The Application of Electrical Resistivity Imaging in Collapse Doline Floors: Divača Karst, Slovenia

Electrical Resistivity Imaging (ERI) is a widely used tool in geophysical surveys for investigation of various subsurface structures. In this study an ERI was conducted in collapse doline floors located in Divača karst, Slovenia. This group of

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  UROŠ STEPIŠNIK (LJUBLJANA) THE APPLICATION OF ELECTRICAL RESISTIVITY IMAGINGIN COLLAPSE DOLINE FLOORS: DIVAČA KARST, SLOVENIA   Abstract:  Electrical Resistivity Imaging (ERI) is a widely used tool in geophysical surveys for investiga-tion of various subsurface structures. In this study an ERI was conducted in collapse doline floors lo-cated in Divača karst, Slovenia. This group of collapse dolines in hinterland of River Reka ponors havefloors inundated and flattened with loamy sediment. Collapse doline development and transforma-tion processes are discussed, and various characteristics and potential formation mechanisms of flatand loamy doline floors are considered. The loamy fills might reflect sedimentationof suspended ma-terialfromfloodwatersthatinundatedthelower partsofthecollapsedolines,orcouldhaveoriginatedin now-demolished cave passages on the slopes that were filled with finer material. Key words:  karst, collapse doline, loamy sediment, Electrical Resistivity Imaging (ERI), Slovenia INTRODUCTIONKarst surface in Slovenia covers an area of about 44% or 9,000 km 2 (Novak1993). It developed mainly in limestone and dolomite bedrock. The Sloveniankarst includes at least 330 major collapse dolines with volumes ranging from0.03 Mm 3 up to 12.6 Mm 3 . The larger collapse dolines are usually situated in areasof karst through flow, mainly in the hinterland of the larger ponors and inthe catchments of major karst springs. Collapse dolines are more common in thehinterland of the ponors of the Matarsko podolje area, in the Ljubljanica Rivercatchment and on the Kras (Karst, Carso) plateau.The article discusses collapse dolines in the southeast part of the Kras pla-teauwhichwillbereferredtoastheDivačakarst.Detailedgeomorphicanalysisof several collapse doline floors in the area is discussed in this article. Thesubsurface structure of the doline floors was established using Electrical Resisti- vity Imaging (ERI) techniques, with subsequent interpretation of the ERI data. A SuperSting R1/IP earth resistivity meter, developed by Advanced Geosciences,Inc., was used for data collection. The data were processed to generate two-di-mensional resistivity models using EarthImager 2D resistivity inversion software. S T U D I A G E O M O R P H O L O G I C A C A R P A T H O - B A L C A N I C A  VOL. XLII, 2008: 41–51 PL ISSN 0081-6434L A N D F O R M E V O L U T I O N I N M O U N T A I N A R E A S  This method has been confirmed to be appropriate for providing a robust visuali-zation of the epikarst structure and the subsurface structure of the collapsedolines (Stepišnik and Mihevc 2008). The research included geomorpho-logical mapping of several collapse dolines and granulometrical and petrologicalanalyses of loamy sediment from the collapse doline floors.ELECTRICAL RESISTIVITY IMAGING Although Electrical Resistivity Imaging has been successfully utilised for char-acterising the subsurface for many years, it has certain limitations. The method waslabour-intensive, interpretation of data was time-consuming and the method itself based on individual subjective interpretation (Roman 1951; Zhou et al. 2002).Development of computer controlled multielectrode resistivity survey systems andthe development of resistivity modelling software have allowed more cost-effectiveresistivitysurveys andbetter interpretationof the subsurface (Locke andBarker1996). These surveys are usually referred to as Electrical Resistivity Imaging (ERI) orElectricalresistivitytomography(ERT)(Zhou etal.2002).Thesefactsallowdatatobe collected and processed quickly so the electrical Resistivity Imaging surveys be-come a valuable tool in subsurface investigations (Zhou et al. 2000).Electrical Resistivity Imaging surveys are typically conducted to determinethe resistivity of the subsurface. Resistivity data can be used to determine the lo-cation of various geologic and soil strata, bedrock fractures, faults and voids. Fun-damental to all resistivity methods is the concept that the current is impressedinto the ground and the effect of this current within the ground can be measured.The effect of potential or differences of potential, ratio of potential differences, orsome other parameter thatis directly related to these variables are the most com-monly measured effect of the impressed current. The principal differencesamong various methods of electrical resistivity lie in the number and spacing of the current and potential electrodes, the variable quantity determined and themanner of presenting the results (EarthImager 2003; Zhou et al. 2000).Generally, carbonate rock has a significantly higher resistivity than loamy material because of its considerably smaller primary porosity and fewer inter-connected pore spaces. Its resistivity value is about 1000 ohm · m (Telfordet al. 1990). Loamy materials can hold more moisture and have higher ion con-centration to conduct electricity; therefore, their resistivity values are below250 ohm · m (Telford et al. 1990). The high contrast in resistivity values be-tween carbonate rock and loamy material favours the use of Electrical resistivi-ty method to determine the boundary between bedrock and overburden orloamy sediment (Zhou et al. 2000). A frequently occurring problem with Electrical Resistivity Imaging is the de-termination which electrode configuration will respond best to the materialchanges in karst features. Each array has distinctive advantages and disadvan-42  tages in terms of sensitivity to the material variations, depth of investigation andsignal strength. The most typical arrays are dipole-dipole array, Wenner array andSchlumberger array. The dipole-dipole array gives good horizontal resolution of data while Wenner and Schlumberger arrays are more directed in vertical resolu-tion. In the application to karst surveys, the dipole-dipole array has provided high-est precision of ground changes sensitivity and had greatest sensitivity to verticalresistivity boundaries (Zhou et al. 2002).COLLAPSE DOLINESCollapse dolines are surface karst depressions of varied shape and size. Vol-umes of larger collapse dolines exceed the volumes of the largest known cavechambers in the area, so collapse doline formation cannot be related solely to a se-ries of collapse processes within cave chambers and eventually on the surface(Habič 1963; Šušteršič 1973; Stepišnik 2004; Waltham et al. 2005; Ste-pišnik 2007). Their origin is related to the concentric removal of material, with as-sociated collapse of underground chambers, or with gradual removal of tectoni-cally fractured carbonate bedrock above active cave passages (Habič 1963;Mihevc 2001;Stepišnik 2004).Althoughcollapsedolineshavecommonlybeendefined as depressions that are formed above cave chambers (Cramer 1941;Gams 1983;Šušteršič 1973;Ford andWilliams 1989),avarietyofspeleoge-netic mass removal processes contribute to their development.Formationofsmaller collapse dolinesisrelatedtocavechambercollapse. Attheinstantofcollapse, aqualitativemodificationtakesplaceasasubsurfacekarstfeature becomes a surface karst feature. From this moment onward a combina-tion of speleogenetic processes and a variety of exogenic geomorphic processesbegin to operate. Development of larger collapse dolines involves gradual re-movalofmaterialabovetheactivecavepassages. Durationoftheprocessdefinesthevolumeofthecollapsedolinesandthedynamicsoftheprocessdefinesthein-clination and morphology of the slopes. Dominance of material removal over therate of weathering of bedrock on the doline margins results in the formation of steep slopes and walls (Stepišnik 2007).Many morphological classifications of collapse dolines have appeared inpublished karstological literature. The most common is a simple subdivision of collapse dolines into “immature” and “mature” or “degraded” (Habič 1963;Šušteršič 1984; Summerfield 1996; Waltham et al. 2005; Waltham2006). However, collapse doline morphology is a result of the establishment of a balance between various geomorphic processes, whose dynamics, extentand duration inside the collapse dolines influence their size and shape.Changes involved depend upon the rates of underground removal of the rock,on slope angles and mechanical properties of the bedrock, which are not uni-form even within a single collapse doline (Stepišnik 2007). Consequently,43  the published, classical, view of collapse dolines, which estimates their age onthe basis of their general morphology, is not appropriate.Collapse doline floors are subjected to a number of processes that result inthe development of a variety of floor morphologies. In collapse dolines undergo-ing continuous removal of material above active cave passages, the floors arerocky with funnel-shaped depressions in accumulated talus. If the process of ma-terial removal is negligible or absent, concave floors occur, and these are filled with the finer fractions of weathered bedrock, commonly covered with soil.Smaller patches of loamy material are not uncommon on the floors of collapsedolines. If their floors lay tens or hundreds of meters above piezometric level andthereisnosoilonthekarstsurface,itmustbeconcludedthatthesoiloriginatedinnow-demolished cave passages on the slopes that were completely filled withfiner material. If weathered material has been completely removed or if the floorlies near the level of the piezometric surface, the lower parts of the collapsedolines are permanently or periodically filled with water or active water flow ispresent. In most cases the floors of such collapse dolines are flooded only occa-sionally, during periods of higher piezometric levels. If floodwaters contain a sig-nificant suspended load, sediment will eventually be deposited from a stagnant water body. Each ensuing flood will result in the deposition of additional loamy sediment layers on the floor. The ultimate outcome of such sedimentation is theestablishment of flat, loamy floors in collapse dolines (Stepišnik 2003; Ste-pišnik 2007). The occurrence of flat collapse doline floors at similar elevationsmight be a result of sedimentation of suspended material from floodwaters thatinundated the lower parts of several neighbouring collapse dolines approxi-mately at the same flooding event (Stepišnik 2004; Stepišnik 2007).DIVAČA KARSTKras is a limestone plateau situated above Trieste Bay in the northern AdriaticSea. Stretching in the Dinaric (northwest–southeast) direction, it is 40 km long and14 km wide and covers about 440 km 2 . With regard to the surrounding regions, it isphysiographically well individualised. Lower flysch regions and the Adriatic Seabound it from the southwest and the northeast, to the northwest it is surrounded by the alluvial Isonzo (River Soča/Isonzo) plain. Towards southeast, the border of Krascan be well defined by the flysch Brkini Hills and the River Reka valley.Divača karst is situated in the south-eastern part of the Kras between the hin-terland of the River Reka ponor and the town of Divača. The bedrock in the areacomprises thickly bedded Cretaceous limestone, dipping approximately 20 de-grees towards the south, bounded in the south and north by Paleogene thin-bed-ded limestone. The Divača karst is positioned northwest of the contact with theEocene flysch bedrock. At the end of extensive blind valley on the eastern flank,the River Reka sinks into the cave system of — Škocjanske jame (5,800 m of pas-44  sages; Central Cave..., 2008) at the elevation 317 m. This large sinking rivercauses substantial oscillations of the water level in the caves and neighbouringkarst surface. In Škocjanske jame water rises up to 90 m (M i h e v c 1984). Afterthe terminal sump River Reka reappears in the Kačna jama (12,750 m of pas-sages; Central Cave 2008) which is situated south of Divača.Surface of theDivača karst is mainlyplanated at theelevation of 430mand isdissected by numerous dissolution dolines, collapse dolines and roofless caves.Dissolution dolines are up to 100 m in diameter and about 10 m deep. Their den-sity can be higher than 200 dolines per km 2 . The volumes vary between somethousands to several tens of thousand cubic meters (Mihevc 1997). On the pla-natedsurfaceitispossibletorecogniseseveralrooflesscaveswhicharedenudedsections of horizontal or subhorizontal epiphreatic cave passages. The largestsection is about 30 m wide and can be recognised for a length of about 600 m(Mihevc 1997). Several authors have investigated the development of collapsedolines and caves on theDivača karst (Radinja 1967;Gams 1983;Gospoda-rič 1984, 1985), classifying collapse dolines by shape and position and according45 Fig. 1. DEM of the Divača karst
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