfallentstehung untersucht und, mittels von der Seismik abgeleiteter geotechnischer Parameter, instabile Zonen im Untergrund identifiziert. Ich habe die wesentlichen Strukturmerkmale von Senken und Erdfällen und die Entwicklung eines mehrfachen Kollaps mit SH-Wellen Reflexionsseismik untersucht. Für den Subrosionshorizont habe ich stark heterogene und zerbrochene Schichten mit klein-skaligen, nicht-tektonischen Stöorungen identifiziert. Die Reflektoren zeigen einen Versatz an den gebrochenen Schichten der Erdfallränder und sie fallen in Richtung des Erdfallzentrums ein. Aufeinanderfolgende Kollapsereignisse treten bevorzugt an den gestörten Erdfallrändern auf und folgen dabei dem lokalen Grundwasser-Drainagesystem. Außerdem untersuche ich die Rolle von Störungen bei der Erdfallentstehung. Ich zeige, dass ein durch tektonische Beanspruchung in Störungsblöcke zerteiltes Gebiet Grundwasserfluss ermöglicht und dadurch die Auslaugung löslicher Gesteine begünstigt. Steil einfallende Störungen können als Barrieren für Grundwasserfluss senkrecht zur Störung fungieren, aber sie können einen Grundwasserfluss parallel zum Streichen der Störung erm¨oglichen. Daher sind tektonisch beanspruchte Gebiete anfällig für verstärkte Auslaugung und die Bildung von Erdfällen. Ich habe zur Unterstützung der seismischen Ergbenisse die Resultate von elektrischer Widerstandstomographie, transienter Elektromagnetik und Gravimetrie verwendet. Mit den elektrischen Verfahren habe ich einen Zusammenhang hergestellt zwischen Zonen geringen elektrischen Widerstands mit tiefen und oberflächennahen Subrosionsstrukturen, Störungen und Brüchen, welche ich in der P- und S-Wellen Reflexionsseismik identifiziert habe. Dadurch konnte ich den vertikalen und horizontalen Wasserfluss nachvollziehen. Insbesondere an Störungen kann das Wasser in die Tiefe migrieren und Gesteine auslaugen, und artesisches, salz-haltiges Grundwasser kann aufsteigen und eine Binnensalzstelle formen. Die einhergehenden Massenumlagerungen im Untergrund generieren eine mit Gravimetrie detektierbare Anomalie im lokalen Schwerefeld. Eine Analyse elastischer Parameter und seismischer Attribute basierend auf Scherwellenseismik und Bohrlochseismik liefert Informationen über subrosions-induzierte instabile Zonen, welche von Bedeutung sind für den Hoch- und Tiefbau. Ich habe geringe Scherwellengeschwindigkeiten und verringerte Scherwiderstände für den Subrosionshorizont und die gestörten auflagernden Schichten festgestellt. Geringe Scherwiderstände weisen auf instabile Zonen hin, welche zum Teil auch hohe Poissonzahlen aufweisen. Anomalien des Schermodul und der Poissonzahl korrelieren mit geringen elektrischen Widerständen, welche auf erhöhte Leitfähigkeiten durch Fluide hinweisen. Eine zu beobachtende Wellenkonversion von S-Welle zu P-Welle im Subrosionshorizont wird vermutlich durch geneigte Schichten und Brüche verursacht. Die seismische Attributanalyse zeigt eine starke Abschwächung hoher Frequenzen und eine geringe Kohärenz benachbarter seismischer Spuren und dies korrelliert mit dem Grad der subrosionsbedingten Störung des Untergrundes. Zusammengefasst zeigt diese Arbeit die Eignung von 2D-Scherwellenreflexionsseismik und Bohrlochseismik für die Untersuchung von Subrosionsgebieten und die Identifizierung instablier Zonen

High-Resolution Shear Wave Reflection Seismics and Borehole Seismics as Tools for the Imaging and the Characterization of Near-Surface Subrosion Structures

Subrosion and the resulting structures, such as sinkholes and depressions, pose a great hazard risk. They can cause damage to buildings and infrastructure, and lead to life-threatening situations. But compared to other natural hazards, like earthquakes and tsunamis, very little is known about the generation processes or the possibilities of forecasting. Germany suffers from a widespread sinkhole problem, because soluble deposits, such as gypsum and anhydrite, are close to the surface in many areas. One such area is the federal state of Thuringia, where the two investigation areas of this thesis, Bad Frankenhausen and Schmalkalden, are located. This thesis is concerned with high-resolution imaging and characterization of near-surface subrosion structures using shear wave reflection- and borehole seismics. The possibilities and limitations of the seismic methods are outlined, and an interdisciplinary geophysical approach for the investigation of subrosion areas is presented. Furthermore, I investigate the impact of faults on sinkhole occurrence, and I identify unstable zones using geotechnical parameters derived from seismic. I investigate the main structural characteristics of depressions and sinkholes, and the development of a multiple collapse event, using SH-wave reflection seismic imaging. A strongly heterogeneous, fractured strata with small-scale intraformational faults is identified as the subrosion horizon. The displaced reflectors at the fractured sinkhole margins dip towards the focal point of collapse structures. Consecutive collapse events are triggered more easily at the margins and they follow the direction of the local drainage system. I investigate the role of faults in sinkhole development using S-waves. I show that if an area is tectonically divided into fault blocks this enables groundwater flow and therefore leaching of soluble rocks. Steep-dipping faults can act as a barrier for horizontal groundwater flow, but can allow water to flow parallel to the fault strike. As a result, areas affected by tectonic deformation are prone to enhanced dissolution and sinkhole formation. Electrical resistivity tomography, transient electromagnetics and gravimetry were used to support the reflection seismic results. The former two methods help to correlate buried and near-surface subrosion structures, and faults and fractures that were observed in P-wave and S-wave reflection seismic sections with low resistivity zones, and therefore show the vertical and horizontal water flow. Especially at faults, water can migrate downward to dissolve soluble rocks, and artesian groundwater can migrate upward to form an inland salt marsh. The mass-movement generates local minima of the Bouguer anomaly that correlate with subrosion structures. An analysis of elastic parameters and seismic attributes derived from VSP and 2D SH-wave reflection seismic delivers information about subrosion-induced unstable zones that are important for engineering and construction. Low shear wave velocities and a reduced shear strength are identified for the subrosion horizon and the disturbed overlying deposits. Low shear strength values indicate unstable zones, which show in part high Poisson’s ratios. Shear modulus and Poisson’s ratio anomalies correlate with zones of low electrical resistivities, which indicates their high conductivity due to fluid flow. Conversion of S-to-P-wave within the subrosion horizon is probably caused by dipping layers and fractures. Seismic attribute analysis reveals strong attenuation of high frequencies and low similarity of adjacent seismic traces, which correlates with the degree of subrosion-induced disturbance of the underground. In summary, this work demonstrates the suitability of 2D SH-wave reflection seismic and VSP to investigate subrosion areas and to identify unstable zones

Porosity estimation of a geothermal carbonate reservoir in the German Molasse Basin based on seismic amplitude inversion

Abstract The Molasse Basin is one of the most promising areas for deep geothermal exploitation in Germany and the target horizon is the aquifer in the Upper Jurassic carbonates. Carbonate deposits can be very heterogeneous even over a small area due to diagenetic processes and varying depositional environments. The preferential targets for geothermal exploitation in carbonate deposits are fault zones, reef facies and karstified areas, since they are expected to act as hydraulically permeable zones due to high porosity and high permeability. Therefore, identifying these structures and characterizing, e.g., their internal porosity distribution are of high importance. This can be accomplished using 3D reflection seismic data. Besides structural information, 3D seismic surveys provide important reservoir properties, such as acoustic impedance, from which a porosity model can be derived. In our study area in Munich we carried out a seismic amplitude inversion to get an acoustic impedance model of the Upper Jurassic carbonate reservoir using a 3D seismic data set, a corresponding structural geological model, and logging data from six wells at the ‘Schäftlarnstraße’ geothermal site. The impedance model and porosity logs were than used to calculate a porosity model. The model shows a wide porosity range from 0 to 20% for the entire reservoir zone and the lithology along the wells reveals that dolomitic limestone has the highest porosities and calcareous dolomite has the lowest porosities. The study area is cut by a large W–E striking fault, the Munich Fault, and the footwall north of it shows higher porosities and more intense karstification than the hanging wall to the south. Considering the entire study area, an increase in porosity from east to west is observed. Furthermore, we identified a complex porosity distribution in reef buildups and pinnacle reefs. The reef cores have mostly low porosities of, e.g., < 3% and the highest porosities of up to 7 to 14% are observed at the reef caps and on the reef slopes. The reef slopes show a characteristic interfingering of the reef facies with the surrounding bedded facies, which indicates a syn-sedimentary reef development with slightly varying build up growth rates. We also assessed the reservoir quality with regard to porosity distribution and determined areas with moderate to good quality for geothermal exploitation by defining porosity evaluation levels. The porosity evaluation maps show that the carbonate rocks of Berriasian to Malm $$\zeta$$ ζ 1 are preferential targets for exploitation, especially in the footwall of the Munich Fault and to the west of the hanging wall, because these areas are characterized by high porosities due to intense karstification of bedded and massive facies, although the latter is mainly restricted to reef caps and reef slopes

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

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

Combination of 2D Shear Wave Reflection Seismics and Travel Time Analysis of Borehole Geophone Data for the Investigation of a Sinkhole Area

In November 2010, a 30 m wide and 17 m deep sinkhole occurred in a residential area of Schmalkalden, Germany. Subsequent geoscientific investigations showed that the collapse was naturally caused by the dissolution of sulfates below 80 m depth. In 2012, the Thuringian State Institute for Environment and Geology (TLUG) established an early warning system including 3C borehole geophones deployed in 50 m depth around the backfilled sinkhole. During the acquisition of two shallow 2D shear wave seismic profiles in the vicinity of the sinkhole, the signals generated by a micro-vibrator at the surface were additionally recorded by the four borehole geophones of the early warning system and a vertical seismic profiling (VSP) probe in a fifth borehole. Travel time analysis of the direct P- and S-wave arrivals enhances the understanding of wave propagation in the area. Seismic velocity anomalies are detected and related to the structural seismic images of the 2D profiles. In case of travel paths perpendicular to faults, the velocity is decreased, whereas the velocity of waves travelling parallel to the strike direction of faults is not significantly lowered. The combination of receivers located at the surface recording reflected seismic waves and distributed downhole receivers recording direct waves, enables analyzing of seismic wave propagation and velocities in more detail and beyond 2D. Therefore, the experiment setup will be further extended in future. The presented method shows the potential to locate instable zones in a sinkhole area. In our further research we propose to evaluate the suitability of the method for the time lapse monitoring of changes in the seismic wave propagation, which could be related to subrosion processes

Combination of 2D Shear Wave Reflection Seismics and Travel Time Analysis of Borehole Geophone Data for the Investigation of a Sinkhole Area

In November 2010, a 30 m wide and 17 m deep sinkhole occurred in a residential area of Schmalkalden, Germany. Subsequent geoscientific investigations showed that the collapse was naturally caused by the dissolution of sulfates below 80 m depth. In 2012, the Thuringian State Institute for Environment and Geology (TLUG) established an early warning system including 3C borehole geophones deployed in 50 m depth around the backfilled sinkhole. During the acquisition of two shallow 2D shear wave seismic profiles in the vicinity of the sinkhole, the signals generated by a micro-vibrator at the surface were additionally recorded by the four borehole geophones of the early warning system and a vertical seismic profiling (VSP) probe in a fifth borehole. Travel time analysis of the direct P- and S-wave arrivals enhances the understanding of wave propagation in the area. Seismic velocity anomalies are detected and related to the structural seismic images of the 2D profiles. In case of travel paths perpendicular to faults, the velocity is decreased, whereas the velocity of waves travelling parallel to the strike direction of faults is not significantly lowered. The combination of receivers located at the surface recording reflected seismic waves and distributed downhole receivers recording direct waves, enables analyzing of seismic wave propagation and velocities in more detail and beyond 2D. Therefore, the experiment setup will be further extended in future. The presented method shows the potential to locate instable zones in a sinkhole area. In our further research we propose to evaluate the suitability of the method for the time lapse monitoring of changes in the seismic wave propagation, which could be related to subrosion processes