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The Lessons Learnt from Geophysical Investigation of Sinkholes in Rock Salt in Thailand

Peangta Satarugsa

Department of Geotechnolgy, Faculty of Technology, Khon Kaen University, Khon Kaen, Thailand

ABSTRACT

Sinkhole is one of geo-hazards that have occurred in many areas worldwide. Knowledge on mechanisms for sinkhole

formation is crucial in planning of land use and land development. This review aims to provide a concise summary of

author’s research efforts to understand the potential causes of sinkholes’ hazard in the areas in Northeastern region of

Thailand. This region of Thailand is considered to be at high risk due to the naturally high abundance of rock salt, soluble

rock, from which large quantities of salt are industrially produced. Salt production in this region has been achieved by two

methods, namely, controlled solution mining and uncontrolled solution mining which is known also as brine pumping

method. In the first part, this review highlights a scientific definition for naturally occurring sinkholes, classification, and

mechanisms with a particular emphasis on the sinkhole that occurs in rock salt. In a second part, it discusses applied

geophysical methods used in an investigation of subsurface cavities in rock salt. In the third part, it summarizes data from

seismic and resistivity surveys used for the purposes of mapping and monitoring of subsurface cavity expansion. In

conclusion, mapping of subsurface cavities is necessary in high-risk areas to provide public protection against sinkhole’s

hazards. Highly detailed subsurface mapping is recommended in the high-risk areas such as those with thick soluble rocks,

located near ground surface. The goal standard for minimizing sinkhole’s hazards is to acquire three-dimensional pictures

of surface cavities and knowledge on expansion rates together with geo-mechanics of the cavities overburden. Additional

research should be directed to determining (1) the best solution for economic, rapid and effective subsurface mapping and

(2) economic and effective underground cavities treatment.

Keywords: Sinkhole, Rock Salt, Geophysical Investigation, Geo-hazard, Thailand

1. INTRODUCTION

Sinkhole is one of geo-hazards that have occurred globally

that could result in loss of human lives and assets

(Waltham et al., 2005). Once people have experienced

sinkhole’s hazard in surrounding areas, most became

anxious and had the desire for knowledge on whether their

lives and assets would be swallowed or sunk into the

ground. It is thus conceivable that knowledge on the

presence of subsurface cavity beneath man-made

structures would help relief such fears of sinkhole hazards.

In addition, knowledge on mechanisms of sinkhole

formation is necessary for land planners and decision

makers in order to create the most appropriate and suitable

programs of land use and land development. This review

is intended to improve our understanding of natural and

man-made sinkholes and geo-mechanisms (Figure 1),

using the northeastern region of Thailand as a case study.

Thailand (Figure 2) contains soluble rocks, limestone,

gypsum and rock salt and dissolution of these soluble

rocks could readily produce natural sinkholes. Apparently,

sinkhole hazard has occurred in Thailand. However, this

review article has a focus on sinkholes in salt in the

Northeastern region of Thailand because human activities

appeared to be involved tremendously in the mechanism of

the sinkholes in this region. The Northeastern Thailand

contains a large volume of rock salt deposits, estimated to

be more than 18 trillion tones (Supavanich, 1986). Total

world salt production in 2007 was about 257 tones (Salt

institute, 2011). Salt is one of mineral resources for human

uses, thus salt products used in Thailand and also exported

to those other countries are extracted mostly from an

underground rock salt in the Northeastern Thailand. There

are two method of salt extraction from the underground in

the Northeastern Thailand, controlled solution mining and

uncontrolled solution mining. An uncontrolled solution

mining, known also as pumping brine, is popularly used

although it is known to cause detrimental effects on the

environment. Approximately, fifty to sixty percents of

total Thailand salt production are produced by an

uncontrolled solution mining. It has been on and off risen

to public concerns in order to help the environment by stop

the producing salt with the uncontrolled solution mining,

but this method still remains in operation. Nevertheless, it

is my belief that if the mechanism of sinkholes is well

understood, a significant change in salt production could

be achieved and the uncontrolled solution mining would be

substituted with the methods that have been proofed to be

minimal detrimental effects on environment.

2. NATURALLY OCCURRING VS.

MAN-MADE SINKHOLES

2.1 Natural sinkhole

2.1.1 Definition

A sinkhole is a natural closed depression in the ground

surface, caused by dissolution of soluble and insoluble

rocks and soils. Three common soluble rocks are rock salt,

gypsum and limestone (including dolomite and chalk).

The rock salt and gypsum are, respectively, about 6,000

and 150 times more soluble than the limestone (Freeze and

Cherry, 1979), also see Table 1. Dissolution of these rocks

produces spaces and caverns develop underground [e. g.,

Figures 1 (a) and (b)]. The most common insoluble rock

Corresponding author: peangta@kku.ac.th

Copyright is held by the author/owner(s)

GEOINDO 2011, December 1-3, 2011

International Conference on Geology, Geotechnology and Mineral Resources of Indochina (GEOINDO 2011)

1-3 December 2011, Khon Kaen, Thailand

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that hosts sinkhole is a lava tube of basalt. A collapsed of

the lava tubes form sinkholes. Piping through dispersive

[e. g., Figures 1 (c) and (d)] and loessial soils can also

produce sinkholes (Lukicet al., 2009). A collapse of an

abandoned undergrounds mine or ruptured sewer pipeline

(e.g., sinkhole in Guatemala City on 2 June 2010) results

in a closed depression in the ground surface, a sinkhole.

This sinkhole is called a man-made sinkhole

[e. g., Figure 1(e)].

Figure 1. Picture of sinkholes (a) and (b) sinkholes

found in the Northeastern Thailand, (c) and (d)

sinkhole from dispersive soils collapsed at the surface.

(Photos by Jon White), (e) man-made sinkhole

occurred in Gautamala (Photo taken from

http://edition. cnn.com/2007/ WORLD/ americas/02/23/

sinkhole.ap/index.html.

Figure 2. Map display surface boundaries where

sinkhole-prone due to rock salt and limestone and

dolomite dissolutions.

In fact, the term “sinkhole” is derived from the evolution

process and thus different terms are used in its description

such as a sink, shake hole, swallow hole, swallet, cockpit,

doline or cenote. The sinkhole has many different

definitions e. g., a closed depression site where water sinks

into the ground, a site where the ground subside by slow

dissolution of its soluble rockhead, an open hole into

which a stream sinks, a downward movement of surface

materials, or a natural depression or hole in the Earth's

surface caused by karst process. A technical term of

“doline” was introduced by geomorphologists for a

specific description of “a closed depression in karst” and

“karts is defined as a landscape that is distinguished by its

underground drainage”. However, a term of “sinkhole”

has been popularly used to describe karstic depression as

well as those other natural and man-made ground subsided

process.

Table 1.Dissociation reactions and solubility of some representative minerals that dissolve congruently in water,

at 25C and 105 Pascal (1 bar) pressure (after Freeze and Cherry, 1979; Ford and William, 2007).

Minerals Dissolution reaction Solubility

(mgL-1)

Common range of

abundance in water

(mgL-1)

Quartz

Calcite

Dolomite

Gypsum

Sylvite

Halite

SiO2 + H2O H4SiO4

CaCO3 + H2O + CO2 Ca2+ + 2HCO3

_

CaMg(CO3)2 + H2O + CO2 Ca2+ + Mg2+ + 4HCO3

_

CaSO4 2H2O Ca2+ + SO4

2-+ 2H2O

KCl + H2O K++Cl- + H+ + OH-

NaCl + H2O Na++ Cl- + H++ OH-

12

60

50

2,400

264,000

360,000

1-12

10-350

10-300

0-1,500

0-10.000

0-10,000

2.1.2 Formation and classification

In the case of soluble rocks, formation of an underground

cavity in rocks requires source of soluble rocks, water that

is unsaturated with soluble minerals composite in the

soluble source rocks, a hydraulic gradient to move the

water through the system, and an outlet for escape of

dissolved water (Johnson, 2005). As the rock dissolves, an

underground cavity is created. Further dissolution

continues with a resultant enlargement of cavity and an

occurrence of additional collapse blocks. The cavity

reduces support to the ground above and can cause a

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1-3 December 2011, Khon Kaen, Thailand

447

gradual or sudden collapse of overlying rocks or sediments

into a sinkhole.

In the case of insoluble rocks i.e., lave tube of basalt; an

underground cavity is developed during a rock forming

(Budetta and Del Negro, 1995). A collapse of a cavity into

a sinkhole occurs as a lava tube roof drops out due to

weathering and erosion or imposed load, producing excess

load beyond roof’s strength.

In the case of dispersive and loessial soils, a collapse into a

sinkhole requires water drainage passing through these

soils. These soils can easily be washed out by water due to

their physical properties. The more water passes through,

the greater erode of the soils. Table 2 shows the sinkhole

classification by Waltham et al. (2005) which illustrate six

types of sinkholes with helpful description of sinkhole’s

features, processes, sizes and hazard.

Table 2. The six types of sinkholes with major parameters for each type (modified from Waltham et al., 2005).

Type I :

Solution sinkhole

Formation process

Host rock types

Formation speed

Typical max size

Engineering hazard

Dissolution lowering the surface

Limestone, dolomite, gypsum, rock salt

Stable landform evolving over > 20,000 years

Up to 1,000 m across and 100 deep

Fissure and cave drain must exist beneath floor

Type II :

Collapse sinkhole

Formation process

Host rock types

Formation speed

Typical max size

Engineering hazard

Rock roof failure into underlying cave

Limestone, dolomite, gypsum, basalt

Extremely rare, rapid failure events into old cave

Up to 300 m across and 100 deep

Unstable breakdown floor, failure of loaded roof

Type III :

Caprock sinkhole

Formation process

Host rock types

Formation speed

Typical max size

Engineering hazard

Failure of insoluble rock into cave in soluble rock

Any rocks overlying limestone, dolomite, gypsum

Rare failure events, evolve over > 10,000 years

Up to 200 m across and 100 m deep

Unstable breakdown floor

Type IV :

Dropout sinkhole

Formation process

Host rock types

Formation speed

Typical max size

Engineering hazard

Soil collapse into soil void from over bedrock failure

Cohesive soil overlying limestone, dolomite, gypsum

In minutes, into soil void evolved over months or years

Up to 50 m across and 10 m deep

The main threat of instant failure in soil-covered karst

Type V :

Suffosion sinkhole

Formation process

Host rock types

Formation speed

Typical max size

Engineering hazard

Down-washing of soil fissures in bedrock

Non-cohesive soil over limestone, dolomite, gypsum

Subsiding over months or years

Up to 50 m across and 10 m deep

Slow destructive subsidence over years

Type VI :

Buried sinkhole

Formation process

Host rock types

Formation speed

Typical max size

Engineering hazard

Sinkhole in rock, soil-filled after environmental changes

Rockhead depression in limestone, dolomite, gypsum

Stable features of geology, evolved over >10,000 years

Up to 300 m across and 100 m deep

Local subsidence on soft fill surrounded by stable rock

2.1.3 Size

Size and depth of sinkholes found in the world are highly

variable. The largest sinkhole is the Qattara Depression in

Egypt. It is about 80 km by 120 km, 133 m deep that

covers area of 19,605 km2. The depression was formed as

a result of salt weathering and wind erosion. The origin of

the Qattara Depression is not a fully karstic formation.

The karstic process occurred at the first stage and later was

superimposed by a fluviatile process (Albrittonet al.,

1990). In Thailand, there are two natural lakes, Nong

Harn, Sakon Nakhon and Nong Han, Udon Thani,

covering an area of 123 km2 and of 36 Km2, respectively.

These two lakes are interpreted to be the result of sinkhole

collapsed because rock salt is located beneath these two

lakes [Rau and Supajanya, 1985; Satarugsaet al., 2004 (a)].

2.2 Man-made sinkholes

Table 3 lists a variety of human’s activities can accelerate

or trigger an underground collapse into sinkholes. The

major critical factor is an increasing water flow

throughsoil and cavity beneath(Waltham et al.,

2005)processes of run-off drainage, broken pipeline, or

unlined ditches

further accelerate soil erosion and cavity expansion,

causing underground structural failure and sinkholes to

occur. For the over-pumping and de-watering of

groundwater, the overburden sediments that cover

underground cavities are buoyantly supported by

groundwater fluid pressure. The water below ground helps

keeping the surface soil in place. However, groundwater

pumping can produce a new sinkhole, if such pumping

results in a lowering of groundwater levels which in turn

causes underground structural failure and an occurrence of

sinkholes.

3. SINKHOLES ON SALT

Rock salt is a rock that is composed mainly of halite

(NaCl). The rock salt is readily and highly soluble in

natural waters (Table 1). A conduit 1 m in diameter can be

dissolved out of salt within just a few years’ time, and

surface lowering by rockhead dissolution can achieve the

rate of > 10 mm/year(Bell, 1975, 1992; Waltham et al.,

2005). Salt outcrop can survive in a dry, a desert, area

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where there is no rainfall i. e., no possibility of dissolution

by water.In the wet area where salt is exposed at surface,

its readily dissolution leaves an insoluble residue as a layer

of collapse breccias. Salt behaves as a ductile material at

normal atmospheric pressure and temperature. With such

properties, void in salt trend to close at the rates matching

dissolution enlargement and therefore large caves rarely

occur in salt. When flow pattern of groundwater toward an

area of rockhead, the results areal subsidence may be in the

range of a few meters deep and in the order of kilometers

across. However, when dense brine almost stagnates on

the rockhead and the impermeable salt cannot be reached

or dissolved by fresh groundwater, areal subsidence would

no longer take place.

Studies of subsidence at salt field area and nearby in

Cheshire Plain, England (Bell, 1975, 1992) have indicated

a rate of areal subsidence on salt under natural conditions

of groundwater circulation to be up to 15 mm/year for a

long-term monitoring and 100 mm/year for a short-term

monitoring. A subsided rate has been found to be up to

900 mm/year in the areas where brine is abstracted from

the well sunk into zones of wet rockhead for the purposes

of salt production. An increasing subsidence rate is caused

by brine abstraction process when water circulation is

greatly increased. Renewal flows of fresh water onto salt

further increases dissolution rates. When the brining

operations have ceased, the subsidence fell to minimal

rates that can be qualified as natural dissolution in the

undisturbed drainage regime. Without disturbance by

brining, solution cavities on salt are self stabilizing.

Table 3. The main processes by which sinkholes

are induced through human’s activities

(partly after Waltham et al., 2005).

Increase water input to soil cover

Uncontrolled run-off drainage from a site or structure

Install drainage ditches that are unlined

Broken pipelines

Soak away drains (dry well) within a soil profile

Unsealed borehole

Irrigation for agricultures

Impoundment of reservoirs and floodwater retention

basin

Increased through drainage due to water table

decline Over pumping for groundwater extraction

De-watering to maintain dry quarry operations or mine

workings

Physical disturbance to the ground Partial removal and consequent thinning the soil profile

Partial or total removal of vegetation on the soil cover

Vibrations from blasting or plant movement

Structural loaded on foundations with in the soil profile

Water table fluctuations

4. GEOPHYSICAL INVESTIGATION

OF SINKHOLES

Geophysical investigation techniques offer inexpensive

and effective methods for acquiring subsurface

information and hence they have been used widely. In

general, geophysical investigation techniques involve

identification of geophysical anomalies - where there are

vertical and horizontal changes in physical properties.

Thus, a successful application of geophysical techniques is

based on the validity of assumptions on subsurface

geophysical properties contrast between target and its

surroundings.

Each geophysical technique requires sufficient contrast in

measurable physical properties and has different

limitations and resolution, depending on an instrument

used. It is necessary to recognize those limitations and

resolutions since a selection of an appropriate geophysical

method is one of the factors in making applied geophysical

techniques successful. Table 4 shows examples of the

selection of geophysical methods for cavity detection.

Table 5 is summarizes recent geophysical methods,

commonly documented in the detection of subsurface

cavities since 2000.

Seismic survey is one of popular and well known

geophysical methods, but an application of seismic survey

for the purposes of the detection of subsurface cavity is

pretty much still in research stage. A few successful

detection attempts with the use of high-resolution seismic

reflection survey can be found in the literature reports (e.g.

Steeples and Miller, 1987; Branham and Steeples, 1988;

Kourkafas and Goulty, 1996; Piwakowskiet al., 1997;

Satarugsaet al., 2001, 2002, 2004(b); Miller et al., 2006).

Application of refraction survey for the detection of cavity

has been less frequent, as compared with the reflection

survey (Mari et al., 1999; Satarugsaet al., 2001, 2004(b);

Ezerskyet al., 2009; Frumkinet al., 2011). Recently, a 2-D

resistiviy imaging has been shown to be a powerful

method to detect a cavity. Under this circumstance, a

cavity can be distinguished on the basis of resistivity

anomaly from background (e. g., Loke, 1999; van Schoor,

2002; Satarugsaet al., 2004(c-e), 2006; Robertet al., 2006;

Frumkinet al., 2011). Integrated geophysical methods as

shown in Table 4 help better identify subsurface cavities

(e. g., Satarugsaet al., 2001, 2002; Ezerskyet al., 2009;

Frumkinet al., 2011).

5. SINKHOLES IN THAILAND: A

FOCUS IN AT-RISK NORTHEASTERN

REGION

5.1 An overview of geology

Thailand (Figure 2) is located in Southeast Asia between

latitudes 5o 37’ N and 20o 27’ N and longitudes 97o 22’ E

and 105o 37’ E and covers area of 518,000 km2. Thailand

can be divided into four geographic regions; the

mountainous highland in the north and northwest, the

Khorat plateau in the northeast, the central plain, and the

southern peninsular. The Northeastern Thailand covering

area of 160,000 km2 is located in the Khorat Plateau and is

composed of two tectonic basins, Khorat and Sakon

Nakorn basins (Figure 2). The two basins cover areas of

46,000 km2 (Satarugsa et al., 2005). Subsurface geology

of the two basins consists of unconsolidated sediments

overlaying deformed claystone and rock salt layers of

Maha Sarakham Formation. The claystone and rock salt

layers were deposited during Cretaceous period and were

later filled with other sediments. The rock salt layers were

deeply buried under the younger rock strata and were

subjected to a tectonic compressive stress, thus the rock

salt and those other strata deformed with different their

viscous properties. The rock salt strata were normally

mobilized and flown upward to form salt domes,

producing thickness of rock salt layers varies significantly.

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Data from hundreds of bore holes drilled into the claystone

and rock salt together with data from several 2-D seismic

sections, reveal that there are many deep and shallow salt

domes varying in sizes and shapes with depths below the

ground surface range from a few meters up to 1,300 meters

(Suwanich, 1986;Satarugsaet al., 2001, 2005).

5.2 Salt production and the risk of

sinkholes’ hazard in the Northeastern

region

Rock salt deposits are one of the important mineral

resources for the northeastern region. Major salt

productions in this region have been achieved by two

methods, a controlled solution mining and an uncontrolled

solution mining (a pumping brine method). An controlled

solution mining is a method that is mined by drilling a

deep (depth >200 m) well below salt roof and injecting

water through the well casings to dissolve the salt, leaving

a residence time long enough for the brine solution to

reach sodium chloride saturation, and later extracting brine

through the well casing. A cavity occurs at the well

location and a size of the cavity is known or can be

calculated. If a sinkhole occurs, it occurs at the well

location. This method needs a long-term monitoring after

mine abandonment, especially groundwater flow system.

In contrast the uncontrolled solution mining, salt is

extracted by drilling a shallow well into a zone of brine

aquifer, and pumping brine. This method greatly increases

water circulation, drawing renewed flows of fresh water

into the salt and therefore increasing dissolution rate. As a

result of such practice, rock salt cavities can be created

along the flow paths and may not be necessary that the

cavities occur only at the well location. The cavities can

spread over a large area. Therefore, the rock salt cavities

can be formed nearby or several kilometers away from the

well. Thus, the collapse of rock salt cavities into sinkholes

(e.g., Figure 1) has been found in several places nearby the

uncontrolled solution mining area. Loss of water from

reservoir overnight being due to the collapse of the ground

into the sinkhole was documented (Solgosoomet al., 1999;

Satarugsaet al., 2001). New sinkholes with depths from 1-

5 m and diameters between 1-8 m have been found while

the existing sinkholes have kept expansion. The result of

the sinkhole’s occurrence causes nearby residents living

with a fear of loss of life and properties. The uncontrolled

solution mining is environmental harmful method. It is no

longer included to be considered as salt-mine activities and

it is recommended to be converted to the controlled

solution mining (Berestet al., 2005).

Brine production in the Northeastern Thailand first began

with natural brine seepage on the ground, brine spring and

scraping of exposed salt on the ground to form brine

solution. Later the brine has been pumped out to increase

production rate, wells were drilled into brine aquifer

without any regards where salt dissolution taken place.

Formerly, it was believed that such brine pumping was

environmentally friendly because it was little different

from brine seepage on the ground and the resultant

dissolution could be spread over a large horizontal area. If

subsidence has occurred, it would appear as a trough of a

large horizontal extension at the surface. This subsidence

would not be recognized in a local area. However, these

lines of thought are completely wrong when applied the

pumping brine method in the Northeastern Thailand.

Indeed, salt producers in Thailand are under a regulation of

the Thailand Salt Production Department. According to the

department’s regulation, salt producers using the pumping

brine method are not allowed to drill brine wells into rock

salt stratum and, hence, the brine is withdrawn from the

underground by a cycle of groundwater flow system. This

improper regulation remains unchanged, although the

number of sinkhole nearby the pumping brine areas

continues to increase. Today, over 500,000 tons of salt is

annually extracted from the pumping brine method. Thus,

it can be predicted that the occurrences of sinkholes will

continue to increase while the size of pre-existing

sinkholes will be expanded excellent potential but not fully

developed. A = primary method, B = secondary method

Table 4. Geophysical methods for detection of subsurface cavity [British Standards Institute,

BSI (1999) and ASTM (1999)].

Geophysical method

Usefulness of method

Physical properties measurement BSI

(1999)

ASTM

(1999)

Seismic refraction 1 B Seismic velocity; largely related to variations in rock

mass strength Seismic reflection 2

Cross-hole seismic 3 B (A)

Resistivity sounding 2 Electric resistivity or conductivity; related to

variations of porosity, fluid conductivity, degree of

saturation

Resistivity profile/ imaging 3

Induced polarization 0

Electromagnetic profiling 3 A

Ground penetrating radar 3 A Same as electrical

Gravity and microgravity 2 A Density; related to lithology and fissuring

Magnetic 1 Magnetic field of ground materials

Downholeself potential 1 Same as electrical

Downhole resistivity 0

Downhole neutron /gamma-ray logs 0 Radioactivity; porosity, density, moisture

Downhole conductivity 2 Same as electrical

Downhole sonic velocity 2 Seismic velocity

0 = not applicable; 1 = limited use; 2 = used, could be used, but not the best approach, or has limitations;

3 = excellent potential but not fully developed.

A = primary method, B = secondary method

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Table 5. Commonly documented geophysical methods for the detection of subsurface cavities.

Geophysical methods Physical anomaly Investigation Results Example of Investigators

Seismic refraction Contrast in velocity

Interpreted geologic cross sections

(2-D prospect), map showing

contours of depths of rock salt map

(3-D prospect)

Satarugsaet al., 2001;

Satarugsaet al., 2004(b);

Ezerskyet al., 2009;

Frumkinet al., 2011

Seismic reflection Contrast in acoustic

impedance

Seismic sections (2-D prospect),

contours to depth of rock salt maps

(3-D prospect)

Satarugsaet al., 2001;

Satarugsaet al., 2004(b);

Miller et al., 2005;

Miller et al., 2006

Krawczyk, et al., 2011

Resistivity sounding Contrast in

resistivity

Interpreted subsurface logging and

interpreted geologic cross section Satarugsaet al., 2001;

Resistivity imaging Contrast in

resistivity

Pseudo-geologic cross section,

3-D earth images

Bataynehet al., 2000;

Satarugsaet al., 2001;

Van Schoor, 2002;

Satarugsaet al., 2002;

Satarugsaet al., 2004(c-e);

Satarugsaet al., 2006;

Eso and Oldenburg, 2006;

Robert et al., 2006;

Rose and Leucci, 2010;

Frumkinet al., 2011

Transient

electromagnetic (TEM)

Contrast in

resistivity/

conductivity

Interpreted subsurface logging and

interpreted geologic cross section

Carlson and Urquhart, 2006

Ezerskyet al, 2009;

Frumkinet al., 2011

Micro gravity Contrast in density Contours of gravity anomaly map Rybakovet al., 2001;

Whitelaw et al., 2008

Ground Penetrating

Radar (GPR)

Contrast in dielectric

constant

/permittivity

GPR cross section

Zisman and Wightman, 2005;

Rose and Leucci, 2010;

Frumkinet al., 2011

Micro magnetic Magnetic

susceptibility Contours of magnetic anomaly map Rybakoet al., 2005

Electromagnetic (EM)

Contrast in

resistivty/

conductivity

Interpreted geologic profile,

contours of conductivity value Satarugsaet al., 2001

5.3 Geophysical investigation

Figure 2 shows a map of potential areas where sinkholes

can take place due to dissolution of rock salt and

limestone in the Northeastern Thailand. For sinkholes

in salt, the areas bound in Khorat and Sakon Nakhon

basins.Figure 3 shows generalized subsurface

stratigraphy of the sinkhole prone areas. Detailed

geological and geophysical mapping have been

conducted since 1990 (e. g., Hinthong and

Cahroenprawat, 1990; Solgosoomet al., 1999;

Satarugsaet al., 2001, 2006). Reports of four

geophysical methods, resistivity both sounding and

imaging, seismic both refraction and reflection, gravity,

magnetic, have been documented for mapping rock salt

and subsurface cavities (Satarugsaet al., 2001, 2002,

2004 (a-e), 2006). Results from those surveys are

described below:

5.3.1 Seismic refraction and reflection

surveys

Satarugsa et al. (2002, 2004(b)) have conducted seismic

refraction and reflection surveys for detection of

subsurface cavities in order to critically assess the

possibility of land subsidence (Figure 4). Surveys have

conducted at and nearby sinkhole areas using high

resolution reflection and semi 3D refraction. The

survey results have indicated that (1) applied seismic

reflection survey is suitable for location of subsurface

cavities under certain circumstances. The presence of

subsurface cavity disrupts a reflected layer in the

seismic section and weakening events with low in

frequency and amplitude. The low frequency and

amplitude found in shot records can be used as indirect

indications for subsurface cavity. Near surface

conditions together with size and depth of the

subsurface cavities appeared to influence to a large

extent success of a survey, (2) applied seismic refraction

survey did not seem to be of use in detection of

subsurface cavities in this study area.

5.3.2 Resistivity survey

Sounding resistivity has been conducted for subsurface

cavities mapping, but the data did not allow conclusive

interpretation (Solgosoom et al., 1999; Satarugsa et al.,

2001). This was found to the case of poor resolution

although sounding resistivity has been used widely in

the investigation of groundwater in the Northeastern

region of Thailand. Results from 2-D resistivity

imaging, conducted at a man-made tunnel with different

electrode configurations have shown that the Dipole-

Dipole and Wenner-Wenner configurations have

provided better tunnel imaging. The tunnel appears as a

lateral anomaly in a homogenous medium. An

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1-3 December 2011, Khon Kaen, Thailand

451

anomalous zone of the tunnel is distinguishable by the

zone with high resistivity, surrounded by the lower

background resistivity. However, location of the tunnel

anomaly appears to be misplaced on the field resistivity

pseudosection. It is different from the synthetic

resistivity pseudosection. This suggests a strong 3-D

effect from nearby structures [Satarugsa et al., 2004(c-

e))]. Results from the nearby sinkhole areas have

shown that a Dipole-Dipole configuration provides a

better result for cavity imaging than the Wenner-

Wenner configuration. An anomalous zone of the

cavity is distinguished by very low and lowest resistivity

zone surrounded by the higher background resistivity

(Figure 5). The 2-D resistivity survey can be used for

mapping a rock salt along the profile where rock salt

imaging is displayed clearly, consistent with the drilling

results. The 2-D resistivity imaging can also be used for

determination of subsurface cavity expansion (Figure 6).

Results from the three repeated measurements along the

profile near active sinkhole together with the sinkhole’s

diameter expansion have shown subsurface deformation

through time (Satarugsaet al., 2006).

Figure 3. Schematic geologic column of the Khorat and SakonNakhon basins where the shallow rock salt

at near surface and their physical properties.

Figure 4. (a) Part of 12 fold stack section, insets at the bottom of figure are enlarged events from the white box

showing a zone of discontinuous reflector, (b) Depth structural contour map of the top of claystone showing 3D

perspective view. The highest and deepest depths are 1 and 9 m below the ground surface, respectively.

A box outlines the area shown in Figure 1 (b) (after Satarugsaet al., 2002).

International Conference on Geology, Geotechnology and Mineral Resources of Indochina (GEOINDO 2011)

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Figure 5. Resistivity image of sinkhole, pseudo geologic section and interpreted subsurface geology and sinkhole.

6. CONCLUDING REMARKS:

LESSONS LEARNT

To-date, mechanisms underlying the occurrence of

natural sinkholes has been well established. Areas at

high risk of ground subsidence and a sinkhole hazard

include those contain soluble rocks, limestone

(including dolomite and chalk), gypsum, rock salt on the

surface or near surface down to 200 m. However, it is

noteworthy that the high abundance of soluble rocks

alone cannot produce the sinkhole’s hazard, as the

formation of sinkhole requires also unsaturated water

and a hydraulic gradient that move the water through the

system in the presence of an outlet to allow an escape of

dissolved water. Areas with an intermediate risk of the

sinkhole’s hazard are those contain loessial and

dispersive soils. Collapsing of soils in such areas

requires water drainage passing through the soils. Areas

considered to be at low risk of the sinkhole’s hazard are

those contain extrusive rocks in which a cavity

formation occurs during rock formation. An expansion

of the cavity by erosion and weathering is very small

relative to the soluble rocks. Nevertheless, should the

sinkhole mechanisms involve also human activities in

addition to that of the nature, the hazard magnitude

would be unpredictable.

To provide public protection against sinkhole’s hazards,

mapping of subsurface cavities need to be undertaken in

areas considered to be prone to the formation of a

sinkhole. Highly detailed subsurface mapping is

recommended in high-areas such as those with thick

soluble rocks, located near ground surface. The goal

standard for minimizing sinkhole’s hazards is to acquire

three-dimensional pictures of surface cavities and the

knowledge on expansion rates together with geo-

mechanics of the cavities overburden. Additional

research should be conducted to determine (1) a best

solution for economic, rapid and effective subsurface

mapping and (2) economic and effective underground

cavities treatment.

Results of applied geophysical surveys for mapping the

subsurface cavities, involving human activities (salt

production industries) have shown the following. (1)

Reflection and semi-3-D refraction surveys can identify

the presence of subsurface cavities. An interpretation

has to be made with caution. Evidence of low

amplitude and frequency appears in a short record can

be used as an indirect clue for subsurface cavity. (2)

International Conference on Geology, Geotechnology and Mineral Resources of Indochina (GEOINDO 2011)

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The 2-D resistivity survey can detect a cavity in the

near-ground surface. A well-defined object as very low

to lowest resistivity structures bound with a higher

resistivity reveals a cavity in the near-ground surface.

(3) The 2-D resistivity survey can be used for mapping a

rock salt along the profile where the rock salt imaging is

clearly displayed, consistent with the drilling results. (4)

The 2-D resistivity imaging can be used to detect the

expansion of a subsurface cavity with repeated

measurements through time. Dissolution of rock salt

can be imaged. Comparing results between seismic and

resistivity survey, the 2-D resistivity techniques provide

rapid, effective and inexpensive for mapping and

monitoring subsurface cavities in the Northeastern

Thailand.

Finally, it is no doubt that the uncontrolled solution

mining, known also as pumping brine, is harmful to the

environment. Dissolution of rock salt is not known. A

resulting subsidence can be both regional horizontal

expression and local (sinkhole) expression. Thus, the

Thailand Salt Production Department should revise

urgently a proper regulation for the pumping brine

method of industrial production of salt otherwise the

lessons learnt could not be accomplished. Subsurface

cavity mapping is not the right approach to resolving a

fear of sinkhole hazards among the Northeastern people.

Figure 6. A series of photographs showing sinkhole expansion together with resistivity imaging

pseudosections acquired across the sinkhole. (a) February 2004, (b) February 2005,

(c) February 2006. (after Satarugsa et al, 2006).

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