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Enviroscan Ukrainian Institute of Speleology and Karstology


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Community news

Speleology in Kazakhstan

Shakalov on 04 Jul, 2018
Hello everyone!   I pleased to invite you to the official site of Central Asian Karstic-Speleological commission ("Kaspeko")   There, we regularly publish reports about our expeditions, articles and reports on speleotopics, lecture course for instructors, photos etc. ...

New publications on hypogene speleogenesis

Klimchouk on 26 Mar, 2012
Dear Colleagues, This is to draw your attention to several recent publications added to KarstBase, relevant to hypogenic karst/speleogenesis: Corrosion of limestone tablets in sulfidic ground-water: measurements and speleogenetic implications Galdenzi,

The deepest terrestrial animal

Klimchouk on 23 Feb, 2012
A recent publication of Spanish researchers describes the biology of Krubera Cave, including the deepest terrestrial animal ever found: Jordana, Rafael; Baquero, Enrique; Reboleira, Sofía and Sendra, Alberto. ...

Caves - landscapes without light

akop on 05 Feb, 2012
Exhibition dedicated to caves is taking place in the Vienna Natural History Museum   The exhibition at the Natural History Museum presents the surprising variety of caves and cave formations such as stalactites and various crystals. ...

Did you know?

That geohydrologic unit is an aquifer, a confining unit or a combination of aquifers and confining units comprising a framework for a reasonably distinct geohydrologic system [22].?

Checkout all 2699 terms in the KarstBase Glossary of Karst and Cave Terms


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KarstBase a bibliography database in karst and cave science.

Featured articles from Cave & Karst Science Journals
Chemistry and Karst, White, William B.
See all featured articles
Featured articles from other Geoscience Journals
Karst environment, Culver D.C.
Mushroom Speleothems: Stromatolites That Formed in the Absence of Phototrophs, Bontognali, Tomaso R.R.; D’Angeli Ilenia M.; Tisato, Nicola; Vasconcelos, Crisogono; Bernasconi, Stefano M.; Gonzales, Esteban R. G.; De Waele, Jo
Calculating flux to predict future cave radon concentrations, Rowberry, Matt; Marti, Xavi; Frontera, Carlos; Van De Wiel, Marco; Briestensky, Milos
Microbial mediation of complex subterranean mineral structures, Tirato, Nicola; Torriano, Stefano F.F;, Monteux, Sylvain; Sauro, Francesco; De Waele, Jo; Lavagna, Maria Luisa; D’Angeli, Ilenia Maria; Chailloux, Daniel; Renda, Michel; Eglinton, Timothy I.; Bontognali, Tomaso Renzo Rezio
Evidence of a plate-wide tectonic pressure pulse provided by extensometric monitoring in the Balkan Mountains (Bulgaria), Briestensky, Milos; Rowberry, Matt; Stemberk, Josef; Stefanov, Petar; Vozar, Jozef; Sebela, Stanka; Petro, Lubomir; Bella, Pavel; Gaal, Ludovit; Ormukov, Cholponbek;
See all featured articles from other geoscience journals

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Your search for scenario (Keyword) returned 76 results for the whole karstbase:
Showing 61 to 75 of 76
Jewel Cave, South Dakota, 2012,
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Wiles, Mike

Jewel Cave was first entered in 1900 and in 1908 became the first National Monument established for the protection of a cave. Over 153 miles have been mapped to date, making it the second longest cave in the world. The cave has a prominent barometric wind, which provides evidence that most of the passages remain undiscovered. Recent geological mapping has documented compelling relationships between the cave and the present-day geologic contacts and structure. It found that there is no mappable paleokarst topography within the Jewel Cave quadrangle, and infers that cave “paleofill” is really a more recent “neofill.” These observations suggest that the cave was formed concurrently with the modern landscape and present-day geologic contacts and structure. A working hypothesis ties the key observations to a plausible scenario to encourage reconsideration of the timing and mechanisms responsible for the origin of the cave.


Glacial processes in caves, 2012,
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Luetscher, M.

Glacial processes are known to impinge on many karst systems, of which the active formation of cave ice represents a salient feature. In temperate environments, the preservation of massive, perennial cave ice deposits, comprising sometimes tens of thousands cubic meters, represents probably the most severe test for models of sporadic permafrost distribution. Additionally, stratified cave ice deposits foster detailed glaciochemical investigations to decipher this environmental archive. Recent investigations have shown that the accessible time window for paleoclimate reconstructions sometimes covers several thousands of years, but understanding the relation between external climate change and the cave ice mass balance still remains challenging. Process-oriented studies suggest that interannual cave ice mass balances respond primarily to modifications in the winter thermal and precipitation regimes. By contrast, cave ice ablation is largely driven by heat exchange with the surrounding rock, which is a function of the external mean annual air temperature. Many mid-latitude, low-altitude ice caves are thus likely to disappear under a warming climate scenario. Yet, traces of former glacial processes can be observed in several temperate cave environments. Cryoclasts, solifluction lobes, sorted sediment patterns, cryogenic calcite, and broken speleothems provide clues for the reconstruction of paleo-permafrost. Because they can be accurately dated with U-series methods, cryogenic cave calcites offer a promising field of investigation for past glacial processes 


Mixing of water in a carbonate aquifer, southern Italy, analysed through stable isotope investigations, 2013,
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Petrella E. , Celico F.

Mixing of water was analysed in a carbonate aquifer, southern Italy, through stable isotope investigations (18O,δ2H). The input signal (rainwater) was compared with the isotopic content of a 35-meter groundwater vertical profile, over a 1-year period. Within the studied aquifer, recharge and flow are diffuse in a well-connected fissure network.

At the test site, the comparison between input and groundwater isotopic signals illustrates that no efficient mixing takes place in the whole unsaturated zone, between the fresh infiltration water and the stored water.

When analysing the stable isotope composition of groundwater, significant variations were observed above the threshold elevation of 1062 m asl, while a nearly constant composition was observed below the same threshold. Thus, temporal variations in stable isotope composition of rainwater are completely attenuated just in the deeper phreatic zone.

On the whole, taking into consideration also the results of previous studies in the same area, the investigations showed that physical characteristics of the carbonate bedrock, as well as aquifer heterogeneity, are factors of utmost importance in influencing the complete mixing of water. These findings suggest a more complex scenario at catchment scale.


Glacial Processes in Caves, 2013,
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Luetscher, M.

Glacial processes are known to impinge on many karst systems, of which the active formation of cave ice represents a salient feature. In temperate environments, the preservation of massive, perennial cave ice deposits, comprising sometimes tens of thousands cubic meters, represents probably the most severe test for models of sporadic permafrost distribution. Additionally, stratified cave ice deposits foster detailed glaciochemical investigations to decipher this environmental archive. Recent investigations have shown that the accessible time window for paleoclimate reconstructions sometimes covers several thousands of years, but understanding the relation between external climate change and the cave ice mass balance still remains challenging. Process-oriented studies suggest that interannual cave ice mass balances respond primarily to modifications in the winter thermal and precipitation regimes. By contrast, cave ice ablation is largely driven by heat exchange with the surrounding rock, which is a function of the external mean annual air temperature. Many mid-latitude, low-altitude ice caves are thus likely to disappear under a warming climate scenario. Yet, traces of former glacial processes can be observed in several temperate cave environments. Cryoclasts, solifluction lobes, sorted sediment patterns, cryogenic calcite, and broken speleothems provide clues for the reconstruction of paleo-permafrost. Because they can be accurately dated with U-series methods, cryogenic cave calcites offer a promising field of investigation for past glacial processes in caves.


Cave aerosols: distribution and contribution to speleothem geochemistry, 2013,
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Dredge J. , Fairchild I. J. , Harrison R. M. , Fernandezcortes A. , Sanchezmoral S. , Jurado V. , Gunn J. , Smith A. , Spö, Tl Ch. , Mattey D. , Wynn P. M. , Grassineau N.

There is developing interest in cave aerosols due to the increasing awareness of their impacts on the cave environment and speleothem; this paper provides the first attempt to synthesize the issues. Processes of cave aerosol introduction, transport, deposition, distribution and incorporation are explored, and reviewed from existing literature. Key issues of specific aerosol processes of distribution and production as well as cave location and morphology effects are highlighted through the presentation of preliminary monitoring data. This study identifies the strong relationship between cave ventilation, cave aerosols and their consequent spatial distribution. The contribution of cave aerosol deposition to speleothem geochemistry is modelled and evaluated using a mass balance framework. As an example, speleothem trace element data from Obir Cave (Austria) are compared with aerosol inputs to evaluate their significance. The mass balance study demonstrates that generally, under normal continuous growth and environmental conditions aerosol deposition will be of only minor importance. However, it highlights specific scenarios in which aerosol contributions will be significant: speleothem hiatuses (or slow growth), high aerosol deposition, and secondary microbiological feedback.


MODELLING THE EVOLUTION OF KARST AQUIFERS IN THREE DIMENSIONS/Conceptual models and realistic scenarios Inaugural dissertation/ zur Erlangung des Doktorgrades Dr. rer. nat. am Fachbereich Geowissenschaften im Institut fur Geologische Wissenschaften der Fr, 2013,
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Hiller, Thomas

This work presents the development of three dimensional karst evolution models for various settings and conditions. As karst aquifers are very sensitive to changes of their hydraulic boundary conditions a comprehensive understanding of the governing processes inside a karst aquifer is indispensable. Especially if a karst aquifer is inuenced by anthropogenic utilization like e.g. the construction of a dam site, the resulting changes inside the aquifer need to be understood as good as possible to prevent any unpredictable incidents. The use of numerical models to simulate the development of a karst aquifer is therefore a suitable tool in the preliminary investigations. It will be shown that simple three dimensional damsite models can be used to evaluate the parameters that control the karst aquifer evolution. Based on these simple models an enhanced three dimensional model of a real damsite is developed. This model is used to simulate the evolution of the aquifer close to this damsite and to expose how the construction of the dam inuenced the nearby bedrock signicantly. It is shown that the karstied zone around the dam site is the reason for the subsidence of an adjacent highway. The presented numerical results can be veried by eld observations. Additionally to the damsite models a three dimensional model approach is presented that describes the formation of large collapse dolines. Collapse dolines are signicant surface features of karst landscapes and their evolution which is usually linked to a subsurface karst system is of high interest in the karst community. To simulate the evolution and interaction of such a doline system, a three dimensional model with several spatially distributed dolines is used. There, based on the concept of a mechanically weakened crushed zone, the evolution over time is presented. The applied collapsing mechanism used in this work also allows to estimate the bedrock removal and surface lowering over time. The determined rates are in good agreement with values reported in literature


Marine seismic-reflection data from the southeastern Florida Platform: a case for hypogenic karst, 2013,
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Cunningham, Kevin J.

Recent acquisition of twenty marine seismic-reflection profiles suggests a hypogenic karst origin for the Key Biscayne sinkhole located on the seafloor of Miami Terrace at the southeastern part of Florida Platform. Analysis of the seismic-reflection data strongly suggest the submarine sinkhole was produced by dissolution and collapse of Plio(?)-Pleistocene age carbonate strata. A complex fault system that includes compres-sional reverse faults underlies the sinkhole, providing a physical system for the possible exchange of groundwater with the sinkhole. One seismic profile is suggestive of a mas-ter feeder pipe beneath the sinkhole. The feeder pipe is characterized by seismic-reflection configurations that resemble megabreccia and stratal collapse. The sinkhole is located at a depth of about 365 m below sea level. The record of sea-level change dur-ing the Plio(?)-Pleistocene and amount of subsidence of the Florida Platform during this span of time indicates that the sinkhole has always been submerged at a water depth of about 235 m or more. Thus, the near-surface epigenic karst paradigm can be ruled out. Possible hypogenic models for sinkhole formation include ascending fluids along the fault system, such as, dissolution related to the freshwater/saltwater mixing at a regional groundwater discharge site, or processes related to gases derived from gener-ation of hydrocarbons within deep Mesozoic strata. Hydrocarbon-related karstification provides several possible scenarios: (1) oxidation of deep oil-field derived hydrogen sulfide at or near the seafloor to form sulfuric acid, (2) reduction of Cretaceous or Paleocene anhydrite or both by oil-field methane to form hydrogen sulfide and later oxidation to form sulfuric acid, and (3) carbon-dioxide charged groundwater reacting to form carbonic acid. Further, anerobic microbes could form methane outside of a hy-drocarbon reservoir that ascends through anhydrite to form hydrogen sulfide and later oxidized to sulfuric acid.


Geochemistry and isotope geochemistry of the Monfalcone thermal waters (northern Italy): inference on the deep geothermal reservoir, 2013,
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Petrini R. , Italiano F. , Ponton M. , Slejko F. F. , Aviani U. , Zini L.

Geochemical investigations were carried out to define the origin of the low- to moderate-temperature thermal waters feeding the Monfalcone springs in northern Italy. Chemical data indicate that waters approach the composition of seawater. Mixing processes with cold low-salinity waters are highlighted. The δ18O and δD values are in the range −5.0 to −6.4 ‰, and −33 to −40 ‰, respectively, suggesting the dilution of the saline reservoir by karst-type freshwaters. A surplus of Ca2+ and Sr2+ ions with respect to a conservative mixing is ascribed to diagenetic reactions of the thermal waters with Cretaceous carbonates at depth. The measured Sr isotopic composition (87Sr/86Sr ratio) ranges between 0.70803 and 0.70814; after correction for the surplus Sr, a 87Sr/86Sr ratio indicating Miocene paleo-seawater is obtained. The dissolved gases indicate long-lasting gas–water interactions with a deep-originated gas phase of crustal origin, dominated by CO2 and marked by a water TDIC isotopic composition in the range −5.9 to−8.8 and helium signature with 0.08 < R/Ra < 0.27, which is a typical range for the crust. A possible scenario for the Monfalcone thermal reservoir consists of Miocene marine paleowaters which infiltrated through the karstic voids formed within the prevalently Cretaceous carbonates during the upper Eocene emersion of the platform, and which were entrapped by the progressive burial by terrigenous sediments.


Molecular divergence and evolutionary relationships among Aemodogryllinae from Southern China, Laos and Thailand (Orthoptera, Rhaphidophoridae), 2013,
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Valerio Ketmaier, Claudio Di Russo, Mauro Rampini, Nadine Bernhardt, Ralph Tiedemann, Marina Cobolli

In this study we screened for sequence polymorphisms at one mitochondrial (Cytochrome Oxidase subunit I) and one nuclear (Internal Transcribed Spacer 1) gene 33 populations of the cave cricket generaDiestrammena, ParadiestrammenaEutachycines and Paratachycines from Southern China (three Provinces: Jiangxi, Guangdong and Guizhou), Laos and Thailand. Twenty-five of these populations were assigned to the genusDiestrammena, subgenus Gymnaeta, while the remaining eight belonged to the genera Paradiestrammena (3), Eutachycines (3) and Paratachycines(2). The degree of troglomorphosis varies among them; some populations are blind and depigmented, some have fully developed eyes, while some others show intermediate characteristics. Phylogenetic searches carried out on the two gene partitions separately revealed multiple cases of incongruence but only three of them were statistically significant and were hence removed from the subsequent analyses based on the combined data set. Our data do not support Diestrammena as monophyletic while representatives of ParadiestrammenaEutachycines and Paratachycineswere clustered together; the validity of some nominal species was confirmed molecularly but we also revealed a large number of deeply divergent lineages. Populations with the same degree of troglomorphosis do not cluster together. We identified five major clades; divergence among them (and in a few circumstances also within them) is always higher than the DNA barcode threshold for intraspecific comparisons in insects. In two circumstances, the same clades (III and V) are co-distributed in geographically distinct areas (Provinces). This geographical distribution might be explained by envisioning an evolutionary scenario based on zones of secondary admixture following epigean dispersal among lineages that diverged in allopatry.


Fault — Dissolution front relations and the Dead Sea sinkhole problem, 2013,
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Ezersky Michael, Frumkin Amos

There are two conflicting models of sinkhole development along the Dead Sea (DS). The first one considers structural control on sinkholes, constraining them to tectonic lineaments. This hypothesis is based on seismic reflection studies suggesting that sinkholes are the surface manifestations of active neotectonic faults that may serve as conduits for under-saturated groundwater, enabling its access across aquiclude layers. Another hypothesis, based on results of multidisciplinary geophysical studies, considers the salt edge dissolution front as themajor site of sinkhole formation. This hypothesis associates sinkholes with karstification of the salt edge by deep and shallow undersaturated groundwater. Our recent seismic reflection and surface wave studies suggest that salt formed along the active neotectonic faults. Sinkholes form in a narrow strip (60–100 m wide) along a paleo-shoreline constrained by faults and alluvial fans which determined the edge of the salt layer. This scenario reconciles the two major competing frameworks for sinkhole formation.


Biological Control on Acid Generation at the Conduit-Bedrock Boundary in Submerged Caves: Quantification through Geochemical Modeling, 2013,
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Herman Janet S. , Hounshell Alexandria G. , Franklin Rima B, Mills Aaron L.

No-mount Cave, located in wekiwa Springs State Park in central Florida, USA, is an aphotic, submerged, freshwater cave in which large colonies of sulfur-oxidizing bacteria live in filamentous microbial mats. Upwardly discharging groundwater enters the cave from the Upper Floridan aquifer, specifically the Eocene-aged Ocala Limestone. we undertook a combined field, laboratory, and modeling study in which we sought to determine the amount of calcite dissolution attributable to the generation of protons by microbially mediated sulfide oxidation. The chemical compositions of groundwater within the limestone formation collected through a newly designed sampling device and of water in the cave conduit were used in geochemical modeling. we used the reaction-path model PHREEqCI to quantify the amount of calcite dissolution expected under various plausible scenarios for mixing of formation water with conduit water and extent of bacterial sulfide oxidation. Laboratory experiments were conducted using flow-through columns packed with crushed limestone from the study site. Replicate columns were eluted with artificial groundwater containing dissolved HS- in the absence of microbial growth. without biologically mediated sulfide oxidation, no measurable calcite dissolution occurred in laboratory experiments and no additional amount of speleogenesis is expected as formation water mixes with conduit water in the field. In contrast, significant calcite dissolution is driven by the protons released in the biological transformation of the aqueous sulfur species. Although a range of results were calculated, a plausible amount of 158 mg Ca2+ released to conduit water per liter of groundwater crossing the formation-conduit boundary and mixing with an equal volume of conduit water was predicted. Our modeling results indicate that significant cave development can be driven by microbially mediated sulfide oxidation under these hydrogeochemical conditions


Insights into Cave Architecture and the Role of Bacterial Biofilm, 2013,
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Caves offer a stable and protected environment from harsh and changing outside conditions. They lend living proof of the presence of minute life forms that delve deep within the earth’s crust where the possibility of life seems impossible. Devoid of all light sources and lacking the most common source of energy supplied through photosynthesis, the mysterious microbial kingdom in caves are consequently dependent upon alternative sources of energy derived from the surrounding atmosphere, minerals and rocks. There are a number of features that can be observed within a cave that may serve as evidence of microbial activity, for example, formation of biofilms comprised of multiple layers of microbial communities held together by protective gel-like polymers which form complex structures. Different bacterial biofilms can develop on the walls of the cave which can be visually distinguished by their colorations. Moreover, the pH generated by the metabolism of bacterial biofilm on the cave environment can lead to precipitation or dissolution of minerals in caves. Caves also offer an excellent scenario for studying biomineralization processes. The findings on the association of bacteria with secondary minerals as mentioned in this review will help to expand the existing knowledge in geomicrobiology and specifically on the influence of microorganisms in the formation of cave deposits. This paper reviews the current state of knowledge of biospeleology of caves and the associated bacterial biofilms. Recommendations for future research are mentioned to encourage a drift from qualitative studies to more experimental studies.


Fingerprinting water-rock interaction in hypogene speleogenesis: potential and limitations of isotopic depth-profiling, 2014,
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Spötl Ch, Dublyansky Y.

Dissolution processes in karst regions commonly involve (meteoric) water whose stable isotopic (O, H, C) composition is distinctly different from that of the paleowaters from which the host rock (limestone, dolostone) formed. This, in theory, should lead to isotopic alteration of the host rock beyond the active solution surface as the modern karst water is out of isotopic equilibrium with the carbonate rock. No such alteration has been reported, however, in epigenetic karst systems. In contrast, isotopic alteration, commonly referred to as isotopic halos or fronts, are known from various hypogene systems (ore deposits, active hydro­thermal systems, etc.). These empirical observations suggest that stable isotope data may be a diagnostic tool to identify hypogene water-rock interactions particularly in cave systems whose origin is ambiguous.

We have been testing the applicability of this assumption to karst settings by studying the isotopic composition of carbonate host rocks in a variety of caves showing clear-cut hypogene morphologies. Cores drilled into the walls of cave chambers and galleries were stud­ied petrographically and the C and O isotope composition was analyzed along these cores, which typically reached a depth of 0.5 to 1.2 m. We identified three scenarios: (a) no isotopic alteration, (b) a sigmoidal isotope front within a few centimeters of the cave wall, and (c) pervasive isotope alteration throughout the entire core length. Type (a) was found in caves where the rate of cave wall retreat apparently outpaced the rate of isotopic alteration of the wall rock (which is typical, for example, for sulfuric acid speleogenesis). Type (c) was observed in geologically young, porous limestone showing evidence of alteration zones up to 5 m wide. The intermediate type (b) was identified in hypogene karst cavities developed in tight limestone, dolostone and marble.

Our data in conjunction with evidence from speleothems and their geochemical and fluid-inclusion composition suggest that the spa­tial extent of the isotopic alteration front depends on the porosity and permeability, as well as on the saturation state of the water. Wider alteration zones primarily reflect a higher permeability. Shifts are most distinct for oxygen isotopes and less so for carbon, whereby the amplitude depends on a number of variables, including the isotopic composition of unaltered host rock, the isotopic composition of the paleofluid, the temperature, the water/rock ratio, the surface of water-rock contact, the permeability of the rock, and the time available for isotope exchange. If the other parameters can be reasonably constrained, then semi-quantitative temperature estimates of the paleowater can be obtained assuming isotopic equilibrium conditions.

If preserved (scenarios b and c), alteration fronts are a strong evidence of hypogene speleogenesis, and, in conjunction with hypogene precipitates, allow to fingerprint the isotopic and physical parameters of the altering paleofluid. The reverse conclusion is not valid, however; i.e. the lack of evidence of isotopic alteration of the cave wall rock cannot be used to rule out hypogene paleo-water-rock interaction.


Nonlinear Flow Process: A New Package to Compute Nonlinear Flow in MODFLOW, 2014,
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Mayaud C. , Walker P. , Hergarten S. , Birk S.

A new MODFLOW package (Nonlinear Flow Process; NLFP) simulating nonlinear flow following the Forchheimer equation was developed and implemented in MODLFOW-2005. The method is based on an iterative modification of the conductance calculated and used by MODFLOW to obtain an effective Forchheimer conductance. The package is compatible with the different layer types, boundary conditions, and solvers as well as the wetting capability of MODFLOW. The correct implementation is demonstrated using four different benchmark scenarios for which analytical solutions are available. A scenario considering transient flow in a more realistic setting and a larger model domain with a higher number of cells demonstrates that NLFP performs well under more complex conditions, although it converges moderately slower than the standard MODFLOW depending on the nonlinearity of flow. Thus, this new tool opens a field of opportunities to groundwater flow simulation with MODFLOW, especially for core sample simulation or vuggy karstified aquifers as well as for nonlinear flow in the vicinity of pumping wells.


Sinkholes, pit craters, and small calderas: Analog models of depletion-induced collapse analyzed by computed X-ray microtomography, 2014,
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Volumetric depletion of a subsurface body commonly results in the collapse of overburden and the formation of enclosed topographic depressions. Such depressions are termed sinkholes in karst terrains and pit craters or collapse calderas in volcanic terrains. This paper reports the first use of computed X-ray microtomography (?CT) to image analog models of small-scale (~< 2 km diameter), high-cohesion, overburden collapse induced by depletion of a near-cylindrical (“stock-like”) body. Time-lapse radiography enabled quantitative monitoring of the evolution of collapse structure, velocity, and volume. Moreover, ?CT scanning enabled non-destructive visualization of the final collapse volumes and fault geometries in three dimensions. The results illustrate two end-member scenarios: (1) near-continuous collapse into the depleting body; and (2) near-instantaneous collapse into a subsurface cavity formed above the depleting body. Even within near-continuously collapsing columns, subsidence rates vary spatially and temporally, with incremental accelerations. The highest subsidence rates occur before and immediately after a surface depression is formed. In both scenarios, the collapsing overburden column undergoes a marked volumetric expansion, such that the volume of subsurface depletion substantially exceeds that of the resulting topographic depression. In the karst context, this effect is termed “bulking”, and our results indicate that it may occur not only at the onset of collapse but also during progressive subsidence. In the volcanic context, bulking of magma reservoir overburden rock may at least partially explain why the volume of magma erupted commonly exceeds that of the surface depression.


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