The Development of Drawdown Dolines and Subsidence Dolines with the Comparison of Their Bedrock Resistivities—A Case Study

Abstract

The effect of the epikarst on the development of drawdown dolines and subsidence dolines is described. For this, the resistivity values of the bedrock determined by Vertical Electrical Sounding (VES) measurements were used. The higher resistivities below drawdown dolines are explained by the deeper position of the piezometric surface, while the low resistivity values below subsidence dolines can be traced back to the more elevated position of the piezometric surface. Resistivities increasing towards the centre of drawdown dolines refer to cavity heterogeneity increasing towards the centre and increasing vertical percolation rate, while decreasing resistivity values indicate an increasing degree of cavity fill above the piezometric level. At their asymmetrical variety, the bedrock of their opposite slope is of asymmetric resistivity, which is explained by the different elevations of the piezometric surface and the latter by different infiltration that took place on slopes with different inclinations. The same resistivity values of the doline with a flat floor indicate the homogeneous cavity formation of the epikarst. This latter results in homogeneous vertical infiltration, which favours horizontal dissolution. The piezometric surface is not deflecting below the subsidence dolines because resistivity differences are small between the bedrock below the doline and the bedrock of its environment. Below subsidence doline, above the piezometric surface, cavities develop (the resistivity is higher relative to the resistivity of the environment), and then the cavities become filled by suffosion (therefore, the resistivity below the doline is lower relative to its environment). The passage below the doline develops into a shaft as a result of its concentrated water supply and the epikarst is separated into parts.

1. Introduction

In this study, conclusions are made on the position and pattern of the piezometric level by bedrock resistivity values. Electric resistivity depends on the quality and water content of rocks. The more porous the rock and the higher its water content, the less the resistivity. Taking these into consideration, the different effects of the epikarst on the development of drawdown dolines (which are features that developed in the karstified rock) and subsidence dolines are described. Thus, conclusions are made on the different development of the dolines belonging to the two doline types.

The epikarst consists of the surface and subsurface parts with cavities of the karstifying rocks [1,2,3]. In the zone of the epikarst, secondary porosity is high (it may reach 10–20%), while it is only 1–2% in the vadose zone [4]. The reason for this is that the infiltrating waters passing through the epikarst become saturated and are not able to exert a dissolution capacity below the saturation level in the vadose zone. Therefore, the infiltrating water swells back and percolates laterally [2]. The surface that was formed by back-swelling is the piezometric surface [2,3]. Its elevation depends on the degree of water supply and drainage relative to each other. Its pattern depends on the distribution of the degree of vertical drainage.

Dolines are common features of karst areas. Different terminology was made during their classification in the literature; probably the most accepted is listed below. Dominant features of karst surfaces are solution dolines and subsidence dolines [5,6]. Solution dolines are depressions that develop on karstic rock. The reason for their development is that the dissolved material is transported out of the rock [4,7]. Solution doline varieties are drawdown dolines (Figure 1), point-recharge dolines and inception dolines [7]. Drawdown dolines most often occur on soil-covered karst, but they may also occur on covered karst [8]. Subsidence dolines develop in the superficial deposit. They have several varieties that refer to their development.

Subsidence dolines (sinkholes) are depressions that develop in the superficial deposit; therefore, the bearing beds extend beyond the margin of the doline. Subsidence dolines may be situated at the plain surface of the superficial deposit, above the plain, covering bedrock, at the plain surface of the caprock, above a filled bedrock depression and in the indentation of the bedrock when the latter is lined with superficial deposit. Below subsidence dolines, there are mostly no depressions on the bedrock [9].

Therefore, with the exception of drawdown dolines situated below the caprock, there is soil on the side slopes of drawdown dolines, and the doline deepened into a karstic rock, while the side slopes of subsidence dolines are constituted by the bearing caprock series, and below them there is no depression on the bedrock with the exception of the above-mentioned things [9,10,11,12]. If there is a depression, as mentioned above, the development of the subsidence doline is younger than the development of the drawdown doline, which is situated below it.

Subsidence dolines are predominantly of small size (their diameter is rarely larger than 10–20 m), but there may be small dolines among drawdown dolines too, but their size often significantly exceeds the size of subsidence dolines (their diameter may be several hundred meters and their depth may be several tens of meters too).

In drawdown dolines, but also in subsidence dolines, there are passages and shafts which are not always visible. These features may also occur in the side of depressions in drawdown dolines, and the bedrock crops out at their opening onto the surface. In the case of subsidence dolines, the shaft of the caprock is connected by passages that develop in the superficial deposit. The shaft size of subsidence dolines is large relative to the size of the depression and it is always at the deepest point of the depression; around it, the bedrock is characterised by a collapse of some meters in thickness [13]. The bedrock may crop out of below the caprock at these features as well (see below).

Subsidence dolines receive meteoric water that falls on the superficial deposit surface of their environment. Therefore, water drainage features (gullies, creeks) are connected to the dolines. At drawdown dolines, these features are absent. Thus, they mainly transmit the meteoric water that fell onto their area and into the karst.

The thinner the superficial deposit, the higher the probability of subsidence doline development [14,15]. The local thinning out of the superficial deposit is favoured by the fact that there is a mound on the bedrock and/or a valley developed on the cover [12,13,16]. The studied dolines of the Bakony Region are mainly above bedrock mounds and of valley floor position (Figure 2). Out of the 30 studied subsidence dolines, 17 dolines are on the valley floor, 20 dolines are above covered mounds, but 11 dolines are on valley floors, where the mound is below the doline and the bearing valley floor.

Their distribution is also different. Subsidence dolines occur independently of drawdown dolines or apparently independently of them. In the latter case, they develop above filled and covered drawdown dolines [8], and in this case, as has already been mentioned, there is a genetic relationship between the two doline types. Drawdown dolines are never formed in subsidence dolines. However, subsidence dolines are often connected to drawdown dolines.

The material gets into the karst in the form of solution from drawdown dolines. Solid material (soil, weathering residue) cannot be or can only be transported subordinately in the lack of suitable water flow and drainage passage. The depressions of subsidence dolines are formed during the transportation of solid material since they develop during the denudation of undissolvable caprock. This may take place by collapse if the superficial deposit is a consolidated caprock doline or caprock collapse sinkhole or unconsolidated, but cohesive dropout doline or cover collapse sinkhole. If it is non-cohesive, suffosion doline or cover suffosion sinkhole develops by suffosion or by water transport [9,10,11,17,18]. Transportation in solution is also possible when the caprock is of calcareous material. However, the caprock may also be curved (sagging sinkhole) into the space that was formed by the transportation of dissolvable bedrock material [18]. When a caprock sagging sinkhole and a cover sagging sinkhole form depends on the place of dissolution that triggers material hiatus and on the character of the superficial deposit.

2. Research Areas

The profiles are in the area of the Hungarian Bakony Regions, Bükk Mountains, the karst area of Western Mecsek and the Romanian Pádis area (Figure 3). The Bakony Region is an area of the Transdanubian Mountains (Hungary) with the largest expansion. It is mainly built up of Triassic dolomite, which is covered by Jurassic, Cretaceous and Eocene limestone in larger and smaller patches [19,20] (Figure 4). The carbonate terrain is covered by the large patches of the Csatka Gravel Formation at several sites, but loess and its clayey varieties occur in large expansions. Karst features of the latter are the subsidence (suffosion) dolines. The resistivities of five covered karst patches of the Bakony Region [16] were studied in a way that the profiles went through the subsidence dolines of the research areas.

The Eleven-Förtés doline group is situated on Mount Kőris in the Northern Bakony, on a low-inclined terrain between the mound of Mount Kőris and Parajos-tető, between Márvány-Valley and Holes Valley at altitudes of 670–680 m. Mount Kőris is a horst (horst group) with great expansion and elevated position, the Mesozoic horsts which are its continuation are situated in a subsided form northwest below basin sediments. In the northern part of the horst area, there is an Upper Triassic dolomite in great expansion at the surface, while Dachstein and Jurassic limestones are widespread in the south. Along faults, the direct environment of Mount Kőris (with Eleven Förtés) gets into an elevated position even within the horst group and thus it is situated above Dachstein limestone and dolomite terrains. In some places, carbonate rocks are covered by loess and its reworked varieties in patches. Its constituting rocks are Lower Jurassic limestone and the overlying superficial deposits. The superficial deposits of the area are clay (loessy, with limestone debris), loess (clayey–muddy) or clay with limestone debris, loess (with sandstone or limestone debris), limestone debris (clayey) and limestone debris. The limestone bedrock is more dissected in the NE direction in the area of the Eleven-Förtés doline group: it is separated into mounds and depressions. In the latter, the superficial deposit is thickening out. The doline group developed in an area with an expansion of 200 × 200 m, which is areic. It is a covered karst area, which is enclosed by the outcropping limestone; thus, a paleokarstic depression with a dissected floor may probably have been filled or covered.

The main feature of this area is a small valley. Covered karst depressions (suffosion dolines, their number is 9) occur both in and outside the above-mentioned valley. Several plugged and filled depressions occur; their inner part is marshy or their area is occupied by a smaller pond.

The Tés Plateau is situated in the Eastern Bakony, with an expansion of 16 km in the EW direction and 8 km in the NS direction, with an altitude of 420–480 m. It is covered by karst, where the bedrock is constituted by limestones of various ages (Upper Triassic Dachstein Limestone, Jurassic limestone, Cretaceous and Eocene Limestone) and dolomite. The Tés Plateau Eastern area can be separated into three smaller areas. Tés-1 includes the floor of Tábla Valley, Tés-3 involves the floor of one of the tributary valleys of Tábla Valley, while Tés-2 includes the interfluve between the two valleys. There are several, large shaft caves (46 caves) in the area of Tés Plateau. These open from the floor of subsidence dolines and are their drainages. The Tés Plateau Eastern area is areic.

In the Bakony Region, between Mount Kőris and Hajag, several blocks are aligned being covered with Cretaceous limestone (these belong to the Albian Stage). One of them is Mester-Hajag, which is a block with a NNW–SSE strike direction with an altitude of 440–500 m between Szekrényeskő-árok and Sötét Valley. Our research area was the northern part of this block (Mester-Hajag North).

In the area of Mester-Hajag, the surface is dissected by limestone mounds with a height of several meters, on which the limestone debris is widespread and at most is covered with soil. The mounds constitute rows. The superficial deposit survived between them but thinned out (since the superficial deposit was partly transported into the surrounding valleys, but the mounds hindered the complete transportation of the superficial deposit). There are 85 small dolines in the area of Mester-Hajag, and no erosion features are connected to them. These are suffosion dolines and are often aligned in rows on lower, covered terrains between the mounds (the lower terrains that bear the dolines are of an expansion of some hundred meters). Only two inactive dolines are known from this area.

In the area next to Fehérkő Valley (its altitude is 300–340 m), a small doline row, constituted of four subsidence (suffosion) dolines, constitutes the karst features between buried mounds or mounds exhumed to a low degree, but some inactive, filled subsidence dolines can be found here. Both Mester-Hajag North and Fehérkő Valley have a surface runoff.

The Homód Valley karst area (with an altitude of 460–480 m) was formed on an inherited, areic valley floor section in the southern part of the Hárskút basin on Middle Eocene limestone. This karst area is areic, where 22 dolines being separated into two groups can be found. Several filled, inactive dolines occur in this area.

The Bükk Mountains (Hungary) are folded mountains with imbricated and nappe structures (Figure 5). Their central part is separated into the Little and Great Plateaus, which are synclinal structures mainly built up of Triassic limestones [19]. Our research area was Zsidó-Rét on the Great Plateau near Tar-kő (mount), along the blue tourist road, where VES measurements were made along two profiles on the floor of a plate-shaped doline.

Pádis constitutes the central part of the Bihar Mountains (Apuseni Mountains, Romania, Figure 6). Its basement is built up of metamorphic rocks overlain by Triassic and Jurassic limestones and dolomite, with Permian sandstones at the margins [21]. The plateau of Pádis is separated into two levels: the upper is constituted by mounds, and the lower is built up of nearly plain, soil-covered, and covered karst terrains between the mounds. The plain terrains are mostly areic. Solution dolines developed both at the upper and the lower levels, which are often coalesced with each other. There are covered karst patches with many subsidence dolines in the karst depressions of the lower level since the streams originating from the sandstone terrain lined them with sandstone debris. The profiles were made on the covered karst of such a low, areic terrain (Răchite) and on the covered floor of an epigenetic valley. In both areas, the profiles went through the subsidence dolines.

The Western Mecsek Karst (Hungary) is built up of Lower and Middle Triassic limestones, which are enclosed by dolomite and sandstone from the south [23]. The karst is inclined in the northern direction and its surface is covered by loess. There are several solution dolines aligned in rows on this karst and many subsidence (mainly suffosion) dolines. These occur in and outside drawdown dolines. A striking characteristic of this karst area is the great density of dolines. According to Lippmann [24], the doline density (including solution dolines) may reach 80 dolines/km². The research area was the Cigány-földek area. The measurement profiles went through the drawdown dolines, crossing subsidence dolines, but they also went through the subsidence dolines of the terrain without drawdown dolines.

3. Methods

Bedrock resistivities below drawdown dolines and subsidence dolines were measured along profiles using the VES method by colleagues of Terratest Ltd., and based on the measurements, geological–geoelectric profiles were made. In the course of Vertical Electrical Sounding (VES) measurements, an electric current is conducted into the rocks through two grounded electrodes, and the other two electrodes measure the potential difference which is created by the current dispersion. The sounding curve is made from this. A theoretical curve is chosen which coincides with the sounding curve. Theoretical curves (and thus, the sounding curve too) show the resistivity of rocks, which depends on rock porosity, the quality of the filling material and water content. By the resistivity data of adjacent measurement sites, a geoelectric–geological profile can be constructed for a given distance. Since drawdown dolines had a larger diameter, the resistivity was measured in the total expansion of the doline, while due to their smaller expansion, resistivity values were measured only at one or some sites of subsidence dolines. Resistivity data were obtained from the environment of subsidence dolines with the lengthening of the profiles.

The average values of bedrock resistivities below drawdown dolines were compared with the averages of bedrock resistivities below subsidence dolines. T-probe was used to study the significance of bedrock resistivities below doline types. For this, the resistivity values of drawdown dolines in the area of Răchite, Pádis and below subsidence dolines of epigenetic valleys as well as the resistivities of the drawdown dolines and subsidence dolines of the Western Mecsek Karst were taken into consideration. The change of resistivity values below drawdown dolines was studied in three soil-covered karsts (Pádis, Bükk Mountains, Western Mecsek karst). Thirty subsidence dolines were studied in five covered karst areas of the Bakony Region (

Table 1. Bedrock resistivity data at the bearing profiles of the subsidence dolines of the Bakony region.

Doline Code	Area	Resistivity [Ohmm]			Profile Length (m)	Measurement Number	Cover Thickness Below Doline [m]	Shaft	Genetics According to Sediment Structure	Reducible Doline Group
		Below Doline	Average Along Profile	Its Largest Difference
I-16	Tés 3	-	346.4	92.00	100	5	0	+	●■	E
I-23	Tés 2	-	350.00	0.00	30	2	0	+	●
I-24	Tés 2	-	275.00	120.00	200	4	0	+	?
I-32	Tés 1	-	252.62	170.00	200	8	0	+	▲
I-33	Tés 1	-	301.28	186.00	260	7	0	+	▲
H-1	Homód Valley	-	307.00	390.00	144	8	0	+	▲■
E-6	Eleven-Förtés	-	758.57	660	105	7	0	+	●■
I-25	Tés 2	230	306.67	90.00	10.8	5	6.5	-	●	B
I-27	Tés 2	360	400.67	178.00	60	3	4.8	-	?
H-2	Homód Valley	200	252.5	140	126	5	7.7	-	●
H-6	Homód Valley	170	278.75	180	300	8	1.5	-	●
E-1	Eleven-Förtés	360	688	650	180	10	4.9	++	●▲
E-2	Eleven-Förtés	650	840	690	87	6	3.9	-	●○
E-5	Eleven-Förtés	560	645	10	26	2	5.3	-	●○■
I-17	Tés 2	310	251.63	80	75	8	0.9	-	●	A
I-18	Tés 3	370	248.2	83	60	5	2.9	-	●○
I-26	Tés 2	340	275.00	200	12	7	13.3	-	●■
I-31 1	Tés 1	430	272.28	125	213.33	7	6.3	+	●▲
H-8	Homód Valley	390	303.33	220	300	9	6.5	-	●○
E-3	Eleven-Förtés	670	550	780	118.33	5	2.7	-	●○
MH18	Mester-Hajag	1500	1700	1000	67.5	5	5.4	-	●	D
MH22	Mester-Hajag	1600	1660	900	52.5	5	5.8	-	?
MB-50	Mester-Hajag	3350	493.89	160	100.67	9	3.2	-	?
F-2 1	Fehérkő-árok	1390	1420	1330	118.00	7	3.6	-	●○
MH10	Mester-Hajag	1800	1271.67	1000	80.00	6	3.9	-	?	C
MH41 1	Mester-Hajag	2000	1862.5	1000	92.5	8	4.0	-	●○
MH52	Mester-Hajag	1400	1262.5	900	80.00	8	9.1	-	?
MB41 1	Mester-Hajag	510	505.6	156	196.67	10	4.6	-	●○
F1	Fehérkő Valley	2500	1987.14	2720	65.78	7	1.2	-	●○
E-8 1	Eleven-Förtés	1100	646	740	72.5	5	8.1	-	●○

++ excavated by wrecking. + there is a shaft ● suffosion ▲ collapse ■ erosion ○ sagging subsidence ? unascertainable ¹ doline with bedrock depression, the others are above bedrock mound.

). Here, the bedrock resistivity values of some dolines were compared with the average resistivities of their environment (of their bearing profiles). Taking the bedrock resistivity averages of the dolines and the average resistivities of their environment into consideration, five groups were made. The karstification characteristics of the areas which involve the dolines of these areas were studied.

4. Results

It has been mentioned that the morphology of drawdown dolines and subsidence dolines is different. At solution dolines, there is a depression on the bedrock. However, there may also be a depression on the bedrock below subsidence dolines in the following cases.

-

The compaction doline developed by the compaction of the sediment filling the bedrock depression [10]. The doline is younger than the depression of the bedrock and developed independently of it.

-

It is also younger than the bedrock depression and developed independently of it even when the bedrock depressions are filled with lenticular bedded series and these are overlain by the beds bearing subsidence dolines (Figure 7).

-

Similarly, the doline is younger than the bedrock depression if the depression of the subsidence doline is constituted by the beds that curved into the bedrock depression (drawdown doline, Figure 8). In this case, the development of the subsidence doline is not independent of the drawdown doline of the bedrock since the caprock subsided during its development. There is a genetic relationship between the development of the two doline types; a precondition of subsidence doline development is drawdown doline development.

Bedrock resistivity data show that at some sites, resistivities are higher than several thousand Ohmm, while at other places, these values are at about 1000–2000 Ohmm or lower. Therefore, this was regarded as the basis for the classification of resistivity values.

In the bedrock of drawdown dolines, the resistivities are high. Their value exceeds 5000 Ohmm (

Table 2. Resistivity values in the bedrock below drawdown dolines.

Area	Number of Profiles Taken into Consideration (Profile)	Number of VES Measurement (Measurement)	Average Penetration of Measurement into the Bedrock Relative to Bedrock Surface (m)	Average Resistivity 1 (Ohmm)	Site of Profile
Apuseni Mountains, Pádis Răchite (Romania)	3	46	4.55–8.58	11,385.44	closed area enclosed by mounds, buried recent depressions on the bedrock, with subsidence dolines at its surface
Apuseni Mountains, Pádis Răchite (Romania)	5	34	4.76–6.96	10,335.83	closed area enclosed by mounds, plain bedrock, with subsidence dolines at the surface
sum and average at Răchite	8	80	-	10,860.63
Hideg Valley, Aggtelek Karst,	1	6	4.4–5.4	5220	solution dolines (soil-covered karst), recent
Czigány-földek, Mecsek Karst inner part of drawdown dolines	4	25	5.16–6.56	4730.78	subsidence dolines in drawdown doline
Czigány-földek Mecseki Karst, margin of drawdown dolines	4	19	5.61–7.12	3818.45	subsidence dolines in drawdown doline
Czigány-földek sum and average	4	44	-	4274.61
Hochschwab	3	19	5.39–11.54	5258.98	large paleodepression, with covered karst subsidence dolines
Totes Gebirge	2	13	0.5–5	4176.4	paleodepression, with covered karst subsidence dolines
Zsidó-Rét, Bükk Mountains	2	6	4.65–5.81	10,000	floor of plate-shaped doline
sum and average	20	168	-	6865.73	drawdown dolines (recent and paleokarstic)

¹ from the average of profiles.

). However, in the bedrock below dolines, the trends of resistivities may be different, which can be put into the following groups.

In the bedrock, resistivities may be high, but the same or similar too (Figure 9).

Resistivities change below the doline. Their values may decrease (Figure 10) or increase (Figure 8) towards the margins.

The value of resistivities is different on the opposite slopes. In this case, they are higher on one slope and lower on opposite slopes (Figure 11).

Below subsidence dolines, resistivities are lower (

Table 3. Resistivity values in the bedrock only in the case of the occurrence of subsidence dolines.

Area	Number of Profiles Taken into Consideration (Profile)	Number of VES Measurement (Measurement)	Average Penetration of Measurement into the Bedrock Relative to Bedrock Surface (m)	Average Resistivity 1 (Ohmm)	Site of Profile
Bakony Mountains, eastern part of Tés plateau	14	104	3–10	315	mostly on valley floor
Bakony, Eleven-Förtés	10	68	5–7	611	inactive depression, subsidence dolines in its area
Bakony, northern part of Mester-Hajag	18	137	3–8	1273	area enclosed by mounds with subsidence dolines
sum and average	42	309	-	733
Cigány-földek Mecsek Karst outside solution dolines	2	11	4.7–6.2	3785.50	subsidence dolines
area next to Răchite	2	8	2.49–6.37	8443.33	covered karst of valley floor with subsidence dolines
Dachstein 2	1	7	10–15	868.56 (453.86)	paleodoline
sum and average	47	328	-	2885.57	-

¹ from the average of profiles; ² its data were ignored from average calculation since the measurement was made in drawdown doline, the value in brackets was calculated without higher resistivities of deeper position.

), particularly in the Bakony Region (Table 1), where there are no drawdown dolines at all. Here, the resistivity values are below 1000–2000 Ohmm, but at some sites, they are predominantly below 1000 Ohmm. (It has to be noted that in areas with drawdown dolines, resistivity values are also higher on the bedrock bearing only subsidence dolines, and they may exceed 2000 Ohmm, but they do not reach the resistivity values of the sites where the bedrock bears drawdown dolines).

The thirty subsidence dolines of the Bakony Region can be put into five groups, which are the following (

Table 4. Average resistivity values of the profiles bearing the dolines by doline groups.

Doline Code Group Code	Area	Resistivities [Ohmm]			Average Profile Length [m]	Average Measurement Number [Measurement]	Average Cover Thickness Below the Dolines of the Group [m]	Shaft (Total) [Shaft]	Doline Case Number [Case Number]	Potential Water Supply into the Karst
		Average Resistivity of Bedrock at the Dolines of the Doline Group	Average Resistivity of Profile Parts Bearing the Dolines of the Doline Group	The Largest Resistivity Difference of Doline Groups
E	Tés, Homód Valley, Eleven-Förtés, Homód Valley	-	370.13	229.71	137	5.86	0	7	n = 7	a lot of
B	Tés, Homód Valley, Eleven-Förtés	367.14	487.37	276.86	112.83	5.57	4.96	1	n = 7	a lot of
A	Tés, Homód Valley, Eleven-Förtés	418.33	323.41	248.0	129.78	6.83	6.23	1++	n = 6	little
D	Mester-Hajag, Fehérkő Valley	1206.25	1318.47	847.5	86.17	6.5	4.5	0	n = 4	little
C	Mester-Hajag,, Fehérkő Valley	1551.67	1255.90	1086.2	97.21	7.33	5.15	0	n = 6	little

Notice: ++ excavated by wrecking.

):

-

Below dolines, resistivities are low (below 1000 Ohmm) and lower than in their environment (group B).

-

Below dolines, resistivities are low (below 1000 Ohmm), but higher than in their environments (group A).

-

Below dolines, resistivities are high (above 1000 Ohmm), but lower than in their environments (group D).

-

Below dolines, resistivities are high (above 1000 Ohmm), but higher than in their environments (group C).

-

Below dolines, resistivities are low (below 1000 Ohmm), but there is no superficial deposit on the floor of the doline, there is a shaft (group E).

Dolines of groups A, B, and E occur in the eastern part of the Tés Plateau, in Homód Valley and in the area of Eleven Förtés. Dolines of groups C and D are in the area of Mester Hajag North and Fehérkő Valley. Dolines of groups A, B, and E developed in areas where no or negligible surface runoff is present.

In these areas, the karst receives more water than in the case of dolines belonging to groups C and D which have a surface runoff.

5. Discussion

Below drawdown dolines, the epikarst contains a lot of cavities [4,7]. As a result of significant cavity formation, the piezometric surface is deeper than at sites where no drawdown dolines occur and thus, the epikarst contains fewer cavities. Therefore, there may be a higher chance of the occurrence of waterless cavities below this doline type, which results in higher resistivity. However, below these dolines, the piezometric level is not only deeper, but it also curves [11,25,26] (Figure 12) due to heterogeneous vertical drainage. Taking into consideration resistivities below drawdown dolines, the piezometric surface may have the following pattern.

The resistivity is higher in the centre of the doline (Figure 8) because the piezometric surface is at the deepest position here. This can be traced back to the fact that water supply is of the highest degree in the doline centre and so passage development too, which results in the high degree of drainage. Thus, both the subsidence and the depth of the piezometric surface are the largest relative to the surface here. Therefore, above the piezometric surface, the number of dry cavities is the largest here, and thus, the resistivity is also the highest (Figure 13a).

When the resistivity is lower in the doline centre than at its margin (Figure 8), the lower resistivity cannot be explained by the higher position of the piezometric surface because drainage of the highest degree should also be below the doline centre in this case. It can be explained by the fact that the cavities above the piezometric surface are filled (Figure 13b).

Asymmetric resistivity can be observed at asymmetric dolines with side slopes of different gradients (Figure 11). This phenomenon can be interpreted by different runoff (on the bedrock) and infiltration proportions. On steeper doline slope bedrock surfaces, more water flows down and less infiltrates, which results in a lower piezometric surface below this slope. Therefore, there are more waterless cavities and this causes higher resistivity. On more gentle side slopes, less water can flow down and more water infiltrates; thus, the piezometric surface will be higher. Therefore, there will be more water-filled cavities; thus, resistivity will be lower (Figure 13c). On dividing walls between dolines, there will also be relatively higher resistivity, less water infiltrates; thus, the piezometric surface will be of lower position (e.g., Figure 10 at VES measurement site R52).

Below some so-called plate shaped dolines with flat floor [8] the resistivities are high and the same in the bedrock (Figure 9). High resistivity refers to many dry cavities above the piezometric level, thus the low position of this level. From the similarities of resistivity values, conclusions can be made on homogeneous cavity formation, and thus homogeneous drainage. This takes place if the dissolution capacity of the water getting into the epikarst increases and its quantity decreases (Figure 13d). The subsidence of the piezometric level is faster than that of the saturation level. The wet part of the epikarst is used up. Material transportation stops when the piezometric level in its whole expansion coincides with the saturation level. This happens first at the main drainage and then spreads towards the sites of drainage of lower degree thus, towards doline margins. The dissolution of the doline floor does not happen towards the doline centre, but it takes place towards the margins. The deepening of the dolines slows down; thus, it gets less and less water and the water arrives at its outer parts. Such dolines are not active, which is proved by their small depth and the ruined state of their margins [8].

The piezometric level below the subsidence dolines that were studied in the Bakony Mountains is not curved since the resistivity differences between the doline and its environment are low. Therefore, below subsidence dolines, the epikarst is immature because the dissolution capacity of the water infiltrating through the superficial deposit decreases. However, from the surrounding covered terrain, a lot of water arrives at the deepest point of the doline, which is small in the beginning. This strengthens the local development of the epikarst. In accordance with this, passages and shafts of the bedrock with different widths can be noticed in subsidence dolines. (At sites where they are invisible, it is not sure that they are absent, but they might be covered with reworked superficial deposits.)

More water arrives at the epikarst from areic surfaces belonging to dolines of groups A, B and E than from the surfaces bearing the dolines of groups C and D. This is proved by the fact that the karstification of the terrains bearing the dolines of groups A, B, and E is more intensive (doline density and size) and greater than in the case of the terrains bearing the dolines of groups C and D. At dolines of groups A, B and E, the epikarst is more mature, but due to the larger degree of water supply, the piezometric level is of mote elevated position; thus, resistivities are lower than below the dolines of groups C and D.

In the case of subsidence dolines with both low and relatively high resistivities, there are dolines at which the resistivity is higher (the dolines of groups A and C) and dolines at which the resistivity is lower (the dolines of groups B and D) than in their environment.

In the case of the dolines of groups A and C, as the decalcification of the cover increases, cavity formation takes place above the piezometric level. In this case, resistivity below the doline slightly increases. Subsequently, suffosion becomes more intensive (doline size increases) and the already developed cavities become filled (or the increase in doline size causes a larger quantity of water to infiltrate and thus contributes to a slight rise of the piezometric level); due to the above things, the resistivity decreases below the doline. The dolines of group B develop from the dolines of group A (Figure 14I), while the dolines of group D are formed from the dolines of group C (Figure 14II). At the dolines of group B, shaft development will be increasingly intensive. Their upper parts do not develop because a rock veneer is formed from the washed-in superficial deposit, but they develop below it. Smaller passages coalesce by dissolutional widening. A blind shaft develops, and its ceiling collapses [13]. The process favours the subsidence, collapse, and suffosion of the cover. The epikarst is separated into parts, but a strong hydrological relationship survives between the epikarst parts and the shaft. According to observations, during rainfall, water arrives at the epikarst parts from the surface through the shaft. Further observations prove that if the intensity of rainfall increases, water flows back from the epikarst into the shaft, which swells back and results in the development of an intermittent lake on the doline floor (Figure 14III-e).

6. Conclusions

In accordance with literary information, below drawdown dolines, the piezometric surface is deeper which is supported by the above-described high resistivities. Taking into consideration the value and pattern of resistivities, conclusions can be made on the different characteristics of cavity formation in the epikarst. From resistivities, the size and different patterns of dry cavities and thus the way of doline development can be interpreted.

The same resistivity refers to a plain piezometric level and laterally similar cavity formation and thus a plain doline floor. Resistivity increasing towards the doline centre refers to a sagging piezometric level, while a resistivity decrease marks the presence of filled cavities above the piezometric level. The increasing cavity formation of the epikarst results in depression development. The different resistivity decrease of opposite slopes can be traced back to a piezometric level with different depths. This is the consequence of the slopes being dissolved to a different degree, which results in the development of asymmetric depression.

The low resistivities of the epikarst below subsidence dolines refer to a high piezometric level and immature cavity formation. The presence of a superficial deposit retards the cavity formation of the epikarst and makes it local (shafts develop below the dolines). All this can be explained by the dissolution of a lower degree since the dissolution capacity of infiltrating waters is partially used up during infiltration through the superficial deposit. However, there will be a water surplus on the doline floor, which not only originates from dolines but also from impermeable environments.

In the Bakony Region, sites with lower resistivity, where karstification is more intensive, are richer in water. Therefore, two doline development lines can be differentiated in the mountains. The development line of terrains is richer in water, where type B dolines develop from type A dolines and then type E dolines are formed from type B dolines. Dolines belonging to the other development line occur in areas that are poorer in water. Here, type C dolines are formed and then from these type D dolines develop. These latter lose their activity more easily (become filled), but if there is more water in their environment, they may be transformed and may get into the previous development line.