Application of Gravity Survey for Geological Mapping and Cavity

Detection: Malaysian Case Studies

Abd Rahim bin Harun

School of Environmental and Natural Resources Sciences, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, Bangi, Selangor,

Malaysia; abdrahim.harun@gmail.com

Abdul Rahim bin Samsudin School of Environmental and Natural Resources Sciences, Faculty of Science

and Technology, Universiti Kebangsaan Malaysia, Bangi, Selangor, Malaysia; abrahim@ukm.edu.my

ABSTRACT The gravity method is one of the geophysical tools used for geological, engineering and environmental investigations where the detection of geological boundaries, cavities, subsurface karstic features, subsoil irregularities, or landfills is essential. Successfully delineation of geological boundary of Bukit Arang Tertiary Basin in the east of Perlis and north of Kedah illustrates its usefulness as an efficient geological mapping tool. In higher accuracy measurements, the microgravity method has been widely and successfully used for locating and monitoring subsurface voids. Since microgravity methods measure gravity variations at the surface, they are directly influenced by the density distribution in the subsurface and particularly by the presence of voids, which create a mass deficit relative to the density of the surrounding terrain. In many cases, deep or small-scale heterogeneities generating low-amplitude anomalies can be detected and the reliability of further interpretation requires highly accurate measurements which are carefully corrected for any quantifiable disturbing effects. Most of the microgravity surveys in Malaysia were carried out by the Mineral and Geoscience Department mainly to detect the occurrence of subsurface cavities in limestone areas of Klang Valley in Federal Territory and Kinta Valley in Perak. Results of the sinkholes occurrences studies at the sites of a proposed public library building in Baling, Kedah and at the compound of the KTM Quarters, Ipoh, Perak showed that the low amplitude of gravity values located exactly or slightly adjacent to the sinkhole positions. Good correlation of borehole data with several results of microgravity survey such as at Mahkota Wira Condominum site, Jalan Sultan Azlan Shah, Ipoh, Perak and at the proposed Light Rail Transit Station in Jalan Ampang, Kuala Lumpur showed that the cavity location can be successfully determined by the microgravity measurement technique. Although the gravity method can be used successfully in locating the subsurface targets but it’s still a non-unique interpretation and the results need to be confirmed either by drilling or by other geophysical methods. KEYWORDS: Gravity survey, geological mapping, cavity detection, Malaysian case studies.

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INTRODUCTION The gravity method is one of the geophysical tools used for geological, engineering and

environmental investigations where the detection of geological boundaries, cavities, subsurface karstic features, subsoil irregularities, or landfills is essential. In Malaysia a number of gravity surveys had been carried out by Loke (1981), Abdul Rahim Samsudin (1990s), Jamaludin (1990s-2000s), Mohd Anuar (1990s-2000s), Abd Rahim Harun (1990s-2000s), by Minerals and Geoscience Department of Malaysia (MGD) and many others. It has been routinely employed by the MGD for the purposes of geological mapping (delineation or map the geological boundaries and geological structures) and cavity detection (determination of the presence and the extent of cavities in the limestone bedrocks). In few cases it was also contributed in groundwater and mineral explorations.

MATERIALS AND METHODS Data acquisitioning for the regional gravity survey is usually using the La Coste &

Romberg Gravity meter. The grid spacing of measurement stations depend on the size of the subsurface structures to be delineate. Normally, the regional type of gravity survey were carried out using 5 kilometers grid spacing with a temporary base station was set up near to the survey area. The height of each gravity station was determined using two unit altimeter (one unit of base altimeter and mobile altimeter respectively) to an accuracy of ±5.0 feet. This was desirable if the reduced gravity data were accurate to within ±0.05 mgal.

As for the microgravity survey, it was carried out usually based on a 5-metre grid using La Coste & Romberg Model D161 Gravity meter with sensitivity of 1 µgal (0.001mgal). Stringent field procedures were followed throughout the survey to maintain the desired quality of the data. A temporary base station was established to monitor and correct for gravity meter drift. Measurements at the base station were repeated every 45 minutes to 1 hour. For every measurement at the base, at least two readings were made within 10 minutes of each other. The two readings must agree within a range of 0.01 mgals. Twenty percent of the total number of stations were repeated to ensure consistency in data quality. Stations with abrupt changes in gravity values were verified by repeating the measurements. The height of each microgravity station was determined by a dumpy level with an accuracy of ± 0.3 cm. The accuracy is desirable if the reduced gravity data are to be accurate to within 0.01 mgals.

DATA REDUCTION AND PROCESSING Measured values are corrected for effects caused by variations in latitude, elevation,

topography, earth tides, and instrument drift. These normal gravity variations and compensating corrections gravity data are discussed in brief below. For more in-depth discussion of gravity data corrections, see Butler (1980) or Telford et al. (1990).

When all of the corrections have been applied to the observed gravity data, the result is the Bouguer gravity value, ∆g which is written as:

∆g = gobs ± ∆gfac ± ∆gb ± ∆gØ + ∆gτ

where

gobs = gravity value at station (relative to base) after drift correction in mgals

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∆gfac = free-air correction ∆gb = Bouguer correction ∆gØ = latitude correction

∆gτ = terrain correction

As for the MGD the gravity data were reduced using the MREDUC software developed by British Geological Survey. To differentiate between shallow and deeper geological features, the reduced data were treated to a regional-residual field separation. This was done using the polynomial surface fitting technique. A second order polynomial surface was generated to represent the regional and subtracted from the Bouguer gravity values to produce the residual anomaly map.

Gravity anomaly from two or more separate but spatially close sources will not be adequately resolved even after the regional-residual separation. The first vertical derivative filter was therefore applied to resolve the causative source(s).

Processing, contouring and plotting of the gravity data were accomplished by using the GEOSOFT Mapping and Processing software.

RESULTS AND DISCUSSIONS The Gravity Method Use for Geological Mapping

Case study 1: Delineation of the tertiary basins in North Kedah and Perlis

A regional the gravity survey used for geological mapping was done by MGD in the project to determine the extent of the Tertiary basins in East Perlis and north of Changlun, Kedah. The survey was conducted in June 2001 with gravity measurements using 500 to 1000-m grid spacing were made over a total of 164 gravity stations, covering about 188 km² (Abd Rahim Harun and Ho, 2001) Figure 1.

In general, the study area is underlain by two lithological units: Kubang Pasu Formation and the Bukit Arang Coal Beds. Rocks of the Kubang Pasu Formation consist of shale, mudstone and poorly sorted argillo-arenaceous sediments varying from muddy siltstone through wacke, subgraywacke and arkose to felspathic quartzite. The formation age is late Devonian to Triassic (Jones, 1978).

The Bukit Arang Coal Beds comprise semi- consolidated clays, silts, sands and gravels of lacustrine-deltaic and possibly fluviatile origin probably laid down in a localized basin in late Tertiary times in low undulating country on the borderland between east Perlis, north Kedah and Thailand (Jones, 1978).

A reduction density of 2.67 g/cc was used for the bouguer plate correction. The bouguer anomaly map (Figure 2) shows the gravity values from 7.50 to 22.0 miligal. Generally, there are two relatively low gravity value zones reflecting the rock type being less dense than in the surrounding area. The low gravity zones are interpreted as due to the presence of the semi-consolidated Bukit Arang Coal Beds. Both zones seem to widen towards the Malaysia-Thai border.

Gravity modeling was done across the two gravity low zones, namely along profile AA’ in Perlis and profile BB’ in north Kedah (Figure 3). The wedge type of gravity modeling was

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applied with density assumption of 2.4 g/cc for Kubang Pasu Formation and 1.5 g/cc for Bukit Arang Coal Beds. The gravity model of profile AA’ indicates that the maximum thickness of The Bukit Arang Coal Beds is 181 metres and profile BB’ indicates the maximum thickness of around 87 metres. Comparison of the Tertiary basins as delineated by Jones and those with the gravity survey is given in Table 1.

Table 1: A comparison of the Tertiary Basins in East Perlis and North Kedah as

delineated by Jones and the gravity survey East Perlis North Kedah

Jones Gravity Jones Gravity

Area

26 km²

25 km²

11.7 km²

6 km²

Depth

183m

181m

Not available

87 m

Figure 1: Location and distribution of gravity stations in East Perlis and North of Kedah

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Figure 2: Bouguer anomaly map for East of Perlis and North of Kedah

AA’ BB’ Figure 3: Gravity model of profile AA’ and BB’

The microgravity method use for sinkhole studies and cavity detection

In higher accuracy measurements, the microgravity method has been widely and successfully used for locating and monitoring subsurface voids. Since microgravity methods measure gravity variations at the surface, they are directly influenced by the density distribution in the subsurface and particularly by the presence of voids, which create a mass deficit relative to the density of the surrounding terrain. In many cases, deep or small-scale heterogeneities generating low-amplitude anomalies can be detected and the reliability of further interpretation requires highly accurate measurements which are carefully corrected for any quantifiable disturbing effects.

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Most of the microgravity surveys in Malaysia were carried out by the Minerals and Geoscience Department mainly to detect the occurrence of subsurface cavities in limestone areas of Klang Valley in Federal Territory and Kinta Valley in Perak. Usually the survey was carried out based on 5 metre grid using the La Coste & Romberg model D161 gravity meter with sensitivity of 1 microgal (0.001 miligal).

Case study 2: The microgravity survey for sinkhole studies at Keretapi Tanah Melayu (KTM) quarters, Jalan Spooner, Ipoh, Perak

In June 1995, the KTM quarters were shocked with the occurrence of several sinkholes. The quarters are located in an area underlain by limestone. The microgravity survey was meant to investigate the cause of sinkhole occurrences and to delineate other high risk areas which are prone to sinkholes. A total of 175 microgravity stations at 5 m grid were established during that period (Figure 4). The microgravity results are presented as Bouguer (Figure 5a) and residual anomaly maps (Figure 5b). The survey detected eight anomalies which are attributed to the presence of cavities in limestone bedrock. Five sinkholes have already developed in the vicinity of the detected anomalies. A probable subterranean channel was delineated cutting through the bedrock. The channel runs almost in a north-south direction to the east of the survey area. This channel possibly provides an avenue for sediments from the overburden to be transported and discharged.

Gravity modeling was undertaken over a few anomalies determine the depth, shape and size of these cavities. The modelling was done assuming a sediment-filled cavity with a density of 2.00 gm/cc. The results are shown in Figure 6a and 6b with one is relatively larger indicates a thickness of almost 8 metres.

Figure 4: Location and distribution of gravity stations in KTM quarters Jalan Spooner,

Ipoh , Perak.

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(a) (b)

Figure 5: Bouguer anomaly map (a) and residual anomaly map (b) of sinkhole occurrence studies in Jalan Spooner, Ipoh, Perak.

(a)

(b)

Figure 6: Gravity modelling of (a) Profile AA’ and (b) profile BB Case study 3: The microgravity survey to investigate the sinkhole occurrence at the site of the proposed public library building, Baling, Kedah

In January 2001, a sinkhole size up to 2.5 metre in diameter occurred in the beginning of foundation work at the site of proposed public library building in Baling, Kedah. A microgravity survey was carried out on a 5-m grid spacing with a total of 87 stations observed

A A’

B

B’

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(Figure 7). The bouguer anomaly map (Figure 8a) shows the combined gravitational effects of both shallow and deeper geological structures. Generally, low gravity values were detected towards the northwest, reflecting the bedrock is deeper towards the northwest as compared to the southwest. 2-D gravity modeling across the deepest part of the channel profile AA’ and profile BB’ were done indicates a channel thickness 6.6 and 3.6 m from the bedrock level (Figure 9). The gravity modeling was done by assuming a density of 2.8 g/cc for the limestone and 1.8 g/cc for the in-filled alluvial materials.

Figure 7: Location and distribution of gravity stations at the site of the proposed

public library building, Baling, Kedah

(a)

(b)

Figure 8: (a) Bouguer anomaly map and (b) residual anomaly map of sinkhole occurrence studies in Baling, Kedah.

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(a) (b) Figure 9: Gravity modelling of (a) Profile AA’ and (b) profile BB’

The sinkholes occurrences studies at the sites of a proposed public library building in Baling, Kedah and at the compound of the KTM Quarters, Ipoh, Perak showed that the low amplitude of gravity values located exactly or slightly adjacent to the sinkhole positions. Process of enhancing the subsurface topography and other shallow features of interest in the bedrock were done by producing the residual map and first vertical derivative map respectively.

Case study 4: The microgravity survey at the proposed Mahkota wira condominium site, Jalan Sultan Azlan Shah, Ipoh, Perak

A microgravity survey to locate cavities in limestone was carried out at the proposed Mahkota Wira condominium site at Jalan Sultan Azlan Shah, Ipoh, Perak (Figure 10 a). The survey was carried out based on a 5 metre grid spacing with a total of 373 stations was established during the survey in year 1995. The survey successfully mapped variations in depth of limestone bedrock. Many significant anomalies are delineated, which are attributed to the possible presence of cavities in the limestone or bedrock topographic depression. 2-D gravity modeling had been done on four selected profiles AA’, BB’, CC’ and DD’ (Figure 10b), indicate the cavities are at depths between 0.7 to 2.2 metres from the level of the bedrock. The heights of the cavities are, however, only between 0.7 to 1.5 metres.

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(a)

(b) Figure 10: (a) Location of the microgravity survey to detect cavity and (b) Residual

anomaly map at Jalan Sultan Azlan Shah, Ipoh, Perak

Case study 5: Microgravity survey at the proposed light rail transit (LRT) station, Jalan Ampang, Kuala Lumpur

A microgravity survey to locate cavities in limestone was carried out at the proposed LRT station site at Jalan Ampang, Kuala Lumpur in year 1994 (Figure 11). The survey was carried out based on a 5-m grid spacing covering of 170 x 100 m² area. A total of 567 stations were established during the survey. Four significant anomalies attributed to the possible presence of cavities in the limestone, were delineated. 2-D gravity modelling indicate the cavities are at depths of 0.9 to 1.7 metres from the level of the bedrock. The cavities are between 0.5 to 0.7 metres in height.

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(a)

(b)

Figure 11: (a) Location of the microgravity survey to detect cavity and (b) Residual anomaly map at Jalan Ampang, Kuala Lumpur.

Figure 12(a): Gravity model of profile AA’at Jalan Sultan Azlan, Ipoh, Perak

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Figure 12(b): Gravity model of profile DD’ at Jalan Ampang Kuala Lumpur

Good correlation of borehole data with several results of gravity modeling (Figure12) such as at Mahkota Wira Condominum site, Jalan Sultan Azlan Shah, Ipoh, Perak and at the proposed Light Rail Transit Station in Jalan Ampang, Kuala Lumpur showed that the cavity location and bedrock topography can be successfully determined by the microgravity measurement technique.

CONCLUSION Although the gravity method can be used successfully in locating the subsurface targets

but it’s still a non-unique interpretation and the results need to be confirmed either by drilling or by other geophysical methods. However, the subsurface structures such as karstic features together with rugged bedrock topography will normally require detailed closely spaced drillings to be carried out. Technically it is more cost-effective to implement geophysical investigations such as gravity survey prior to drillings. This will help in the planning of positioning the boreholes and minimize the number required in any site investigation.

ACKNOWLEDGEMENT

The authors would like to thank Minerals and Geoscience Department of Malaysia for their support in sharing their findings. Particular thanks must also go to Ahmad Zulkifli, Jamaludin Othman, and Sukri Ghazali for their help in actual gravity data acquisition and processing.

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