Introduction
Sinkholes are caused by dissolution and subsurface erosion of soluble rocks
such as salt, sulfate and carbonate in the presence of water (e.g.
groundwater). Fractures and faults can serve as fluid pathways that allow the
water to flow through the subsurface and thus generate cavities. A number of
authors have pointed out that there is a causal connection between
strike-slip faults and sinkholes .
Strike-slip fault zones with different internal types of fault sense
(reverse, normal and strike-slip) are most likely to produce a fine mosaic
of small fault blocks that allow groundwater to move freely, thus creating an
area of subsidence. Over time, different dissolution structures can occur, e.g. depending on the solubility of the rocks, the hydraulic gradient and
the type of the overburden (e.g. soft sediments or solid rock). The two main
features that can evolve close to the surface are collapse or depression
structures. The former occurs if the overburden is thin enough. The latter is
due to slow dissolution
. Sinkholes can cause
damage to buildings and infrastructure and may even lead to life-threatening
situations if they occur, for example, in urban areas .
Geological map (a) of Schmalkalden and the surrounding areas (after
), showing the Heßleser Fault zone (HFZ) crossing the
position of the sinkhole (red square). In the lower-left corner a map of
Germany shows the position of Schmalkalden (red dot). Our interpretation of the fault movement (b)
is that they were reactivated in the Mesozoic, which led to the formation of a dextral
strike-slip fault zone, which includes the Stahlberg Fault zone (SFZ) and the
Viernauer Fault zone (VFZ). The HFZ connects the two major faults at an acute
angle of 30∘. We interpret this as a Riedel R-shear that is also dextral
in movement (see text). For an explanation of the stratigraphic
abbreviations, see .
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Photographs showing the sinkhole in Schmalkalden that opened up on
1 November 2010. It is 26 to 30 m in diameter and 12 to 17 m in depth
.
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To determine the causes and the main controlling factors of the sinkhole
formation in the urban area of Schmalkalden, a number of investigations were
conducted on behalf of the Thuringian State Institute for Environment and
Geology (TLUG), including investigation of possible man-made underground
cavities, boreholes, micro-gravimetry, 2-D compression wave (P wave)
reflection seismic and hydrological investigations (e.g. chemical
composition of the four aquifers) .
The P-wave reflection seismic was unsuccessful at imaging the first ca. 30 m
below surface due to a relatively poor resolution, but for instance shear
waves (S wave) are able to image the near-surface in high resolution
. Interpretation of near-surface faults and
structures from the surface down to ca. 100 m depth is important for
understanding the local geology and the dissolution-induced structures and
processes in general. Therefore, the Leibniz Institute for Applied Geophysics
(LIAG) carried out 2-D SH-wave reflection seismics in this area.
Study area
Geological evolution
Schmalkalden is located in southern Thuringia, Germany. The deeper bedrock
below the research area consists of metamorphic gneiss and micaceous shale,
which were deformed during the Variscan orogeny. Later this bedrock was
uplifted and formed the Ruhla–Schmalkalden Horst (RSH).
During the upper Permian, the Zechstein Sea transgressed, but due to the
horst location, the sediment strata on the RSH are much thinner than
elsewhere in the basin. The climatic conditions of the Permian led to a
deposition of very thick evaporite. The Zechstein deposits are
subdivided into seven sequences, which begin with reef dolomite rocks
surrounding the RSH and forming the Werra Formation. The upper part of the
Werra Formation is characterized by red claystones which indicate the end of
the reef growth . Sulfate rocks occur above this and form the
main horizon prone to dissolution. The following Staßfurt Formation
consists of sulfates, claystones and dolomites, whereby over 50 % of the
sulfate rocks consist of gypsum, which is why this formation is categorized
as part of the dissolution horizon. This formation is followed by the Leine
Formation, which contains claystones and carbonates. The upper part of the
Zechstein deposits is represented by claystones, sandstones and dolomites of
the Leine, Aller, Ohre and Friesland formations, and finishes with sand-
and claystones of the Fulda Formation .
In the study area, the Zechstein Formation is followed by terrestrial
sediments of the Triassic, i.e. the Calvörde and Bernburg formations of
the Lower Buntsandstein (Fig. ). Because of intense erosion
due to fault movement, mostly since the Upper Cretaceous, which also led to
the uplift of the Thuringian Forest, these formations are also the youngest
beds to outcrop in the region, except for some Quaternary deposits.
distinguished six tectonic phases between the Lower
Carboniferous and the Tertiary, the last two of which are the most important
for this work. From the upper Permian to the Lower Cretaceous the area of
Schmalkalden was subject to an extensional stress regime, while from the
Upper Cretaceous to the Tertiary it was dominated by a compressional stress
regime.
Faults
Thuringia is crossed by several major NW–SE-striking faults
. Schmalkalden is located to the south of the
Stahlberg Fault zone (SFZ). The SFZ is downthrown to the south-west and
uplifts basement rocks to the north-east. However, this was accompanied by
dextral strike-slip movement, as can be seen from the jogs in the fault trace
(Fig. ). Together with the Viernauer Fault (VFZ) to the
south, the SFZ and the VFZ formed a dextral strike-slip fault zone. The
Heßleser Fault zone (HFZ) connects the two major faults at an acute angle of
30∘. We interpret this as a Riedel R-shear (for definition of a
Riedel shear, see ), also dextral in movement
(Fig. ). Most probably this movement took place during the
Upper Cretaceous/early Tertiary inversion phase in Europe
. The south-eastern part of the HFZ cross-cuts
the town of Schmalkalden. The fault zone contains several smaller fault
branches that strike NW–SE .
Stratigraphy of the five boreholes (TLUG; personal communication, 2017) near the sinkhole
(f is fault, fz is fault zone). The numbers below the profiles show the final
depth below surface of each borehole. The stratigraphic units were used for
the interpretation of the seismic profiles.
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Sinkhole
On 1 November 2010 at 03:00, a large sinkhole
opened up in the residential area of Schmalkalden (Fig. ). The
sinkhole was 26 to 30 m in diameter, 12 to 17 m in depth and the
crater had a volume of 4000 to 4200 m3 . Directly
after the collapse, groundwater fountains were observed along the sinkhole
margins and the sinkhole filled with water, which subsequently slowly seeped
away into the ground. The crater became wider due to instability of the
sinkhole margins. The bedrock, which was visible within the crater, was
strongly fractured and showed small-scale faults and folding of layers. The
damage caused by the collapse, such as cracks in houses and streets, was
mainly concentrated on the areas north and north-east of the sinkhole. The
houses directly besides the sinkhole were temporarily evacuated for safety
reasons. New cracks formed due to slope movements along the south-west
dipping layers, caused by the reduced bedrock stability. To stop slope
movement the sinkhole was quickly filled with gravel. On behalf of TLUG,
investigations, e.g. archive research, were conducted to determine whether
the sinkhole had anthropogenic causes, such as man-made cavities or large
underground facilities. However man-made cavities were not found below the
sinkhole, and in the vicinity only very small constructions were found, which
were not nearly large enough to generate such a large sinkhole .
As a result, the sinkhole is probably of natural origin. Although this is the
first sinkhole in the urban area of Schmalkalden, several saltwater springs
can be found in the surrounding area, which also indicate the long-lasting
dissolution processes in this region .
Location map of the SH-wave reflection-seismic profiles
(yellow lines), the boreholes (orange dots), the sinkhole (red square) and
the main fault branches of the Heßleser Fault zone (black lines) (ArcGIS,
Open Source Map).
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Boreholes
To investigate the stratigraphy in the vicinity of the sinkhole in detail,
five boreholes with depths of 143 to 167 m were drilled
(Fig. ). In the following, the stratigraphy of the 05/2011 borehole is
shown as an example (TLUG, personal communication, 2017). The first 3.55 m consists of
anthropogenic deposits, and from 3.55 to 13 m depth, Quaternary terrace
gravels and sandy colluvial deposits are found. Between 13 and 28 m depth
there are red sandstones of the Triassic Buntsandstein (Calvörde
Formation) followed by deposits of the Permian Zechstein. The Zechstein is
subdivided into seven formations (see previous section) of which the Fulda
Formation (z7) from 28 to 43.45 m depth is the youngest and consists
of silty sandstones and claystones (also called “Bröckelschiefer”). Below
z7, from 43.45 to 58.60 m, the Aller, Ohre and Friesland formations
(z4–z6) and the upper Leine Formation (z3Tb) are found with disturbed clay-
and sandstones. Between 58.60 and 86.65 m are disturbed dolomitic lime-
and claystones of the lower Leine Formation (z3Ca and z3Ta). The Staßfurt
Formation (z2) from 86.65 to 106.85 m depth consists of claystone and
gypsum. The oldest Zechstein sequence, the Werra Formation (z1), is found
between 106.85 and 150.00 m and consists of a gypsum and anhydrite
dissolution breccia, claystone and carbonate. The top of the bedrock, the
Paleozoic Hohleborn Formation, was drilled in four other locations
(Fig. ).
Some important results were the discovery of faults and the verification that the
dissolution horizon is situated between the base of the Werra Anhydrite (z1)
and the base of the Leine Carbonate (z3).
Results
Seismic interpretation of S1
SH-wave reflection-seismic profile S1 of ca. 350 m length, was
carried out north of the sinkhole (Fig. ). At ca. 10 to 25 m
depth, a continuous reflector with high amplitude can be traced throughout
the entire profile (Fig. ). The strong impedance contrast represents
the boundary between the Triassic sandstones of the Calvörde Formation
(suC) and the Permian claystones of the Fulda Formation (z7). This reflector,
which can be found in all SH-wave reflection-seismic profiles, was
used as a marker horizon. In contrast, the area beneath shows a mostly
discontinuous reflection pattern with no remarkable reflector, although
lateral amplitude variations are observed due to strongly fractured strata
within the seven Zechstein formations.
In the area north-west of the sinkhole, shallowly dipping reflectors form a
bowl-shaped structure of ca. 150 m length and 50 m depth within the
Triassic and Permian deposits. In this area the Calvörde Formation shows
local thickening. Numerous faults in the Zechstein formations with
small-scale vertical offsets of ca. 1 to 3 m were identified.
A low-reflectivity zone (LRZ) can be observed at ca. 70 to 100 m depth
between 175 and 275 m profile length. This zone shows an almost
transparent reflection pattern compared to the neighbouring reflectors at the
same depth and is located within the Staßfurt Formation. This correlates
with the dissolution horizon within the Zechstein formations of Werra
Anhydrite to the Leine Carbonate (z1-z3).
The near-surface in all profiles is represented in detail, with a resolution
of less than 1 m at depths down to ca. 15 m and a resolution of ca. 1 to
3 m at 50 m depth.
Uninterpreted reflection-seismic profile S2 (a), with stratigraphy
derived from the 03/2010 borehole (b) and with interpretation (c). The
stratigraphic units are explained in Fig. . The profile was
surveyed south of the sinkhole. Steep-dipping normal and reverse faults
within the Permian deposits were identified and in the north-east a
bowl-shaped structure (red dashed circle) is visible, which is interpreted as
a dissolution-induced depression.
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Uninterpreted reflection-seismic profile S3 (a) with stratigraphy
derived from the 04/2011 and 02/2011 boreholes (b) and with interpretation (c).
The stratigraphic units are explained in Fig. . The profile was
surveyed from south to north across the sinkhole area, leaving out the
sinkhole itself. Just as in profiles S1 and S2, steep normal and reverse
faults can be seen, but no bowl-shaped structure is present. Instead the
southern sinkhole margin is visible as a transparent area (blue dashed
circle), with near-surface reflectors dipping towards the
sinkhole.
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Uninterpreted reflection-seismic profile S4 (a) with stratigraphy
derived from the 01/2010 borehole (b) and with interpretation (c). The
stratigraphic units are explained in Fig. . The profile was
surveyed north of the sinkhole, parallel to S1(1). The normal faults seen in
S1 were also identified in S4, while two almost bowl-shaped structures (red
dashed circles) are visible in the south-west and the north-east.
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Seismic interpretation of S2
SH-wave reflection-seismic profile S2 of ca. 400 m length was
carried out south of the sinkhole (Fig. ). The marker horizon
that represents the base of the Calvörde Formation is clearly visible at
ca. 10 to 15 m depth and is traceable throughout the entire profile
(Fig. ). The Permian deposits below show the same discontinuous and
disrupted pattern as in profile S1, and numerous fractures were identified.
In the north-eastern area, dipping reflectors form a bowl-shaped structure of
ca. 100 m length and 40 m depth within the Triassic and Permian deposits.
Below this depression a steep normal fault dipping to the south-west was
identified by the reflection-seismic profile and the 03/2011 borehole.
South-west of the seismic section other normal and reverse faults with
vertical offsets of ca. 5 to 10 m were imaged.
In the Staßfurt Formation, which includes the dissolution horizon, a
large LRZ is observed between 175 and 300 m profile length at ca. 70
to 100 m depth below the depression.
Seismic interpretation of S3
SH-wave reflection-seismic profile S3 of ca. 370 m length (including
a 30 m gap around the sinkhole) was carried out from south to north,
passing the sinkhole (Fig. ). The reflection pattern is similar
to S1 and S2.
The flat-lying, mostly continuous reflectors of the Quaternary and the marker
horizon of the Triassic Buntsandstein can be precisely identified
(Fig. ). The reflection pattern of the Permian is discontinuous due
to vertical displacements of reflectors. Several near-surface normal and
reverse faults were identified with fault offsets of ca. 10 to 20 m.
Bowl-shaped structures, as seen in S1 and S2, are not shown, but between
ca. 100 and 120 m profile length, an almost transparent area at ca. 40 m
depth can be observed. This part of S3 was surveyed alongside the filled
sinkhole margin. No layers can be identified within the sinkhole zone; the
reflectors south of it dip towards the sinkhole. South of this structure,
between 80 and 120 m profile length, a northward-dipping steep normal
fault was identified in the 04/2011 borehole.
Small LRZs are observed between 240 and 270 m profile length at 70 to 95 m depth.
Seismic interpretation of S4
SH-wave reflection-seismic profile S4 of ca. 190 m length, was
carried out north-west of the sinkhole (Fig. ). The Quaternary
and the Triassic deposits were identified using the marker horizons and the
stratigraphy of the 01/2010 borehole, which was projected onto the seismic line
(Fig. ).
In the north-east and the south-west, within the discontinuous and displaced
Zechstein formations, faults with vertical offsets of 5 to 10 m were
imaged, which are probably the same as those seen in S1, since S4 runs
parallel to the western part of S1. In the same areas, two almost bowl-shaped
structures can be identified down to ca. 20 m depth, but they are not as
good as visible as the depressions of S1 and S2. In the Zechstein formations
z2 to z3 a large LRZ is observed between 60 and 110 m profile length at
ca. 60 to 90 m depth.
Map showing the complex fault geometry around the sinkhole. The
faults identified in the SH-wave seismic profile were extrapolated to the
surface. Blue and red balls represent normal and reverse faults,
respectively. The white areas within the balls display the position of the
hanging wall. Note the high fault density north of the sinkhole area with
fault blocks less than 10 m wide. For borehole information (orange dots),
refer to Fig. 3.
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Geological interpretation
Combining the information from the geological map, the seismic profiles and
the boreholes the following interpretation can be drawn. The subsurface below
Schmalkalden has been affected by tectonic movements since at least the
Mesozoic, when a NW–SE striking, dextral strike-slip fault zone containing
the SFZ and the HFZ formed. The latter crosses the subsurface of the town of
Schmalkalden. In the area of the sinkhole, the strike-slip fault created a
zone of reverse, normal and strike-slip faults, as shown in the seismic profiles
(Fig. ).
We observe local thickening of the Triassic Calvörde Formation, which can
only be accounted for by syntectonic sedimentation during the Triassic.
Normal faults generated additional accommodation space for the terrestrial
fluvial sediments. This was part of the multiphase tectonic deformation
which has been recorded in southern Thuringia . The
extensional stress regime during the upper Permian to the Lower Cretaceous
and the compressional stress regime during the Upper Cretaceous to the
Tertiary produced the dextral strike-slip zone and the various strike-slip
faults within it. The fine mosaic of fault blocks is mainly attributed to
this latter phase.
Fault inventory
The complex 3-D structure of the faults is difficult to decipher with 2-D
seismic lines, even if they are densely spaced. Nevertheless, large (5 to
10 m) displacements of the reflectors were found at several locations along
the profiles, and they are interpreted as near-surface normal and reverse
faults; e.g. below the western margin of the depression structure, steep,
north-east-dipping reflectors are visible in profile S1 at a depth of 50 to
100 m. The 05/2011 borehole proved the existence of a fault in this zone,
which was interpreted as a north-east-dipping normal fault.
Figure shows an overview of all fault positions extrapolated
to the surface. The high density of faults means that fault blocks are less
than 50 m wide and sometimes less than 10 m, especially directly north of
the sinkhole. In general, there is a mixture of apparent normal (25 faults)
and reverse (10 faults) faults. All faults have apparent dip angles
greater than 70∘ (apparent angle because they are measured in a
2-D section). This means the seismic profile is close to the true dip
direction of the faults, because otherwise the faults would not be so steep.
Consequently, the faults strike roughly NW–SE, similar to the strike of the
HFZ (Figs. and ). There is a tendency for normal
faults on the south-west end of the profiles to dip north-east and vice versa.
Note also that a few faults are outside of the presumed fault zone.
Interestingly, reverse faults cannot be followed from one seismic section to
another (e.g. S1 and S4, Fig. ), meaning that either the
strike length of the fault is less than the distance between seismic sections
(e.g. less than 50 m), which is most unlikely, or that a reverse fault in
one profile correlates with a normal fault in another profile. The
significance of this is described in detail in the next section.
Diagram demonstrating how jogs in a strike-slip fault can cause
both constraining and releasing bends. After right-lateral movement, normal
and reverse faults are created at the releasing and constraining bends,
respectively. The strike of the new faults will be close to that of the
strike-slip fault, depending on the original jog angle. Effectively, it
creates a system of normal and reverse faults with similar strike, along the
strike of the strike-slip fault.
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Discussion
The fact that steep normal and reverse faults occur side by side, as seen in
the four seismic sections, could be due to two different mechanisms, both
related to strike-slip faults. It is possible that some of the normal faults that were
generated under extension during the deformation phase from the upper Permian
to the Lower Cretaceous were inverted later by compression during the Upper
Cretaceous to the Tertiary . For instance, fault bends may
switch from transtensional to transpressional systems or vice versa if the
original fault bend was at a low angle relative to the maximum horizontal
stress . Alternatively, if there are jogs along the strike
of a strike-slip fault, this will produce constraining and releasing bends
, which will cause, after movement, reverse and normal faults,
respectively, with strikes similar to the strike-slip fault, along the strike
of the strike-slip fault Fig. .
The high fault density and the complex fault geometry in the research area
(Fig. ) did not allow a direct spatial correlation of
the faults, e.g. connecting the faults that were identified in two 2-D
seismic profiles. Only a high-resolution 3-D shear-wave reflection-seismic
survey could deliver more or less unquestionable spatial correlations, but
such a technique is still in development. Nonetheless, we were able to
identify the 2-D fault geometries and displacements from the 2-D reflection-seismic profiles.
The presence of a fault or a fault zone is not the only condition that has to
be fulfilled for the occurrence of a sinkhole like that in Schmalkalden.
Faults can be classified as open or sealed faults. Fault seal due to clay
smear or mineralization could hamper dissolution, because it reduces fluid
pathways . On the other hand, an open fault can act
as a fluid pathway.
During displacement along a fault, the hanging wall in particular undergoes
deformation caused by the fault morphology and the resulting strain
variations. The variations in strike of secondary faults are the direct
result of these strain variations . These small strike
variations can be observed in Schmalkalden, but besides the faults and
fractures visible in the seismic sections, subseismic-scale deformation will
also have occurred. Displacements along faults can result in a high fracture
density around and between faults, creating a damage zone that has the
potential to increase the fluid flow due to enhanced permeability. The key
factors for the increase in fracture density are the change in mechanical
rock properties, interactions between faults and a change in fault geometry
. The SH-wave reflection-seismic profiles carried out in this study identified a complex local and
regional fault system with a dense fracture network, which enables the
groundwater to circulate through the evaporites and therefore it enhances
dissolution and subsidence due to an increase in permeability. Different
joint sets were observed in outcrops in the vicinity of the sinkhole
:
(1) steep joints and fracture with a NW–SE strike,
(2) flat joints and fractures with a NW–SE strike,
(3) NE–SW-striking joints and fractures,
(4) young NNE–SSW-striking joints and
(5) young NNW–SSE-striking joints.
The sinkhole of Schmalkalden is located at the meeting point of three
groundwater catchment areas: the first is to the north and belongs to the
Gespringe Spring. The second is ca. 200 m south of the sinkhole and belongs
to the confluence of the Schmalkalden River (Fig. ) with the
Stille River. The third is to the west and belongs to the Mittelschmalkalde
River. Four groundwater levels are found, in the Quaternary
gravel, the Lower Triassic sandstone, the Leine Carbonate (z3) and the
Paleozoic bedrock. The latter consists of deep thermal, mineralized water,
which is undersaturated with regard to sulfates. This is an indicator of a
short residence time and a high hydraulic gradient. The more undersaturated
the water, the more sulfates can be dissolved. The main groundwater level
situated in the Zechstein formations actively leaches the soluble Permian
deposits of the Werra to Leine formations (z1–z3). This Zechstein water
ascends along faults and fractures and mixes with water from the upper
groundwater levels, since no widespread vertical separation of groundwater
levels is available due to tectonics and dissolution . The
groundwater movement follows the morphological gradient, and at the steep
faults and the intersections of faults the artesian-confined groundwater can
migrate upward and leach the soluble Permian deposits. A tracer test revealed a
hydraulic gradient of 100 to 150 mh-1 ,
which is a typical value for fractured and karstic aquifer .
Groundwater table contour map after reveals a
confluence zone of three different groundwater bodies. Three groundwater
bodies show different flow routes towards the sinkhole area, and the
change in groundwater flow can be seen to correlate with the
faults.
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Groundwater table contour plans reveal a change in flow direction around the
area of the HFZ (Fig. ). To the north, the groundwater contour lines
run from north to south with a flow direction from east to west and from
west to east towards the Werra River. In the area of the HFZ, however, the
contour lines run approximately east to west with a flow direction from north to
south and from south to north towards the Schmalkalde River. This change in
groundwater flow direction may be another reason for the occurrence of the
sinkhole and can be correlated with the faults discovered in this study,
because steep-dipping faults are assumed to be barriers for horizontal
groundwater flow perpendicular to the faults but serve as conduits for
horizontal flow along the faults . The NW–SE-striking
fault branches of the HFZ hamper the groundwater flow coming from the east
from the Thuringian Forest towards the Werra River; as a result the water
flows from north to south along the faults towards the Schmalkalde River and
thereby passes through the sinkhole area.
Several other studies regarding sinkhole distribution have shown the
clustering of sinkholes along fault lineaments
and the decreasing number of sinkhole
occurrences with increasing distance from the fault . A
lineament of sinkholes can also be used to find hidden faults .
This is the case in Schmalkalden, since the only sinkhole in the urban area
formed within the strongly fractured HFZ. A single fault within the soluble
rocks might not have influenced the groundwater flow direction and the upward
migration of artesian-confined groundwater that much and might not have
triggered a collapse due to dissolution and subsurface erosion. Nevertheless,
since the soluble rocks are located within a strike-slip fault zone with a
typically strongly fractured underground and increased permeability, as
described above, the dissolution process is greatly enhanced.
We assume that elongated cavities formed along the fault planes, and at fault
intersections these cavities become larger and migrated upward over time. As
the sustainability of the overburden is exceeded, the cavity collapses and a
sinkhole forms. Additionally, the dissolution-induced depressions within the
Quaternary and Triassic deposits and the Zechstein evaporites that are
displaced along steep-dipping normal faults and show local thinning,
are a result of the leaching processes which occur along the faults. The
dissolution rate is most probably very slow. For northern Germany, which is
affected by the dissolution of salt, the dissolution rate is mostly less than
1 mm per year , and since sulfate rocks have a even lower
solubility, the dissolution rate in Schmalkalden is probably much lower than
this. Such a determination requires a numerical modelling approach that
includes information, e.g. groundwater flow models, mass balance
calculations, geomorphological analyses, geochemical investigations of the
soluble rocks and the groundwater and fracture evolution
. To date, a numerical model does not exist for the
sinkhole of Schmalkalden.
Possible indicators of unstable zones and dissolution processes are the LRZs
observed in the seismic data. They are visible at the depth of the
dissolution horizon and below dissolution-induced features such as
depressions. A LRZ can indicate an area in which the evaporites
have already been leached or are being leached due to the contact with
groundwater. This can also be the reason why the dissolution horizon itself
does not generate a strong impedance contrast.
Dissolution creates structures at a subseismic scale which cannot be
detected because the resolution limit of the SH-seismic at such depths
is about 3 to 4.5 m. In previous work by and ,
carried out in Hamburg and Bad Frankenhausen in Germany, respectively,
similar dissolution-induced features were observed. The seismic sections show a
lateral and vertical variable reflection pattern with discontinuous
reflectors and small-scale fractures formed by dissolution of soluble rocks
near the surface.
Besides reflection-seismic surveys, other geophysical investigations were carried out
in Schmalkalden on behalf of TLUG, e.g. gravimetry. Gravimetry is sensitive
to density and due to the fault-enhanced dissolution the density varies
vertically and laterally. The micro-gravimetric surveys show a negative
anomaly with a WNW–ESE extension which crosses the sinkhole and the
surrounding area . This anomaly can be linked to the fault
system that crosses the sinkhole area. The source of the gravimetric anomaly
is located at a depth of 50 to 100 m, which coincides with the
dissolution horizon .
We assume that the dissolution processes are still ongoing, because the key
factors that lead to leaching soluble rocks are still present, e.g. the
complex strike-slip fault system and the resulting fractured subsurface with
its fluid pathways, the soluble rocks and the confluence zone of three
groundwater catchment areas. In the seismic section, several depressions were
identified that might develop into sinkholes in the future if the cavities
reach the surface, e.g. the large depression north-west of the sinkhole,
which also coincides with the WNW–ESE-trending negative gravimetry anomaly.
In summary, we have discussed the discovery of steep, normal and reverse
faults in the seismic sections, which are part of a strike-slip fault zone.
The faults are positioned very closely, forming a dense fracture network, not
only on the seismic scale but also at the subseismic scale. They act as
fluid pathways and lead to fault-enhanced dissolution. We describe a LRZ
as a possible indicator of an unstable zone and we show that gravimetry
measurements confirm our results.
Altogether, shear-wave reflection seismic measured at Schmalkalden has proven
to be a suitable method to image and analyse near-surface dissolution
structures at high resolution. This is also the case in Hamburg
, Bad Frankenhausen in Germany and at the Dead
Sea in Jordan . The method is applicable to other study areas if its limitations are kept in mind, such as the lower penetration depth
with respect to P-wave refection seismic, especially in dissolution areas, and
the limits of resolution. We suggest it should be used to support P-wave
reflection seismics to obtain more structural information in areas where the
P wave has insufficient resolution.