Assessment of Subsurface Profile at SILC, Nusajaya by 2D Resistivity Method

Siti Norsalkini Mohd Akip Tan1, a, Edy Tonnizam Mohamad1 ,b , Rosli Saad2,c

and M.M.Nordiana2,d 1Department of Transportation & Geotechnic, Universiti Teknologi Malaysia (UTM)

2School of Physics, Universiti Sains Malaysia (USM)

anorsalkini@gmail.com, bedytonizam@gmail.com, crosli@usm.my, mmnordiana@gmail.com

Keywords: Subsurface, Geophysics, 2D Resistivity, Sedimentary, Weathering

Abstract : An assessment of subsurface profile of a study area at Nusajaya. Johor was conducted

using 2D resistivity method. The aims of the study are to determine the subsurface profile hence

contouring the appearance of outcrop by 2D resistivity imaging. Subsurface structure can be

delineated based on the physical and chemical parameter which is apparent resistivity of the

medium. A survey line of 80m and minimum 2m electrode spacing was adopted using Pole-dipole

array. Data acquisition on the survey line was completed and processed by using Res2Dinv

software. The inversion model resistivity shows sandstone contains iron mineral (30-250 ohm-m)

and weathered sandstone (500-1000 ohm-m). The lower part of the layer represents sandstone and

siltstone exhibit high resistivity values of 1500-5000 ohm-m. The range from 30- 1000 ohm-m

correspond to the grade IV and V of sandstone in the actual profile. Meanwhile, the high range of

the resistivity value matched the material with weathering grade II and III in ground mass. The

deterioration of most physical properties was associated with the increase number of weathering

grade.

Introduction

Subsurface exploration in engineering projects can be divided into direct assessment and indirect

assessment. Direct assessment such as geotechnical and geological techniques can be applied

simultaneously with indirect assessment like geophysical method in order to ease the process of

subsurface exploration with low cost and effective method. Since geophysics in civil engineering is

an application of the principles and methods of physics in the measurement of subsurface

characteristics and properties, the techniques are used to establish ground properties and profile for

the engineering and development processes.

The behaviour of subsurface is complex dependent upon numerous aspects. The understanding of

the nature of rock type and its weathering profile is crucial and very significant in site investigation

processes. Thick profile of weathered zone can be found in tropical region. Ibrahim Komoo [1]

recommended that civil engineering practitioners should give more attentions to tropical terrains

such as in Malaysia. Because there are prominent differences of climate environment where heavy

rain pours conditions, wide variation of temperature and high humidity happens all year round. This

study focus on sedimentary area where the occurrence of discontinuity such as bedding thickness,

folding, foliation and the inhomogeniety of rocks can manipulate its physical and mechanical

properties. The weak and weathered material is refer as weak rock, where Santi and Doyle [2] noted

that weak materials are intact, unweathered to slightly weathered materials that show significant

deterioration, particularly near the ground surface or along fractures. Problems in evaluating the

rock arise in the weathering zone II-V in term of opt for the most appropriate technique and cost

estimation.

The information gathered from in situ observation was used to determine the behaviours of the

subsurface. The study was carried out at an outcrop in Southern Industrial Logistic Cluster (SiLC)

Nusajaya, Johor with coordinate N 12834.90 E103 35 35.00 consisted of sedimentary rock

such as sandstone and siltstone (Figure 1).The slope at the survey area had been cut for the

construction of industrial building (Figure 1 (c)). Geology characteristics that can be seen at the

outcrop are sandstone, siltstone and clay, and some mineral like quartz with reddish-brown, white

and yellow color (color effect from weathering process).

This site is mainly underlain by sedimentary material such as sandstone, shale, claystone and

siltstone. Physical characteristics of the sandstone in the study area are generally light grey to

yellowish in colour and have very fine to medium sized grain [3]. Information at this area is Triassic

age; interbedded sandstone, siltstone and shale with lithology boundary with acidic to intermediate

volcanic; mainly pyroclastics, rhyolitic to dacitic composition (Geological map of Peninsular

Malaysia, Mineral and Geoscience Department Malaysia, 1985[4]).

The objective of this study is to contour the appearance of structure layer based on the parameter

and characteristic of the structure .Thus, 2-D resistivity imaging method is deployed in order to gain

continuous and precise information of subsurface in different representative geological situations,

and for evaluating the behaviour by the interpretation methods. Outcrop of the site are compared to

the 2D profiling to gain the subsurface structures in natural geological environments. Throughout

the study, 2-D profiling with Pole-dipole array is adopted for this study due to the good horizontal

and vertical resolution [5].

2D Resistivity Imaging

Electrical resistivity method measure the distortions in the electrical potential on the ground

Figure 1: Location of Study Area (a) in Peninsular Malaysia, (b) google map of the study

location & (c) the outcrop at study area ( Mineral and Geoscience Department Malaysia,

1985 [4]).

PENINSULAR

MALAYSIA

JOHOR

Legend:

Sedimentary (Triassic-

Jurassic Formation)

(a) (b)

(c)

surface based on current flow changes passes through an anomaly. It is not directly measure the

current and this makes the interpretation of electrical data is more difficult based on different

circumstances[6]. Table 1 shows the common resistivity value of some typical rocks and soil

materials[7].

Table 1. Resistivity values of common rocks and soil materials[7].

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

Methodology

2D resistivity survey was conducted using ABEM SAS4000 Terrameter with ES10-64C selector,

smart cables with 5m takeouts and stainless steel electrodes. A survey line was conducted at the

study area. The survey used Pole-dipole array with 2m spacing between electrodes. Coordinate of

the study area are taken by 12 channels Garmin global positioning systems (GPS) instrument. After

the data acquisition, the raw data of the apparent resistivity are process by using Res2Dinv

software. The measured data was inverted to resistivity image to simplify the interpretation of the

result. Some gridding and contouring was done to the data based on actual site condition.

Subsurface outcrop was map by using Surfer software.

Result and Discussion

The result of weathering grade is presented with inversion resistivity profile (Figure 2) trends

South-North direction to a length of 80m and imaging depths of 15m. The interpretation of the

resistivity image was corresponded to the actual rock mass profile. It was found that the ground

mass was classified from grade II to grade V sandstone and grade III siltstones, which was from

moderately to completely weathered materials. This weathering classification was indicated by in

situ observation on the mass. The bulk of material commonly is coarse. Some of the material has

been weathered to clay, which undergone leaching processes. Thin beds of clay are found between

sandstone and shale especially at the upper part of the horizons. For the most part the sand is of

white, cream, buff, or pale-yellow colour, with only minor dark-brown to black variations.

Generally the colour index is directly related to the clay content, whether this is due to the presence

of organic matter or to iron staining. The dark-brown to black colours referred to section A, D and F

in Figure 2 could be due leaching and redeposition of oxides of iron, alumina at lower levels [3].

The inversion model resistivity displays upper part as sandstone contains iron mineral (30-250

ohm-m) and weathered sandstone (500-1000 ohm-m) alternately. The lower part of the layer

represents sandstone and siltstone exhibit high resistivity values of 1500-5000 ohm-m. The range

from 30- 1000 ohm-m correspond to the grade IV and V of sandstone in the actual profile as shown

in Figure 2. Meanwhile, the high range of the resistivity value matched the material with

weathering grade II and III in ground mass. The data show wide range of value was because the

ground mass comprises of different type of rock. The results somehow have shown a good

agreement, whereby the resistivity profile and actual ground mass profile matched each other.

Figure 2: The inversion resistivity model at Nusajaya, Johor matched to actual outcrop of study

area.

Conclusion

The ground mass have been characterised by resistivity interpretation result and on site

observation on the weathering. Clearly, the relationship between weathering grade shows a good

agreement and their properties are vital in order to predict the performance of the subsurface

material. Surface observation can be used to support the imaging technique interpretations in

geophysicals survey. Hence, the interpretation of the resistivity result can ease the process of site

investigation in civil engineering works.

Acknowledgement

The authors express their sincere thanks to all technical staffs of Department of Transportation &

Geotechnic, Universiti Teknologi Malaysia (UTM), all technical staffs and postgraduate of School

of Physics, Universiti Sains Malaysia (USM) for the data acquisition during the study.

References

[1] Ibrahim Komoo, Weathering As An Important Factor In Assessing Engineering Properties Of

Rock Materials. Forum on Soil and Rock Properties, Geological Society Malaysia. Universiti

Malaya, Kuala Lumpur, (1995).

[2] Santi, P. M. and Doyle, B C., 1997, The Locations And Engineering Characteristics Of Weak

Rock In The U.S-Characterization of Weak and Weathered Rock Masses, Association of

Engineering Geologists Special Publication No.9: Association of Engineering Geologists, Denver,

CO, pp. 122, (1997)

[3] Burton C.K., Geology and Mineral resources, Johor Bahru- Kulai area, South Johore.

Geological Survey of Malaysia, Map Bulletin 2; p. 56, 1973

[4] Mineral and Geoscience Department Malaysia, Geological Map of Peninsular Malaysia, 8th

Ed. Ministry of Natural Resources and Environment, (1985).

[5] Nordiana M.M, Rosli S. Theoretical model for 2D resistivity mapping with Enhancing

Horizontal Resolution (EHR) technique. Electronic Journal of Geotechnical Engineering (EJGE) ;

17(Bund. D): 483-493,(2012)

[6] Loke MH. Instruction manual for the 2D resistivity forward modeling program Res2Dmod , p.

1-11, 1994.

[7] Keller GV. Frischknecht FC. Electrical methods in geophysical prospecting. Pergamon Press

Inc., Oxford; 1996.