Sinkhole Development Research Articles

Assessing Collapse Risk in Evaporite Sinkhole-prone Areas Using Microgravimetry and Radar Interferometry

Geophysical and remote-sensing methods were applied to better understand sinkhole precursor movement and assess the potential for sinkhole development in evaporitic areas. The approach is illustrated with two examples over bedded salt deposits and a salt dome in Texas, USA. Large sinkholes (90 to 200 m in diameter) formed over Permian bedded salt near Wink in western Texas in June 1980 and May 2002, and on the flank of a coastal-plain salt dome in Daisetta in May 2008. Residents, government officials, and industry representatives wish to better understand the potential for sinkhole formation and growth in both areas. At Wink, limited spatial and temporal data on vertical ground movement from standard surveying has been greatly extended by satellite-based radar interferometry, which was used to delineate areas having recent movement and determine rates of movement. Results from interferometry guided site-specific investigations that included acquisition of high-resolution gravity data, which identified shallow-source mass deficits that indicate potential for continued subsidence or sinkhole formation. At Daisetta, interferometry was used to determine that no detectable subsidence preceded sinkhole collapse (indicating sudden collapse once the upward-migrating void reached a depth that allowed the cohesiveness of overlying semiconsolidated sediments to be overcome), and gravimetry was used to identify other areas where shallow mass deficits exist across the salt dome. Data from both areas can be used to construct risk maps, design comprehensive subsurface investigations, and develop monitoring programs based on repeat radar interferometry and geodetic GPS measurements.

Dynamic dissolution of halite rock during flow of diluted saline solutions

The dynamic dissolution pattern of halite salt rocks taken from coreholes near the Dead Sea was studied in laboratory‐scale experiments. When unsaturated solution (with respect to halite) flowed through salt cores, dissolution developed along preferential flow pathways in a channel structure. The channel structure was related to the salt's properties and internal heterogeneities, flow velocity and impact of gravity. Preferential dissolution pathways developed in areas of minimum resistance to flow, such as large‐pore networks and cracks. Nevertheless, in many cases no structural heterogeneity was observed along the dissolution channels prior to the experiments. The initial formation of channels took place above a critical flow velocity; below this threshold, dissolution developed as a slowly propagating front. In these cases, salt re‐precipitation resulted in clogging and cessation of flow through a few of the salt cores. Solution density was found to be important, as evidenced by the fact that more channels developed upward than downward, due to gravitational fractionation. The development of dissolution channels could have very important implications for the overall permeability of the salt layer in general, and the use of salt formations for industrial waste storage and the development of sinkholes along the Dead Sea shore in particular.

The effectiveness of electrical resistivity imaging in sinkhole investigations

Electrical resistivity methods are generally used to explore buried sinkholes and to predict dropout, sinkholes is analysed. A sinkhole of known geology is used to illustrate the limitation of vertical electrical sounding technique. Approximated geological sections of the sinkhole are modelled to investigate the effectiveness of the resistivity imaging which is considered to be two-dimensional exploration approach. The current study demonstrates that the resistivity imaging method is able to constrain the subsurface geological information for engineers, and to decide if soil improvement is needed for these regions of e.g. karstic terrain. If appropriate array distances, comparable to diameter and the depth of sinkhole are selected, the stratigraphy and the structure of the sinkholes can be mapped effectively, and to assess the potential for the future sinkhole development is monitored easily. Key words: Resistivity imaging, sinkholes.

Identification, remediation, and analysis of karst sinkholes in the longest railroad tunnel in South Korea

Tunneling in karst terrains presents numerous challenges to the engineer including the unpredictable occurrence of cavities and well-developed hydraulic conduits that could lead to sinkhole development. During construction of the longest railroad tunnel in South Korea, excessive water inrushes and support collapses accompanying the formation of a couple of sinkholes and subsidence on the ground surface. Such cases of sinkhole formation during tunneling in South Korea have been rarely reported. Various characterization methods were used at the tunneling site to identify the causes of the sinkholes and the subsidence on the ground surface. Especially noteworthy in this regard is that the massive limestone in this area contains a high degree of lime purity, as high concentrations of lime are very corrosive. A limestone micro-scale cavity readily developed along the fault planes and joints due to weathering and chemical corrosion. The data obtained from various types of geological and geophysical exploration methods identified the formation of micro-scale cavity networks that served as conduits, carrying reservoir water from the surface to the underground tunnel. Additional micro-scale vertical cavity networks were detected for the unexcavated tunnel route through geophysical and horizontal probe core drilling methods. Ground reinforcement was applied to sinkhole and subsidence at the ground surface and the cavity inside the tunnel for remediation. After the installation of underground waterproof grouting around the tunnel and of grouting walls around the sinkholes, the groundwater level recovered and the ground surface settlement ceased. Seepage analyses were performed to provide an optimal drainage design for long-term stability. Based on this case study and the lessons learned, we suggest a risk reduction system for safe tunneling in karst terrains.

Three-dimensional seismic geomorphology and analysis of the Ordovician paleokarst drainage system in the central Tabei Uplift, northern Tarim Basin, western China

High-quality three-dimensional seismic data acquired in the central Tabei Uplift, Tarim Basin, western China, provide a rare opportunity to characterize in exceptional detail the three-dimensional geomorphology of a deeply buried (5500–6500 m [18,045–21,325 ft]) Ordovician unconformity and the related paleokarst drainage system. An integrated approach was applied that emphasized integration of seismic data with available conventional core, wireline logs, and age-equivalent outcrops. The exceptional quality of the seismic data allowed a seismic detection limit of karstified features of less than 75 75 m (246 246 ft) horizontally and 6 m (20 ft) vertically. Interpreted geomorphologic and depositional elements include fluvial channels and canyons, fluvial valleys, sinkholes, and tower karsts and hills. The modern tower karst-drainage system in Guilin, China, is very similar to the mapped Ordovician karst-drainage system and is used as a modern analog. The interaction between the surface karst-drainage system and the shallow-subsurface cave-passage system is evidenced by the observation that surface canyons appear to initiate in areas associated with intense sinkhole development. Also, surface river valleys tend to correspond to dip-oriented surface depressions partly related to near-surface cave collapse. During burial into the deeper subsurface, the combination of intrastratal collapse (karstified strata) and suprastratal collapse (postkarst-deposited strata) created large damage zones hundreds of meters thick and kilometers wide. Coalesced-collapsed paleocave systems can be interpreted from the unique circular pattern of faults (observed in map view) that are associated with seismic bright spots.

TEM study of the geoelectrical structure and groundwater salinity of the Nahal Hever sinkhole site, Dead Sea shore, Israel

Since the 1990s a large number of sinkholes have appeared in the Dead Sea (DS) coastal area. Sinkhole development was triggered by the lowering of the DS level. In the literature the relationship between the sinkholes and the DS level is explained by intrusion of relatively fresh water into the aquifer thereby dramatically accelerating the salt dissolution with creation of subsurface caverns, which in turn cause sinkholes. The main goal of our project was detection and localization of relatively fresh groundwater. During our study we used the transient electromagnetic method (TEM) to measure the electrical resistivity of the subsurface. As a test site we selected Nahal Hever South which is typical for the DS coast. Our results show that resistivity of the shallow subsurface reflects its vertical and lateral structure, e.g., its main hydrogeological elements explain the inter-relations between geology, hydrogeology, and sinkholes. The TEM method has allowed detailed differentiation of layers (clay, salt, etc.) in the subsurface based on their bulk resistivity. The 10 m-thick salt layer composed of idiomorphic crystals of halite deposited during the earlier Holocene period was extrapolated from borehole HS-2 through the study area. It was found that in Nahal Hever the typical value of the bulk resistivity of clay saturated with the DS brine varies between 0.2 and 0.3 Ωm, whereas saturated gravel and sandy sediments are characterized by resistivity between 0.4 and 0.6 Ωm. The high water salinity of the aquifer (enveloping the salt layer) expressed in terms of resistivity is also an important characterization of the sinkhole development mechanism. The electrical resistivity of the aquifer in the vicinity of the salt unit and its western border did not exceed 1 Ωm (in most cases aquifer resistivity was 0.2–0.6 Ωm) proving that, in accordance with existing criteria, the pores of the alluvial sediments are filled with highly mineralized DS brine. However, we suggest that the criterion of the aquifer resistivity responsible for the salt dissolution should be decreased from 1 Ωm to 0.6 Ωm corresponding to the chloride concentration of approximately 100 g/l (the chloride saturation condition reaches 224 g/l in the northern DS basin and 280 g/l in the southern one). Based on TEM results we can reliably conclude that in 2005, when most of sinkholes had appeared at the surface, salt was located within a very low resistivity environment inside sediments saturated with DS brine. Intrusion of relatively fresh groundwater into the aquifer through the 600 × 600 m2area affected by sinkholes has not been observed.

The formation of cover collapse sinkholes in North of Hamedan, Iran

Most sinkholes in the Hamedan region central plain in western Iran are developed in partially unconsolidated sediments overlying Oligo-Miocene limestone. Analyses of hydrologic and hydrogeologic conditions from 1988 to 2008 show that conditions were favourable for sinkhole development. These sinkholes developed in groundwater discharge areas near the base of karstic Oligo-Miocene limestone in zones with major groundwater circulation, thick overburden and a close proximity to deep-source carbon dioxide gas. High carbon dioxide partial pressures, caused by rising gases from pneumatolytic and hydrothermal phase sources, renew the aggressiveness of the water in the vicinity of the discharge zone. The acidic water dissolves limestone, especially along the fractures and joints. Moreover, the presence of large conduits below the surface allows for the transport of unconsolidated sediments and the continual development of sinkholes.

Geologic and hydrogeologic factors affecting sinkhole (obruk) development in Central Turkey

Konya region is located near Tuzgolu (salt lake) in the center of Turkey. The surface of the study area which is between Cumra-Karapinar and Aksaray-Sultanhani-Obruk plains in the Konya region is 2,100 km2. Obruk landforms are concentrated in a 100 km2 section of the study area which contains approximately 50 obruks. The climate is typically continental and annual average precipitation varies between 314.2 mm/year in Sultanhani, 298.4 mm/year in Karapinar, and 333 mm/year in Besagil. Evaporation value is 1,444 mm/year. In the study area, agricultural activities are carried out on 5,115 ha of land, which is irrigated from the 109 wells drilled and operated by several soil-water cooperatives. The static water levels in the wells vary between 75–91 m. Even though Karapinar and Sultanhani plains are closed basins, there is groundwater connection between them. Since the groundwater flow in this area is canalized between some lithological and tectonic barriers, the underground formations are eroded by groundwater flow moving between the limestone. Over time, dissolution occurs within the limestone formations and they eventually collapse. Generally speaking, circular obruks are formed in horizontal layers and oval ones are observed in layers with inclination. Some obruks reach the groundwater table. The main composition in the study area is, from the oldest to the youngest, Paleozoic schist–fillit–quartzite–crystallized limestone and Mesozoic ophiolites, Cenozoic limestone with pebble, marl, clay, calcite, silica, dolomites and tufa, marl layers with gypsum and salt, lacustrine limestone with tufa layers and andesite near the volcanites, sediments of clay, sand, pebble, and silt of Plioquaternary and basalt flows. Samples collected from all of the sampling stations were analyzed and hydrochemical graphs were prepared. According to hydrochemical evaluations, pH is between 7.1 and 8.2, EC is between 840 and 1,965 μmho/cm, and hardness varies between 40 and 81 FS°, and the water is classified as C3 S1 category according to US Salinity Laboratory interpretations. With respect to irrigation water criteria, the water is of good quality, but not good as drinking water. In the study, the effects of dissolved CO2 in groundwater on karstification were also investigated. Water from wells drilled at Yarimca, Acikkuyu, and Badirik plains contains large amounts of CO2. As is commonly known, those waters which contain CO2 more than 50 mg/l are considered to be corrosive and can cause dissolution of limestone. Through the preparation of topographical elevation maps of the groundwater, water level maps were drawn and the flow direction of groundwater, recharge area of groundwater, groundwater hydraulic balance and level fluctuations of the groundwater, as well as the locations of possible exploitation sites were determined.

Middle Miocene aquatic and wetland vegetation of the paleosinkhole at Tarnów Opolski, SW Poland

I analyzed spores, pollen and organic-walled algal remains in Middle Miocene deposits that filled a paleosinkhole at Tarnow Opolski, in the Upper Silesian Upland, Poland. Middle Triassic limestone bedrock in the study area underwent intense karstification under relatively warm and humid climate, resulting in development of numerous sinkholes. As the corrosion reached non-karstified marls, flowing water in the vadose zone was plugged and meteoric waters thus accumulated, leading to pond development. This palaeobotanical study was undertaken to “reconstruct” the fossil freshwater algae community and vegetation in and around the water body. Chlorophyta such as Botryococcus, filamentous algae from the family Zygnemataceae, mainly Mougeotia, Spirogyra and Zygnema, and desmids were important components of the algal assemblage. Most of the identified algae prefer mesotrophic to eutrophic conditions, and are characteristic of stagnant or slow-flowing shallow water. Among the free-floating and rooted plants were Salviniaceae/Azollaceae (probably Salvinia), Nuphar, Potamogeton, and carnivorous taxa such as Utricularia and probably Aldrovanda. The water body was surrounded by swamp vegetation composed of herbs such as Typha, Sparganium, Alismataceae and others, and riparian forests. The upper portion of the sediment section contains abundant pollen of swamp forest taxa such as Taxodium/Glyptostrobus, Nyssa and Alnus. The paleobotanical remains from the sinkhole at Tarnow Opolski suggest it was a shallow water body that was probably prone to rapid warming. Presence of resting cells, e.g. zygospores of Zygnemataceae and desmids, suggests that the pond may have dried periodically. Presence of Nuphar, Utricularia and Aldrovanda pollen, however, indicates long periods with standing water.

Human-induced hydrogeological changes and sinkholes in the coastal gypsum karst of Lesina Marina area (Foggia Province, Italy)

In 1930, a canal was excavated to connect the Lesina Lagoon with the Adriatic Sea, modifying the track of the previous existing Acquarotta canal. The 1100 m long stretch of the canal situated next to the coast exposed unknown highly cavernous gypsum bedrock underlying a loose sandy cover. During the last two decades, a large number of cover collapse and cover suffosion sinkholes have formed along two bands situated next to the canal, impinging the adjacent Lesina Marina residential area. The area affected by subsidence has increased exponentially from 1999 to 2009. The tight spatial correlation between the sinkholes and the canal reveals that the subsidence phenomenon has been induced by the local hydrogeological changes caused by the canal in the coastal evaporite aquifer: (1) Local lowering of the average water table. (2) Deflection of the groundwater flow lines towards the canal and increase in flow velocity. (3) Amplification of the groundwater level oscillations, largely controlled by the tidal regime. (4) Local reversals in the groundwater flow, changing the canal temporarily from effluent to influent. These changes in the hydrogeological functioning of the system have favoured both internal erosion and karstification processes. Hydrochemical evidence reveals that gypsum dissolution is a currently active process favoured by fresh water and sea water mixing and cation-exchange processes. Most likely, the partial replacement of a concrete lining in the canal by pervious gabions in 1993 provided more adequate conditions for the evacuation of the sediments filling the karst conduits, accelerating internal erosion and sinkhole development.

Salt-dissolution-induced subsidence in the Dead Sea area detected by applying interferometric techniques to ALOS Palsar Synthetic Aperture Radar images

This paper discusses the interpretation of ground motions detected in the dried up Lynch Strait, Dead Sea area, by applying radar interferometric techniques to ALOS Palsar Synthetic Aperture Radar images. Four ALOS scenes spanning from December 15, 2007 to May 17, 2008 have been processed leading to the generation of five interferograms. Three ground deformation zones have been detected. One of them shows surface displacement which could be related to an earthquake (ML 3.1) that took place on April 13, 2008. High rates of subsidence have been measured in the northern Lynch Strait. They suggest that these subsidence phenomena follow the same trend of rapid increase as sinkholes. Additional measurements should be carried out in order to refine this observation. The comparison between sinkholes' distributions in the Lynch Strait with that of Ghor Al Haditha, six kilometers eastward, supports the idea that the earthquake that hit the southern Dead Sea on April 23, 1979 (M 5.1) reactivated faults and fractures in the Lynch Strait triggering the development of sinkholes and subsidence in the frame of the Dead Sea recession.

The Dead Sea sinkhole hazard new findings based on a multidisciplinary geophysical study

A geophysical study has been carried out in the Dead Sea (DS) coastal area of Israel and Jordan with the goal of better understanding the development of sinkholes in the area. The following surface geophysical methods have been applied: (1) Seismic Refraction method for mapping the buried salt formation; (2) Microgravity and Magnetic Resonance Sounding (MRS) for detecting cavities in the subsurface; (3) Transient Electromagnetic method (TEM) for estimating groundwater salinity. It has been found that: (1) sinkholes have formed within a strip 50-100 m wide along the salt formation edge; (2) a zone with a large density of cavities with a total volume of tens of thousands of cubic meters have been inferred in sinkhole sites; (3) cavities underlying sinkholes are filled with unconsolidated sediments locally reducing hydraulic conductivity. Further development of new cavities has not been detected; (4) groundwater salinity variations along the DS shore are insignificant and changes in the electrical conductivity of the groundwater obtained using TEM were in most cases related to changes of porosity caused by collapse of subsurface sediments. (5) the applied geophysical methods provide valuable data on the development of sinkholes in the DS coastal areas.

The Impact of Some Lithological and Structural Variableson Geoelectrical Properties of Mishraq SulphurField (M-1) Northern Iraq

This work includes the preparation of aerial photogeological map and the study of many geological profiles. It also involves the study of stratigraphical division and correlations of the exposed bedrocks at Butmah-East and Raven anticlines, particularly the Fat'ha Formation near Mosul Dam northern Iraq. It is evident that the sinkholes persisting through the area in question were initiated primarily within the upper part of the upper member of Fat'ha Formation. These were produced from the dissolution of thick gypsum beds in addition to the structural interplay and intersections of lineaments and faults. It can be concluded that the recent sinkhole in the tourist city is a product of faults crossing and the study could predict a new sinkhole development about 200m to the southeast of the present one.

Prediction of cavern configurations from subsidence data

An analytical method has been developed to predict the location, depth and size of caverns created at the interface between salt and overlying formations. A governing hyperbolic equation is used in a statistical analysis of the ground survey data to determine the cavern location, maximum subsidence, maximum surface slope and surface curvature under the sub-critical and critical conditions. The regression produces a set of subsidence components and a representative profile of the surface subsidence under sub-critical and critical conditions. Finite difference analyses using FLAC code correlate the subsidence components with the cavern size and depth under a variety of strengths and deformation moduli of the overburden. Empirical equations correlate the subsidence components with the cavern configurations and overburden properties. For the super-critical condition, a discrete element method (using UDEC code) is used to demonstrate the uncertainties of the ground movement and sinkhole development resulting from the complexity of the post-failure deformation and joint movements in the overburden.

Salt karst and tectonics: sinkholes development along tension cracks between parallel strike‐slip faults, Dead Sea, Jordan

AbstractThis work deals with the tectonic interpretation of an alignment of more than 300 sinkholes stretching along the Jordanian coast of the Dead Sea, Ghor Al Haditha area. Its dimensions are 6 km long with a width of 600 m. Sinkholes appeared during the last decades as a consequence of the very rapid lowering of the lake level. The linear shape was inferred from ground collapse inventories carried out between 1991 and 2008. The lineament is replaced and analyzed in its structural setting at regional and local scales. Its direction (N 24° E) is sub‐parallel to the ones displayed by many focal mechanisms, especially the one associated with the earthquake of the 23 April, 1979 (mb = 5·1; N 20° E ± 5°), which is representative of all focal mechanisms calculated on a fault plane compatible with the general direction of the Jordan‐Dead Sea Transform fault system for the east coast of the Dead Sea area.The alignment of sinkholes is constituted by 13 minor linear segments separated by as many empty spaces. Four minor linear units present an en‐echelon arrangement from which one can deduce the presence of a local extensional stress field. In this context, the sinkhole locations provide information of subsurface discontinuities interpreted as hidden fractures. In a close future, such results could support the work of decision‐makers and engineers in the projected development of the area. Copyright © 2009 John Wiley & Sons, Ltd.

The evolving landscape and climate of western Flores: an environmental context for the archaeological site of Liang Bua

The rapidly changing landscape of the eastern Indonesian archipelago has evolved at a pace dictated by its tropical climate and its geological and tectonic history. This has produced accelerated karstification, flights of alluvial terraces, and complex, multi-level cave systems. These cave systems sometimes contain a wealth of archaeological evidence, such as the almost complete skeleton of Homo floresiensis found at the site of Liang Bua in western Flores, but this information can only be understood in the context of the geomorphic history of the cave, and the more general geological, tectonic, and environmental histories of the river valley and region. Thus, a reconstruction of the landscape history of the Wae Racang valley using speleothems, geological structure, tectonic uplift, karst, cave, and terrace development, provides the necessary evidence to determine the formation, age, evolution, and influences on the site. This evidence suggests that Liang Bua was formed as two subterranean chambers ∼600 ka, but could not be occupied until ∼190 ka when the Wae Racang wandered to the southern side of the valley, exposing the chamber and depositing alluvial deposits containing artifacts. During the next ∼190 k.yr., the chambers coalesced and evolved into a multi-level and interconnected cave that was subjected to channel erosion and pooling events by the development of sinkholes. The domed morphology of the front chamber accumulated deep sediments containing well stratified archaeological and faunal remains, but ponded water in the chamber further prevented hominin use of the cave until ∼100 ka. These chambers were periodically influenced by river inundation and volcanic activity, whereas the area outside the cave was greatly influenced by glacial phases, which changed humid forest environments into grassland environments. This combined evidence has important implications for the archaeological interpretation of the site.

Sinkholes in the salt-bearing evaporite karst of the Ebro River valley upstream of Zaragoza city (NE Spain): Geomorphological mapping and analysis as a basis for risk management

A detailed sinkhole map has been produced in a stretch of the Ebro Valley (40.8 km 2) including the western sector of Zaragoza city (NE Spain). During the last few decades, around 70% of the original sinkhole area has been filled with anthropogenic sediments causing the disappearance of 137 ha of wetlands. The interstratal karstification of salts (halite and glauberite) and a WNW–ESE-trending joint set have played a major control in the development of sinkholes. Three morphometric types of sinkholes have been differentiated, each attributed to a specific subsidence mechanism inferred from the paleosinkholes exposed in the surrounding of Zaragoza city; sagging of bedrock and cover, collapse of bedrock and cover, and collapse of cover material related to the downward migration of particles through dissolutional conduits. Each type of sinkhole is characterised by a distinctive behaviour in terms of controlling factors, spatio-temporal distribution and kinematics, and consequently the proposed differentiation may have a practical utility. The vast majority of the subsidence damage identified in the area occurs within the boundaries of pre-existing sinkholes identifiable in old aerial photographs and topographical maps. This fact demonstrates that the application of preventive planning strategies based on detailed geomorphological maps would have allowed avoidance of most of the large financial losses caused by subsidence in the area, of the order of hundreds of thousands of euros per year.

The San Vittorino Sinkhole Plain: relationships between bedrock structure, sinking processes, seismic events and hydrothermal springs

The San Vittorino Plain is located along the Velino River Valley within an intermontane depression, filled with continental deposits. This plain is characterized by the presence of numerous sinkholes, often filled with mineralized spring waters. From a structural point of view, the study area is characterized by four tectonic units with different paleogeographical evolution and deformation styles. From the innermost unit they are: the Reatini Unit, the Navegna Unit, the Salto Unit, and the Nuria Unit. The main tectonic lineaments are represented by the Anzio-Ancona line, the Olevano-Micciani line, the Fiamignano-Micciani Fault and the Cerquara-Piedimozza Fault. The development of sinkholes in the study area is mainly due to deep suffusion and deep piping processes, linked to fluid upwelling along fracture-fault systems and to dissolution (by H2S and CO2) in the continental deposits. The sinkhole distribution suggests that they are mainly formed in a collapsed sector of the Mt. Paterno block whose structural setting, characterized by several splays, with marly-clayey and siliciclastic deposits at the footwalls, favoured the development of confined aquifers. The rise of groundwater to the surface along the network of faults and fractures crossing the carbonate bedrock is likely controlled by hydrostatic pore pressures, which may be strongly increased by earthquake shaking. The surface distribution of sinkholes in the San Vittorino Plain, showing differences within the study area, is strictly linked to the bedrock structure and the related hydrogeological setting, as demonstrated by the detailed survey carried out in the area.

Study of the factors affecting the karst volume assessment in the Dead Sea sinkhole problem using microgravity field analysis and 3-D modeling

Abstract. Thousands of sinkholes have appeared in the Dead Sea (DS) coastal area in Israel and Jordan during two last decades. The sinkhole development is recently associated with the buried evaporation karst at the depth of 25–50 m from earth's surface caused by the drop of the DS level at the rate of 0.8–1.0 m/yr. Drop in the Dead Sea level has changed hydrogeological conditions in the subsurface and caused surface to collapse. The pre-existing cavern was detected using microgravity mapping in the Nahal Hever South site where seven sinkholes of 1–2 m diameter had been opened. About 5000 gravity stations were observed in the area of 200×200 m2 by the use of Scintrex CG-3M AutoGrav gravimeter. Besides the conventional set of corrections applied in microgravity investigations, a correction for a strong gravity horizontal gradient (DS Transform Zone negative gravity anomaly influence) was inserted. As a result, residual gravity anomaly of –(0.08÷0.14) mGal was revealed. The gravity field analysis was supported by resistivity measurements. We applied the Emigma 7.8 gravity software to create the 3-D physical-geological models of the sinkholes development area. The modeling was confirmed by application of the GSFC program developed especially for 3-D combined gravity-magnetic modeling in complicated environments. Computed numerous gravity models verified an effective applicability of the microgravity technology for detection of karst cavities and estimation of their physical-geological parameters. A volume of the karst was approximately estimated as 35 000 m3. The visual analysis of large sinkhole clusters have been forming at the microgravity anomaly site, confirmed the results of microgravity mapping and 3-D modeling.

Identification of sinkhole development mechanism based on a combined geophysical study in Nahal Hever South area (Dead Sea coast of Israel)

Seismic refraction, magnetic resonance sounding (MRS), and the transient electromagnetic (TEM) method were applied to investigate the geological and hydrogeological conditions in the Nahal Hever South sinkhole development area at the Dead Sea (DS) coast of Israel. Microgravity and MRS results reliably reveal large karst cavity in the central part of investigated area. The map of the seismic velocity shows that sinkholes in Nahal Hever can be divided into two major groups: sinkholes close to the salt edge and sinkholes over compact salt formations between a few tens to a hundred meters from the major cavern. The present study shows that the formation of sinkholes of the first group is caused by soil collapsing into the cavern. In the area occupied by sinkholes of the second group, karst was not detected either by MRS or by seismic diffraction methods. TEM results reveal shallow clay layer saturated with DS brine underlain sinkholes of this group. It allows suggestion that the water drainage and intensive water circulation during rain events wash out fine rock particles from the unsaturated zone into the pre-existing cavern, initiating the formation of sinkholes of the second group. Karst development takes place at a very low bulk resistivity (<1 Ω m) of the DS aquifer, attesting to the fact that pores are filled with a highly saline solution. Refilling of the karstic cavities with collapsing and flushed soil slows down sinkhole development in the area. The sinkhole formation cycle at the site is estimated at 10 years. Sinkhole development throughout the studied area is triggered by a drop in the level of the DS, which reduces the head of the confined aquifer and the strength of the overlain sediments.