Identification of Contact Zone Using 2D Imaging Resistivity with EHR Technique

Identification of Contact Zone using 2D Imaging Resistivity with EHR technique

Postgradute Student M.M. Nordiana Geophysics Section, School of Physics, 11800 Universiti Sains Malaysia, Penang, Malaysia e-mail: [email protected]

Senior Lecturer (Dr.) Rosli Saad Geophysics Section, School of Physics, 11800 Universiti Sains Malaysia, Penang, Malaysia e-mail:[email protected]

Postgraduate Student Nur Azwin Ismail Geophysics Section, School of Physics, 11800 Universiti Sains Malaysia, Penang, Malaysia e-mail:[email protected]

Postgraduate Student Nisa’ Ali Geophysics Section, School of Physics, 11800 Universiti Sains Malaysia, Penang, Malaysia e-mail: [email protected]

ABSTRACT This paper illustrates findings of subsurface investigation and 2D imaging resistivity with Enhancing Horizontal Resolution (EHR) technique on a selected site at Batang Merbau, Tanah Merah, Kelantan (Malaysia). The purpose of this study is to characterize interbedded sedimentary with regards to the rock type. Four survey lines were employed at the study area. Results are presented in inversion model resistivity form. The results of the study showed that the study area has a maximum of three main zones, alluvium with resistivity value of 10-800 Ωm, sandstone with resistivity value of 1000-3000 Ωm and granitic bedrock with resistivity value of >3000 Ωm. There is a contact zone between granite and sandstone which produces fractures. Application of 2D imaging resistivity with Enhancing Horizontal Resolution (EHR) technique was successfully used in subsurface investigation. KEYWORDS: Subsurface, 2D imaging resistivity, Enhancing Horizontal Resolution (EHR) technique, contact zone, fracture

INTRODUCTION In nature, rocks may be brought into contact through deposition, intrusion, faulting and shearing. The fundamental contacts, unconformities, intrusive contacts that form are normal depositional contacts, unconformities, intrusive contacts, faults and ductile shear zones. The ways in which rock bodies fit together is deduced from geologic mapping, supplemented wherever possible by drilling and geophysical data (Davis, 1984).

Geotechnical investigation techniques such as borehole, excavation and geological mapping etc are not only destructive methods, but they are also not cost effective in obtaining this information;

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Vol. 17 [2012], Bund. B 198 furthermore, these techniques usually provides a localized insight into the subsurface conditions. Geophysical techniques on the other hand are non-invasive, cost effective and can give a detail picture of the subsurface geometry with maximum time and cost effectiveness. 2-D resistivity imaging (Griffths and Barker, 1993; Loke and Barker, 1996) is now gaining importance in subsurface investigations. (Bichler and Bobrowsky, 2004; Lapena and Lorenzo, 2003). In this method, high resolution imaging of the electrical properties of the subsurface can be obtained at a reduced cost.

The study adopted 2D imaging resistivity with Enhancing Horizontal Resolution (EHR) technique for the investigation. About four survey lines were conducted at Batang Merbau, Tanah Merah, Kelantan (Malaysia) to determine the subsurface resistivity distribution in the area.

GENERAL GEOLOGY The Colluvium/Terrace member refers to the sequence of gravel beds, lateritic layers and residual sediments. In Malaysia, this unit is exposed to the south of Rantau Panjang area, southern part of Pasir Mas district, Tanah Merah district and Machang district. These sediments are composed mainly of sand, gravelly sand, clayey sand, gravel, gravel beds and laterite, friable to very firm, abundant mottle, red to reddish brown, poorly sorted and moderate amounts of iron concretions. The gravel beds unit is locally exposed at Ban Ai Su Re. Gravel beds with sand lenses, semi- consolidated, commonly characterize this unit. Class of gravel beds vary from gravel to boulders. They are made up of sandstone, quartz, chert, granite and schist. The sand layers are characteristically reddish brown to red, medium to coarse grained, poor to moderately sorted, moderately cemented and subangular to subrounded. This unit is 15-20 m thick and dips to the south (185°/ 5°-10°, dip direction/ dip angle).

Lateritic layers unit is exposed mainly at undulating landforms of Waeng district, Sungai Padi district and western part of Sungai Kolok district. There are mainly characterized by sand, silty sand, gravelly sand and gravel, yellow color. Abundant mottles, strong brown to red and iron concretions are also common. The thickness of this unit varies from 2-6 m. The residual sand unit overlies the Lateritic layers unit with gradational contact. This very thick sequence (>15 m) is underlain by Triassic Kemahang Granite/ Sukhirin granite at the northern part of Sukhirin district. The sequence consists of weathered porhyritic biotite-hornblende granite and is characterized by sand, gravelly sand, loose to friable, moderately sorted. The thickness ranges from 5 to approximately 25 m (Malaysian and Thai working group, 2006).

Tanah Merah is composed of andesite flow, andesitic tuff and agglomerate but predominantly pyroclastics. It is mostly extrusive in nature, probably contemporaneous with the deposition of the associated sedimentary rocks. The most impressive andesite in the Transect area is located in the Tanah Merah area, to the east of the Temangan Ignimbrite. The volcanic rocks comprise strongly sheared, highly altered andesite, andesitic tuff and agglomerate. Agglomerate is characterized by the abundance of rhyolitic clasts within thin to medium layers (30-40 cm). The colours of these rocks are generally pale brown, pale purple grey to light greenish grey with white spots (1-5 cm in diameter). Volcanic clasts are white or yellowish grey in colour.

MacDonald (1967) assigned this rock unit as Carboniferous to Triassic in age. Aw (1990) reported that the main volcanic activities occurred during Permo-Triassic period.

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CONTACT BETWEEN GRANITE AND SANDSTONE This is nonconformity. The contact is between an igneous and sedimentary rock formation. Granite should be on the bottom and sandstone on top, if this is not the case then some type of over-turning has taken place. The depositional environment allowed sand to acculate over the top of the granite and later consolidated into a rock. This is called a contact. Granite will change into sandstone when it is eroded into sand and then settle into a pile someplace, and then be exposed to large amounts of pressure to create a sedimentary rock. The arrangement of how the layers of sandstone over massive granite, with an inclined planar contact between the two have at least three possibilities or three hypotheses to evaluate. Hypothesis 1: The granite was intruded as a mass of magma into its present position below the sandstone.

sandstone sandstone

granite

Hypothesis 2: The contact between the sandstone and granite is a fault, a surface along which the two rock masses have moved relative to each other into their present position.

sandstone granite sandstone granite

Hypothesis 3: The contact between the sandstone and granite is an unconformity, a surface of ancient erosion at which granite was weathered and eroded and then sand was deposited on top of it from the sandstone. This particular kind of unconformity would be nonconformity, a surface of erosion between an overlying sedimentary rock and an underlying unlayered rock.

sandstone sandstone granite granite

If Hypothesis 1 were correct, we might expect to see that the sandstone had been baked by the heat from the granitic magma, and perhaps recrystallized. If Hypothesis 2 were correct, we might expect to see ground-up or pulverized rock (called "fault gouge") and linear tracks of deformation showing the direction of fault movement (called "slickensides"). If Hypothesis 3 were true, we might expect to see pebbles of granite in the basal sandstone (Noblett, 1986). Ductile shear zones produce contacts entirely different from contacts marking ordinary faults. Unlike ordinary brittle fault surfaces, ductile shear zones commonly do not display any physical break. Instead, differential translation of rock bodies, separated by the shear zone, is achieved entirely by ductile flow. Markers pass through ductile shear zones without necessarily losing their continuity, but the effects of the shearing are reflected in distortion of the markers and in the transformation of the original rocks to sheared rocks like mylonites. Distortion is expressed both in shape and orientation changes. Vol. 17 [2012], Bund. B 200

Ductile shear zone contacts are most commonly formed in igneous or metamorphic environments, where elevated temperature and/ or confining pressure render one or both wall rocks ductile. However, ductile shear zones can also form during soft-sediment deformation of unconsolidated sands and mud. In such environments, in such environments, the ductile response is due to the water-rich nature of the sediments. Geologic mapping has revealed that ductile shear zones are tabular to curved, centimeters to kilometers thick, and marked by penetrative foliations. Shear zones can accommodate either shortening or stretching of crustal rocks. Diagrams drawn by Ramsay (1980) illustrate two of the possibilities (Figure 1). Older rock can be brought up against younger, higher level rock during crustal shortening (Figure 1A); relatively young, high-level rock can be sheared down onto deeper, older rocks during crustal extension (Figure 1B). Properties of ductile shear zone contacts are predictable from the fundamentals of strain analysis (Ramsay, 1980). If a body of rock is cut by a shear zone of some uniform thickness, the original bedrock caught within the zone of shear will be distorted such that it lengthens in one direction and shortens in another. The direction of stretching corresponds to the plane of flattening in the shear zone, a plane whose physical manifestation is a penetrative foliation, inclined at an acute angle to the slip (or shear) planes.

Figure 1: A) Ductile shearing that accomplishes layer-parallel shortening. B) Ductile shearing that accommodates layer-parallel stretching. (Modified from Ramsay, 1980). Vol. 17 [2012], Bund. B 201

THEORY OF 2D IMAGING RESISTIVITY Electrical Imaging System is now mainly carried out with a multi-electrode resistivity meter system (Figure 2). Such surveys use a number (usually 25 to 100) of electrodes laid out in a straight line with a constant spacing. A computer-controlled system is then used to automatically select the active electrodes for each measure (Griffith and Barker, 1993). Throughout the survey conducted in the proposed site, the Pole dipole protocol has been used with the ABEM SAS4000 system with Enhancing Horizontal Resolution (EHR) technique (Nordiana et al., 2011). The data collected in the survey can be interpreted using an inexpensive microcomputer.

Station 3

Laptop C1 P1 P2 C2 3a 3a 3a computer Resistivity meter Station 2

C1 P1 P2 C2 2a 2a 2a

Station 1 Data C1 P1 P2 C2 E l e c t r o d e s level a a a

n = 1 .• ...... 1 n = 2 • ...... 2 • n = 3 3 ...... • ...... n = 4 4 • . . . . n = 5 5 • . n = 6 6

Figure 2: The arrangement of electrodes for a 2-D electrical survey and the sequence of measurement used to build up a pseudosection.

The resistivity method basically measures the resistivity distribution of the subsurface materials. Table 1 shows the resistivity values of some of the typical rocks and soil materials (Keller and Frischknecht 1996). Igneous and metamorphic rocks typically have high resistivity values. The resistivity of these rocks is mainly dependent on the degree of fracturing. Since the water table in Malaysia is generally shallow, the fractures are commonly filled with ground water. The greater the fracturing, the lower is the resistivity value of the rock. As an example, the resistivity of granite varies from 5000 ohm-m in wet condition to 10,000 ohm-m when it is dry. When these rocks are saturated with ground water, the resistivity values are low to moderate, from a few ohm- m to a less than a hundred ohm-m. Soils above the water table is drier and has a higher resistivity value of several hundred to several thousand ohm-m, while soils below the water table generally have resistivity values of less than 100 ohm-m. Also clay has a significantly lower resistivity than sand.

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Table 1: Resistivity values of common rocks and soil materials in survey area.

Material Resistivity (ohm-m) Alluvium 10 to 800 Sand 60 to1000 Clay 1 to 100 Groundwater (fresh) 10 to 100 Sandstone 8 - 4 x 103 Shale 20 - 2 x 103 Limestone 50 – 4 x 103 Granite 5000 to 1,000,000

STUDY AREA

The study location is situated at Batang Merbau, Tanah Merah, Kelantan with the coordinate of 5o 50’ 45.0” N and 102o 01’ 49.6” E (Figure 3). There were four survey lines were carried out on the survey area with Enhancing Horizontal Resolution (EHR) technique. The length of each line is 200 m.

Survey area

Figure 3: 2D Imaging resistivity survey lines with EHR technique at Batang Merbau, Tanah Merah, Kelantan (Malaysia).

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Photo 1: Study area at Batang Merbau, Tanah Merah, Kelantan.

METHODOLOGY

Four resistivity survey lines with EHR technique (L1-L4) were carried out on the study site (Figure 4). The length of each survey line is 200m. Each of L1 to L4 running from west-east which means that the 0 m is at west and 200 m is at east. The minimum electrode spacing for all the survey line was 2.5 m and the survey were used pole-dipole array with minimum current is 2mA and maximum was 20mA. A total of 41 electrodes were used. The first electrode was located at 0 m (beginning of the line) while the last electrode was located at 200 m (end of the line). The inter electrode spacing of 5 m was adopted for the study. After the data acquisition complete, the first electrode was shifted to the right by 2.5 m while the last electrode was located at 202.5 m and the process of data acquisition was repeated in the same line. Processing was performed using Res2Dinv software. RESULTS AND DISCUSSION

Generally the area was divided into three main zones (Figure 4), alluvium with resistivity value of 10-800 Ωm, sandstone with resistivity value of 1000-3000 Ωm and granitic bedrock with resistivity value of >3000 Ωm. There is a contact zone between granite and sandstone which produces fractures. The approximate depth to the granitic bedrock is about 30 m. Various linear features suspected to be fractures were also delineated on the model sections. These linear features were observed to coincide with the granite and sandstone boundary and are also characterized with low resistivity value. The presence of these geologic weak zones (i.e. fractures) within the subsurface will further supported with the hypothesis 2.

Vol. 17 [2012], Bund. B 204 Fracture and Contact zone

GRANITE L1

Fracture and Contact zone

SANDSTONE GRANITE

SANDSTONE

Fracture and Contact zone

SANDSTONE

GRANITE L4

Figure 4: Resistivity section of survey line L1, L2, L3 and L4 at Batang Merbau, Tanah Merah, Kelantan (Malaysia). CONCLUSSION Contacts are simple to distinguish in theory, but where structures are complex, deformation multiple and exposures poor, recognizing the nature of contacts can be a very difficult and challenging assignment. Of all the missions of geologic mapping, proper interpretation of contacts is of the greatest importance. Accurate reconstruction of geologic history depends on correct interpretation of contacts. The whole study area is alluvium with sandstone and granitic bedrock. Conclusively, the application of 2d Imaging resistivity with Enhancing Horizontal Vol. 17 [2012], Bund. B 205

Resolution (EHR) technique integrated with geological information is a powerful tool in the subsurface study. ACKNOWLEDGEMENTS

The authors wish to express their gratitude to the Department of Irrigation and Drainage, Kelantan for their permission to conduct the research. The authors thank the technical staffs of the geophysics laboratory and all geophysics postgraduate students, School of Physics, Universiti Sains Malaysia for their assistance during the data acquisition. REFERENCES 1. Aw, P. C. (1990) “Geology and Mineral Resources of the Sungai Aring Area, Kelantan,” Geological Survey Malaysia Memoir, Vol. 12, pp 116. 2. Bichler, A. and Bobrowsky, P. (2004). "Three-dimensional mapping of a landslide using multi-geophysical approach," The Quesnel Forks landslide. Landslide, Vol. 1, pp 29-40. 3. Davis, G. H. (1984), “Structural Geology of rocks and regions,” Chapter 7 Contacts, pp 203. 4. Griffith D. H. and Barker R. D. (1993), “Two dimensional resistivity imaging and modeling in areas of complex geology,” Journal of Applied Geophysics, Vol. 29, pp 211-226. 5. Keller G. V. and Frischknecht F. C. (1996), “Electrical methods in geophysical prospecting,” Pergamon Press Inc., Oxford. 6. Lapena, V. and Lorenzo, P. (2003). "High resolution geoelectrical tomographies in the study of Giarrossa landslide (southern Italy)," Bull. Eng. Geol. Environ, Vol. 62, pp 259-268. 7. Loke, M. H. and R. D. Barker (1996), "Rapid least-squares inversion of apparent resistivity pseudo sections by a quasi-Newton method," Geophysical Prospecting, Vol. 44, pp 131-152. 8. MacDonald, S. (1967) “Geology and mineral resources of North Kelantan and North Terengganu,” Geological Survey Malaysia Memoir, Vol. 10, pp 202. 9. Malaysian and Thai working groups, (2006), “Geology of the Batu Melintang-Sungai Kolok Transect area along the Malaysia-Thailand border. A joint project carried out by Minerals and Geoscience Department Malaysia and Department of Mineral Resources, Thailand. The Malaysia-Thailand Border Joint Geological survey Committee (MT-JGSC),” pp 23-24. 10. Noblett, J. B. (1986), “The Garden of the Gods and basal Phanerozoic nonconfomrity in and near Colorado Springs, Colorado in Beus, S.S., ed., Centennial Field Guide Volume 2,” Rocky Mountain Section of the Geological Society of America, pp 335- 338. 11. Nordiana, M. M., Rosli, S., Nawawi, M. N. M, Adiat, K. A. N., Azwin, N. I., Hidayah, N. E. I. and Anderson, A. B. (2011), “Assessing the efficacy of Enhancing Vol. 17 [2012], Bund. B 206

Horizontal Resolution (EHR) technique using 2D resistivity method,” Electronic Journal of Geotechnical Engineering, Vol. 16, Bund R, pp 1405-1413. 12. Ramsay, J. G. (1980), “Shear zone geometry: a review,” Journal of Structural Geology, Vol. 2, pp 83-99, Pergamon Press, Ltd., Oxford.