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Identification of sinkhole development mechanism based on a combined

geophysical study in Nahal Hever South area (Dead Sea coast of Israel)

Article in Environmental Geology · September 2009

DOI: 10.1007/s00254-008-1591-7

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ORIGINAL ARTICLE

Identification of sinkhole development mechanism basedon a combined geophysical study in Nahal Hever South area(Dead Sea coast of Israel)

Michael Ezersky Æ Anatoly Legchenko ÆChristian Camerlynck Æ Abdallah Al-Zoubi

Received: 23 April 2008 / Accepted: 30 September 2008 / Published online: 25 October 2008

� Springer-Verlag 2008

Abstract Seismic refraction, magnetic resonance sound-

ing (MRS), and the transient electromagnetic (TEM)

method were applied to investigate the geological and

hydrogeological conditions in the Nahal Hever South

sinkhole development area at the Dead Sea (DS) coast of

Israel. Microgravity and MRS results reliably reveal large

karst cavity in the central part of investigated area. The map

of the seismic velocity shows that sinkholes in Nahal Hever

can be divided into two major groups: sinkholes close to the

salt edge and sinkholes over compact salt formations

between a few tens to a hundred meters from the major

cavern. The present study shows that the formation of

sinkholes of the first group is caused by soil collapsing into

the cavern. In the area occupied by sinkholes of the second

group, karst was not detected either by MRS or by seismic

diffraction methods. TEM results reveal shallow clay layer

saturated with DS brine underlain sinkholes of this group. It

allows suggestion that the water drainage and intensive

water circulation during rain events wash out fine rock

particles from the unsaturated zone into the pre-existing

cavern, initiating the formation of sinkholes of the second

group. Karst development takes place at a very low bulk

resistivity (\1 X m) of the DS aquifer, attesting to the factthat pores are filled with a highly saline solution. Refilling

of the karstic cavities with collapsing and flushed soil slows

down sinkhole development in the area. The sinkhole for-

mation cycle at the site is estimated at 10 years. Sinkhole

development throughout the studied area is triggered by a

drop in the level of the DS, which reduces the head of the

confined aquifer and the strength of the overlain sediments.

Keywords Dead Sea � MRS � TEM � Pseudo-sinkholes �Seismic refraction � Sinkholes

Introduction

The study area is relatively highly populated and located

within the Dead Sea (DS) basin, a single geological and

geographical unit shared by Jordan, Israel, and the Pales-

tinian Authority. Since the 1990s thousands of sinkholes

have developed in alluvial fans and other unconsolidated

sediments along the coastlines of the DS. The most

alarming cases were sinkholes that occurred recently

somewhat 40 m east of the main route, highway #90, in the

Mineral Beach area and in land used by farmers (agricul-

tural land) on the Jordanian side.

There are two principal competitive geological models

explaining sinkhole development. These models provide

alternate interpretations of the cause of void formation at

depths of tens of meters within the sediments or salt layer.

The piping model (Arkin and Gilat 2000); explains the

gravel holes forming in the frontal areas of young alluvial

M. Ezersky (&)Geophysical Institute of Israel, 6, Haba’al Shem-Tov Str.,

PO Box 182, 71100 Lod, Israel

e-mail: [email protected]

A. Legchenko

Institut de Recherche pour le Développement (IRD-LTHE),

BP53, 38041 Grenoble Cedex 9, France


C. Camerlynck

Université Pierre et Marie Curie-Paris 6, UMR 7619 Sisyphe,

4, Place Jussieu, 75252 Paris Cedex 05, France


A. Al-Zoubi

Al-Balqa Applied University,

Salt 19117, Jordan


123

Environ Geol (2009) 58:1123–1141

DOI 10.1007/s00254-008-1591-7

fans. They are typically funnel-shaped with a surface

diameter ranging from 1 to 30 m. The funnel pipe diameter

is proportionally smaller and may not exceed several

meters. The pipe depth may not exceed 15 m. Pre-existing

unique flow lines (which can be recognized in the con-

glomerate beds by iron and limonite staining deposited by

the flow water) form the focus for developing sinkholes.

Fine material is washed out along the flow path in places

where the flow changes from laminated to turbulent. Fines

are washed out, and a hollow is formed. The process

continues in an upward direction forming a pipe. As the

pipe approaches the surface, sudden collapse occurs

forming a funnel-shaped hole.

Other one is the salt dissolution model (Yechieli et al.

2002). The chemical mechanism (salt dissolution) model

requires the concurrence of three factors: (1) lithological (a

salt layer close to the surface), (2) hydrological (unsatu-

rated groundwater flowing in contact with the salt layer),

and (3) tectonic (fractures or faults allowing the unsatu-

rated water to flow in contact with the salt layer).

The latter model is widely accepted as the main mech-

anism of sinkhole formation. None of the models, however,

fully explains the sinkhole formation mechanism. Flushing

of the silt suspension into the DS is likely to pollute the

seawater. This has not been confirmed by observation in

Israel, but similar phenomena have been noted by Taq-

ieddin et al. (1999) along the Jordanian coast. The energy

of the flowing water is presumably insufficient for the

transport of such considerable soil mass (Yechieli et al.

2002; Frumkin and Raz 2001). The chemical salt dissolu-

tion from below is also problematic, because (1) no fresh or

under-saturated saline water was found in most boreholes

drilled at the sinkhole sites, and (2) the salt dissolution

model does not provide information about the internal

deformational erosion processes (caused, for example, by

sagging) discussed in the recent genetic sinkhole classifi-

cations (Gutierres et al. 2008).

Sinkhole formation involves two types of processes: (1)

dissolution and (2) deformation-internal erosion (Gutierres

et al. 2008). Dissolution alternate the shape of interfaces,

forms the cavities, whereas deformation causes fractures,

faults, etc. Suffusion increases the porosity and so on.

Thus, processes involved in sinkhole formation cause

changes in the properties and structure of the subsurface

material. Some of these changes can be detected by geo-

physical methods such as gravimetry, seismics, electrical

resistivity, etc. It was shown (Ezersky et al. 2006) that the

geophysical methods applied are very sensitive to the

anomalies that correspond to changes produced by disso-

lution and subsidence processes features. Subsidence, for

instance, of the salt top, was detected by the high-resolu-

tion reflection method in the Nahal Hever South (NHS)

site. Cave was detected using seismic diffraction imaging.

Prominent bowl-shaped resistivity anomalies of 300–

1,000 X m were detected at the Ein Gedi site above largedissolution cavities (Ezersky 2008).

Within the framework of the NATO project related to

the sinkhole occurrence, a field study applying hydrogeo-

logical and geophysical methods has been conducted with

the aim of developing an integrated approach for assess-

ment of the natural hazards caused by the development of

sinkholes in the DS region of Israel and Jordan (Ezersky

et al. 2005). According to the main working hypothesis,

sinkhole formation is related to pre-existing karst cavity/

cavities developed within the salt. Sinkholes are formed by

cover collapse and suffusion into the karst. The formation

of karstic cavities is connected with the hydraulic con-

ductivity (permeability) of aquifers and the salinity of the

solution filling the pores.

The main objectives of the investigation include the

following: (1) salt layer mapping, (2) determination of the

resistivity structure of the subsurface associated with the

salinity of the groundwater (Yechieli et al. 2001), and (3)

characterization of the hydrogeology (the groundwater

level, water content, and hydraulic conductivity). The main

techniques used to study the hydrogeology of the sinkhole

development sites were the transient electromagnetic

(TEM) method in mapping mode [also referred to as the

time domain electromagnetic (TDEM) method] in combi-

nation with magnetic resonance sounding (MRS). The

seismic refraction method was used for mapping of the

subsurface salt layer.

Investigated area

The DS is the terminal lake of the Jordan River system,

located 420 m below sea level in an extremely arid envi-

ronment with an annual precipitation of 50–100 mm. Since

the early 1960s, large amounts of fresh water have been

diverted from Lake Kinneret and the Jordan River, result-

ing in a rapid drop in the DS level at rates of about 0.5 m/

year since the 1960s till 1980s and 0.8 m/year in the 1980s.

The drop in the DS level is accompanied by lowering of

groundwater level in the DS coastal area. Sinkhole devel-

opment sites are distributed along the DS shoreline

(Fig. 1a).

The hydrogeological hypothesis suggests that the low-

ering of the DS is accompanied by a corresponding

lowering of the groundwater level owing to a good

hydraulic connection between the DS and the western

aquifer system of the DS shore. The drop in the DS level

permits the intervention of low salinity groundwater into

coastal area (Yechieli and Gavrieli 2000). This water

causes dissolution of salt layers, up to 20 m thick, which

exist in the shore area at a depth of 25–50 m below the

1124 Environ Geol (2009) 58:1123–1141

123

surface. For a long period of time, these salt layers were

surrounded by very saline groundwater similar in compo-

sition to the DS water, whose total dissolved solids

[TDS] = 340 g/l. DS water also has very high (up to

224 g/l) chloride concentration, which constitutes 98% of

the anions (Cl-) with little sulphate and carbonate. It has to

be noted that chloride concentration is therefore the main

parameter used for characterization of the DS shore

groundwater (Yechieli 2000). Migration of the aforemen-

tioned fresh or unsaturated (in relation to Cl-) groundwater

into the coastal area has lead to their contact with the salt

layer and initiated the salt dissolution process (Frumkin

and Raz 2001; Yechieli et al. 2002).

The question is how the water comes into contact with

salt, which is supposed to be enveloped in clay. The

groundwater is characterized by very high salinity, similar

to DS water. Based on electrical resistivity (Fig. 1b),

hydrogeologists have identified several conventional inter-

faces separating zones of different salinity. These are brine,

zone III (with electrical resistivity q\ 1 X m) east of area,toward the DS, and fresh (or brackish) water, zone I

(q[ 3 X m), west of the area. The intermediate zone II isidentified as diluted brine (with a resistivity of 1 X m \q\ 3 X m) (Kafri et al. 1997). The hypothetical model ofsinkhole formation suggests that these interfaces migrate

with the drop in the DS level. Zones designated as I–III in

1970 occupy positions I0–III0 in 2003. Consequently, the salt

edge comes into contact with unsaturated water with respect

to Cl. The studied NHS area is located in the central part of

the northern DS basin in Israel (Fig. 1a).

Geology

The stratigraphy of the NHS test site is composed of alluvial

fan sediments to a depth of 18 m, a 5-m-thick marl layer,

and an 11-m-thick salt layer. A clay and gravel formation

underlies the salt layer. The section visible at the scarp of

the sinkholes consists of sand and gravels with clay inter-

calations. Iron oxide mineralization is also visible (Arkin

and Gilat 2000). Three boreholes (HS-1, HS-2, and HS-3)

were drilled in the area.1 The stratigraphy of the HS-2 and

HS-3 boreholes is presented in Fig. 2a. Boreholes HS1 and

HS2 (drilled practically in the same location) detected the

salt layer at the depth interval of 24–35 m. In borehole HS-

3, 40 m away, a cavity filled with a dense mud was detected

at a depth of 24–28 m, which is assumed to be dissolution

cavity at the depth of the dissolved salt unit that appeared in

the HS-2 borehole. It should be noted that the surface at the

HS-3 borehole does not collapse till 2008. The water table

in the area was measured at a depth of 17.2 m in borehole

HS-1 in 1999 and at 18.5 m in borehole HS-3 2 years later.

The salinity of the groundwater is 76 g/l at the water table

Fig. 1 a Distribution of the sinkhole development sites along the Dead Sea shoreline in Israel and Jordan; b presumed hydrogeological model ofsinkhole formation in the western shore of the Dead Sea, with water resistivity zonation

1 Yechieli, GSI report no. GSI/08/2002 (in Hebrew).

Environ Geol (2009) 58:1123–1141 1125

123

and quickly increases with depth. At the salt depth, the

water contains 188–200 g/l Cl (i.e., *83–88% of saturateddegree). Eighteen sinkholes have formed in the southern

and central part of the area until 2005, and recently three

sinkhole swarms have developed rapidly in the northern

part of the NHS area (Fig. 2b).

Chronological summary of the occurrence of sinkholes

in Nahal Hever

Based on the time of their occurrence, four groups of

sinkholes can be identified: before 1999, 1999–2002,

2003–2005, and recent ones (Fig. 2b). It seems that the first

southern sinkholes (nos. 1–7) are aligned along short lines

(200–250 m long) with close azimuths of 300�. Sinkholesof second group (nos. 9–12) are concentrated at the center

of area. Sinkholes of third group (nos. 14–17) are dissi-

pated at the southern part of the area. Sinkhole 5 is shown

in Fig. 2c. Later (1999–2002), subsidence number 3 and

sinkholes 9–12 were formed in the central part of the area

following the alignment of sinkholes 1–4 (Fig. 2b, d). After

2002, sinkholes 14–17 developed in the southern and

eastern part of the area. In December 2005, observations

revealed that in the southern part of the area the develop-

ment of sinkholes 14 and 15 was slowing down. Vast

vegetation developed at the bottom of sinkhole 15

(Fig. 2e). Other sinkholes at the center of the area contin-

ued to grow, and new sinkholes in the northern parts began

to collapse (Fig. 2f).

Features of the Nahal Hever South area revealed

by previous geophysical studies

In March 1999, when seven small sinkholes (1–2 m in

diameter and 0.5–1.0 m deep) existed at the area, a micro-

gravity study (Rybakov et al. 2001) revealed there a large

negative residual gravity anomaly with a size of approxi-

mately 50 9 80 m2 and an amplitude of -0.15 mGal. This

Fig. 2 a Geological section based on boreholes HS-2 and HS-3 situated 40 m apart and align in the north direction; b sinkhole distributionthroughout the Nahal Hever South area according to the time of their occurrence; c–g images of sinkholes (see text for explanations)

1126 Environ Geol (2009) 58:1123–1141

123

anomaly was interpreted as a subsurface dissolution caverns

within the salt layer, covered from above by a thin salt plate

shaped like a peak-cap (Fig. 3b). We will consider further

development of the anomaly in the ‘‘Pre-existing cavities’’

section.

Methods

Seismic refraction method

Seismic refraction allowed us to recognize and delineate a

salt layer. However, identification of the salt layer with the

seismic refraction method poses two important problems.

The first is the determination of the correct velocity crite-

rion, the second the generation of a proper geological

model (Ezersky 2006). A salt velocity criterion of Vpmin =

2,900 m/s (within the DS graben area) is accepted as the

statistically substantiated lower limit of compressional

wave velocity (Vp) for the central and southern parts of the

DS’s western shore. The geological model adopted for

geophysical mapping is illustrated in Fig. 3a.

This model is based on previously available seismic

refraction and geological data of the DS area (Ezersky

2006). The high velocity unit comes in contact with the

relatively low velocity unit at depths of approximately 20–

50 m. There are lateral velocity variations along the

refractor. Unconsolidated sediments (alluvium) overlay

this unit. Mapping of the salt–alluvium boundary is the

main objective of this study. Salt edge coming in contact

with sediments has been found to be shaped either like a

‘‘peak-cap’’ (Fig. 3b) or like a ‘‘wall’’ (Fig. 3c). Peak

thickness found at boreholes EG-7 and EG-13 varies within

a range of 2–4 m.

We interpreted seismic refraction data using the general

reciprocal method (GRM). The GRM is suitable for map-

ping subsurface layer-boundaries, because it is based on the

principle of refraction migration, permitting determination

of the detailed geometry of the refractor and providing

information about lateral velocity variations (Palmer 1986).

We applied the GRM to map the salt–alluvium boundary.

Data acquisition was carried out with the 48-channel

StrataView seismic recorder of Geometrics (Canada). We

used a 48-geophone spreads with 2.5, 5, and 10 m sepa-

rations between geophones. The DIGIPULSE source was

applied at five points on every refraction line, according to

the GRM technique: at both ends of the spread (zero-off-

sets), within the spread (split), and at a certain distance

from each end (offsets). The shot offset distance was

approximately half of the spread length. Geophones of

Fig. 3 a Hypothetical geological model of the salt unit; b, c verified models of the salt edge ‘‘peak-cap’’ and ‘‘wall’’ type, respectively; 1 salt, 2clay, 3 sandy sediments

Environ Geol (2009) 58:1123–1141 1127

123

10 Hz (vertical) and 8 Hz (horizontal) were used as seis-

mic sensors. The data processing was carried out using the

REFRINT software (Shtivelman 1995), which implements

two methods of interpretation: the GRM and the slope

intercept method (SIM).

Because the studied area is characterized by a smooth

topography, the accuracy of the depth of the interface was

determined by the dip of the refractor top. Where the

maximum angle of the refractor dip is less than 20�, theGRM provides estimates of refractor velocities within 5%

accuracy (Palmer 1986). Accuracy is lower when interfaces

are sharp. The accuracy of the location of a vertical

boundary between various refractor blocks (edge of the salt

layer) generally depends on velocity contrast, the depth to

the refractor, and the separation between geophones. For

our model, accuracy did not exceed two to three separa-

tions between geophones. Determination of the depth of the

horizontal interface (salt layer top) is less exact and

depends on the velocity contrast between layers and the

accuracy in the determination of the velocity of each layer

located above the refractor.

According to our experience, the difference between

refractor depth based on seismic measurements and the salt

top depth found in boreholes accuracy is higher than 10%.

To increase the reliability of the survey, we performed

seismic measurements along five of the 15 intersecting

pairs of lines that have been studied in the NHS area

between 2005 and 2007.

Magnetic resonance sounding

MRS is a method sensitive specifically to groundwater

(Legchenko et al. 2004). To perform the MRS measure-

ments, the protons of hydrogen in the groundwater

molecules are energized by a pulse of alternating current

generated in the surface loop of the device. The magnetic

resonance signal is returned by the protons after the exci-

tation current is turned off. One sounding consists of

measuring the MRS signal for different values of current in

the loop. Two main parameters are derived from MRS

measurements (Lachassagne et al. 2005): (1) the MRS

water content (w), which is closely related to the amplitude

of the MRS signal; (2) the MRS relaxation time T1.

Assuming a horizontal stratification, the inversion of

sounding data provides an estimate of the water content

w(z), and the relaxation time T1(z) as a function of depth (z)

(Legchenko and Shushakov 1998). The combination of the

MRS water content and relaxation time enables estimation

of the soil’s hydraulic conductivity (permeability) as

KMRS ¼ CpwT21 ;, with Cp being an empirical constant.Hydraulic conductivity is a scale-dependent parameter.

Taking into account that MRS results are averaged over a

large area defined by the loop size, pumping tests, which

also provide results averaged over a large volume, are used

for calibration. These MRS parameters can be linked to the

following hydrogeological parameters (Lachassagne et al.

2005): porosity (u, dimensionless); Darcy’s permeability[L2] [or hydraulic conductivity K (L/T)]; transmissivity T

[L2/T], which is equal to the coefficient of hydraulic con-

ductivity of an aquifer multiplied by its thickness measured

perpendicularly to the direction of the flow, most fre-

quently along the Z axis (L is distance and T is the time).

MRS is an efficient tool for characterizing aquifers as

well as for locating water-filled voids in the subsurface

(Vouillamoz et al. 2003). The relaxation time T1 is the most

reliable parameter for identifying water in voids. The MRS

signal generated by water-filling cavities is characterized

by a long relaxation time (T1 ? 1,000 ms); the signalreturned by water in a porous medium has a shorter

relaxation time (T1 \ 400 ms).In practice, a karst aquifer is generally composed of karst

conduits and caverns (T1 ? 1,000 ms) and of porousmedium (T1 \ 400 ms). The MRS signal measured fromsuch an aquifer depends on the ratio between bulk water in

the karst conduits and water in the rock. A high value of the

apparent relaxation time T1ap ([500 ms) can be consideredas a karst signature. Inversion of MRS data for T1 values can

be used to detect karst conduits within the porous water-

saturated rock also when the apparent T1ap \ 500 ms.All MRS measurements were carried out using the

NUMISplus MRS system developed by IRIS Instruments

(France). In the DS area, interpretation of MRS data

requires knowledge of the electrical conductivity of the

subsurface, and therefore, each MRS measurement was

accompanied by Transient EM measurements.

Transient electromagnetic method

TEM [also referred to as the time domain electromagnetic

method] is sensitive specifically to bulk resistivity (con-

ductivity) of the studied medium, especially in the range of

low resistivity (Kaufman 1978). This method is used

extensively in Israel for the localization of the fresh water–

saline water interface in coastal areas (Kafri et al. 1997;

Yechieli 2000; Yechieli et al. 2001). Generally, resistivity

of the subsurface depends on some parameters, most

important of which are salinity of the fluid in the pores and

porosity. Kafri et al. (1997) have shown that a bulk resis-

tivity of 1 X m or less is typical of the concentrated brinein the DS region, reflecting sediments that contain DS brine

or slightly diluted brine. The TEM method usually identi-

fies the sharp interface unambiguously. Consistent with the

hydrogeological model presented in Fig. 1b, 1 X m resis-tivity isolines form the top of the DS brine layer. In

addressing the DS sinkhole problem, it is important to

know the location of this interface and its configuration

1128 Environ Geol (2009) 58:1123–1141

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relative to the salt edge to estimate the possibility of salt

dissolution (Yechieli et al. 2001). Therefore, the 1 X msurface is an important reference to define the structure and

geometry of the upper surface (top) of sediments saturated

with the DS brine.

Quantitative interpretation of the results is based on the

modified Archie’s Law (Archie 1942), which establishes

that in partially saturated soil with ionic pore water con-

ductivity, bulk electrical resistivity depends on porosity,

pore volume occupied by the fluid, and the resistivity of the

fluid filling the pore space. In case of partially saturated

medium, bulk resistivity is expressed as

qx ¼ aqwS�n/�m ð1Þ

where qw is the resistivity of the solution filling the pores,/ is the porosity, S = Vel/VR is a fraction of the total porevolume filled with the same solution (referred to also as

degree of soil pore filling), Vel is the volume of the solution

in the pores, VR is the total pore volume per unit volume of

soil, and n is an empirical parameter termed the saturation

exponent (usually with a value of approximately 2).

Parameters a and m appear to depend on the cross-

section geometry of the pores along the flow path. The

value of parameter a varies mostly within range of 0.6–1.4.

Parameter m varies within the range of 1.37–1.95. In case

of complete pore space saturation, parameter S = 1 and

Eq. 1 becomes the conventional Archie’s Law:

qx ¼ aqw/�m ð2Þ

The TEM method can be used for vertical sounding or

profiling. A procedure commonly applied for ground

exploration with transient techniques involves laying a

square loop in the vicinity of the area to be examined and

performing soundings or profilings (McNeill 1980b). In our

study, coincident loop configuration was used (Barsukov

et al. 2006) when the same loop serves both as transmitter

(Tx) and receiver (Rx). Because sinkhole development

seems to be related to dimensional variations in water

salinity and to the geometry of the 1 X m interfaces, ideally3D measurements should be performed, but the necessary

technology is still under development, and both fieldwork

and data processing are very costly. In our case, dimensional

multisoundings (quasi-3D) served as a substitute for

3D measurements, and results must be interpreted with

some caution. Evaluation of the commercially available

equipment worldwide has shown that the TEM FAST

48HPC is a highly efficient system packaged in a light

portable unit that uses a single Rx/Tx loop. The system

supports the coincident loop configuration that accelerates

data acquisition in the field. The specifications of the TEM

FAST 48 HPC system can be found in the TEM FAST

manual (AEMR 2005a). The principles of the method are

described by Barsukov et al. (2006).

The TEM measurements were interpreted using two

commercially available 1D inversion software packages.

TEM-RESEARCHER is designed for modeling and

inversion of large TEM sounding data sets (AEMR 2005b).

For more accurate estimations of the selected inversion

results, we used the IX1D software from Interpex (2006).

Results

Mapping of compact salt by seismic measurement

The salt location map based on seismic compressional

wave velocities is shown in Fig. 4. A map based on five

refraction lines shows the velocity distribution throughout

the central part of the area in the year 2000 (solid thick

line). A map based on 15 seismic refraction lines (50–100

m apart), added in 2005–2007, has extended the previously

mapped area and enabled comparison of the salt distribu-

tion in 2000 and 2007. The low-velocity zone (Vp \2,750 m/s) associated with either a no salt or a dissolved

salt area has a complicated distribution. Borehole HS-3

crosses a dense mud 5 m thick at a depth of 24–29 m

(Fig. 2a), located within this low velocity zone. As a result,

we can identify an area with a velocity of less than

2,750 m/s with no salt (Ezersky 2006). Borehole HS-2,

drilled in a zone with a velocity of more than 2,900 m/s,

crosses a salt layer 11 m thick at a depth interval of 24–

35 m. We can therefore identify the compact salt unit with

a velocity greater than 2,900 m/s. The seismic velocity

map in Fig. 4 shows that sinkholes in the southern part of

the area (nos. 6, 7, 14, 15, 16, and 17) are located within

the high-velocity area (2,900–3,520 m/s), which differs

substantially from the northern part of the area (sinkhole

nos. 4, 9, 10, 12, and above), as well as from other sites

surveyed along the DS coast, where sinkholes are typically

arranged to the west of the salt edge in zones interpreted as

salt dissolution areas (Ezersky 2006; Ezersky et al. 2007).

A portion of the sinkholes discovered in 2000 (1, 2, 3, 13)

was located close to the salt edge, along both sides of it.

Magnetic resonance sounding results

The MRS study, aimed at characterizing the aquifer in the

sinkhole development area, was performed at the NHS site

in 2005 (Legchenko et al. 2008a). A total of 14 MRS

soundings were carried out with a loop of 100 9 100 m2.

The location of the MRS stations is shown in Fig. 5. In

NHS, MRS revealed two different zones: (1) in the central

and northern part there is an aquifer, which has karstic

features, and 2) a low permeability material at a depth of

30–35 m in the southern part.

Environ Geol (2009) 58:1123–1141 1129

123

The long relaxation times (T1 [ 1,000 ms) are theunambiguous signature of bulk water in the subsurface

(Vouillamoz et al. 2003). Depending on the rocks, the bulk

water may be interpreted as a karst aquifer or as water-

saturated gravel. Considering the geology, the aquifer

detected by MRS6 and MRS9 stations was interpreted as a

karst aquifer with water-filled cavities (Legchenko et al.

2008b). The MRS indicates that the karstic caves are

located at the depth of 25–35 m, corresponding to the

position of the salt layer revealed by borehole HS-2 and

seismic investigation results. The map of maximum T1value distributions (Fig. 6 a) delineates the karst zone. 3D

targets (like karst cavities) cannot be accurately resolved

using a 1D survey and interpretation design. Consequently,

the white dashed line in Fig. 6 delineates approximate limit

of karstified salt. In the northern part of the investigated

area, the limit of the karst zone was not defined. In the

southern part, compact clay-type material with low water

content and a short T1 was detected at MRS2, MRS11,

MRS12, and MRS15 stations. Thus, the MRS data reliably

confirm the absence of an aquifer in the southern part of the

area to a depth of 30–35 m, and no cavities were detected

in that sector.

In 2007, MRS measurements conducted in the same

positions as in 2005 enabled the estimation of changes in

hydrogeological parameter values. These changes are

shown in Fig. 7 as maps of variations of hydraulic con-

ductivity (Fig. 7a) and T1 relaxation time (Fig. 7b).

For calibration of MRS transmissivity and hydraulic

conductivity, three boreholes (AR-3, EG-6, and EG-8) with

pumping tests carried out by the Geological Survey of

Israel (GSI)2 in Ein Gedi-Arugot area have been used. In

these boreholes, the pumping tests reveal the transmissivity

Fig. 4 Map of the salt edgebased on seismic refraction lines

acquired in 2000–2007: areas

with a velocity higher than

2,900 m/s are identified as

compact salt, whereas areas

with a velocity lower than

2,750 m/s are interpreted as

loose water-saturated sediments

2 Volman et al. (2003), Stage B, Report GSI/42/2003 (in Hebrew).

1130 Environ Geol (2009) 58:1123–1141

123

of 0.96E-2, 1.8E-2, and 1.9E-2 m2/s, respectively. The

average value yields 1.58E-2 m2/s. MRS station is located

at about 300 m from these boreholes. After boreholes, the

empirical constant is set at Cp = 7.5E-8 and the MRS

transmissivity is estimated as 1.6 ± 0.8E-2 m2/s. Three

boreholes (HS-1, HS-2, and HS-3) were drilled by GSI in

Nahal Hever. Very low yield was observed during the

pumping tests and consequently the transmissivity was not

measured. Two MRS stations around these boreholes

reveal the transmissivity of 3.5E-5 and 4.5E-4 m2/s. Hence,

MRS results in the studied areas are qualitatively in

agreement with the borehole data.

Figure 7 shows that the relaxation time at the MRS6

station decreased from 1,050 ms in 2005 to 300 ms in

Fig. 5 Location of the MRSstations, boreholes, and known

sinkholes in 2005

Fig. 6 Maps of maximum T1 relaxation time (a) and transmissivity (b), and the approximate limit of karstified salt (white dashed line)

Environ Geol (2009) 58:1123–1141 1131

123

2007, and hydraulic conductivity value decreased by a

factor of 6 in the same period. This is unambiguous evi-

dence that the karst cavities were filled by collapsed

sediments after the sinkhole formation. Figure 7 shows

also that the values of hydraulic conductivity (K) in the

central part of the area (interpreted as the DS aquifer)

decreased during the same 2 years by a factor of 5–10,

whereas at the margins of the sinkhole formation area,

these parameters remained at a low level, close to the

sensitivity of the equipment.

The top of the aquifer derived from MRS data inversion

is presented in Fig. 8. MRS stations MRS12 and MRS15

reveal an absence of aquifers. For this raison, the top of the

aquifer does not appear on the maps at southern MRS

stations.

The conventional 2D map in Fig. 8a shows the topog-

raphy of the top of the water table. Above the main cavern

(MRS6 station), the water top is deeper (coinciding with

the top of the aquifer). The same map in 3D presentation

shows this more conspicuously in Fig. 8b. Based on these

Fig. 7 Changes in hydraulic conductivity (K) map (a) and T1 relaxation time map (b) between October 2005 and March 2007

Fig. 8 The top of the MRS detected water in 2D (a) and 3D (b) in Nahal Hever South

1132 Environ Geol (2009) 58:1123–1141

123

maps, we can conclude that the water above the karst

cavities is drained from the unsaturated zone into the

cavern.

TEM FAST results

In 2005, TEM measurements were performed at 88 stations

of 25 9 25 m2 and 50 9 50 m2 over an area of 600 9

600 m2. Figure 9 shows the TEM location map, together

with sinkholes and boreholes.

Generally, square loops (mainly 25 9 25 m2 side) were

used for the fast mapping of the area to determine lateral

and vertical resistivity distribution. The measurements at

these stations were performed in May 2005. The number of

stations were placed around sinkholes (i.e., the sinkholes

were located within the loops).

Figure 10 shows examples of the inversion logs (also

referred to as inversion sections) of the two main types of

transient curves of the NHS area. Station TEM24 is located

in the northern sector of the area and the TEM14 in the

southern part of the area (see Fig. 9 for locations). The

inverse resistivity sections are shown in Fig. 10. Both

sections were acquired with loops of 25 9 25 m2. The

most important parameters of the inverse logs are the Hup(uppermost 1 X m interface), Hlw (lower 1 X m interface),and qh (specific resistivity of the lower half space). Theseparameters are highly stable and only minimally affected

by equivalence. The parameters of the low resistivity layers

are among the main objectives of our study (Yechieli et al.

2001). The examples of TEM soundings show that, despite

the fact that resistivity generally decreases with depth there

are differences in its structure in different parts of the area.

Let us consider the dimensional distribution of ground

resistivities throughout the NHS area.

Inverse resistivity sections in the northern part

of the NHS area

The TEM24 inverse resistivity section comprises three to

four layers whose resistivities gradually decrease with

depth from several tens of ohm meter (X m) to slightly lessthan 1 X m (Fig. 10a). TEM24 resistivity section isslightly affected by equivalence. TEM revealed the 1 X minterface at a depth range of 22–28 m, which closely cor-

responds to the water table determined in 2005 by the HS-2

borehole at -415 m.

Inverse resistivity sections in the southern part

of the NHS area

The inverse resistivity section of the TEM14 station,

comprising sinkhole 14 within the loop (Fig. 10b), consists

of four layers. There are two very low resistivity layers

with resistivities of 0.5 X m. The lower one is located in

Fig. 9 Map of TEM stationlocations at the Nahal Hever

South site

Environ Geol (2009) 58:1123–1141 1133

123

the lower part of section, below 26 m. This layer charac-

terizes the confined aquifer. The other one is located in the

uppermost part over the water table, at a depth range of 2–

4 m. The presence of a low resistivity layer with a resis-

tivity of B0.5 X m in the uppermost part of the sectionabove the water table (which is located at a depth of 20–

22 m) is typical for the southern part of the area.

We performed TEM soundings of sinkholes within loops

of 25 9 25 m2. Another example of such a TEM inverse

resistivity log is at sinkhole 7, shown in Fig. 11. The

transient curve is shown in Fig. 11a and the inverse resis-

tivity log in Fig. 11b.

The structure of the resistivity log at the TEM7 station is

similar to that at the TEM14 station. It comprises four well-

determined and slightly equivalent layers, two of which are

of very low resistivity. One low resistivity layer is located

in the uppermost part of section, above the water table.

Figure 11c, d shows that the surface expression sinkhole 7

has remained almost unchanged between 1999 and 2007,

except for a slight increase in its depth by approximately

1 m.

Configuration of the 1 X m interface

We generated maps for the 1 X m upper and lower inter-face throughout the NHS area. Figure 12 shows the upper

1 X m interface map and the sinkholes on the surface. Theupper 1 X m interface occupies the southeastern part of theNHS area. This interface is located at high elevations

(-395 to -400 m) below sinkhole nos. 6, 7, 14, 15, 16,

and 17, which are located just above the top of the upper

low resistivity layer. In the central and northern parts of the

area, the interface drops sharply to an elevation of -415 to

-420 m, coinciding with the lower 1 X m interface. Themap presented in Fig. 12b shows distribution of bulk

resistivity through NHS area. It is seen that resistivity

gradually increases from slightly less than 0.5 X mat eastern margins of the area to 1 X m at its border.Figure 13 presents the topography of the lower 1 X minterface in a 3D presentation, showing its spatial geome-

try. This interface descends from the southeast, where it is

located close to the surface, toward the northwest. The

prominent feature of this map is the large funnel-shaped

hole in the central part of the flat. The hole is approxi-

mately 200 m in diameter. Most of sinkholes are located in

the vicinity of this hole.

The configuration of the 1 X m interface is shown alsoin the resistivity section crossing the area in the

Fig. 10 Examples of TEM24 and TEM14 resistivity sectionsobtained in the northern (a) and southern (b) sectors of the area.The main parameters of the interpretation are shown: Hup, uppermost1 X m interface; Hlw, lower 1 X m interface; qh, specific resistivity ofthe half space; dashed lines are equivalent logs

Fig. 11 Transient curve (a) and inverse resistivity log (b) of the TEM7 station (25 9 25 m2 loop, 1.49% misfit error); c, d sinkhole 7 in 1999and 2007

1134 Environ Geol (2009) 58:1123–1141

123

northwest–southeast direction. This section, in combina-

tion with the MRS data, enables the interpretation of the

Nahal Hever hydrogeological situation and the mecha-

nism of sinkhole formation (Fig. 14). The edge and

greatest depth of the salt layer was revealed by the seis-

mic refraction study. The water table of the DS aquifer

was determined by the MRS method at a depth range of

22–28 m. In the northwestern part of the salt layer,

resistivity is from 0.5 to 0.9 X m, whereas in the south-eastern part it is located in a zone of considerably lower

resistivity of less than 0.5 X m. The prominent funnel-

shaped depression in the 1 X m interface is located at thecentral part of the section.

A similar slighter depression was revealed by MRS at

the top of the aquifer (Fig. 8b). A karstic cave was detected

by MRS in 2005. Finally, there is a very low resistivity,

shallow layer in the southeastern part of section, discussed

above. Note the inclination of the 1 X m interface from theedges toward the center of the section. Figure 14 shows

that the resistivity of the DS aquifer at an elevation range

-410–to -420 m, close to the salt edge, is still less than

1 X m.

Fig. 12 (a) The elevation mapof the upper 1 X m interfaceand (b) resistivity map of theupper low resistivity layer both

generated from 1D TEM

resistivity logs through the

Nahal Hever South site.

Numbered figures are sinkholeson the surface, black points areTEM stations

Fig. 13 3D presentation of thelower 1 X m interfacegenerated from 1D TEM

resistivity logs through the

Nahal Hever South area.

Numbered features aresinkholes visible at the surface

Environ Geol (2009) 58:1123–1141 1135

123

Discussion

Pre-existing cavities

The first sinkholes were discovered in the NHS in October

1996. Back analysis of aerial photographs enables us to

determine that the first sinkholes appeared in 1993.3 Until

1999, only seven sinkholes developed in an area of

400 9 500 m2. A cluster of sinkholes has been formed at

the location of the main microgravity anomaly (1 in

Fig. 15a) detected in March 1999 by Rybakov et al. (2001).

It comprises several large segments that can be seen in the

photographs shown in Fig. 15b (marked with number 6).

Smaller clusters (2–4 in Fig. 15a) are also presented

around the main anomaly. In August 2002, a subsidence

has appeared at the location of anomaly 2. Over a period

of 2 years, it transformed into a large subsidence of 30 m

in diameter and 1.0–1.5 m in depth (Ezersky et al. 2006).

Its salt contour is shown in Fig. 15a, dividing the area

into two parts: a compact salt area and a no-salt one (5 in

Fig. 15a). It can be seen in Fig. 15a that anomaly 1

coincides with the salt layer. We interpreted it as a karstic

cavity formed under a salt peak-cap (Fig. 3b). Using 3D

forward modeling, the volume of the cavity was estimated

as *35,000 m3 (Eppelbaum et al. 2008). Thus, themicrogravity anomaly delineates a cavity with a complex

structure comprising segments separated by the salt

partition.

MRS results confirmed by boreholes (HS-2 and HS-3)

reliably show the presence of a karst cavity at the center of

the NHS area in 2005 (Legchenko et al. 2008b). Two

stations, MRS6 and MRS3, are shown in Fig. 15a. The

surface area affected by the sinkholes is relatively large.

MRS results also confirm that the karstified zone is larger

than the 200 9 200 m2 area. MRS6 and MRS9 results

show that the karst volume for the two soundings is

equivalent, as detected also by the MRS8 and MRS10

stations. These observations suggest that the karst zone

consists of many cavities and channels rather than one

large cavity. The volume of the karst was estimated based

on the volume of water-filled pores. With an uncertainty of

±50%, the karst volume at NHS in vicinity of the MRS6

station has been estimated at 27,000 (±13,500) m3 (Leg-

chenko et al. 2008b).

Thus, when process of sinkhole development had just

begun, large karst cavities already existed in the NHS area.

These cavities have a complex structure and are filled with

water or clayey mud, as demonstrated by the HS-3

borehole.

Origin of sinkholes

Analysis based on a combination of the map of sinkhole

distribution (Fig. 2b) and the seismic velocity map

(Fig. 4) shows that the sinkholes in the Nahal Hever area

can be divided into two major groups: (1) central and

northern sinkholes located close to both the cavern shown

in Fig. 15 (sinkholes 1, 2, 3, 4, 5, 11, and 12) and to the

salt edge (additional northern sinkholes 19–21), and (2)

sinkholes over a compact salt formation (6, 7, 14, 15, 16,

and 17) at some tens to a hundred meters from the major

cavern.

Fig. 14 NW-SE TEMresistivity section throughout

the studied Nahal Hever area.

The cave was revealed by MRS

mapping

3 Krovi et al., GSI report no. GSI/20/04.

1136 Environ Geol (2009) 58:1123–1141

123

First group of sinkholes

MRS results reliably revealed a large karst zone at the

central part of the investigated area, located along the salt

edge. Sinkholes in this zone (first group), following the

classification of Gutierres et al. (2008), can be related

partially to cover collapse sinkholes and partially to cover

suffusion ones (Fig. 2c, d). Sinkholes of the first type are

formed by the collapse of soil (cover) into the cavity. They

have scarped edges at the time of formation and are gen-

erally several meters in diameter. Sinkholes of the second

type develop through the downward movement of cover

particles into the cavities. According to Gutierres et al.

(2008), cover suffusion sinkholes are commonly bowl-

shaped holes, and their diameter can range from a few to

ten meters. The development of cover suffusion sinkhole 5

can be seen by comparing its initial shape in January 1999

(Fig. 2c) with its shape in January 2001 (Fig. 16c).

Second group of sinkholes

The situation of the sinkholes in second group is more

complicated. One presumption is that these sinkholes are of

a ‘‘collapse chimneys’’ origin, i.e., collapse of the surface

soil into small dissolution holes developed at the salt top.

However, the bulk resistivity of the confined aquifer is very

low (\0.5 X m, Fig. 14), and consequently groundwater ishighly mineralized (more than 210 g/l chloride at the HS-1

borehole). Moreover, there is 5-m-thick clay covering the

upper salt surface (Fig. 2a), which hampered the contact of

salt top with groundwater. Thus, possibility of salt top

dissolution in this area is seems to be doubtful. The seismic

refraction method shows high compressional velocities of

3,600–3,800 m/s under sinkholes of second group (Fig. 4).

Usually, seismic velocities in the sinkhole formation areas

(close to the salt edge) are of 2,900–3,100 m/s as a result of

heightened porosity. This prevents identifying sinkholes as

cover collapse into dissolution holes at the surface of the

salt layer [for instance, collapse chimneys as described by

Gutierres et al. (2008)].

It would be suggested that sinkholes in the second group

are linked to the faulting in the area, owing to their

arrangement along the lineament (Fig. 2a), but the azimuth

of the lineament (300�) does not coincide with the dia-grams showing the fault azimuth distribution in the area,

which is 10�–30� and 340�–350� (Abelson et al. 2003).Moreover, sinkhole development is concentrated within a

narrow zone, whereas the faults are spread through the

entire DS area. It is more likely that the linear arrangement

of sinkholes in the second group is linked to the subsurface

stream.

MRS detected no karst under the sinkholes in this group.

Sediments in the southern part of the area have low

hydraulic conductivity and are characterized by MRS as

clayey and silty sediments.

TEM mapping shows features of the subsurface geology

below sinkholes in this group. It is seen from Fig. 12b that

the sinkholes of the second group are underlain by a layer

of very low resistivity (0.25–1.0 X m) located some 3–5 mbelow the surface, at an elevation of -400 m. The top

Fig. 15 a Residual gravity anomaly map of 1999 and sinkholeclusters formed during some years (Rybakov et al. 2001, from SEG

permission); b sinkhole cluster (1) image comprising number of

segments (6) in May 2007; 1–4 sinkhole clusters, 5 no salt area, 6chambers, 7 seismic line, 8 DIGIPULSE seismic source (see Fig. 14afor location)

Environ Geol (2009) 58:1123–1141 1137

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elevation map of this layer is shown in Fig. 12a. This

phenomenon can also be seen in Fig. 14 at the southeastern

section of the area. Yechieli (2000) suggests that this layer

is composed of clay and silt that only a few decades ago

was saturated with DS brine when the DS level was at an

elevation of -390 m. Nowadays, close to the sea saline

relicts of the low hydraulic conductivity are found that are

not still flushed of the DS brine by meteoric water. Thus,

the origin of the low-resistivity layer corresponds to a

clayey unit saturated with residual DS brine. Note that the

layer under the southern sinkholes is of very low resistivity,

which supports the idea that the layer is not affected by

either subsidence or collapse—otherwise, layer resistivity

should be higher. This indicates that the layer forms an

aquiclude for recharged water coming from above. The two

maps in Fig. 12 show the clear boundary of this layer

delineated by the 1 X m resistivity isoline. It can be seen inboth maps that there is an obvious qualitative correlation

between the distribution of sinkholes in the second group

and the boundaries of the upper low-resistivity layer.

Moreover, the larger sinkholes 5, 14, and 15 are arranged

along the 1 X m boundary of this layer. The smallersinkholes 6, 7, 16, and 17 are located in the center and at

the margins of the low-resistivity layer.

This correlation, based on TEM data, supports the sug-

gestion that sinkholes in the second group may be related to

the type of pseudo-sinkholes formed by a mechanism

similar to that described by Arkin and Gilat (2000).

Recharged water flushes out the fine particles, as shown in

Fig. 14, and transports it to the karstic cavity. Is it possible

to transport particles to a distance ranging from tens to a

hundred meters under the conditions of the DS? The

possibility of transporting the particles through the area is

usually questioned because of the arid climate and pre-

cipitation of only 50–100 mm/year.

Previous studies have considered the possibility that the

flushed fine particles have been transported to the DS,

500 m away (Arkin and Gilat 2000). Subsurface water

channels have been observed in the open sinkholes

(Fig. 16a–c). Signs of iron oxide mineralization, visible in

the walls of the sinkholes, also attest to subsurface water

flow (Arkin and Gilat 2000).

Turbulent flow has been observed in the region during

periods of short rainfall, causing flood events (Frumkin and

Raz 2001). Comparison of the image of sinkhole 5 taken in

February 1999 (Fig. 2c) and in January 2001 (Fig. 16d) can

indirectly characterize the intensity of the surface streams.

Higgins (1984) concluded that piping is characteristic for

arid and semiarid climates, where the soil contains a clay–

silt component and where water throughflow arises, creat-

ing pipe-like openings in the subsoil. These openings can

later collapse to form a ‘‘pseudo-karst’’ or surface gulley

system. The rainfall is intercepted by any opening in rel-

atively level upland or terrace surfaces. Diverted water

moves downwards until (in one possible scenario) it is

blocked by an impermeable layer and moves laterally. An

additional condition for this scenario is a lateral opening

that allows water to flow laterally.

On one hand, clay revealed by TEM provides the

impermeable layer. This layer is lifted over the ground-

water table, dropping annually by 1 m. Currently, the

difference between the elevation of the top of the clay layer

and the groundwater table is approximately 20 m. On the

other hand, as result of drying clay consolidates, i.e., there

Fig. 16 Subsurface channelsvisible in sinkholes (a–c) andsurface water channel (d)

1138 Environ Geol (2009) 58:1123–1141

123

is a decrease in volume. This phenomenon, based on a

decrease in the effective pressure (Terzagi 1925), is dis-

cussed by Baer et al. (2002) with respect to the subsidence

of the DS shores. Heterogeneity of the subsurface mani-

fested in interbedded clay–silt–sand layers causes

heterogeneous deformations and consequently a lateral

opening. It was probably such a lateral opening (fracture)

that was revealed by GPR in subsided alluvium in the NHS

(Ezersky et al. 2006). The seismic diffraction method,

based on dissipation of the seismic energy, has shown

strong heterogeneity of the shallow subsurface in the NHS

area (Pelman et al. 2007).

We have considered alternative origins for sinkholes in

the second group: collapse chimneys or flushing. Our

suggestions need to be checked by numerical hydrogeo-

logical modeling and by TEM 3D forward modeling.

The two groups of sinkholes differ in their arrangement

(Fig. 2b), aspect, and long-term development.

We studied ten sites distributed along entire western DS

coastal area, from north to south, using the seismic

refraction method (Ezersky et al. 2007). We established

that at all sites the sinkholes developed along the western

salt edge by cover collapse into the salt karst. (Formation

of the second group of sinkholes, located within a range of

tens to hundreds of meter from the salt edge, is not typical

for the studied sites of the western DS coast and it is evi-

dently characterized by features of geological structure of

subsurface, as discussed above.) Nevertheless, water cir-

culation plays an important role in the process of sinkhole

formation. The MRS map in Fig. 8 shows that water in the

phreatic zone is drained into the main cavern. Similar

results are produced by the TEM map of the upper and

lower 1 X m interfaces (Figs. 12a, 13). The funnel-shapeddeep depression in the central part of the lower interface

(Fig. 13) clearly connects the phreatic and the confined DS

aquifers. The hole in the 1 X m interface seems to serve asthe channel for recharged water, for subsurface material

collapsing into the karst, and for flushed-out fine particles

refilling the dissolution caverns.

Long-term sinkhole development

Two MRS measurements, performed in 2005 and 2007,

showed reliably that the karst cavern (MRS 6 station; see

Fig. 15a for location) was refilled with fine soil material.

The maps shown in Fig. 7a, b indicate that I, hydraulic

conductivity, and relaxation time have decreased drasti-

cally during the period associated with a decrease in

porosity. The MRS 3 station (comprising cluster 1 in

Fig. 15a) produced T1 = 900 ms in 2005 and T1 =

300 ms in 2007. Thus, in the last 2 years, porosity under

two stations decreased as result of refilling of the existing

cavities. When the cavern was refilled by sediments, water

circulation in the vicinity of the main cavern slowed

down.

All sinkholes of the NHS area of 400 9 500 m2 con-

tinued developing until the cavern was refilled with

collapsing and flushing soil, causing the sinkholes in both

groups to slow down their development. This observation

suggests that the flushed out sinkholes are closely linked to

the pre-existing karst caverns. If flushed particles were

transported to the DS (Arkin and Gilat 2000), a drop in the

DS level would accelerate sinkhole development, but this is

not the case. The development of sinkholes has slowed

after the cavern was refilled with sediments.

As mentioned before, MRS has shown that in time the

karst volume is refilled with sediments as a result of surface

collapse and cover suffusion into the karst cavities. This

sharply reduces the hydraulic conductivity of the sediments

and slows down sinkhole development at the site. These

conclusions are supported by GSI data acquired in the past

10 years.4 GSI has been monitoring sinkhole appearance

systematically, and results obtained for the Nahal Hever

area confirm our ground observations: since 2004, the

number of new sinkholes has been diminishing in the area

of 400 9 500 m2 under investigation (Fig. 17). At the

same time, sinkhole formation has accelerated north of this

area.

General discussion

Based on these geophysical observations, we can now

advance a feasible scenario for the development of sink-

holes in the NHS area:

• A karstic dissolution cave of complex geometrydevelops near the western salt edge over a long period

of time. The cavity comprises communicating cham-

bers with salt partitions.

• The ceiling of cavern shaped like peak cap (Fig. 3b) issupported by the head of the confined aquifer, allowing

it to reach a considerable span. There is a 5-m-thick

clay layer bridging the cave and strengthening the

ceiling.

• A drop in the DS level decreases the head, increasingsupport for the overlaying sediments. Drying of

the sediments decreases their strength and causes the

collapse of the cover into existing karst cavities in the

salt formation (the first group of sinkholes). Part of

sinkholes in this group is formed by cover suffusion.

The process extends over a period of time because of

the bridge effect.

• Open pits and karst caverns intensify the local watercirculation, causing washing out of fine soil materials,

4 Abelson et al., GSI report no. TR-GSI/07/2007 (in Hebrew).

Environ Geol (2009) 58:1123–1141 1139

123

which then triggers the development of sinkholes at a

distance of tens to the first hundred meters from the

main cavern (the second group of sinkholes).

• When the main cavity is filled by soil, water circulationslows down and development of the sinkholes (in both

groups) slows down. A complete cycle of sinkhole

development takes place over approximately 10 years,

with the active phase lasting 5–6 years (Fig. 17).

Conclusion

Seismic refraction, MRS, and the TEM method were

applied to investigate the geological and hydrogeological

conditions in the sinkhole-affected NHS area along the DS

coast of Israel. Our geophysical results suggest that the

development of sinkholes in the NHS area is triggered by

the lowering of the DS level and caused by pre-existing

karst cavities in the salt formation.

Two groups of sinkholes have been identified: (1)

sinkholes over the large karst cavern, formed by the simple

collapse of the rock into the cavern; (2) sinkholes in the

vicinity of the large cavern (tens to one hundred meters

away), caused by flushing out of fine particles from the

ground into the cavity.

Our proposed scenario for the development of the

sinkholes assumes the existence of karst caverns before the

sinkhole formation. These caverns initiate the development

of the sinkholes, and when they fill up with sediments,

development of the sinkholes slows down.

Acknowledgments Our study was sponsored by the NATO‘‘Security through Science’’ Program (project SfP no. 981128). We

are also grateful to the Israel Ministry of Infrastructure for supporting

the study. The authors wish to thank Dr. U. Frieslander for his con-

tribution to the project. We are grateful to the GII staff for the

efficient organization of the fieldwork. We wish to thank Drs. Y.

Yechieli and M. Abelson, who are in charge of the sinkhole problem

at the Geological Survey of Israel, for providing geological materials

about the investigated areas. We wish to thank Dr. P. Milanovich and

Prof. A. Frumkin for their help in interpreting the data. We are

grateful for the help of Dr. K. Chalikakis (IRD) in the carrying out the

fieldwork. We are grateful to Mr. Y. Goldman and Mr. E. Raz for

assistance in data collection. Finally, the original manuscript was

substantially improved owing to the constructive criticism and edition

of the anonymous reviewer.

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Identification of sinkhole development mechanism based �on a combined geophysical study in Nahal Hever South area �(Dead Sea coast of Israel)AbstractIntroductionInvestigated areaGeologyChronological summary of the occurrence of sinkholes in Nahal HeverFeatures of the Nahal Hever South area revealed �by previous geophysical studies

MethodsSeismic refraction methodMagnetic resonance soundingTransient electromagnetic method

ResultsMapping of compact salt by seismic measurementMagnetic resonance sounding resultsTEM FAST resultsInverse resistivity sections in the northern part �of the NHS areaInverse resistivity sections in the southern part �of the NHS areaConfiguration of the 1 &OHgr; m interface

DiscussionPre-existing cavitiesOrigin of sinkholesFirst group of sinkholesSecond group of sinkholesLong-term sinkhole development

General discussion

ConclusionAcknowledgmentsReferences

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