An Evaluation of Geoelectrical Imaging on Dipping Beds Muhammad Safwan Bin Zulkifli Petroleum Geoscience, Geoscience and Petroleum Engineering Department, Universiti Teknologi PETRONAS. Email address: [email protected] This paper was submitted in partial fulfilment of the requirements for the Bachelor of Technology (Hons) (Petroleum Geoscience) Universiti Teknologi PETRONAS (September 2013). Electronic reproduction, distribution, or storage of any part of this paper without the written consent of Universiti Teknologi PETRONAS is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. _________________________________________ Abstract

INTRODUCTION Geoelectrical imaging is one of the method in mapping the Earth subsurface especially in shallow part based on the resistivity of subsurface. In many geological situations, 2D electrical imaging surveys can give useful results that are complementary to information obtained by other geophysical method such as seismic method. Seismic method is a very powerful and expensive tool in imaging the

Geoelectrical resistivity method is a type of

subsurface structure but it will have difficulty

geophysical method which use current source to

(without using advanced data processing technique)

determine the subsurface resistivity distribution by

in mapping discrete bodies such as boulders, cavities

making the measurement on the ground surface. A

and pollution plumes. 2D electrical surveys should

lot of protocols used in this method and each

be used in conjunction with seismic surveys as they

protocol will give different results due to the

provide

resolution capability. In this project, three protocols

subsurface (Loke, 2000). Generally, geoelectrical

which are Wenner array, Schlumberger array and

imaging has played an important role in addressing

Dipole-dipole array will be used to investigate the

a wide variety of hydrogeological, environmental

imaging capabilities of each protocols at various

and geotechnical issues. This survey method has

angle of dipping beds and to describe the

been used for decades in hydrogeological, mining

geoelectrical responses in vertical and horizontal

and geotechnical investigations. More recently, it

direction of dipping bed at various angles. A

has

physical tank experimental that consists of a marble

(Aizebeokhai & Loke, 2010).

slab which represent the subsurface bedding layers

The main purpose of Geoelectrical resistivity survey

will be executed in order to know at what dipping

is to determine the distribution of subsurface

angle each protocols will give the best resolution.

resistivity by taking measurements of the potential

The result of the physical tank experimental will be

difference on the ground surface. From these

compared with the result from forward modelling in

measurements, the true resistivity of the subsurface

order to get the imaging capabilities of the protocol

can be estimated. The geological materials has their

used.

own resistivity values and they are related to various

been

complementary

used

for

information

environmental

about

survey

geological parameters such as the mineral and fluid

content, pore water salinity, temperature, porosity

are 10 Ωm and 500 Ωm. This is to make sure the

and degree of water saturation in rock (Geoelectrical

result contains the least background noise. Thus, the

Imaging, n.d). Related to the resistance is the

most resemble result to the model could be obtained

resistivity which is a characteristic of a material

and help in interpreting the result from the laboratory

rather than that of a particular specimen (Layugan,

data.

n.d).

Two resistivity values are used in this modelling; 10

Generally, the resistivity measurement are normally

Ωm and 500 Ωm. This is to make sure the protocols

made by injecting current into the ground through

only give the response to these two resistivity values

two current electrodes, and measuring the resulting

and thus could avoid from mapping any background

voltage difference at two potential electrodes. From

noise. Below are the forward models with dipping

the current and voltage values, an apparent resistivity

angles of 0, 27 and 90 degree respectively. 500 Ωm

value is calculated based on the principle of Ohm’s

and 10 Ωm resistivity value represent the dipping

Law.

green block and background respectively.

Geoelectrical resistivity imaging survey consist of several protocols where the common uses are dipoledipole array, Wenner array and etc. The choice of survey protocols is really depend on the geological structure, the goal and also the limitation of the Figure 1: 0˚ block Resistivity Model

survey. Surprisingly, different protocols used in a same study area could give different result of imaging. This is because of the different resolution, vertical or horizontal, those protocols give especially when dealing with dipping layers of bedding. Thus, the aim of this project is to investigate the

Figure 2: 27˚ block Resistivity Model

geoelectrical responses result from various angles of dipping beds. The lab dipping test results will be compared with the forward resistivity modelling and field resistivity survey. The geoelectrical responses will be measured from a different types of protocols Figure 3: 90˚ block Resistivity Model

which specifically focus on Wenner array, dipoledipole array, and Schlumberger array. Evaluation and comparative analysis will be carried out in vertical and horizontal directions in 2D approach.

2.

Laboratory Data Acquisition

A laboratory experiment of geoelectrical resistivity was done in order to see the response of each

METHODOLOGY 1.

Forward Resistivity Modelling

protocols towards the dipping beds at different angle. A marble slab with a dimension of 3.5cm x 40cm x

Forward modelling is a great tool in getting the idea

98cm was immersed in a 0.5m height tank filled fully

of how the resistivity response looks like before the

with tap water. A string was used to hang the marble

real experiments is done. The forward resistivity

slab to a hanger so that it will be easier to change the

model only contain two values of resistivity which

dipping angle for the next test. There are 3 sets of

dipping angle, which are 0˚, 27˚ and 90˚ degree to

The three figures above show the results tested by

the water surface were tested.

the three protocols at 0 degree angle of slab model.

A total of 61 cooper electrodes were used and the

The response for all protocols are about the same but

electrode spacing is 1cm at short setup and 2 cm at

Wenner and Schlumberger array give the best

long setup. Three main geoelectrical resistivity

response base on the resistivity contrast. The

protocols will be used in this laboratory test. They

resistivity contrast give an indication of the

are Wenner array, Schlumberger array and Dipole-

boundary between two different resistivity bodies.

Dipole array. Below is the picture of experiment setup.

ii.

27˚ Dipping Slab Model

Figure 8: Inverse resistivity section of Wenner array

Figure 9: Inverse resistivity section of Schlumberger array

Figure 4: Physical data acquisition experiment setup

RESULT AND DISCUSSION

Figure 10: Inverse resistivity section of Dipole-Dipole array

The result interpreted is based on the resistivity contrast between two bodies. A good imaging capabilities will give a sharp contrast between two bodies and able to map the model slab. 1.

Forward Resistivity Modelling i.

0˚ Dipping Slab Model

Through the observation, none of the protocols could give about the same image of the resistivity model. The resistivity contrast is very poor and did not show a good boundary between block model and the background body. All of them are out shape from the original model. However, if compare to these three protocols, Wenner array gives the nearest angle of dipping which is 30˚. Whereas, Schlumberger and Dipole-Dipole array give an angle of 35 and 40

Figure 5: Inverse resistivity section of Wenner array

respectively. Even though the resistivity contrast is poor, Wenner array could map the nearest and le to the model. Figure 6: Inverse resistivity section of Schlumberger array

Figure 7: Inverse resistivity section of Dipole-Dipole array

iii.

90˚ Dipping Slab Model

Figure 11: Inverse resistivity section of Wenner array

The resistivity contour is at the top of the section, located at the location of the marble, 0.21m below water surface. Wenner array give a good resistivity Figure 12: Inverse resistivity section of Schlumberger array

contrast as it could give the resistivity between marble and water. The thickness of the resistivity contour are smaller compare to the other protocols which means nearer to the real marble thickness.

Figure 13: Inverse resistivity section of Dipole-Dipole array

Thus, Wenner array is the best protocol to map 0 degree angle of dipping marble slab.

Based on the resistivity contrast, Dipole-Dipole is the best protocol to map structure of 90 degree. From

ii.

27˚ Dipping Marble Slab

the resistivity section, we can see clearly the boundary between the green block model and the surrounding. Thus, Dipole-Dipole has a good ability to detect the resistivity changes in horizontal.

Figure 17: Inverse resistivity section of Wenner array

In conclusion, Wenner array and Dipole-Dipole array are having a good imaging capability for horizontal and vertical structure respectively. For dipping angle of 27 degree, Wenner array is the best

Figure 18: Inverse resistivity section of Schlumberger array

protocol compare to the other two protocols. They could give high resistivity contrast at the boundary of two bodies in different resistivity value and map

Figure 19: Inverse resistivity section of Dipole-Dipole array

about the same angle to the slab model. The three figures above show the result for 27 degree 2.

Physical Data Acquisition i.

0˚ Dipping Marble Slab

angle of dipping marble slab. From the observation, there are no protocols that could give contour the same angle as the marble slab. They are nearly map as horizontal structure. Thus, the author interpret that all of these protocols could not give a good

Figure 14: Inverse resistivity section of Wenner array

resistivity response horizontally and vertically towards the dipping marble of 27 degree.

iii.

90˚ Dipping Marble Slab

Figure 15: Inverse resistivity section of Schlumberger array

Figure 20: Inverse resistivity section of Wenner array Figure 16: Inverse resistivity section of Dipole-Dipole array

The three figures above show the results tested by the three protocols at 0 degree angle of marble slab.

Figure 21: Inverse resistivity section of Schlumberger array

REFERENCES Aizebeokhai, A. P., Olayinka, A. I., & Singh, V. S. (2010). Application of 2D and 3D geoelectrical Figure 22: Inverse resistivity section of Dipole-Dipole array

resistivity

imaging

for

engineering

site

investigation in a crystalline basement terrain, A very clear of resistivity contrast between the

southwestern

marble and water has shown by the Dipole-Dipole

Department of Physics, Covenant University.

Nigeria.

Ota,

Nigeria:

array. The highest resistivity value indicates the

Aizebeokhai, A. P. (2010). 2D and 3D Geoelectrical

resistivity of marble. From the figures, it shows that

Resistivity Imaging: Theory and Field Design.

the location of marble is accurate in the resistivity

Ota, Nigeria: Department of Physics, Covenant

section of Dipole-Dipole. The contour thickness in

University.

Dipole-Dipole section also about the same thickness

Apostolopoulos,

G.

(2008).

Combined

of the marble. Thus, the best protocol for this angle

Schlumberger and dipole-dipole array for

is Dipole-Dipole array.

hydrogeologic applications. Athens, Greece: School of Mining Engineering and Metallurgy.

CONCLUSION AND RECOMENDATION

Basir, J & Zaiton, H. (2011). Lower Carboniferous

Wenner and Dipole-Dipole array are the most

(Tournaisian) radiolarians from Peninsular

suitable protocols to be used in imaging horizontal

Malaysia

and vertical structure respectively if structure is the

Pengajian Sains Sekitaran dan Sumber Alam,

only factor of choosing. They are supported by the

Universiti Kebangsaan Malaysia

result in the forward resistivity model. Wenner array

Geological

and

their

Imaging.

significance,

(n.d).

Pusat

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has a very good vertical imaging resolution whereas,

http://www.geoelectrical.com/coursenote.zip

Dipole-Dipole give a very good horizontal imaging

Layugan, D.B. (n.d). Geoelectrical Sounding

resolution. However, for dipping angle of 27 degree,

and Its Application in the Theistareykir High

there are no protocol that can give a good contrast in

Temperature

resistivity. It is recommended that more test is

National Energy Authority.

needed in order to know the best protocols for every individual dipping angle from 0˚ to 90˚.

Area,

NE-Ireland.

Iceland:

Loke, M.H. (2000). Electrical Imaging Surveys for Environmental

and

Engineering

Studies.

Retrieved from www.geotomosoft.com. ACKNOWLEDGEMENT

Loke, M.H. (2013). Tutorial: 2-D and 3-D Electrical

The author would like to express his utmost

Imaging

appreciation to his Supervisor, Mr. Khairul Ariffin

www.geotomosoft.com.

Mohd Noh who was very motivative, inspiring and

Open

helpful during the project.

Technique:

Surveys.

Energy

Retrieved

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(n.d). Resistivity

from

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(Schlumberger Array. Retrieved July 4, 2013, from http://en.openei.org/wiki/DC_Resistivity_Surv ey_(Schlumberger_Array)