The Unicorn Cave, Southern Harz Mountains, Germany: from Known Passages to Unknown Extensions with the Help of Geophysical Surveys

Journal of Applied Geophysics 123 (2015) 123–140

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Journal of Applied Geophysics

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The Unicorn Cave, Southern Harz Mountains, Germany: From known passages to unknown extensions with the help of geophysical surveys

Georg Kaufmann a,b,⁎,RalfNielbockb, Douchko Romanov a a Institute of Geological Sciences, Geophysics Section, Freie Universität Berlin, Malteserstr. 74-100, Haus D, 12249 Berlin, Germany b Gesellschaft unicornu fossile e.V. Im Strange 12, 37520 Osterode/Harz, Germany article info abstract

Article history: In soluble rocks (limestone, dolomite, anhydrite, gypsum, …), fissures and bedding partings can be enlarged with Received 24 February 2015 time by both physical and chemical dissolution of the host rock. With time, larger cavities evolve, and a network Received in revised form 30 September 2015 of cave passages can evolve. If the enlarged cave voids are not too deep under the surface, geophysical measure- Accepted 6 October 2015 ments can be used to detect, identify and trace these karst structures, e.g.: (i) gravity revealing air- and sediment- Available online 9 October 2015 filled cave voids through negative Bouguer anomalies, (ii) electrical resistivity imaging (ERI) mapping different fi fi Keywords: in llings of cavities either as high resistivities from air- lled voids or dry soft sediments, or low resistivities fl Unicorn Cave from saturated sediments, and (iii) groundwater ow through electrical potential differences (SP) arising from Geophysical survey dislocated ionic charges from the walls of the underground flow paths. Gravity We have used gravity, ERI, and SP methods both in and above the Unicorn Cave located in the southern Harz Electrical resistivity Mountains in Germany. The Unicorn Cave is a show cave developed in the Werra dolomite formation of the Self-potential Permian Zechstein sequence, characterised by large trunk passages interrupted by larger rooms. The overburden Modelling of the cave is only around 15 m, and passages are filled with sediments reaching infill thicknesses up to 40 m. Karst We present results from our geophysical surveys above the known cave and its northern and southern extension, and from the cave interior. We identify the cave geometry and its infill from gravity and ERI measurements, predict previously unknown parts of the cave, and subsequently confirm the existence of these new passages through drilling. From the wealth of geophysical data acquired we derive a three-dimensional structural model of the Unicorn Cave and its surrounding, especially the cave infill. © 2015 Elsevier B.V. All rights reserved.

1. Introduction rocks, which is often very heterogeneously distributed and results in preferential flowpathsinthesub-surface,withcavesaslarge-scaleendmem- Soluble rocks such as limestone, dolomite, anhydrite, gypsum, or salt bers of the sub-surface voids (e.g. Worthington et al., 2001). can be dissolved physically by water, and for limestone and dolomite in Geophysical prospecting offers a variety of methods for detecting addition chemically by water enriched with carbon dioxide. In addition, subsurface voids. The idea behind most of these geophysical methods mechanical enlargement by erosion through moving water can enlarge is a material property of the void that is significantly different from cavities at later stages. The dissolution in a karst rock is driven by water the surrounding host rock and thus makes a material contrast. This ma- flowing through the karst aquifer. Here, the hydraulic properties depend terial contrast can then be detected using a specific geophysical tech- on the history of the rock: in telogenetic rocks, which experienced deep nique (e.g. Butler, 1984; El-Qady et al., 2005; Dobecki and Upchurch, burial during their formation and thus a strong reduction in primary po- 2006; Nyquist et al., 2007; Mochales et al., 2008; Kaufmann, 2014). rosity (e.g. Bathurst, 1979; Palmer, 2007; Ford and Williams, 2007), flow Caves are voids in the host rock. Air- and water-filled voids have a is focussed on fissures and bedding partings in the host rock created by much lower density (0–1000 kg/m3 for air or water) than the host tectonic activity. In eogenetic rocks, which experienced no deep burial rock (2000–2900 kg/m3 for soluble rock). A sediment infill of a cavity and thus less compaction, the host rock has kept its primary porosity has often a lower density than the host rock, too. This difference in den- (e.g. Vacher and Mylroie, 2002) and flow is guided through the still sity is used in a gravimetric survey, a method to map the gravitational existing primary pore space. The enlargement of either fractures or attraction of the surveyed area. A void in the subsurface is responsible pores by dissolution creates a large secondary porosity typical for soluble for a mass deficit; thus, the resulting Bouguer anomaly derived from the gravity survey will be negative. The resulting Bouguer anomaly, however, is ambiguous because gravity as an integral method provides ⁎ Corresponding author at: Institute of Geological Sciences, Geophysics Section, Freie only information about the bulk composition of the subsurface. There- Universität Berlin, Malteserstr. 74-100, Haus D, 12249 Berlin, Germany. E-mail addresses: [email protected] (G. Kaufmann), fore, additional information is needed to model the Bouguer anomaly [email protected] (R. Nielbock), [email protected] (D. Romanov). with appropriate structures.

The electrical conductivity of the host rock is mainly determined by 2. Location fluids circulating in fractures and fissures of the rock. Thus, rock resistivity is determined by the amount of fracturing and the interconnections. In this section, we review the tectonic history of the Harz Mountains, An air-filled cavity is less conductive electrically than the surrounding during which the soluble rocks of the Zechstein period have been de- host rock. A water-filled cavity is more conductive; it should be imaged posited. We then describe the locality of the Unicorn Cave along with as a less resistive region in the survey. Electrical resistivity imaging (ERI) a hypothesis of its evolution in time. maps the resistance of the subsurface; air-filled voids should be visible as highly resistive areas, and water-filled voids should be low- 2.1. Harz Mountains resistance areas. In limestone or dolomite rock, however, the rock itself has a high resistivity (about 1000–2000 Ωm, Telford et al., 1990), which The Harz Mountains are a mountain ridge in Northern Germany, makes the contrast between void and rock fairly small. about 110 × 30–40 km in size, with the Brocken (1141 m) the highest Groundwater percolating through the unsaturated zone can drag peak (Fig. 1). The Harz Mountains with its old magmatic and metamor- electrical excess charges with it. These charges will induce an electrical phic rocks (e.g. Hohl, 1985; Schönenberg and Neugebauer, 1987) are potential difference, which can be mapped with the self-potential (SP) surrounded by soluble rocks in the north (mainly limestone) and method. Groundwater-induced self-potential is often in the range of a south (mainly anhydrite and dolomite). few tens of millivolt. The basement rocks of the Harz Mountains originate from the geosyn- In this paper, we present results from geophysical surveys above and cline of the Iapetus Ocean (around 700–500 Ma, during the Cambrian, in the Unicorn Cave (Southern Harz Mountains, Germany), a shallow Ordovician and Silurian periods) and got folded as part of the Variscian cave located in dolomitic rocks of the Zechstein period. We aim to iden- Mountain Range. The Variscian Mountain Range was subject to erosion, tify the cave and its huge sediment infill in the geophysical signals and almost denuded when the Permian period (300–250 Ma) started. (gravity, ERI, SP) and will then use the expertise gained from geophys- The area was now on the equator, and the shallow tropical Zechstein ical surveys above the known cave to infer possible continuations be- Ocean transgressed and buried the vegetation, resulting in the thin cop- yond the known parts. We then prove the existence of a southward per shale layer below the Zechstein sequence. The periodically retreating continuation by drilling. We finally derive a three-dimensional model and transgressing Zechstein Ocean covered the area between 257– ofthecaveanditssedimentinfill, based and tested against the geophys- 251 Ma, resulting in a sequence of up to seven depositional cycles, com- ical results. prising dolomites, carbonates, anhydrites, halides (group of minerals

Fig. 1. Topographical map of the Harz Mountains and its surrounding. Cities are shown as grey dots, landmarks as grey diamonds, rivers in blue. The grey rectangular outline identifies the working area. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) G. Kaufmann et al. / Journal of Applied Geophysics 123 (2015) 123–140 125 with halogen atom, e.g. halite, sylvite), and shales, which nowadays dom- Unicorn Cave (Fig. 4) is a roof collapse in the southern part, which leads inate much of the sub-surface in North Germany. Each cycle ended with to the large room Blaue Grotte. From here, small continuations lead to the deposition of aeolian material, creating a cover of marls, shales and the southernmost room, the Van Alten Kapelle. Further to the south, pos- clays. After the final disappearance of the Zechstein ocean, the area was sible continuations are completely covered with infill. These infillings covered by erosional material from mountain ridges, creating the upper comprise dolomitic blocks from roof collapse, lutite from dolomite Permian and Triassic rocks (Buntsandstein formation, mainly sand- weathering, and fluvial and glacial deposits. Towards the north, a large stones). Between 200–100 Ma (Jurassic to Cretaceous periods), first trunk passage leads to the room Leibniz-Halle, the largest room of the Pangaea broke up into Laurasia and Gondwana (150 Ma), and later on cave (Fig. 5, about 15 × 40 m wide and 5 m high). From here, a side pas- Gondwana broke up and the modern-day continent and ocean distribu- sage towards the East, the Jacob-Friesen-Gang, ends in sediments very tion was shaped. The Cretaceous Ocean partly covered the Harz region, close to the cliff line, indicating an older, possibly Pleistocene, entrance. depositing rocks such as the Turonian limestone. The Alpine Mountain Following the main passage further north, two smaller rooms follow, Belt rose (100 Ma), and in its wake the Harz was uplifted, but erosion the Schiller-Saal and the Weisser Saal. From the latter room, an artificial again lowered its elevation. A final uplift phase in the Tertiary period tunnel leads to the surface, which is nowadays used as main entrance caused the asymptotic uplift of the Harz, with the northern parts rising for the show cave. Several small passages continue from the Weisser above the Cretaceous sediments. Saal towards the North, all, however, end in thick sediment infill. The The tectonic uplift of the Harz region tilted the lithological units, and entire cave floor consists of sediment deposits, which are rich in fossil parts of the buried Zechstein sequences (Werra, Staßfurt, Leine, Aller) bone fragments. One of these fragments have been misinterpreted by were exposed along the southern Harz foreland (Fig. 2). The salt in the Leibniz (1749) as remnants of the unicorn, a legendary animal, giving stratigraphic sequences exposed on the surface has been quickly dis- theUnicornCaveitsname. solved, but is still present in the buried Zechstein sequences beneath Several dynamic probes with in situ sampling have been lowered large parts of the North German Basin, with prominent layers of several into the cave sediments to reveal its composition and thickness hundred meters thickness. Along the southern Harz foreland, however, throughout the known cave (see thin blue lines in Fig. 4). The sediment the disappearance of the salt has left layers of dolomite, anhydrite and thickness is substantial, from 15 m in the Weisser Saal (north) to more gypsum exposed on the surface. Here, the high solubility of gypsum than 30 m in the Leibnizhalle (south). The sediments consist of se- and anhydrite is responsible for widespread karstification of the region. quences of dolomitic host rock, either as collapsed blocks or as Around the village of Scharzfeld, however, the Zechstein ocean was dissolutional residue (lutite), gravel layers transported into the cave very shallow. On this shallow part, called Eichfeld-Schwelle, reefs devel- by fluvial processes and in parts glacial material resulting from glacial oped, and thus the region is characterised by dolostones and limestones. advances from the higher Harz Mountains during the Pleistocene ice ages (Möller, 1986; Baier, 2004). Note that the sediment infill shown 2.2. Unicorn Cave in Fig. 4 is based on the publication from Paul and Vladi (2001) and is underestimating the real thickness of the sediment infill, as discussed Uplift and erosion caused widespread incision of the rivers coming below. down from the Harz Mountains. The incising rivers have uncovered A borehole (see thick blue lines in Fig. 4) was lowered from the sur- the Paleozoic basement of the Harz, the Graywacke. The Eichfeld- face above the Leibnizhalle, and core samples were collected during the Schwelle has been dissected into several isolated plateaus. One of drilling campaign. From the surface till 14.5 m depth, the Werra dolomite these plateaus is the region of the Brandköpfe (Fig. 3), with dolomite host rock was passed, then between 14.5 and 18 m depth the drill crossed rocks exposed along a steep cliff, and Graywacke as local basement the Leibnizhalle. Then it continued through the cave sediments till a rocks. depth of 47.2 m. In this depth, the Werra dolomite was reached again. The Unicorn Cave is developed in the dolomite rocks of the plateau The borehole has reached a final depth of 53.6 m, with the lowest dolo- Brandköpfe (Nielbock, 1990, 2008, 2010). The natural entrance to the mitic rock rich in fossils. This lowermost dolomite is characteristic in

Fig. 2. Simpliﬁed geological cross section of the Zechstein Ocean (after Herrmann, 1969). Shown are three Zechstein depositional cycles (Werra, Stassfurt, and Leine series), the copper shale as ﬁrst deposition in the Zechstein ocean, and the shallow reef in the Scharzfeld area, covered mainly by dolomite. The dashed line indicate a hypothetical present-day land surface. 126 G. Kaufmann et al. / Journal of Applied Geophysics 123 (2015) 123–140

Fig. 3. Topographical map of the plateau Brandköpfe, with the mapped part of the Unicorn Cave marked in red. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

the southern Harz region for the transition from Werra dolomite to developed in dolomitic hostrock, thus an estimate of the age of the Graywacke, thus the drill hole ended just above the transition to the cave is fairly difficult, as even a minimum estimate from speleothem de- Palaeozoic Graywacke (Paul and Vladi, 2001). A simplified lithology of posits cannot be obtained due to the absence of speleothems in the cave. the borehole is shown in Fig. 5: The Unicorn Cave in its southern part is However, fluvial sediments recovered in several locations of the known entirely located in the Werra dolomite, with an overburden of 14.5 m at cave indicate moving water at some stage of the evolution, possibly a the location of the drill hole. The cave sediments beneath the Leibnizhalle cave stream. We speculate that the upstream end of the Unicorn Cave are more than 35 m thick, and mainly composed of dolomitic blocks, extents towards the northern end of the dolomitic plateau Brandköpfe, which are likely a result of rock and wall collapse in the Unicorn Cave. the so-called Rottsteinklippen (see results section for more evidence). In between, muddy clay deposits with river gravels have been found, Here, surface water from the higher Harz Mountains runs down the which are evidence of surface water passing through the cave during ear- small creek Bremke. The collected surface water originates in a catch- lier periods. The upper sequences also reveal glacial deposits, likely to be ment comprising insoluble rocks, thus the water is free of dissolved cal- carried into the cave during cold spells in the Pleistocene period, during cium and thus able to dissolve the dolomitic host rock. As shown in which a side passage of the Unicorn Cave was opening into a valley, evi- Fig. 6a, we assume that this surface water hits the dolomite outcrop dent from archeological remains in the cave from that period. Similar sed- and starts enlarging fissures and bedding partings in the Werra dolo- iment lithologies have been found from the dynamic probes in the mite. The sub-surface flow direction is towards the river Paleo-Oder as northern part of the Unicorn Cave (Schillersaal, Weisser Saal), but here a base level, which is the same river as the present-day Oder, but yet the sediment thickness is considerably smaller. In the room Weisser not very much incised. Enlargement of the fissures and bedding partings Saal, a dynamic probe reached through the cave sediments into the creates a river cave, connecting the possible sink at the Rottsteinklippen Graywacke, with only 15 m thickness of cave sediments. and a possible resurgence in the valley of the Paleo-Oder. As the Paleo- From the evidence of the thick, but heterogeneously distributed sed- Oder cuts into the bedrock, the base level drops and the flow path iment cover, we follow Paul and Vladi (2001) and speculate about the through the dolomite has to adapt to the changing boundary conditions evolution of the Unicorn Cave in time (Fig. 6): The Unicorn Cave is and incised the cave passage (Fig. 6b and c). Note that the insoluble G. Kaufmann et al. / Journal of Applied Geophysics 123 (2015) 123–140 127

Fig. 4. Map view (top) and cross section (bottom) of the Unicorn Cave in red, and sediment infill as indicated in the legend. The thin blue lines indicate the location of the dynamic probes, the thicker blue line the position of the borehole (adopted from Paul and Vladi, 2001). The newly discovered southern extensions are also shown: elevations marked as green inverted triangles are relative to a survey station at the entrance to the Blaue Grotte. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) basement rock, the Paleozoic Graywacke, dips towards the Paleo-Oder 3. Methods and thus the Unicorn Cave cannot incise too much along its northern end. Once the small creek Bremke providing the surface water for the Geophysical methods as exploration tools for karst caves and man- cave also starts incising below the dolomite outcrop, the active phase made cavities are based on changes in physical properties of the subsur- of the Unicorn Cave comes to an end. The diminishing water supply re- face, such as voids (gravimetric methods), sediment fillings (gravimet- sults in more sedimentation in the cave, which becomes clogged with ric, electrical and magnetical methods), and disruptions in structure sediments and debris from roof and wall collapse. Once the river and layering (seismic and geo-radar methods). We applied gravimetry, Bremke incises completely below the dolomite, the active phase of the ERI and SP measurements in and above the Unicorn Cave. The surface is Unicorn Cave ends (Fig. 6d). Later on, most parts of the southern contin- characterised by a hilly relief, with a steep dolomitic cliff along the east. uation of the Unicorn Cave are lost to erosion of the plateau Brandköpfe, The plateau above the cave is covered with forest, the soil layer is rather and only a small part remains south of the Blaue Grotte. thin (10–20 cm), followed by the Werra dolomite. After heavy rain, the This hypothesis, explaining the fluvial gravels in the cave as well as soil is often very saturated with water. Roads are forest roads, either the thick sediment cover, filling the incised cave passage, explains the with gravel cover or simple forest tracks. present-day trunk passage of the Unicorn Cave as a remnant of a once active river cave, and is in accordance with modern speleogenetic models of the evolution of vadose passages on inclined strata (e.g. Worthington, 3.1. Gravity 2005; De Waele and Parise, 2013). However, we note that it is in contrast to the hypothesis of Sobotha The gravimetric survey was carried out with a Lacoste –Romberg (1966), who argues that the cave passage, though also evolved in parts type D gravimeter. The total precision estimated from repeated mea- from an active cave stream, evolved upwards by corrosion. We also em- surements was better than 0.03 mGal. The coordinates of the survey lo- phasize that the theory of ghost-rock karstification (Dubois et al., 2014), cations were determined with a mobile global positioning system (GPS) a two-stage evolution first by chemical dissolution under low hydraulic device to 1 m accuracy, sufficient for latitude correction. Elevation, how- gradient, creating voids filled with residual material, then flushing out ever, was determined with levelling to achieve the centimeter-scale ac- of these residual material by fluvial processes, leaving air-filled cave curacy needed for the free-air and Bouguer corrections. The raw survey voids, cannot be ruled out at our location. However, the presence of data were processed with the GRAViMAG software, developed at the both fluvial and glacial sediments are indicative for a single-stage Geophysics Department of the Free University of Berlin. There are four karstification. main processing steps for deriving the Bouguer anomaly: (i) repeat With our hypothesis of the evolution of the Unicorn Cave in mind, measurements at a defined base station, re-visited roughly every we are now ready to define the sub-surface in vicinity of the cave in 30 min to monitor instrument drift of the measurements; (ii) correct terms of signals detectable by geophysical prospection. The properties the tidal effects by reducing the data with theoretically derived earth involved are density, magnetic susceptibility, electrical resistivity, and tides using Eterna software (Wenzel, 1996); (iii) tie the relative gravity dielectric constant. The expected signals will be a combination of the measurements into the regional gravity network through a known sta- air-filled cave and the sediment infill of the remainder of the cave pas- tion with absolute gravity in the village of Scharzfeld; (iv) use latitude, sage. We base our discussion on our first surveys in the area, starting free-air, and Bouguer corrections, including a topographical Bouguer with four gravity profiles and one ERT profile (Kaufmann et al., 2011), correction (where needed) derived from either the shuttle radar topog- and extending the ERT survey towards the north (Kaufmann et al., raphy mission (SRTM) digital elevation model (Jarvis et al., 2008), or 2012), which resulted in a first preliminary interpretation. local DEM model, when higher accuracy is needed. 128 G. Kaufmann et al. / Journal of Applied Geophysics 123 (2015) 123–140

Fig. 5. Left: Photo of the Leibnizhalle with the distant Blaue Grotte as natural entrance (Ralf Nielbock). Right: Lithological proﬁle of borehole traversing the Unicorn Cave in its southern part, traversing the cave room Leibnizhalle.

Densities range from 0 kg/m3 for the air-filled cave passages, about diffusion constant and thus the electrical charge. (iii) Electro-chemical po- 1500 kg/m3 for the cave sediments, 2800 kg/m3 for Werra dolomite as tentials arise on the side of conductive bodies like ores, which reside with- host rock, and about 2600 kg/m3 for the underlying Graywacke. in the transition between unsaturated and saturated zone, thus inducing a redox reaction along the surface of the conductive body. 3.2. ERI We expect streaming potentials from surface water being channeled into wider dissolutionally opened fractures, thus inducing a preferential Electrical resistivity imaging (ERI) was performed using a Campus groundwater flow towards the vadose zone of the Unicorn Cave. Tigre multi-electrode instrument. We used both 32- and 64-electrode setups, mostly with 3–5 m spacing, in a Wenner setup. For data processing, the Res2DInv package from Loke and Barker (1995, 1996) was used, 4. Results applying a robust inverse method. Prior to inversion, bad data points have been eliminated with the help of the software. The groundwater In this section, we present results from our geophysical surveys table in the region is at least 100 m below the surface, as monitored above and in the Unicorn Cave. We follow a strategy, which will pick from nearby wells and springs around the Brandköpfe plateau. Thus, up the signal from the cave and then uses the knowledge gained from ERI resistivity maps characterise the unsaturated zone. The Wenner the surveys above the known parts to extent the interpretation beyond setup was chosen for its ability to focus on vertical changes in resistivity, the known passages into the northern and southern parts of the plateau thus giving us the chance to distinguish between cave void and cave Brandköpfe. sediments. The geographic coordinates along the profiles were either Note: All elevations reported are referenced to the WGS84 ellipsoid. available from the gravimetric survey or sampled with a GPS unit, and To obtain elevations above the Amsterdam tide gauge (NHN), subtract the Wenner profiles were processed including the topographic −44.90 m from the reported elevations. elevation. The known and surveyed parts of the Unicorn Cave are located un- Electrical resistivity of the dolomite outcropping in the survey area derneath the south-eastern part of the plateau Brandköpfe (see map was 500–2000 Ωm, a fairly high value. For air-filled voids, we expected outlined in red in Fig. 7). The passages are mainly oriented in NE–SW di- similarly high values. The electrical resistivity of a sediment-filled void, rection, but all major rooms (Blaue Grotte, Leibnizhalle, Schillersaal, however, can be higher or lower, depending on the degree of water sat- Weisser Saal) clearly reveal a second NW–SE direction. While the first uration of the sediments. NE –SW direction probably marks fissures oriented in the main drainage direction, from the Rottsteinklippen towards the Paleo-Oder, the second 3.3. SP NW–SE direction is the main direction of faults in the Paleozoic rocks of the Harz Mountains. Self-potential surveying has been carried out with non-polarisable Gravity measurements have been carried out above the southern copper–copper sulfate electrodes and a multi-meter. One electrode serves part of the Unicorn Cave, the area south of the roof collapse of the as basis, from which potential differences are mapped across the survey room Blaue Grotte, and in the area close to the Rottsteinklippen along area. The base electrode has been located outside of the sampling profiles. the northern end of the plateau Brandköpfe (see yellow dots in Fig. 7). For all electrodes, a hole was dug out to achieve good coupling conditions The distance between gravity stations is generally about 3–5m. between electrodes and soil. During the SP measurements, very wet con- ERI measurements have been made along several profiles, all in ditions facilitated good coupling. Potential differences can have different Wenner setup. Profiles 7, 9, and 12 all cross the known passages of the causes: (i) streaming potentials are generated from the coupling of fluid Unicorn Cave, while profiles 1–5 and 10 and 11 try to trace the northern flow and a conductive interface, e.g. the surface of pores or fractures. end of the known cave. Profiles 8, 13, 14, and 15 extent our knowledge While the solid wall is often negatively charged, the fluid has a positive into the northern part of the plateau, while profiles 20–26 have been excess charge to compensate. Beyond a boundary layer at the solid– carried out to identify possible extensions in the south of the known fluid interface, the excess charges are dragged with the fluid and induce Unicorn Cave. a current density. (ii) Diffusion potentials result from differences in con- Additionally, SP measurements south of the room Blaue Grotte have centration due to the concentration gradient, which depends on the been carried out along the three southern gravimetric profiles. G. Kaufmann et al. / Journal of Applied Geophysics 123 (2015) 123–140 129

Fig. 6. Hypothetical model of the evolution of the Unicorn Cave. Time progresses from (a)–(d). Soluble dolomite in blue, insoluble basement in grey, air-filled cave white, sediment infill in brown, and inflowing water as blue arrow. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

4.1. Known cave (D-E) The last region of minima, with values around −0.9 mGal, is located beyond the known cave passages. Here, the Unicorn Cave con- In Fig. 8, the Bouguer gravity map over the Unicorn Cave is shown. tinues towards the south with yet unexplored passages, as we The map is based on eight gravity profiles, one along the main cave di- discuss later. rection (roughly north–south), the other seven perpendicular to the main strike of the cave passage (roughly west–east). As reference den- 3 sity, a value of 2800 kg/m has been chosen, the typical average density In Fig. 9, three ERI sections are shown, which all cross the Unicorn of the Werra dolomite. Thus Bouguer gravity values below zero repre- Cave in different parts. The sections show resistivities obtained with a sent mass deficits relative to the dolomitic host rock. The Bouguer robust inversion (color-coded), the incorporated surface topography map is an extended version of earlier works (Kaufmann et al., 2010, (black line), and the root-mean-square (RMS) value for the model. 2011, 2012), with four new gravity profiles added in the southern part. Profile 12, measured with 64 electrodes in Wenner setup and 5 m Clearly, the known passages of the Unicorn Cave have been identified electrode spacing, is oriented along the main direction of the known with gravity, visible in the broad negative pattern with values below cave passages, and starts south of the known parts. Resistivities range −0.2 and −0.5 mGal. As we discuss later in more detail, this broad min- from 100 Ωm, characteristic for wet soil, to more than 6000–7000 Ωm, imum is a result of both the air-filled cave and the sediment infill, both representing air-filled cave voids. Three maxima in resistivity, A, B, and having lower densities than the Werra dolomite. The signal of the cave C, can be associated with the rooms Blaue Grotte, Leibnizhalle, and void quickly vanishes, if the gravity stations are not above the cave, as it Schillersaal, the latter only faintly visible, as the ERI profile here is not di- can be seen along the northern part of the Bouguer anomaly. rectly above the cave. The sediment infill D below the air-filled passages Besides the broad negative pattern in Bouguer gravity, five distinct is also characterised by higher resistivities (around 1000–2000 Ωm). The peaks with values below −0.7 mGal are visible in the Bouguer map low-resistivity feature E south of the known cave parts might indicate a (see bold letters in Fig. 8): connection to the surface, with water seeping into a karst void. Pro file 9, with 32 electrodes each 3 m apart, starts on the eastern side (A) The two small minima coincide with mapped chimneys in the of the main passage of the Unicorn Cave and crosses the side passage room Leibnizhalle, which can be traced several meters upwards Jakob-Friesen-Gang. This side passage, today ending in gravel and from the cave room, and which are also identified in a Lidar scan sand infill, was open to the valley in the east during ice-age periods, pro- of the main passages of the Unicorn Cave (Tanner et al., 2012). viding another natural entrance to the Unicorn Cave. The passage is (B) The large minimum with about −0.9 mGal is caused by the break- clearly visible in the high resistivities above 6000 Ωm in the section (A). down in the Blaue Grotte, and possibly indicates an extension of Profile 7 crosses the Unicorn Cave from west to east in the northern the void space towards the west. parts, but the steep cliff in the east only allowed a short 32 electrode pro- (C) The pronounced minimum of −1.2 mGal identifies a former steep file with 3 m electrode spacing. Thus the small northern continuations of shaft entrance to the Van Alten Kapelle, through which animals fell the Unicorn Cave underneath the profile (Weisser Saal) are below the into this part of the cave. Today, this part is called animal graveyard maximum depth monitored with the ERI section characterised by and it contains numerous animal fragments, including bones of 1000–2000 Ωm typical for the dolomitic host rock, but is not visible in cave bears, among other species. the resistivity section. 130 G. Kaufmann et al. / Journal of Applied Geophysics 123 (2015) 123–140

Fig. 7. Location map of the plateau Brandköpfe, with topography (greyshaded), Unicorn Cave (red), boreholes (black diamonds), gravity points (yellow dots), outcrop in the south (white inverted triangle), and ERT lines (blue lines, number identifies start of profile). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

4.2. Northern extensions karstified fracture in the dolomite, filled with wet sediments. Unfortu- nately, the depth of the expected continuation of the cave void (see We proceed looking just north of the known passages of the Unicorn symbol x in Fig. 10) could not be reached with the three ERI profiles, Cave. At its northern end, the Unicorn Cave ends in several small pas- as to the east the ERI profile cannot be extended beyond the steep cliff. sages, the longest one being the Hubertusgang, which has been dug ERI profiles 10 and 11 extend the search further north, but again do out of the cave sediments for archeological prospection. None of the not reach the desired target depth of the proposed continuation. The small passages heading north resembles the large trunk passage en- spurious high resistivities in profile 10 of more than 8000 Ωm are not countered further south, and we speculate that the main cave passage discussed, as the RMS value of this profile is with RMS =10toolarge. is drowned in sediments from here on, and the small passages accessi- We now move on to the northern end of the Werra dolomite around ble from the room Weisser Saal are just dissolutionally enlarged roof fis- the Rottsteinklippen. In Fig. 11, four ERI profiles in the northernmost sures of the sediment-filled passages. section of the plateau Brandköpfe are shown, all oriented in west–east We test this hypothesis with several ERI profiles oriented perpendic- direction perpendicular to the assumed unknown northern extension ular to the proposed continuation, with 32 electrodes and an electrode of the Unicorn Cave. All four profiles have been carried out with 64 elec- separation of 3 m, restricted by the steep cliff to the east. As the steep trodes and 5 m distance between electrodes to reach deeper into the cliff violates the half-space assumption for interpreting the 2D ERI sec- Werra dolomite. tion, we stopped our measurement lines in about 10 m distance from Profile 14 is located in the same area as the previously discussed pro- the cliff. Resistivity sections for profiles 1,2 and 4 are shown in Fig. 10. files 1, 2, 4, 10, and 11, but twice as long. As expected, the depth resolved All three profiles show a bimodal distribution of resistivities, with low encompasses the presumed location of the northern extension of the Uni- (100–800 Ωm) values in the west, and values around 1000–2000 Ωm corn Cave, and indeed a highly resistive region with 4000– 6000 Ωm(A)is in the east, reflecting the Werra dolomite. The low resistivities are found along the eastern part of the profile. As in the shorter profiles, a found in a topographic low within the forest, but the measured ERI sig- low-resistivity zone is present again in the western part of the profile. nal reaches significantly deeper than the soil zone, which is only about Profile 15 is located further north, with no clear indication of a cavity 10–15 cm thick here. This low-resistivity zone possibly indicates a structure. In profile 13 even further north, a highly resistive circular G. Kaufmann et al. / Journal of Applied Geophysics 123 (2015) 123–140 131

Fig. 8. Known cave: Bouguer anomaly over Unicorn Cave and its southern extension, shown as color-coded map. Topography below as grey-shade, and outline of Unicorn Cave in red. Letters discussed in text. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

Fig. 9. Known cave: ERT proﬁles crossing the Unicorn Cave. The RMS value is given in percent. Letters discussed in text. 132 G. Kaufmann et al. / Journal of Applied Geophysics 123 (2015) 123–140

Fig. 10. Northern extension: ERT profiles just north of the mapped passages of the Unicorn Cave. Symbol X marks presumed location of northern extension of cave. The RMS value is given in percent. Letters discussed in text. feature (B, 4000 Ωm) is present again, but a bit below the expected depth density producing the signal. Here, more gravity measurements are of the cave continuation, possibly indicating dry cave sediments. Profile 8 needed. is the northernmost profile, just along the rim of the Rottsteinklippen, where according to our hypothesis the former cave entrance with the 4.3. Southern extensions sinking stream should be located. Here, the highly resistive anomaly of about 4000–6000 Ωm (C) is found, possibly the former entrance. We examined in more detail the part south of the large roof collapse We have also carried out gravity measurements along the northern of the room Blaue Grotte, as we expect a continuation of the known pas- part of the plateau Brandköpfe, and the resulting Bouguer anomaly is sages beyond the breakdown. As we have already indicated, the shown in Fig. 12. The Bouguer data show a broad area with negative Bouguer anomaly shown in Fig. 8 reveals two large minima (D and E), values, thus a mass deficit relative to the Werra dolomite, but no clear and a broad area of moderate negative values extending south of the pronounced minima as in the south. This might point to cavities known cave passages. We have modelled these gravity data in completely filled with sediments, which then produce a smaller (Kaufmann et al., 2011) with a 2 1/2 D Talwani model with finite strike Bouguer signal. The low Bouguer values below −0.9 mGal in the west- length and later on in (Kaufmann et al., 2012) with the 3D modelling ern part of the surveyed area are not really clear to us, as we suspect the package PREDICTOR (Kaufmann et al., 2015), and we concluded that dolomite to thin out here and thus the Graywacke rock with its lower the Bouger anomaly is best explained with a sediment-filled trunk G. Kaufmann et al. / Journal of Applied Geophysics 123 (2015) 123–140 133

Fig. 11. Northern extension: ERT profiles between the northern end of the Unicorn Cave and the Rottsteinklippen. The RMS value is given in percent. Letters discussed in text. passage capped by a thin air-filled cave void and enlarged chimneys this enlarged fault zone into greater depth and thus results in a resistiv- explaining the peaks in the Bouguer gravity. ity signal of wet to very wet sediments. Note that the soil and sub-soil With these interpretation in mind, three drill holes were lowered during our measurements has been almost saturated with water due into the Werra dolomite south of the Blaue Grotte (see diamonds in to heavy rain falls. Fig. 8) in May 2014. All boreholes passed through 11–12 m of dolomitic We have complemented the ERI measurements with two more pro- rock, and then dropped into air-filled cavities, between 0.5–1.5 m in files of 32 electrodes each, but an electrode spacing of 4 m to provide height. Connection between the cavities has been proven with a better depth coverage (Fig. 14). Profile 24 is the southernmost profile borehole cam, which also revealed a thick sediment infill, which is made, and it crosses just north of a small dolomite outcrop. In this dolo- also present in the known parts of the Unicorn Cave. mite outcrop, a large vertical fissure is exposed, which is significantly After the successful identification of the southern continuation of the enlarged by dissolution and continues into the sub-surface. This struc- Unicorn Cave, we extended our geophysical measurements further with ture is visible in profile 24 (A) as a low-resistivity zone with values of seven ERI profiles above the newly discovered cave void. ERI profiles 20, 200–500 Ωm, crossing the entire ERI section until 25 m depth. Profile 21, and 22 are shown in Fig. 13.Allthreeprofiles have been carried out 26 crosses the profiles 20–22 in an oblique direction from NE towards with a 32 electrodes setup and 3 m electrode distance from east to west. SW and ends at the exposed dissolutionally enlarged fissure. On this However, the newly discovered cave voids (symbols A) could not be pro file, the high resistivity area with values above 6000 Ωmidentifies seen in the resistivity distribution, as resistivity measurements of the the air-filled void encountered by drilling, and the low-resistivity struc- ERI sections did not extend deep enough. Instead, an interesting low- ture further west, with very low values down to 10 Ωm, indicating resistivity structure (B) with values around 100–200 Ωm has been iden- water seeping into the fissure. tified west of the new cave voids, reaching deep into the Werra dolo- We then mapped the area south of the collapse Blaue Grotte with mite, identified as the 1000–2000 Ωm structure in the ERI sections. SP measurements on the same positions as the ERI profiles 20, 21, This low-resistivity structure seems to become shallower on the north- and 22 (Fig. 15). The base electrode has been positioned just east of ernmost profile 20. We speculate that this signal results from a near- the survey area. The resulting potential difference shows a clear neg- vertical fault in the Werra dolomite, which is enlarged by dissolution ative potential down to −10 mV, which closely matches the low in and filled with sediments. Surface water can easily penetrate through Bouguer gravity, and is centered around the three drill holes. The 134 G. Kaufmann et al. / Journal of Applied Geophysics 123 (2015) 123–140

Fig. 12. Northern extension: Bouguer anomaly close to the Rottsteinklippen, the northern end of the plateau Brandköpfe.

Fig. 13. Southern extension: ERT proﬁles south of the Blaue Grotte, the southern end of the Unicorn Cave. The RMS value is given in percent. Letters discussed in text. G. Kaufmann et al. / Journal of Applied Geophysics 123 (2015) 123–140 135

Fig. 14. Southern extension: ERT profiles south of the Blaue Grotte, the southern end of the Unicorn Cave. The RMS value is given in percent. Letters discussed in text. negative potential continues to the west into the region, where ERI 4.4. Cave sediments measurements revealed the low-resistivity zone, which we interpret as dissolutionally widened fissure, filled with wet soil. We speculate In this last part of the results section, we examine the cave sediments that the SP potential anomaly is caused by water seeping from this in the known part of the Unicorn Cave. As the sediment infill consists of fissure towards the cavity, therefore providing evidence of the sub- sequences of dolomitic blocks, fluvial gravel and even glacial infill, its surface karst features. composition and structure is heterogeneous and thus can be mapped

Fig. 15. Southern extension: SP anomaly south of the Blaue Grotte, the southern end of the Unicorn Cave. 136 G. Kaufmann et al. / Journal of Applied Geophysics 123 (2015) 123–140 with geophysical methods. We choose ERI to identify wet and dry sec- very wet from dripwater, especially in the Weisser Saal. Again, the sed- tions as well as air-filled voids. iment cover in this part seems to be rather homogeneous. The three profiles measured in the cave are indicated on the map in We have now discussed all results from our geophysical surveys, Fig. 16 as green lines, together with the three ERI profiles 7, 9, and 12 on both from the surface and within the Unicorn Cave. We have been the surface discussed earlier. able to map the known parts of the Unicorn Cave with gravity and ERI, The main ERI profile 1 in the cave has been measured with 64 elec- and especially the Bouguer gravity signal revealed the importance and trodes and 4 m spacing (Fig. 17), starting in the Hubertusgang in the significant thickness of the sediment infill of the cave passages. Geo- north and traversing the entire cave til the Van Alten Kapelle in the physical surveys south of the known and mapped passages indicated south. The ERI section essentially reveals two different resistivities: values an extension of the Unicorn Cave, which we found by drilling three around 100–400 Ωm, representing wet softer sediments, such as cave boreholes, all of them reaching a new void in around 12 m depth, clays and gravels, and higher resistivities around 100–2000 Ωm, which with 1–1.5 m air-filled cavity and sediment-filled bottom. North of the we identified as breakdown areas with larger dolomitic blocks. With Unicorn Cave, ERI profiles found highly resistive anomalies close to this identification in mind, the profile reveals softer sediments beneath the hypothetical former sink of the cave. the room Weisser Saal (A), then more blocky collapse areas beneath the two large rooms Schillersaal (B) and Leibnizhalle (C), and wetter, soft sediments in the area of the Blaue Grotte (D), with its daylight open- 5. Interpretation ing through the large roof collapse. Profile 3 with 32 electrodes and 1.5 m spacing between them tra- In the previous section, we presented the geophysical results obtained verses the Jakob-Friesen-Gang from the old entrance (E) into the in and above the Unicorn Cave, and we associated the different signals ei- Leibnizhalle (C), and is characterised by wet, soft sediment infill, ther with features of the known cave system or with possible extensions. which might be no thicker than 5 m and therefore close to the Werra In this section, we derive a three-dimensional model of the Unicorn Cave dolomite in the central part. The sediment cover in this side passage and its southern extensions, based on the geophysical results and bore- seems to be rather homogeneous. hole data. We use this three-dimensional structural model to predict geo- Profile 1 with 32-electrodes and 2 m spacing examines the northern physical signals, which then can be compared to our field data. We will room Weisser Saal (F) and the continuation into the Hubertusgang (G). use the program package PREDICTOR (e.g. Kaufmann et al., 2012, 2015) Again, the low resistivities indicate soft sediments, which are in parts developed by the authors.

Fig. 16. Cave sediments: Location map of the Unicorn Cave (red outline), with topography (greyshaded), and ERT lines (blue lines for surface profiles, green lines for profiles in cave, number identifies start of profile). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) G. Kaufmann et al. / Journal of Applied Geophysics 123 (2015) 123–140 137

Fig. 17. Cave sediments: ERT proﬁles in the Unicorn Cave. The RMS value is given in percent. Letters discussed in text.

As starting point we extend the cross section of the Unicorn Cave to The boundary between these two lithological units, the Werra dolo- include the new cave voids found during the drilling campaign. In Fig. 4, mite and the Graywacke, can be found in two boreholes from the Uni- the three boreholes are shown on the cross section in the left part of the corn Cave reaching the Graywacke. We extrapolated these two figure. Black colors indicate Werra dolomite, red colors air-filled voids, locations linearly in north–south direction to estimate the lithological and cave sediments are marked with a diagonal line patterns. On top boundary, which dips from north to south, as suggested in the literature of the three boreholes, a hypothetical air-filled void is shown in red (e.g. Paul and Vladi, 2001). colors. We have levelled the top of all borehole locations, the entrance Removing the topography from the structural model (Fig. 18,lower to the room Blaue Grotte, and the small collapse passages south of the left figure) reveals the location of the Unicorn Cave within the Werra room Blaue Grotte (green triangles in Fig. 4) to derive the height differ- dolomite. The known passages have been derived from a digitised ver- ences between these sites. It is clear that the elevation of the newly sion of the map and cross section of the Unicorn Cave (Paul and Vladi, found air-filled cavity is located below the level of the small collapse 2001), while the southern extension is based on the location of the passages in the Blaue Grotte, but the elevation of the newly found cav- three boreholes and assumed to extend further, as the Bouguer gravity ities correlates with the elevation of the known trunk passages of the data suggest. Also shown are two borehole locations in the Unicorn Unicorn Cave, thus indicating a real southern extension. Cave and the three boreholes in the south. Based on this knowledge from drilling, we assembled a three- The two borehole locations in the known cave enable us to estimate dimensional structural model of the Unicorn Cave and its southern the sediment thickness underneath the known passages of the Unicorn extension within the PREDICTOR software (e.g. Kaufmann et al., Cave (Fig. 18, lower right figure). The sediment infill increases from 2012, 2015). The PREDICTOR package defines a 3D structural model about 15 m in the north to more than 40 m in the south, and we as- from a given digital topography, extended into depth with different sumed this thick sediment infill also for the southern extension. lithological layers. Each of the layers, having different physical prop- Based on the 3D structural model defined above, we run the predic- erties, can accommodate 3D structures such as caves, voids, or other tion part of the PREDICTOR package to simulate Bouguer gravity data for objects. The numerical model is then sub-divided into three parts: the structural model. The result of this simulation, together with the ob- the assemblage of the model, the solution of governing equation, served Bouguer gravity, is shown in Fig. 19. The predicted Bouguer grav- and the prediction of geophysical signals. Details can be found in ity signal results in a similar broad-scale signal as the observations, Kaufmann et al. (2015). visible in the similarities in the shape of the Bouguer gravity below For our purpose, we use a digital elevation model derived from SRTM −0.5 mGal. The modelled signal of the two chimneys (A) close to the data (e.g. Jarvis et al., 2008), and define two lithological units, the Werra Jakob-Friesen-Gang is present, but less pronounced, as our structural dolomite and the Graywacke below (Fig. 18,topfigure). The topography model with its resolution of 2 m is too coarse here. The large Bouguer represents the higher elevation of the plateau Brandköpfe, and the steep minimum (B) resulting from the rooms Leibnizhalle and Blaue Grotte incised valley in the foreground. The red hut marks the location of the (and of course their sediment infill) are present in the prediction, Haus Einhorn, the ticket-selling booth for the show cave. though the observed minimum just west of the mapped passage has 138 G. Kaufmann et al. / Journal of Applied Geophysics 123 (2015) 123–140

Fig. 18. Interpretation: PREDICTOR model of the Unicorn Cave. (a) Model domain and surface area, the house is close to the artificial entrance of the cave. Along the bottom (dark grey colors) the Graywacke is visible. (b) Unicorn Cave and its southern extension (light grey) and Graywacke (dark grey) as structural units, the thin cylinders indicate boreholes (blue, Werra dolomite; pink-cave sediments, light blue-dolomitic blocks, red-Graywacke) (c) as before, but including sediment infill of the cave (white). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) G. Kaufmann et al. / Journal of Applied Geophysics 123 (2015) 123–140 139

Fig. 19. Interpretation: observed (left) and predicted (right) Bouguer anomaly. Letters discussed in text. not been modelled. The former shaft (C) has not been resolved, again Acknowledgements the 2 m resolution is too coarse. The southern continuation (D–E) is well predicted, both in shape and in amplitude. We would like to thank Oliver Kaufman and an anonymous referee, We conclude that the 3D structural model derived with the help of whose comments helped to improve the manuscript. the PREDICTOR package is capable of reproducing the observed broader The geophysical surveys have been part of the B.Sc. thesis of Yosri picture of the Bouguer gravity data, thus the sub-surface model of the Hassanein and Ragnar Frentz and the Diploma thesis of Grit Jahn, all su- southern extension of the Unicorn Cave is in agreement with observa- pervised by GK. Field support from Grit Jahn, Julio Galindo Guerreros, tions. Small-scale features below 2 m, however, cannot be resolved Ron Freibothe, Sebastian Schmidt, Lucinda Gürlich, Yosri Hassanein, properly. Ragnar Frentz and Douchko Romanov is also greatly acknowledged. The ERT profiles 12 (on the surface) and 1 (in the cave) have been measured during a student fieldwork trip with the help of Henri Brasse, 6. Conclusions whom we would like to thank. This work has been funded by the DFG under research grant In this paper, we have compiled a large dataset of geophysical sur- KA1723/6. veys in and above the Unicorn Cave and its surrounding in the southern Harz Mountains. We have shown that the known and surveyed cave References passages can be traced both with gravity and ERI measurements, which respond to the lower density and the different resistivity of Baier, S., 2004. Frühweichselzeitliche Feinlaminierte Sedimente der Einhornhöhle bei cave voids and cave sediments. The Bouguer gravity data revealed the Scharzfeld/Harz (Diploma thesis), Johannes-Gutenberg-Universität Mainz, Germany fi (150 pp.). large sediment in ll in the Unicorn Cave, with sediment thickness Bathurst, R., 1979. Carbonate Sediments and Their Diagenesis. Elsevier. reaching 30–40 m in the southern parts. Boreholes both into the Butler, D., 1984. Microgravimetric and gravity-gradient techniques for detection of sub- known cave sediments and through the known cave have been used surface cavities. Geophysics 49 (7), 1084–1096. De Waele, J., Parise, M., 2013. Discussion on the article ‘coastal and inland karst morphol- to calibrate and verify the indirect geophysical signals. ogies driven by sea level stands: a GIS based method for their evaluation’ by Canora F, With the knowledge gained from geophysical surveys over the Fidelibus D and Spilotro G. Earth Surf. Process. Landf. 38, 902–907. known cave, we proceeded north and south of the known passages, Dobecki, T.L., Upchurch, S., 2006. Geophysical applications to detect sinkholes and ground subsidence. Lead. Edge 336–341 (March). and in the south we predicted a passage continuation based on negative Dubois, C., Quinif, Y., Baele, J.-M., Barriquand, L., Bini, A., Bruxelles, L., Dandurand, G., Bouguer-gravity anomalies and changes in electrical resistivity. This Havron, C., Kaufmann, O., Lans, B., Maire, R., Martin, J., Rodet, J., Rowberry, M., southward extension has been found in 2014 with three boreholes Tognini, P., Vergari, A., 2014. The process of ghost-rock karstification and its role in – lowered into the Werra dolomite, with an air-filled cave void found the formation of cave systems. Earth Sci. Rev. 131, 116 148. El-Qady, G., Hafez, M., Abdalla, M., Ushijima, K., 2005. Imaging subsurface cavities using 11–12 m below ground. These newly discovered cave voids have then geoelectric tomography and ground-penetrating radar. J. Cave Karst Stud. 67 (3), been mapped with additional gravity, ERI, and SP measurements, all of 174–181. which revealed the southern cave extension. Ford, D.C., Williams, P.W., 2007. Karst Hydrogeology and Geomorphology. Wiley, Chichester, England. North of the Unicorn Cave, the geophysical surveys have been less Herrmann, A., 1969. Einführung in die Geologie, Morphologie und Hydrogeologie des promising, however a possible northward continuation of the Unicorn Gipskarstgebietes am südwestlichen Harzrand. In: Herrmann, A., Pfeiffer, D. (Eds.), Cave, as proposed by the hypothesis of its evolution described earlier Der Südharz — seine Geologie, seine Höhlen und Karsterscheinungen 9. Verband der deutschen Höhlen- und Karstforscher, pp. 1–10. in this paper, seems to be likely. Here, more geophysical surveys will Hohl, R., 1985. Die Entwicklungsgeschichte der Erde. 6th edition. Verlag für Kunst und hopefully provide evidence for unknown cavities in the future. Wissenschaft. 140 G. Kaufmann et al. / Journal of Applied Geophysics 123 (2015) 123–140

Jarvis, A., Reuter, H., Nelson, A., Guevara, E., 2008. Hole-filled Seamless SRTM Data V4. Nielbock, R., 2010. Die Einhornhöhle. Die Welt der Einhörner, Höhlenbären und International Centre for Tropical Agriculture (CIAT) (http://srtm.csi.cgiar.org). Neandertaler. Verlag Friedrich Pfeil, München. Kaufmann, G., 2014. Geophysical mapping of solution and collapse sinkholes. J. Appl. Nyquist, J.E., Peake, J.S., Roth, M.J.S., 2007. Comparison of an optimized resistivity array Geophys. 111, 271–288. with dipole–dipole soundings in karst terrain. Geophysics 72 (4), 139–144. Kaufmann, G., Romanov, D., Nielbock, R., 2010. Geophysikalische Untersuchungen an der Palmer, A.N., 2007. Cave Geology. Cave Books. Einhornhöhle. Harz. Mitt. Verb. dt. Höhlen- u. Karstforscher 2, pp. 72–77. Paul, J., Vladi, F., 2001. Zur Geologie der Einhornhöhle bei Scharzfeld am südwestlichen Kaufmann, G., Romanov, D., Nielbock, R., 2011. Cave detection using multi-geophysical Harzrand. Ber. Naturhist. Ges. Hannover 143, 109–131. methods: the Unicorn Cave, Harz Mountains, Germany. Geophysics 76 (3), B71–B77. Schönenberg, R., Neugebauer, J., 1987. Einführung in die Geologie Europas. 5th edition. Kaufmann, G., Jahn, G., Galindo-Guerreros, J., Nielbock, R., 2012. Geophysical exploration Rombach. of cave sites: the case of the Unicorn Cave, Scharzfeld/Harz, Germany. Braunschweiger Sobotha, E., 1966. Die Einhornhöhle im Harz als Musterbeispiel für die. Mitt. Verb. dt. Naturkundliche Schriften 11, pp. 69–80. Höhlen- u. Karstforsch. 12, pp. 63–65 Kaufmann, G., Ullrich, B., Hoelzmann, P., 2015. Two iron-age settlement sites in Germany: Tanner, D., Dietrich, P., Nielbock, R., Röhling, H.-G., Krawczyk, C., Vogel, D., 2012. from field work via numerical modelling towards an improved interpretation. Auswertung der Kluft- und Störungssysteme der, Niedersachsen, mit Hilfe eines Archaeol. Discov. 3, 1–14. terrestrischen LIDAR-Scans. GeoTop 2012 — Landschaften und ihr Geopotential 79, Leibniz, G., 1749. Protogaea sive de prima facie telluris et antiquissimae historiae vestigiis pp.. 59–65 in ipsis naturae monumentis dissertation. C. L. Scheid, Göttingen. Telford, W.M., Geldart, L.P., Sheriff, R.E., 1990. Applied Geophysics. Cambridge University Loke, M., Barker, R., 1995. Rapid least-squares inversion of apparent resistivity Press. pseudosections using a quasi-Newton method. Geophys. Prospect. 44 (1), 131–152. Vacher, H., Mylroie, J., 2002. Eogenetic karst from the perspective of an equivalent porous Loke, M.H., Barker, R.D., 1996. Practical techniques for 3D resistivity surveys and data medium. Carbonates Evaporites 17, 182–196. inversion. Geophys. Prospect. 44, 499–523. Wenzel, F., 1996. The nanogal software: earth tide data processing package eterna 3.30. Mochales, T., Casas, A.M., Pueyo, E.L., Pueyo, O., Roman, M.T., Pocovi, A., Soriano, M.A., Bull. Mar. Terr. 124, 9425–9439. Anson, D., 2008. Detection of underground cavities by combining gravity, magnetic Worthington, S., 2005. Evolution of caves in response to base-level lowering. Cave Karst and ground penetrating radar surveys: a case study from the Zaragosa area, NE Sci. 32 (1), 3–12. Spain. Environ. Geol. 53, 1067–1077. Worthington, S., Ford, D., Beddows, P., 2001. Characteristics of Porosity and Permeability Möller, U., 1986. Kartierung der Höhlensedimente in der Einhornhöhle/Scharzfeld Enhancement in Unconfined Carbonate Aquifers due to the Development of (Diploma thesis), Technical University Clausthal, Germany (40 pp.). Dissolutional Channel Systems. In: Gunay, G., Ford, D., Williams, P., Johnson, K. Nielbock, R., 1990. Die Einhornhöhle — Ein quartärwissenschaftliches Kleinod im Südharz. (Eds.), Present state and future trends of karst studiesTechnical Documents in Mitteilungen des Verbandes der deutschen Höhlen- und Karstforscher 36, pp. 24–27. Hydrology 49 (I). UNESCO, Paris, pp. 13–29. Nielbock, R., 2008. Die Harzer Dolomiten — Natur- und Landschaftsinterpretation unter und übertage. In: Röhling, H.-G., Zellmer, H.H. (Eds.), GeoTop 2008 Landschaften lesen lernen 56. Deutsche Gesellschaft für Geowissenschaften, pp. 146–151.