Determine Sinkholes and Overburden Thicknesses in Selected Covered Carbonate Karst Terrains ( Kinta Valley ) , Perak , Malaysia by Combining Wenner E . R . Tomography , Geological and Satellite Images Techniques

This paper discusses the application of Wenner’s Electrical Resistivity Tomography, geological, and satellite-imaging techniques to the process of determining sinkholes and overburdened thickness in covered carbonate karst terrains. The study area is located in (Kinta Valley), at a place called Kampung Kunsila estate, situated northwest of Kampar City and south of Ipoh, the capital of Perak, Peninsular Malaysia. Several active sinkholes were discovered and identified during site field inspections. The sinkholes ranged from narrow-to-medium and medium-to-wide sizes, some with exposed throats. On average, the diameter of the sinkholes in the study area ranged from 1 m to more than ~ 30 m, some having approximately the corresponding range in depth. Surface water induces erosion along the frame of the sinkhole, which causes enlargement to several meters long. Some of the sinkholes were selected for the application of high-resolution geophysical technique in order to realize the objectives of the study. Wenner’s Electrical Resistivity Tomography (ERT) was used along many profiles to capture the image of the subsurface across and in proximity to various types of sinkholes as a primary stage. This method will allow the recognition of their shapes and estimation of their depths. In addition it will assist to know the origin of these sinkholes. The resistivity data from the multi-electrode measurements were collected at several profiles were plotted in graphs/maps. The resistivity maps showed images of many active karstic cavities and sinkholes having different shapes and origins. Some of these were identified as karstic features (sinkholes and cavities) as well water, clay or air infill. Additionally, the transmission of the material from the top (soil surface) down to the bedrocks via the solution showed enlarged joints, drainage systems or pipes, which is expected. The interpretation of the resistivity data was used to generate a geological model of the specified area. The geological model derived from the interpretation of the geophysical data consisted of a basal limestone unit, which was widely karstified and constituted the bedrock of the study area. The overburdened layers consist of sand, containing lenses of clay and covered by soil or sandy clay and friable sand. There are also rock fragments scattered throughout. This karstified limestone bedrock is intervened with sinkholes and cavities, in-filled with clay or sandy clay and sand. This is invariably interpreted as a karstic process. A certain variety of sinkholes with differing origins was identified in the study area via the information from the geological and geophysical survey data records. The first type of identified sinkholes was a few meters in diameter and depth and was referred to as cover-layers or material subsidence. The second type was wider than the first type in terms of diameter and depth, referred to as cover-collapse sinkholes; and the third kind of sinkhole is a slump depression produced on the ground surface. Finally, the fourth type is an opening with a funnel shape depression or sink, referred to as pipes. The field study identified the type of sediments in the subsurface’s layers, and estimated its approximate depth and thickness. Moreover, the depth of the limestone bedrock was generated for all sites. The tomography data from different profiles were interpreted using the existence of the extracted boring samples to improve the results www.ccsenet.org/jgg Journal of Geography and Geology Vol. 6, No. 4; 2014 200 of the ERT. When comparing the interpretations from individual profiles with the boring data, a discrepancy is detected in the depth values, speculated to be resulting from variations between the normal subsurface geology and the data from the electrical resistivity survey profiles. Most of the data points contain errors of less than 1 m, with some data points containing errors of less than 3 m. This study also discovered that this area and other areas with similar specifications are hazardous vis-à-vis constructing accommodation for habitation or animal sheds. There is a possibility of collapse or subsidence of the soil cover or the material and underlying layers due to the load at any time in the near future. This study also identified an area of ongoing subsurface dissolutional erosion that may eventually lead to a collapse in the future. Two solution methods are recommended to use in the plan to minimize the risk of problem areas in this site.


Introduction
Karst in carbonate terrains is a distinctive landscape and hydrology, which occurs from a mixture of high-rock solubility and a well-developed secondary porosity.The carbonate rocks (limestone, dolomite, marble) are often characterized by a group of landforms, which includes an extensive range of closed surface depressions, underground drainage, well-developed systems, and sinkholes.These depressions form the main focus of this paper.
Sinkholes appear due to many reasons, such as being the result of the runoff of heavy rains channeled towards the bedrock from the flow of groundwater; irregular distribution of unconsolidated material over the bedrock surface; or subsidence movement.Many carbonate rocks contain various amounts of insoluble materials and solution-enlarged joints.When the soluble components are dissolved and removed by groundwater, the insoluble materials remain, which include sedimentary material such as gravel, sand, silt and clay.Sediments such as alluvial or marine sediments may also mantle the bedrock.Regardless of their purported origin, the sediments protect the limestone bedrock and prevent any solution-related features in/on it.
Due to the elevation of the limestone/soil interface being location dependent, defining the depth to the carbonate bedrock with the boring data is time-uncontrollable and expensive, and may be misleading due to insufficient data density.On the other hand, the boundary between rock and soil is often sharply defined Sowers, G.F. (1996).The overburdened soil differs from the underlying carbonate rock, whether it is residual soils produced by mechanical and chemical breakdown of rocks from the ground surface, or it is transported from its origin.This contrast provides the basis for applying geophysical techniques to represent sinkholes, the carbonate bedrocks, and the overburdened soil surface in covered karst terrains.Two-dimensional (2D) electrical resistivity profiles were applied in the proximity of active and non-active sinkholes at the Kunsila estate site, situated in northwest of Kampar and south of Ipoh, as shown in Figure 1.Forty-one steel electrode arrays were employed in these surveys, with a spacing of 5 m.The length of the electrical survey lines is 200 m, and it depends on the size of area and the placement of the wire arrays.The space between the two lines was on average 10 m at this site.
The electrical resistivity data acquired from the survey is constructive for several reasons, among them the fact that karstic features can be delineated by a completed closely spaced parallel resistivity profile; and the subsurface can be immediately imaged across and in proximity to many sinkholes in order to determine the karstic features.The provided images were reliable, and support the interpretation about the study area, if it is underlain by several varieties of karst features (sinkholes, cavities and voids).The survey shows sinkholes, in many different sizes and shapes can be recognized in the study area.It is capable of characterizing the subsurface bedrock in covered karst terrains.
In this study, it is recognized that the subsurface limestone bedrock was widely karstified by providing reliable images of the karstified limestone bedrock.On top of identifying the type of sediments in the subsurface layering, which includes estimating its approximate depth and thickness, it also indicated that the surface was overlaid in various places by friable sand and the remnants of limestone rock and other rock fragments.This is due to the high resistivity recorded in most of the overburdened cover.Furthermore, it also allows an understanding and prediction of whether or not an area in the vicinity to this study area might be a threat in the near future.

Carbonate Karst Terrains
Karst in carbonate terrains is a type of landscape found on carbonate rocks (limestone, dolomite, marble), and is characterized by a group of landforms, including an extensive range of closed surface depressions, underground drainage well-developed system, and a lack of surface streams Cooper, A. H. et al. (2011).Limestone or dolomite carbonate karst terrains are generally active and diverse in character, and exhibit low solubility, high mechanical strength and in some cases, low ductility.The dissolution process happens very slowly; a few millimeters may be dissolved within a hundred years, which has the potential to create a cavity of 1 m or more across in more than a hundred years.Its highly irregular depths of bedrock, residual red clay-rich soil, and surface drainages usually characterize the carbonate karst terrains, which fade away in the underground.
Karst develops on soluble rocks, both at the surface and subsurface, due to the rate of dissolution processes that work in rapidly developing limestone rocks.This depends on a number of factors, such as the power of rainfall, availability of surface water, and its form to revive as well as groundwater.Other factors include the distribution of soil-cover, temperature and biological activity, diffusion rate, autogenic content, structural weakness and lithology of the carbonate sub-layers.
Most sinkholes in carbonate karst rocks are formed via the dissolution processes with acidic water, which occurs when rainwater absorbs carbon dioxide from the air and decomposing organic material in the soil.It becomes more acidic, and penetrates into the cracks, which dissolves the rocks.Carbonate karst can be a part of the global carbon cycle, in which carbon is exchanged between the atmosphere, surface and underground water and carbonate minerals.The dissolution of carbonates via the presence of acid in water combines the carbon derived from the rock and from dissolved CO2 as aqueous HCO3.
The deposition of dissolved carbonate minerals is accompanied, and usually generated by the release of some of the carbon as CO2.When the bedrock is water-saturated, the dissolution continues along the bedding planes through the horizontal cracks between rock layers and joints, or fractures in the rock itself.These conduits enlarge over time, and the water moving through the combination of gravity and hydraulic pressure and it will further enlarge the conduits via a combination of dissolution and abrasion of the surrounding rocks.The communication varieties among chemical, physical and biological processes have a broad range of geological effects, including dissolution, precipitation, and sedimentation and ground subsidence.Diagnostic features such as sinkholes (dolines), sinking streams, caves and large springs are the result of the dissolutional action of circulating groundwater, which may be dispersed to entrenched effluent streams.
Initially, most of the underground water moves by laminar flow within narrow fissures, which gradually become enlarged at or below the water table to form subsurface caves.Once a certain conduit-size threshold is exceeded, which is typically around 10 to 20 millimeters, the flow becomes turbulent.The caves contain a variety of dissolution features, sediments and speleothems (deposits with various forms and mineralogy, chiefly calcite), all of which may preserve a record of the geological and climatic history of the area, surface and subsurface, due to the solution and associated processes.
Most sinkholes were created on soluble carbonate rocks (marble, limestone and dolomite) because of greater dissolution related with the difference in composition.The sinkholes developed at both the surface and subsurface are due to the solution and associated processes.Yassin, R.R. (2002) has documented three main and most imperative situations that may lead to development of sinkholes in karst region under tropical climate: Firstly, due to discrepancy of dissolution in areas, the power of rainwater is greatest at the surface and the first 10 m from it; so the surface of exposed or uncovered carbonate outcrops has eroded, thus lowering the ground in the area.This generally results in sinkholes with shallow depression that may extend up to several tenths of meters.The materials then move away within slowly enlarging fissures, whereby initial openings and cracks are produced by the continuation of the dissolving process on the soluble rocks.This is due to activity of the depressive waters.An internal karst conduit system into the underlying carbonate occurs and results in the growth of cavities in the rock.If the ceiling or the upper limit of the cavity is not strong and eventually collapses, a sinkhole may form at the ground surface.
Secondly, in the subsurface, dissolution and downward gravitational movement of the overlying material due to deformation and internal erosion occurs.Rainwater with a high amount of carbon dioxide finds its way through the cracks and causes fractures in the underlying porous limestones rocks, which converts the calcium carbonate by a chemical process into dissolved bicarbonate.The dissolved substance is then washed away.Consequently, the original minute-line cracks or the joints in the rock are gradually widened and enlarged; hence, it produces a pattern of fissures.If rainwater penetrates deeper into the subsurface, it forms cavities by corrosion.
When this rainwater starts to flow through the underground cavities, it accumulates and fills the holes.Due to the mechanical process of erosion, a destructive action in breaking down and carrying away the rock in a system of linked cavities, cracks and channels.The water can build up pressure, forming dams that can occasionally cause water to flow uphill.Watercourses can develop consisting of caverns, passages and channels or conduits and the faster the water flows the deeper the underground cavern becomes completely filled with water.The dissolution process then depends on the rate at which the water percolates or seep into cavern through the roof.If the land above the cavern is covered with forests, the flow of water with high carbon dioxide content will be much stronger than on under grazing land or a totally exposed surface.Collapse of the cavity roofs caused by dissolution over voids (by the upward diffusion) also results.Deep-seated dissolutional voids may be associated with this phenomenon.Also, a pipe that may reach several hundred meters in height and with sharp-edged depressions up to a few tens of meters in diameter generally results.
Thirdly, the situation caused by tectonic activity and earthquakes in karst terrain regions plays a major role in development of sinkholes in both surface and subsurface.Tectonic fractures occur and these enlarge the networks of open fissures.Collapse or subsidence of the ground surface occurs as the soil slumps, widening both joints and fractures.Constructive flow paths enlarge selectively into caves.The enlargement of a cave or caves and the cavity produced on bedding planes, opening or interconnecting of voids in a block of karstic limestone, may also occur.(1996).The opinions that, the early stage of pinnacle development is due to the differential resistance of physical weathering of the rocks and the role of geological structures (bedding planes, joints, fault etc.) lead to the formation of pinnacles under the cold climates of Tibet to developed.The depth of limestone with few meters from the ground surface can be identified from the depth of pinnacles which determined in ex-mines and sinkholes that distributed in the study area, figure 5.
Figure 5. Pinnacles identified in ex-mines and sinkholes that distributed in the study area presenting that the depth of limestone with few meters from the ground surface The authors researched many studies on the acidity of the rain water done in last fourteen years.The most imperative conclusion drawn is that the enhanced rainfall and elevated acidity of rain water in the last few decades of the 20th and 21st centuries in Malaysia and other countries in Southeast Asia are due to increase amounts of liquefied carbon dioxide and other pollutions.The air pollutant index (API) levels recorded a reading ranging between (112 to 300) in the sky from various states in Malaysia peninsular.The API reading between 0 and 50 is considered good, 51 to 100 is measured (moderate), 101 to 200 (unhealthy), 201 to 300 (very unhealthy), and 301 and above (hazardous).
These are the consequence of volcanic and tectonic activity in the Pacific fire ring, the volcanoes in the Philippines such as Bulusan and Mayon volcanoes in the south east of Manila.In Indonesia such as the Kelud volcano located in East Java, Mount Sinabung volcano in North Sumatra, Mount Merapi volcano located on the border between Central Java and Yogyakarta, Indonesia, being the most active.A spectacular sub-marine volcanic eruption spews out huge columns of ash, smoke, gas and vapors thousands of feet into the Pacific Ocean sky.Liquefied carbon dioxide is also due to land clearing activities such as the burning of large jungle and forest areas those practiced by farmer in Indonesia on a large scale also results in the emission of large volumes of smoke and haze pollutants into the atmosphere.Exhaust emissions from vehicles and waste emissions from factories significantly contribute to air pollution as well.All these events have a large impact on the development and quick dissolving process of carbonate rocks.
The authors also believe that most of the biggest cave and channels in the marbleized limestone of Kinta valley is due to reaction of sulfuric acid (H2SO4) with carbonate rocks this can also be one of the corrosion factors in karst formation, this mechanism may also play a role, as O2-rich surface waters seep into the ground, its brings oxygen which reacts with sulfide present with Cassiterite into the ground surface of Kinta valley area, the oxidation of sulfide leading to the formation of sulfuric acid.Sulfuric acid then reacts with calcium carbonate causing increased erosion within the limestone formation.
The cover layers of alluvial deposits over marbleized limestone of Kinta valley contains soil-piping or channels feature.The Tin (Cassiterite) was accumulated in this alluvial channels or pipes having been washed down from  Many empty (air-infill) sinkholes were discovered at this site, and the emptying or infilling of these sinkholes forms the foundation of the stripping of topsoil and granules of sand, which drops down (in almost all of the study area) for reasons such as heavy runoff rains in the study area.Also, due to the difference in the topography of exposed rock, or due to the subsidence movement in the area, leading materials are channeled towards the sinkhole in order to fill the void in the layer beneath, or to fill the cavity through the existing joint/fracture in limestone bedrock.These sinkholes are only a few meters in diameter and depth, and their small size is due to the fact that the cavities in the limestone cannot progress to considerable sizes before they are filled in with sand.
In some of the sinkholes, it is observed that one-side remains steep, and the opposite side forms a gentle slope.
The sinkhole looks like a funnel that has been cut in half along its length, with a curvature of soil being apparent along the sidewall.This arch forms over the throat of the sinkhole and represents the roof of the void.The area above the soil arch, throat and the steep side is the most stable.
Over time, the sides of the sinkhole will continue to fall and fill in the hole.The sinkhole may reach a point that it will appear as a depression on the land surface, or it may be indistinguishable on a surface relief from the surrounding area.However, if there is a constant supply of water entering a sinkhole, a sinkhole can theoretically stay open for many years.
Several varieties of sinkholes were found and identified surrounding the site during groundwork inspection, some of it shown in figure 10.A total of 18 sinkholes were identified and given specific letters (A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q and R).These sinkholes were not spotted in the satellite image of Perak at a scale 1/5000.Moreover, they were not observed in the satellite image of Google Earth for many reasons; first of all, the chosen satellite image may be older than the age of the sinkholes, meaning that these sinkholes were the result of recent developments.Secondly, sinkholes with smaller sizes were engulfed by plants, which in this case, compromised their visibility vis-à-vis satellite imagery.Additionally, the lake, which is a former tin mine, is clearly visible in the study area in this satellite image of google earth.A few of these sinkholes were selected to be analyzed by a high-resolution geophysical technique in order to achieve the case objectives.ERT in Wenner configuration was functional, which allows the imaging of the subsurface across and in proximity to many sinkholes as a primary stage to speculate on the origin of these sinkholes, recognize their shapes, and estimate their depths.Furthermore, additional research was done by Louis IF et al. (2002).Moreover, other research was completed by Yassin, R.R. ( 2002) and Zhou, et al. (2000).Finally, earlier research was completed by Yahia et al. (1992).These reports enabled the determination of the electrical variables associated with the nature of sediments.Conclusions were made based on the variations in electrical resistivity values related with the nature of sediments.
The geological classification permits the successful imaging of the bedrock and subsurface karstic features, because soil, sand, clay, carbonate rock and air-filled cavities can normally be differentiated and mapped.

Clay
Are usually distinguished by low apparent resistivity's and variables, which are dependent on moisture, mineral content, purity, and unit shape/size, usually from 5 ohm-m to less than 60 ohm-m.In this case, the clay is divided into many types with different colours, which is used in this resistivity section as:

Sand
Is usually characterize by medium apparent resistivity and variables, depending on the moisture content, purity and unit size, usually from 70 ohm-m, to less than 160 ohm-m.The sand is also divided into many types; its colouring scheme described below: • Sand, distinguished by medium apparent resistivity, is typically dark green colour in this study.
• Sandy clay, distinguished by its low medium apparent resistivity, is typically given light green colour in this study.
Weathered limestone rock

•
Comparatively weathered limestone rock typified by high apparent resistivites, typically more than 200 ohm-m, to less than 400 ohm-m, is typically given gray colour in this study.

Intact limestone rock
• Is distinguished by higher apparent resistivity, naturally from more than 400 ohm-m to more than 3000 ohm-m, and varies depending on layer thickness, its impurities and moisture content.It's given a light purple colour in this study.
Intact pure marbleized limestone or dolostone rocks • Is distinguished by higher apparent resistivity, naturally from more than 4000 ohm-m to more than 8000 ohm-m, and varies depending on layer thickness, its impurities and moisture content.It's given a dark purple colour in this study.

Air-filled cavities or voids
• Are generally characterized by very high apparent resistivity, usually more than 3000 ohm-m to less than 6000 ohm-m, but varies depending on the conductivity of the nearby strata and size/shape of void or cavity.Classically, it takes a black colour in this study.
Hence, electrical resistivity values were resolute for each rock unit.The results are tabulated in Table 1.This table was suitable for investigation karst features and its deposits within karst terrains.Also in the same time was suitable for detecting any mineral deposits within the sediments in the area but it need experience for that.
Table 1.Describes the range of resistivity values with the expected geological unit deposits to and to define the cave, cavity and sinkhole with air-infill The geological classification are utilized in this geoelectrical survey that was described above it permits the successful imaging to mapping and differentiated bedrocks and subsurface karstic features.Because there is crossing point between the value of resistivity data of intact limestone and the value of resistivity data to define karst features in fill with air such as cave, cavity and sinkholes.The table above can use for twice time, one with resistivity from 3000 -6000 Ω-m to define the cave, cavity and sinkhole with air-infill, table 1.Also to define rock fragments of limestone with friable and coarse grains sand which containing high porosity with air.In the second time with resistivity from 3000-6000Ω-m as intact limestone, table 2. without mention to cavity and sinkhole with air-infill.no.2&8, with a depth of >12.4m.Sinkhole in-fill with soft moistures clay extended from electrode no.14 to 17&18, with a depth of ~9 m.It seems that the water drain through channel connect with wetter area in the subsurface beneath electrode no.7 at a depth of ~12m.It's possible that the source of this water is meteoric or supply fresh water for palm trees irrigation accumulates in the subsurface and led the area to subsidence with mostly about 0.45m.
Through the interpretation of E. R. Tomography section in profile no.2, figure (13-B), main location with subsidence appearing on section extends in between electrode no.2 &19, representing a semi-tabular shaped anomaly, reaching a depth of >14m contains clay saturated with water.This anomaly extended from profiles no.1 to profiles no.5, widening in the direction of profile no.4, and appears in this profile between electrode no.2 & 20 to reach maximum depth of > 17m.This anomaly was sliming in profile no.5.Moreover, the depth of this anomaly was increased from the depth of 12.4m in profile#1, to reach a depth of 19.8m in profile no5.
This anomaly contains lenses of clay saturated with meteoric water.Deposits with several categories of resistivity values also appear in this feature.The first value, mostly with very low to low resistivity values, represents soft and moisturizing clay and less moisturizing clay, while materials with medium resistivity values represent the remnant of dispersive surficial soil or silty clay.Moreover, materials with medium-higher resistivites represent silty sand and sand.This subsidence area affected the surface of the study area and is apparently detected throughout the site inspection.
In profile no.2 as well delineated cover soil -collapse sinkhole extended mostly in between electrodes no.23&24 and electrodes 25, in -fill with water and moisturized clay.The mouth of sinkhole was open and connected to the surface, this observed in site work inspection.With depth starting from the upper most surface of 1.25m down to 13.50 m in the subsurface.The extremely high resistivity (3000-6000) Ohm-m in the upper part of the sinkhole it's due to vacant with air or unfilled.Also in this profile determined two lenses of clay crammed in the sand.The first lens extended in between electrodes 26&28; define in the depth approximately between 6 -13m in the subsurface.This lens with moderate size connected to the sinkhole mentioned before.The second lens with big size extended in between electrodes no.29&30 and electrode 37; define in the depth approximately between 6.36 -17.5m in the subsurface.In the center of these two lenses, low resistivity values were detected, representing clay.Surrounded by materials with less great or below average resistivites i.e. soil or silty clay.
Through the interpretation of E. R. Tomography in profile no.3, shown in figure (13-C), it is concluded that when the geophysical data interpretation corresponds with the geological site inspection, many subsurface karst features in the shape of sinkholes and cavities with semi-round or oval-shapes will appears along this profile.Some of that water in-fill, others with clay soft or moisturized in-fill or soil, silty clay, sandy clay and sand in-fill.While some of it were empty air in-fill.Additionally, one void was located along this profile, air in-fill.
The first karst feature appears in the subsurface as a cavity at approximately the central point of this profile, almost between electrode no.20 &23, mostly covered by friable sand and rock fragments on the surface.Start with a depth of ~2 m from the ground surface to reach a depth of ~12.4m, being approximately of ~15 m wide.In cross-section, it appears in an oval-shape, with semi rounded walls and a relatively rounded base.At the center of this cavity, low resistivity values were detected, representing clay in-fill and other materials with less greater or below average resistivites i.e. soil or silty clay, surrounded by materials with greater resistivites i.e. silty sand.However, this cavity is active due to the existence of the amount of soil and some clay in the center, and the cover material was very thin, and might collapse in the near future under any load, to form a sinkhole ,figure (13-C).
The second karst feature appears as a cover-collapse sinkhole, detected almost beneath electrodes no.24 and in between electrodes no.28&29 were approximately of ~22.5m diameter.The mouth of this sinkhole was open and connected to the surface, starting with the depth of 1.25m in the ground's surface to reach a depth of ~12.5 m.In the cross-section, it is oval in shape, with semi-rounded walls and a relatively round base.This sinkhole is almost in-filled with very low resistivity values material representing moisturized clay.Other materials low resistivity values representing clay with less resistivities i.e. soil or silty clay, are surrounded by materials with greater resistivities i.e. silty sand, figure (13-C).However, this sinkhole is very active due to the existence of large amounts of clay and other material i.e. moisturized clay, soft clay, soil or silty clay and silty sand may be subjected to piping under any load.
The third karst feature appears in the subsurface as a cover-collapse sinkhole in the right flank of this profile in between electrodes no.29 & 30 to electrodes 31.This sinkhole is ~7.5m diameter in a semi-round shape.The mouth is open and connected to the surface, starting with the depth of 1.25m in the subsurface to reach a depth of ~9m.In the cross section, it appears to have semi-rounded walls and a relatively semi-rounded base.Low to medium low categories of resistivity values were observed in this sinkhole, representing clay and remnants of dispersive surficial soil or silty clay, and are also patterned with greater medium resistivites i.e. silty sand.The upper most of this sinkhole show very high resistivity values were illustrated because this sinkhole is air in-filled till a depth of ~3m from the surface, figure (13-C).
Many empty sinkholes (air-infill) found at this site it was assumed that non-filling of the sinkhole was the foundation stripping of the portion of the top soil and granules of sand that were channeled down, due to the runoff of heavy rains over the surface in the study area, channeling the material from the sinkhole towards the smaller-sized cavity to fill via existing conduits.These sinkholes are a few meters in diameter and depth.
The fourth karst feature appears in the subsurface as a cover-collapse sinkhole on the right flank of this profile from electrode 33 and in between electrodes no.34&35.This sinkhole with open mouth connected to the subsurface, with ~7.5 m diameter and ~12.4 m deep.The high resistivity value is due to the fact that this sinkhole's air is infilled till the depth of~3m from the surface, then in-filled with water.After the dissection of this sinkhole, the cross section appeared to have a pentagon shape, with a near-sharp and irregular wall, and a relatively semi-round base.Several categories of resistivity values were found in this sinkhole, first, very lowlow resistivity values, representing soft and moisturizing clay, less moisturized clay and material with medium resistivity values, representing remnant of dispersive surficial soil or silty clay, and also material with greater resistivites i.e. silty sand and sand, Figure (13-C).
The water remained in the sinkhole, suggesting that the flush or the migrates was stopped after the conduit or the pipe that were formed due to solution-widened joints became choked or blocked with clay and other materials in these drainage, due to the fact that the size of the cavities in the limestone bed rock are infilled or loaded with clay and sandy material.
Small sinkhole observed in the surface in-filled with friable sand and remnant rock fragments from electrode no.28 to in between electrodes no.29&30, with ~7.5 m wide in funnel-shaped, and reaches a depth of ~9.5m.The center with very high resistivity of more than 4000 Ohm-m, consider that void air in-fill were present at the subsurface, measuring ~2.5m wide and ~3.25m high, from a depth of ~ 1.25m to reach a depth of ~4.5m.
The interpretation of E. R. Tomography section in profile no.4,shown in figure (13-D) displays beside the semi-tabular shaped anomaly which present a wetter area of clay saturated with water which determine and mentioned before in left flank, extended almost between electrodes no.2&20 to reach maximum depth of > 17m in this profile.The upper most overlaying surface from the depth of 2m to nearly 5m, in the right flank of this profile in the proximity in between the electrodes no.17&18, electrode no.20&26, electrode no.27&28, electrode no.29&31and in between the electrode no.33&39 is characterized by high resistivites between 200 ohm-m to 3000 ohm-m and more, interpreted as stripped sand dry and friable, with remnants of limestone rock or other rock fragments, extracted by earth moving equipment from the pits of mine, due to previous mining and excavation operations.
In this stripped sand, several small collapses or subsidences were visible and identified in the ground surface of this profile.A few represent cover-collapse sinkhole with small diameter of ~2m to maximum ~3m, with a depth approximately between ~3m and ~4m.It's found between electrode no.17&18, electrode no.25&26, electrode no.28&29, electrode no.35&36.When the geophysical data corresponds to the geological site inspection, it will match the interpretation.These small sinkholes comprises of sand and other remnant of rocks.Some of the profile characterized mostly by high resistivites of >4000 ohm-m, due to the extra size of gaps with air in-fill between the sand and the rock fragments.
Underneath this friable sand and rock fragment there is layer of sand.Longitudinal anomaly represent longitudinal channel extended in the subsurface between electrodes no.23 &39,from depth almost between 4 to 6m down to 19m, in the left flank of this profile consider it due to movement of subsurface water from the sinkhole in right flank of this profile only.This channel in-fill with material of very low resistivity values represents high moisturized clay.The other material of low resistivity values represents stiff clay.Surrounded by materials with less resistivites i.e. soil or silty clay and materials with greater resistivites i.e. silty sand.
The interpretation of E. R. Tomography section in profileno.5,shown in figure (13-E) displays the enormous lens or cavity in pear shape appearing in a semi-tabular shaped anomaly that mentioned the four profile before , continuing to the left flank from electrode no. 8 to in between electrode no.14&15, at about ~ 32.50 m wide.This lens started in the subsurface, from a depth of ~2m, reaching a depth of ~21m.Several categories of resistivity values were observed in this lens, first mostly very low -low resistivity values, representing soft and moisturizing clay and less moisturized clay, while material with medium resistivity values represents the remnant of dispersive surficial soil or silty clay.This cavity or lens combines with the main horizontal trend anomalies.The considerations led to the conclusion that these cavities are recently developed and support the assumption that the origin of this cavity was the presence of conduit feature, likely widened in the study area, and had been rapidly filled with clay and other materials, figure (13-E).In the near -surface of the study area between electrodes no.7&12 in this profile shows an area with subsidence, and it is assumed that this is due to the presence of this lens or cavity body in the subsurface which led the surface to plunging down due to the weight of material that fills this depression.This depression packed with stripped sand dry and friable, with remnants of limestone rock or other rock fragments.
Another lens in oval shape displays in a horizontal trend anomalies, connecting to the first lens in between electrode no.14&15, to electrode no.19, at about ~ 22.50 m wide.This lens started in the subsurface, from a depth of ~6m, reaching a depth of ~14m.Several categories of resistivity values were observed in this lens, first mostly low resistivity values, stiff clay, packed in material with medium resistivity values represents the remnant surficial soil or silty clay.
Channel pipe was observed in between electrodes no.28&30.This pipe is in-filled with sand and proceeded as a conduit to flush material and meteoric water from the top surface down to the limestone bedrock.This conduit reaches a depth of~19.8m, feeding the subsurface with meteoric water.
Moreover the upper most overlaying surface from the depth of 1.25m -6m, and the proximity in between the electrodes no.7&12, electrode no.14&24, electrode no.25&28 and electrode no.30&39 is characterized by high resistivites between 200 ohm-m to 3000 ohm-m and more, interpreted as stripped sand dry and friable, with remnants of limestone rock or other rock fragments, extracted by earth moving equipment from the pits of mine, due to previous mining and excavation operations, and is found covered along the profiles of the study area .
In this sand, several small collapses or subsidences were visible and identified in the ground surface of this profile one in between electrode no.8&9, and the others in between electrode no.16&18, electrode no.23&24, electrode no.31&32 and electrode no.39&37.These represent cover soil -collapse sinkhole and pipes with small diameter of ~1m to not more than ~5m, with a depth approximately between ~3m and ~5m from the ground surface.When the geophysical data corresponds to the geological site inspection, it will match the interpretation.These small sinkholes from the bottom half comprises of sand and other remnant of rock fragments.The top half was characterized mostly by resistivites of >3000 ohm-m, representing an air in-filled.Also depression observed in ERT section extended in between 7&12 from the surface to a depth of 5.50m packed with sand dry and friable and remnants of limestone rock or other rock fragments.Appears due to the wetter area in the subsurface which led the surface to plunging down due to the weight of material that fills this depression.The data base of karst features from (2-D) E R Tomography sections in (Kampung Kunsila estate) north west Kampar city were summarized in table 3.
Jointed limestone with high resistivity were detected at the shallowest subsurface below the tubular anomaly, extended in between electrode no.14&32 in profile no.1, in between electrode no.13&31 in profile no.2, between electrode no.14&34 in profile no. 3, and between electrode no.14&29 in profile no.4 and profile no.5.Intact or compact limestone bedrock, characterized by higher resistivity, was observed beneath Jointed limestone, figure (13-A).The approximate depth mentioned in the Table-4, and can also be detected along other profiles in Figure (13-B, C, D, E), extended with different lengths and depths.Furthermore, pinnacles were clearly detected in many E. R. Tomography profiles sections.In profile no.1, it is detected beneath electrode no.27& 28 at a depth of~10.5m,shown in figure (13-A), and in profile #2,it is detected beneath electrode no.21&22, and reaching a depth of ~12m, shown in figure (13-B).Furthermore, in profile #3, it is located beneath electrode no.18&19, and reaches a depth of ~12.5m.Beneath electrodes no.23&24, it reaches a depth of ~14.5m, while beneath electrodes no.31&32, and it reaches a depth of ~11m, shown in figure (13-C).Moreover, it is clearly observed that profile no.4 reaches a depth of ~15.5 m beneath electrodes no.22&23, shown in figure (13-D).The smoothing that was observed in the subsurface limestone bedrock is the result of employing the RES2DINVver.3.54.Software, which assumes that the subsurface is uniformly layered; and consequently lateral smoothing will form in a non-layered stratum.

Description of Sinkholes Determine in the Study Area
Several varieties of sinkholes were discovered in the study area through the information and data record of geological and geophysical survey.Diagrammatic cross-sections illustrating the type of sinkholes in the study are presented in figure 14(A-C).
The first kind of sinkholes discovered is a few meters in diameter and depth, and are referred to as a soil coversubsidence or collapse.This type of sinkhole is formed where overburden is relatively thin in some regions; the limestone rocks were covered by a thin layer, more permeable and composed of a greater percentage of silt and sand, with a possible thickness between 15m to 30 m, with the slight presence of clay below it.
These layers are covered with soil.These layers are design with lacks of cohesiveness between the particles to form a significant "bridge" across the void where it occurs.The limestone rocks, which are sensitive to both physical and chemical and the natural dissolving processes, will eventually break down.When it collapses, a cavity or void usually forms.The dissolving granules of sand will drop down to fill this cavity or void.The small diameter and depth of these kinds of sinkholes are due to the fact that the open cavities were smaller, and cannot expand in order to reach the considerable size before they were in-filled with sand as illustrated in figure (14-A).
Figure 14(A-C).Diagrammatic cross-sections illustrating the type of sinkholes in the study area, A: The soil cover-collapse sinkhole was within overlain cover soil as a result of a thick section of weakly unconsolidated sand and clay.B: The cover layer -subsidence sinkhole as a result of a bridge collapses and thick section of clay.
C: the cavern roof collapse within the cap rock that is overlain by a thick section of unconsolidated or weakly compacted sands and mudstones The second kind of sinkholes, depending upon the size of the cavity, is referred to as cover layers-subsidence sinkholes.Occasionally, if the sediments were more clay-based, the upper cover will lean towards becoming more inclined.This bridge will provide some dependence to the clay above.However, if this bridge collapses, the material acts as a conduit to any vacant cavity in the limestone bedrock.Some of the sinkholes appear due to an essential variance in the behavioral strength of the soil above the cavity, and the fact of whether this subsides slowly or collapses suddenly.The size of the sinkhole depends upon the size of the cavity, and whether or not it spreads efficiently and intersects the water table.The dissolution processes is possibly reduced in order to reduce the perpendicular difference between the phreatic water table and prospective meteoric surface, and consequently, reducing the percolation processes, as illustrated in figure (14-B).
The third kind of sinkholes discovered in the study area is a sinkhole-like slump depression produced at the ground's surface.This kind is distributed in different sizes and diameter from 5 m to more than 35 m in the study area.Mostly this kind of sinkholes formed after the Indonesia tsunami in 26 December 2004 in which it cannot be observed in satellite images before 2004.These kinds of sinkholes are present in weak sediments or when unconsolidated layers lie above the limestone cap rock.
The displacement may result in plastic deformation for the space located immediately higher than the collapsed area, depending on the lithology and the amount of water associated with these sediments.Due to the upper strata collapsing on the roof of the cavern, which is part of an extensive cavern, the network itself will collapse.
The clay-based sediment shall be transported and deposited a considerable distance away from the cavern via the continuous removal of material entering from the channel or conduit area, allowing the upward development of the conduit, diverting the occurrence of collapse to the surface, as illustrated in figure (14-C).
The fourth type of sinkholes discovered were openings with a funnel shape depression or sink that might develop in the future, generally referred to as piping.This occurs when the rain in-fill is on a slightly impermeable surface layer.During the infiltration process, the water passes through the soil and migrates towards a widened fracture caused by the dissolution process.Water then moisturizes the soil in the area around the opening hole of the conduit or fracture.As a result, the moistened soil begins to lose its cohesive properties, and begins to break apart and falls into the opening drain.The water began to erode the sediment from around these opening and migrate or move down toward the drain, with the sediments under the influence of gravity; it leaves a void or open area in the soil.Around the opening, a funnel-shape depression or sink has the potential to develop in the near future.The downward passage of the conduit or fracture can be enlarged with the passage of time by erosion or dissolution.
For piping to occur, two basic geologic conditions must be met: first, there must be a considerable quantity of materials (sediments or rocks) between the ground surface and the water table; and second, the condition of that sediments or rocks above the water table is capable of maintaining open fractures.Most of the site in the study meets these conditions, as there are high quantities of sediments between the ground surface and the water table.
The piping process without the effect of the water table is illustrated in the diagrammatic cross-section in figure (15-A).
The piping processes include infiltration and dissolution, and both can be active along the same fracture system.Although these processes are in hydraulic communication specified by a sufficient time, the two individual processes can be combined to produce a collapsed-pipe sinkhole.The emergence of a sufficient volume of material dissolution processes can take place in widening conduits or fractures in the cap rocks of limestone, which is overlain on the water table.However, if these processes result in fractures below the water table, it could allow meteoric water to penetrate downward along these fractures, and freely mix with phreatic water.The piping process when the water table has been lessened to a lower position below the top of the cap rock of the limestone is illustrated in the diagrammatic cross-section in figure (15-B).The electrical resistivity data acquired at this site was processed to generate the 2D resistivity models of the subsurface.Ten boreholes were opened, with a maximum depth of 12 m.The borings are completed in the unconsolidated material of soil, clay, sandy clay and sand over karstified limestone.This boring was used to define the limestone/clay boundary and a few of the sinkholes spatially.The depth of the boreholes was approximately ranging from 3, 6, 9, 10 to 12 m.The depth of the boreholes to bedrock was defined as the depth that refuses auguring in the borings.The mistake found in the boring data is the result of the borings recording the top of the weathered limestone zone, instead of the top of the intact or unweathered limestone bedrock.A number of borings have been by residual boulders in the overburdened region, or by limestone fragments.
The limestone dipped approximately toward the southwest in this site.When comparing the interpretations from individual profiles with the boring data, it was discovered that more of the data points have fewer errors of less than 1 m, and some of the data points have errors of less than 3 m.When the boring penetrate deeper in order to reach one of the limestone pinnacles beneath electrode no.27 in profile no.1, the auger became very problematic in the borings when it reaches a depth of 9 m, which represent the transitional zone that contains limestone rock fragments and sand.

The Effect of Alteration in the Environment on the Future of Land Use in This Karst Region
The alterations in Karst Region's environment occur on both the surface and subsurface.The actions that are responsible for these alterations are possibly direct or indirect in nature.The variation in karst environments may maneuver the karst situation toward or away from its boundaries Williams, P.W. (1993).
There is an urgent need for a better understanding of the association between primary actions that impact the karst situation in both, short-or long-term periods, such as agriculture, irrigation and mining, on top of soil erosion and its movement, the rates of sedimentation, and groundwater contamination.The presence of underlying fissures and joints is likely to facilitate the migration of soils from the surface.These features can deepen and disturb the surface of the soil.Sources of contamination include fertilizers and manure disbursed to fields for crop production.The failure to supervise areas and unfortunate management in storing the manure and fertilizers, and the application of liquid fertilizers using irrigation techniques may increase the erosion of the surface's soil.In addition, the inappropriate tillage techniques, such as before heavy rainfall may facilitate the emergence of sinking streams, cavity and sinkholes.This can cause rapid subsurface runoff, and likely lead to increased sediment loads due to erosion in the subsurface emergent streams.Leaving a crop residue cover on the soil surface can increase infiltration, and cause the loss of soluble nutrients to groundwater.This study found that this area and other areas with similar specifications is hazardous for constructing accommodation for habitation, or creating animal sheds by farmers, as well as manure storage facilities, feed storage, and poultry production.There is a possibility of collapse or subsidence of the soil cover or material, or its underlying layers due to the load at any time in the future.This study also identified the presence of an area undergoing subsurface erosion that may eventually collapse in future.

Results and Discussion
This paper demonstrates that ERT is a useful geophysical tool that is appropriate for imaging the bedrock and overburden layers to determine the subsurface karst features in covered karst terrains.The electrical resistivity data acquired from the survey is constructive for several reasons: 12.1 Geophysically a.The complete data analysis and suitable design of profiles arrangement are important factors ensuring the achievement of the project objectives.Karstic features can be delineated by completed closely spaced parallel resistivity profiles and imaging the subsurface immediately across and in-proximity to many sinkholes, in order to determine the karstic features such as (sinkholes, voids and cavities).
b.It provided dependable images and supported the interpretation that the study area is underlain by several varieties of karsts features (sinkholes, cavities and voids).
c.It can be useful for characterizing the subsurface bedrock in covered karst terrains.In this study, it is recognized that the subsurface limestone bedrock was widely karstified and provided dependable images of the karstified limestone bedrock.
d.It identified the type of sediments in the subsurface layer, including estimation for its approximate depth and thickness.
e.It indicated that the surface was overlain in various places by friable sand and remnant of limestone rock and other rock fragments due to high resistivity recorded in most of the overburden cover.
12.2 Geologically a. Several varieties of sinkholes with different origins were discovered in the study area through the information, geological and geophysical survey data records.The first types of sinkholes discovered were a few meters in diameter and depth, referred to as cover-layers or material subsidence.The second type was wider than the first type in terms of diameter and depth, and is referred to as cover-collapse sinkholes.The third type of sinkhole resembles a slump depression produced on the ground surface, and the fourth type is an opening with a funnel-shape depression or sinks, which is referred to as pipes.
b.Many empty (air-infill) sinkholes were found at this site, which supports the assumption that the emptying or infilling of these sinkholes was the foundation of the stripping of the portion of the topsoil and granules of sand that drops down (in almost all of the study area).This is likely due to the fact that there are heavy runoff rains in the study area.Also, there is a difference in the topography of the exposed rock, or because of the subsidence movement in the area.The result is the channeling of materials towards the sinkhole in order to fill the void in the layer beneath, or to fill the cavity through an existing joint/fracture in limestone bedrock.These sinkholes measure only a few feet in diameter and depth, respectively.Their small size is due to the fact that the cavities in the limestone cannot develop to considerable sizes before they are in-filled with sand.Several small sinkholes empty and sand in-fills were identified during the site ground work inspection presented in Figure 16.
c.The overlying sand found in many places on the surface in the area of study, is mostly characterized by high resistivity; interpreted as sand dry and friable with remnants of limestone rock or other rock fragments, had been stripped by earth-moving equipment from the pits of mine, due to ex-mining excavating operation.
d. Through sinkhole observation, standing water remained or stayed in the sinkhole days after the feature formed, suggesting that the flush or the migrates were stopped.This occurs when the conduit or the pipe that was formed due to solution-widened joints becomes choked or blocked with clay or other material to these drainage, because the size of the cavities in the limestone bedrock are in-filled or loaded with clay-base and sandy material.Otherwise, in the rainy season, the meteoric water flushing down through the sinkhole will cause the water table to increase, if the sinkhole spreads laterally and intersects the water table.
The dissolution is reduced because there is less vertical difference between the water table and meteoric water surface and here, the flush or migrates will be less.
e.The geological model interpreted from the geophysical data consists of a basal limestone unit, which constitutes the bedrock of the study area, the overburdened layers consisting of sand, containing lenses of clay and covered by soil or sandy clay and friable sand and rock fragment in certain places.Intervened by sinkholes and cavities filled with clay or sandy clay and sand; it is interpreted as a karstic processes.
f. ERT provides a sufficient resolution to pinpoint the locations of filled sinkholes and other geophysical anomalies, and identified an area of ongoing subsurface erosion that may eventually collapse in the near future.A potential collapse may be due to the presence of longitudinal anomalies in the shape of lenses or cavities in-filled with meteoric water and thick clay.Other materials of sandy or silty clay might also collapse when subjected to a piping under load, and threaten the site in the future.
g. Sinkholes in many different shapes and sizes were recognized in the study area.They are generally circular in outline, conforms to similar shapes such as elliptical or irregular configuration, or look like a funnel.A tunnel or throat is visible within the hole, representing a soil pipe that leads to the bedrock drain.
h.The group includes 10 small sinkholes found in the study area, representing the daughters, while the big sinkhole represents the mother.Due to the interconnected nature of the karst features system, this group of small sinkholes can also bond to form a larger sinkhole, with its shape changing over time, from steep to nearly vertical sidewalls.The portions of the sidewalls can break off over time and fall into the sinkhole.As this process continues, the sinkhole gets larger.If water continues to be added into the sinkhole, it can also get deeper. i.
In accordance to classification of karstic ground conditions dependent on the morphological features characterisation by Waltham, A.C., & Fookes PG (2003), the karst level found in the study area ranges between mature karst KIII to complex KIV.
Figure 16.Several of small sinkholes describe as empty, sand in-fills and crammed with vegetations were identified during the site ground work inspection

Recommendation
This paper focuses on the usefulness of ERT as a geophysical tool to capture the images of the bedrock and overburdened layers in order to determine the subsurface karst features located in selected area, situated in Kinta Valley, named Kampong Kunsila estate, in the northwest of Kampar and south of Ipoh, the capital of Perak, Peninsula Malaysia.In this study of covered carbonate karsts terrain, several varieties of sinkholes were discovered through the information and data record of geological and geophysical surveys; and there are more than a few speculated origins for these sinkholes, which are assumed to be due to cover-layers or material subsidence, or due to a cover-collapse sinkholes, or due to a sinkhole-like slump depression, or piping.Also, an ERT with a, 5-m electrode spacing provides sufficient resolution to pinpoint the locations of filled sinkholes and other geophysical anomalies.The ERT identified an area of ongoing subsurface erosion, which may eventually collapse in the near future due to the presence of longitudinal anomalies in shape of lenses or cavities in-filled with meteoric water.These sinkholes contain thick clay and others materials consisting of sandy or silty clay that could collapse when subjected to piping under load, creating a latent site threat in this case in the near future.
The failure to supervise areas and unfortunate management in storage of manure and fertilizers, and the application of liquid fertilizers via irrigation techniques may increase surface soil erosion.In addition, the inappropriate tillage techniques, such as before heavy rainfall may facilitate the emergence of sinking streams, cavities and sinkholes.This study discovered that this area was hazardous to farmers seeking living spaces, or animal's shades or horses stables over the area.This is due to the fact that a collapse or subsidence for the cover soil or material and underlying layers (due to load) might occur at any time in the near future.This study will enable an understanding that an area with specification approximating to this study area will create future threats.The variety of sinkholes identified and delineated in this site during the geological and geophysical survey is presented in Figure 17.displayed in five traverses or profiles across and in-proximity to many sinkholes located at the site.
The electrical resistivity data were collected and assumed to identify the origin of these sinkholes and their shapes; on top of estimating a sinkhole's and the bedrock's (limestone) depth.Furthermore, it generates the geological model of the area under study and the type of overburdened sediments and its thickness.Several varieties of sinkholes were discovered in the study area via the information and data record of geological and geophysical survey, more than a few causes of origins were speculated for these sinkholes.This study found that this area and other areas with similar specifications are hazardous for constructing accommodation for habitation, or creating animal sheds, as well as manure storage facilities, feed storage, poultry farms, etc.There is a possibility of collapse or subsidence of the soil cover or material and underlying layers due to the load at any time in the future.This study also identified the presence of an area of ongoing subsurface dissolutional erosion that may eventually collapse in the near future.Two solution methods are recommended to use in the plan to minimize the risk of problem areas in this site. Figure

Figure 3 .
Figure 3. Satellite image of Kinta valley, Perak, present the location of the study area

Figure 9 .
Figure 9. Land Photographs viewing some features in the upper most layer of the subsurface due to the effects of meteoric water in a several points the study area Figure Figure process w

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Highly moisturized soft clay is usually distinguished by insufficient low apparent resistivity with water filled porosity or very high mineralized.And is typically given pink colour in this study.• Soft clay with pond water content is usually distinguished by extremely low resistivity and has very high conductivity or highly mineralized.And is typically given dark blue colour in this study.• Moderate moisturized soft clay is usually distinguished by Very low apparent resistivity and has very high conductivity or moderate mineralized.And is typically given light blue colour in this study.
•Clay with low-moisturized are usually distinguished by low apparent resistivity or with low mineralized content.And is typically given yellow colour in this study.