Comparison of Schlumberger and Modified Schlumberger Arrays Ves Interpretation Results

Rjeas Research Journal in Engineering and Applied Sciences 1(3) 190-196 Rjeas Emerging Academy Resources (2012) (ISSN: 2276-8467)

www.emergingresource.org

190

COMPARISON OF SCHLUMBERGER AND MODIFIED SCHLUMBERGER

ARRAYS VES INTERPRETATION RESULTS

Akintorinwa, O. J1 and Abiola, O2 1Department of Applied Geophysics,

Federal University of Technology, Akure, Nigeria 2 Department of Geology, Adekule Ajasin University,

Akungba-Akoko, Ondo State, Nigeria Corresponding Author: Akintorinwa, O. J ___________________________________________________________________________

ABSTRACT Schlumberger array is the most commonly used among other arrays for vertical electrical sounding (VES) and it requires large spacing at both ends for deeper subsurface information. The problem of limited space for spreading in built up areas could lead to incomplete information from deeper depth. In this research, the interpretation results of the VES data acquired using the conventional Schlumberger and modified Schlumberger arrays were compared with view to assessing the effectiveness of the Half Schlumberger array as an alternative to the convectional Schlumberger array at sites with space constraints. Fourteen VES data which were distributed across different lithologies using the Schlumberger and modified Schlumberger arrays with AB/2 varying from 1 to 65m were used for the study. The resulting VES curves from both arrays were interpreted using the partial curve matching technique and computer iteration. The resulting field curves from both arrays were the same and the geoelectric sections across different directions for both arrays display the same geoelectric/geologic sequences with similar parameters. The coefficiency of correlation R from the crosspots of the interpretation parameters are approximately equal to 1, indicating a perfect correlation between the interpretation parameters for both Schlumberger and modified Schlumberger arrays, hence, modified Schlumberger array can be said to be a good alternative to the convectional Schlumberger array especially in a space constraint area. Emerging Academy Resources KEYWORDS: Schlumberger, VES, computer iteration, crosspots ________________________________________________________________________________________INTRODUCTION An alternative method of investigating the subsurface is by drilling, which is expensive and provides information only at discrete location and it is limited to some terrain. Geophysical survey, though sometimes are prone to major ambiguities of interpretation provides a relatively rapid and cost effective means of obtaining subsurface geology. Geophysical methods can provide reasonable and genuine information which could assists in the effective identification and location of subsurface geological structure like faults, fracture zones, fissure zones, weathered rock materials, shear zones, and fresh basement. Information concerning the lithology, stratigraphic sequence and hydro-geological characteristics of the subsurface material can be provided through the aid of electrical resistivity prospecting method. This geophysical method determines the variation in the subsurface distribution of electrical properties within the homogenous entity of the earth. Electrical Resistivity method has been widely used in prospecting for groundwater, foundation studies, dam site location, mineral

exploration, pollution plumes and road failures. The problem of limited space for spreading could lead to wrong judgment and recommendations because of incomplete information about the subsurface resulting from short spreading. In electrical resistivity method, there are many arrays which can be used. This includes Schlumberger, modified Schlumberger, Dipole-Dipole, Wenner, Pole-Pole and others. Most commonly used among the arrays mentioned above for vertical electrical sounding (VES) is the Schlumberger array, which requires large spacing at both ends for deeper information. The modified or the so called Half Schlumberger array which enables vertical electrical sounding with the movement of current electrode (A) while the other current electrode (B) is fixed orthogonally at a large distance away and relative to the centre of the potential dipole (M-N) (Frohlich and Rosenbach, 1986) together with Schlumberger array were used for vertical electrical sounding and the results were compared for correlation. This research attempts a comparative study of the Schlumberger and Half Schlumberger arrays in terms of deduced geoelectrical parameters,

Research Journal in Engineering and Applied Sciences (ISSN: 2276-8467) 1(3):190-196 Comparison of Schlumberger and Modified Schlumberger Arrays VES Interpretation Results

191

in vertical electrical sounding, in a typical basement complex environment Description of the Case Study Area The case study covers some parts of Akure and its falls within longitudes 50 7 E and 50 14 E and latitudes 70 13N and 70 19N (Fig. 1). The areas are characterized by fairly populated vegetation with trees and herbs growing favorably. The areas lie within the tropical rainforest climatic region of Nigeria. It is characterized by two distinct seasons, the wet season (between April and October) and the dry season (between November and March). The mean annual rainfall is about 1660mm, while the average daily temperature of 290C. (Adeleke and Goh Cheng Leong, 1978). The case study areas are underlain by rocks of the Precambrian Basement Complex of Southwestern Nigeria (Rahaman, 1989). The crystalline rocks are porphyritic granite, migmatite gneiss, biotite granite, charnockite and quartzite. Although the basement is concealed within the VES stations, several outcrop of this rock were seen closed to some VES stations. MATERIALS AND METHODS OF STUDY Vertical Electrical Sounding (VES) measures vertical variation of ground resistivity with depth in respect to a fixed station. This is carried out by gradually increasing the inter electrodes spacing at about a fixed centre of array (Telford et al., 1990). Fourteen vertical electrical sounding (VES) were carried out across the case study area which were distributed across the geological units as showing in Fig. 1 using both Schlumberger and modified Schlumberger arrays. The Omega Resistivity meter was used for data collection. GARMINs Global Positioning System (GPS) 12 Personal Navigator was used to record the geographic coordinates of the VES Stations. Schlumberger array utilizes four electrodes system which are arranged linearly with different inter-electrode spacing (Fig. 2a). The electrodes are arranged such that the distance AB between the current electrodes is greater or equal to five times the distance MN, between the potential electrodes. The potential electrodes are fixed about the data station in which the current electrodes are spread until the required maximum separation is attained. For this study, the current electrode spread AB/2 was varies from1m to 65m. The apparent resistivity values a were calculated by multiplier the resistance R measured with the geometric factor G (Equation 1).

[G =l

L2

2] 1

Where L = AB/2 and l = MN/2 In modified Schlumberger array, one of the current electrodes, C2 is kept at infinity (100m) distance perpendicular to the centre of the spread (Fig. 2b), hence not collinear with the other three electrodes, it is at right angle to the other collinear electrodes. The current and the potential electrodes are maintained at the same relative spacing and the whole spread is progressively expanded about a

fixed central point. Current electrode spread AB/2 was also varies from1m to 65m. The apparent resistivity values a were calculated by multiplier the resistance R measured with the geometric factor G (Equation 2).

[G =lL2

] 2

The apparent resistivity measurements at each VES stations for both the Schlumberger and modified array were plotted against electrode spacing on bi-logarithmic graph sheets. The resulting curves were then inspected visually to determine the nature of the subsurface layering delineated by both array. Partial curve matching was carried out for the quantitative interpretation of the curves (Koefoed, 1979 and (Orellana and Mooney, 1966). The results of the curve matching (layer resistivities and thicknesses) were fed into the computer as starting model parameter in an iterative forward modeling technique using WINRESIST computer software (Vander Velper, 1988). From the interpretation results (layer resistivities and thicknesses), geoelectric sections along the different direction were generated. The results were also used to generate layer parameter charts. DISCUSSION OF RESULTS Field Curve Characterization The results of the research were presented as field curves, tables, geoelectric sections, histograms and cross plots. The summary of the VES Interpretation results for the Schlumberger and modified Schlumberger are presented in Tables 1 and 2. Maximum of four layers were delineated by the field curves generated from both arrays. The same curve types were identified by both arrays at each of the sounding stations (Table 1, Table 2 and Fig. 3). A-type curve constituting 71%, H-type curve constituting 21% while the HA-type curves constituting 7% of the total curves as identified by both arrays. The predominant curve type by both arrays is the A-type curves (Fig. 4). Geoelectric Sequence Three geoelectric sections were produced from the interpretation of Schlumberger and modified Schlumberger array along S N, W E and NE SW directions. Fig. 5a and 5b shows the geoelectric along N S for both arrays and three geoelectric sequences were delineated by both arrays; topsoil, weathered layer and the fresh basement rock. The resistivity of the topsoil as delineated by the Schlumberger and modified Schlumberger arrays ranges from 61 105 and 69 99 -m and thickness ranges from 0.8 2.3m and 0.9 1.9m respectively. The ranges of this geoeletric parameters were correlated and indicate composition of clay/sandy clay. The weathered layer composed clay/sandy clay/clayey sand with resistivity varies from 33 -.270-m and 34 - 224 -m with thickness ranges from 1.5 18.7m and 1.6 19.1m for Schlumberger


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and modified Schlumberger array respectively (Fig. 5a and 5b); this indicate similar ranges of geoelctric parameters for the weathered layer from both arrays. The resistivities of basement rock delineated by both arrays are very similar and it ranges from1423 - 1958 -m and 1575 - 2671 -m respectively (Fig. 5a and 5b). The geoelectric along E W direction for both arrays delineates three geoelectric sequences (Fig. 6a and 6b); topsoil, weathered layer and the fresh basement rock. The topsoil resistivity ranges from 51 163 and 46 163 -m with thickness ranges from 1.2 1.6m for both arrays, hence composed of clay/sandy clay/clayey sand. The weathered layer composed of clayey sand/laterite with resistivity varies from 265 -887-m and 264 - 938 -m and thickness ranges from 1.7 26.3m and 1.7 22.3m for Schlumberger and modified Schlumberger array respectively (Fig. 6a and 6b). These ranges of geoelectric parameters for the weathered layer were virtually the same for both arrays. The resistivities of basement rock are very similar and it ranges from 917 - 9833 -m and 916 - 7339 -m respectively (Fig. 6a and 6b). Three geoelectric sequences were delineated by both arrays along SW - NE direction (Fig. 7a and 7b); topsoil, weathered layer and the fresh basement rock. The topsoil ranges from 33 125 and 30 125-m and thickness ranges from 1.0 2.1m and 0.9 2.5m for Schlumberger and modified Schlumberger arrays respectively. The ranges of this parameters correlates with each other and indicate composition of clay/sandy clay. The weathered layer composed sandy clay/clayey sand with resistivity varies from 78 -.130-m and 83 - 130-m and thickness ranges from 5.7 14.6m and 5.6 14.5m for Schlumberger and modified Schlumberger array respectively (Fig. 7a and 7b). The resistivities of basement rock delineated by both arrays are very similar and it ranges from1337 - 18351-m and 1337 - 22602-m respectively (Fig. 7a and 7b). Synthesis of Schlumberger and Modified Schlumberger Arrays Results Fig. 8 shows that topsoil resistivity and thickness determined from Schlumberger and modified Schlumberger were virtually the same and the crossplots of the of the topsoil geoelectric parameters (resistivity and thickness) (Fig. 9) shows a good linear relationship between topsoil parameters determined from the two arrays with coefficiency of correlation (R) of 0.96 and 0.98 respectively. Fig. 10 (a and b) shows that, the weathered layer parameters determined from the Schlumberger and modified Schlumberger are significantly over lap. The crossplot of the weathered layer resistivities and thicknesses determined from both array (Fig. 11) shows a good relationship with coefficiency of correlation of 0.997 and 0.988 respectively. Fig. 12a shows that the bedrock resistivity values from both arrays are very similar. The coefficient of correlation R of 0.936 was obtained from the crossplots of the

bedrock resistivity values determined from the two arrays (Fig. 12b) and it indicates a good linear relationship. The coefficient of correlation R for the cross plotting of the interpreted parameters (Layer thickness and Resistivity) for the three delineated layers (topsoil, weathered layers and bedrock) from the Schlumberger and modified Schlumberger arrays were approximately equal to 1. This indicated that, the results of VES Interpretation by adopting Schlumberger and modified Schlumberger arrays are perfectly correlated. CONCLUSION This research involved the correlation of VES Interpretation results from the data acquired using Schlumberger and modified Schlumberger arrays. The interpretation results from both arrays were correlated with each other i.e. the field curves and geoelectric sections. The cross plots of the interpreted parameters (layer thickness and resistivity) for the delineated layers were produced and the coefficiency of correlation R for the relationship between the two arrays was also determined. The results show a perfect relationship between the interpretation parameters for both Schlumberger and modified Schlumberger arrays (Anjorin and Olorunfemi, 2011), hence, modified Schlumberger array can be said to be a good alternative to the convectional Schlumberger array especially in a space constraint area. REFERENCES Adeleke, B.O. and Goh Chen Leong. (1978): Certificate Physical and Human Geography West African Ed. Oxford University Press, Ibadan, Nigeria. Anjorin, M. P. and Olorunfemi, M. O. (2011): Pacific Journal of Science and Technology, Volume 12. Number 2. Frohlich, R.K. and O.K. Rosenbach. 1986. Geoelectrical DC Equipment GGA 30/31. Technical Bulletin No. 22, 23 pp. Bodenseewerk Geosystem. Owoyemi, F.B. 1996. A Geological-Geophysical Investigation of Rai-Induced Erosional Features in Akure Metropolis. Unpubl. M.Tech thesis, Federal University of Technology: Akure, Nigeria. 11-18. Rahaman, M.A., (1989): Review of the basement geology of Southwestern Nigeria In: Kogbe C.A. (ed Geology of Nigeria Rock View (Nig) Limited, Jos, Nigeria pp. 39 56. Telford, W.M., L.P. Geldart, R.E. Sheriff, and D.A. Keys. 1990. Applied Geophysics (Second Edition). Cambridge University Press: Cambridge, UK. 344-536.


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Vander Velper, B.P.A. (1988): Resist version 1.0, Msc. Research project, ITC, Delf Netherland APPENDIX

Fig. 1: Geologic Map of Some Parts of Akure (after Owoyemi, 1996) Showing the VES Stations.

L

2l

A M N B

(a)

2l

N

B

(b)

C 1

(Fixed)

L/2

P1 P2

C 2

(Mobile)

C 1

MA

C 2

P1 P2

Fig. 2: Typical Four Electrodes Array (a)

Schlumberger Array and (b) Modified Schlumberger

(a)


194

(b)

(c) Fig. 3: Typical Curve Types (a) A-Type (b) H-Type and (c) HA-Type

0

2

4

6

8

10

12

H A HACurve Types

Freq

uenc

y (U

nit)

Schlumberger Array

Modified SchlumbergerArray

Fig. 4: Histogram of the Curve Types for both Arrays

(a)

TopsoilWeathered layerBedrock

(b) Fig. 5: Geoelectric Section along the N - S Direction (a) Schlumberger Array and (b) Modified Schlumberger Array

(a)

(b) Fig. 6: Geoelectric Section along the E - W Direction (a) Schlumberger Array and (b) Modified Schlumberger Array


195

(a)

(b) Fig. 7: Geoelectric Section along the SW - NE Direction (a) Schlumberger Array and (b) Modified Schlumberger Array

0

50

100

150

200

250

300

350

1 2 3 4 5 6 7 8 9 10 11 12 13 14VES Stations

Res

istiv

ity (o

hm-m

)

Schlumberger

Modified Schlumberger

0

50

100

150

200

250

300

350

1 2 3 4 5 6 7 8 9 10 11 12 13 14VES Stations

Res

istiv

ity (o

hm-m

)

Schlumberger

Modified Schlumberger

0

0.5

1

1.5

2

2.5

3

3.5

4

1 2 3 4 5 6 7 8 9 10 11 12 13 14

VES Stations

Thic

knes

s (m

)Schlumberger

Modif ied Schlumberger (a) (

b)

Fig. 8: Histogram of the Topsoil (a) Resistivity and (b) Thickness

R2 = 0.9228

0

50

100

150

200

250

300

350

0 50 100 150 200 250

Topsoil Resistivity (ohm-m) Schlumberger Array

Tops

oil R

esis

tivity

(ohm

-m)

Mod

ified

Schl

umbe

rger

Arr

ay

(a)

R2 = 0.9522

0

1

2

3

4

0 0.5 1 1.5 2 2.5 3 3.5 4

Topsoil Thickness (m) Schlumberger Array

Tops

oil T

hick

ness

(m)

Mod

ified

Schl

umbe

rger

Arr

ay

Fig. 9: Crossplot of the Topsoil Parameters (a) Resistivity and (b) Thickness

TopsoilWeathered layerBedrock


196

0100200300400500600700800900

1000

1 2 3 4 5 6 7 8 9 10 11 12 13 14

VES Stations

Wea

ther

ed L

ayer

R

esis

tivity

(ohm

-m)

Schlumberger

Modif ied Schlumberger

0

5

10

15

20

25

30

1 2 3 4 5 6 7 8 9 10 11 12 13 14

VES Station

Thic

knes

s (m

)

Schlumberger

Modified Schlumberger Fig. 10: Histogram of the Weathered Layer (a)

Resistivity and (b) Thickness

R2 = 0.9938

0100200300400500600700800900

1000

0 200 400 600 800 1000

Weathered Layer Resistivity (ohm-m) Schlumberger Array

Wea

ther

ed L

ayer

Res

istiv

ity (o

hm-m

) M

odifie

d Sc

hlum

berg

er A

rray

(a)

R2 = 0.9764

0

5

10

15

20

25

30

0 5 10 15 20 25 30

Weathered Layer Thickness (m) Schlumberger Array

Wea

ther

ed L

ayer

Thi

ckne

ss (m

) M

odifie

d Sc

hlum

berg

er A

rray

(b) Fig. 11: Crossplot of the Weathered Layer Parameters

(a) Resistivity and (b) Thickness

0

5000

10000

15000

20000

25000

1 2 3 4 5 6 7 8 9 10 11 12 13 14

VES Stations

Res

istiv

ity (o

hm-m

)

Schlumberger

Modif ied Schlumberger

Fig. 12a: Histogram of the Bedrock Resistivity

R2 = 0.8769

0

5000

10000

15000

20000

25000

0 5000 10000 15000 20000

Bedrock Resistivity (ohm-m) Schlumberger Array

Bedr

ock

Res

istiv

ity (o

hm-m

) M

odifie

d Sc

hlum

berg

er A

rray

Fig. 12b: Crossplot of the Bedrock Resistivity

Table 1: VES Interpretation Results for Schlumberger VES Resistivity (m) Thickness (m) Curve type

1 2 3 4 H1 h2 h3 h4 1 68.5 118.1 1543.4 - 1.4 18.7 - - A 2 105.2 269.6 1423 - 2.3 9.6 - A 3 212.2 130 3217 0.7 3.6 - H 4 61 33 1958 0.8 1.5 - H 5 88 78 421 12370 1.3 2.4 4.5 HA 6 107 231 12374 3.3 4.3 - A 7 171 95 1217 0.7 12.3 - H 8 163 265 917 1.6 1.7 - A 9 83 431 1469 1.3 8,2 - A

10 51 887 9833 1.2 26.3 - A 11 33 90 3817 1.0 8.1 - A


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12 89 122 18351 1.5 5.7 - A 13 65 78 1337 1.4 10.8 - A 14 125 130 2567 2.1 14.6 - A Table 2: VES Interpretation Results for Half Schlumberger VES Resistivity (m) Thickness (m) Curve type

1 2 3 4 H1 h2 h3 h4 1 66.5 117.3 1575.3 - 1.3 19.1 - - A 2 98.8 224.3 5833.5 - 1.9 7.8 - A 3 302.0 128.8 3488 - 0.3 4.0 - H 4 70 34 2671 - 0.9 1.6 - H 5 89 84 367 12947 1.1 2.6 4.4 - HA 6 107 243 8584 3.3 4.4 - A 7 172 87 906 0.8 12.2 - H 8 163 264 916 1.6 1.7 - A 9 73 484 1343 1.3 7.8 - A

10 46 938 7339 1.2 22.3 - A 11 30 88 4027 0.9 8.0 - A 12 88 120 22602 1.6 5.6 - A 13 66 83 1337 1.2 11.8 - A 14 125 130 3157 2.5 14.5 - A

Comparison of Schlumberger and Modified Schlumberger Arrays Ves Interpretation Results

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