2. METODOLOGY

2.1 Soil Electrical Resistivity

2.1.1 Equipment Specification

Fig 1. Geopulse Tigre Resistivity Meter

2.3.1.1 Product

Geopulse Tigre Resistivity Meter

2.3.1.2 Company

Campus International Products Ltd.

2.3.1.3 Transmitter

Maximum power output : 18W

Current range : 0.5mA – 100mA

Square wave repetition : 2.8 – 8.4 sec

(commutated DC)

Number or reading averaged : 1 to 16

2.3.1.4 Receiver

Input voltage range : 0 – 180V

Input impedance : 22MΩ

Display : 80 characters alpha-

numeric liquid crystal

Power supply : Rechargeable sealed

lead acid 7 Amp/h @12v

2.3.1.5 General

Weight : 6 kg

Method : Wenner (ASTMG57-95a)

Computer : PC compatible

Operator : 4 person

2.1.2 Work Procedure

2.1.2.1 General

Practically, the electrodes (potential and current) are spread always in line, by using Wenner configuration. In this method, low frequency AC is injected and generated potentials in the earth’s surface are measured. Resistivity of the earth can be calculated then. By using inversion algorithm, the layering of sub surface can be determined based on resistivity structure as a function of depth.

2.1.2.2 Apparent resistivity

It is necessary to consider that measured resistivity on the surface is an apparent resistivity, based on hypothesis that medium is a single layer homogeneous and isotropic. This term can be arrange by following equation:

ρ=2 π ΔVI

1

( 1r1− 1r2 )−( 1r3− 1r4 )

=(2 π ΔVI ) p

Where the parameter p has to do with the electrode geometry. By measuring ΔV and I and knowing the electrode configuration, we obtain a resistivity ρ . Over homogeneous isotropic ground this resistivity will be constant for any current and electrode arrangement. That is, if the current is maintained constant and the electrodes are moved around, the

potential ΔV will adjust at each configuration to keep the ratio ( 2π ΔVI ) constant.

If the ground is inhomogeneous, however, and the electrode spacing is varied, then the ratio will change, in general. This results in a different value of p for each measurement. Obviously the magnitude intimately involved with the arrangement of electrodes.

C2C1 P2P1

a a a

This measured quantity is known as the apparent resistivity,ρa . Another term which is

frequently found in the literature is the so-called surface resistivity. Although it is diagnostic, to some extent, of the actual resistivity of a zone in the vicinity of the electrode array. This apparent resistivity is definitely not an average value. Only in the case of homogeneous ground is the apparent value equivalent to the actual resistivity. Obviously it is equal to the true surface resistivity only when the ground is uniform over a volume roughly of the dimensions of the electrode separation.

2.1.2.3 Electrode spreads

The most commonly used point-electrode systems are the Wenner and Schlumberger arrays. At this point is used Wenner spread which the electrodes are uniformly spaced in a line. From above equation, r1=r4=a and r2=r3=2a. Thus the apparent resistivity is:

ρa=2 πa ΔVI

For depth exploration using Wenner spread, the electrodes are expanded about a fixed centre, increasing the spacing a in steps. For lateral exploration or mapping, the spacing remains constant and all four electrodes are moved along the line, then along another line, and so on. In mapping, the apparent resistivity for each array position is plotted against the centre of the spread.

Fig. 1 Wenner Spread

3. RESULTS OF FIELD SURVEY AND LABORATORY TEST

3.1 Results of Soil Resistivity Measurements

3.1 Electric Soil Resistivity

Soil resistivity measurements have threefold purpose. First, such data is used to make sub-surface geophysical survey as an aid in identifying structure of the subsurface layer, depth to the bedrock, and other geological phenomena. Second, soil resistivity directly affects the design of a grounding system. Third, resistivity has a direct impact on the degree of corrosion in underground pipelines. A decrease in resistivity relates to an

increase in corrosion activity, and therefore dictates the corrosion-protective treatment should be used.

Soils constitute the most complex environment known to metallic corrosion. Corrosion of metals in soil can vary from fairly rapid dissolution to negligible effects. Moisture in soils will probably have the most profound affect when considering corrosivity than any other variable. No corrosion will occur in environments that are completely dry. Water is required in soils for ionization of the oxidation process and ionization of soil electrolytes. Flowing water is a more severe environment than stagnant water.

Most all soils are heterogeneous. This results in different environments interacting on different parts of the metal surface, and produces differences in electrical potential. Differences in oxygen, acidity, and salt content also give rise to corrosion potential.

Soil resistivity (conductivity) are extremely important as a indicator to corrosion rate of the medium. Lower resistivity (high conductivity) can generate high corrosion rates. Metals that are buried will generally be anodic in a low resistivity soil, and cathodic at an adjacent high resistivity soil. Soil heterogeneity in conjunction with specific resistivity, is the most important aspect of soil corrosion. The following table may serve as a simple guide in predicting the corrosivity of a soil with respect to resistivities alone:

Table 1. Soil Resistivity and Anticipated Corrosivity

Soil Resistivity (m) Anticipated Corrosivity

< 7

7 ~ 20

20 ~ 50

50 ~ 100

> 100

Very Severe

Severe

Average

Mild

Unlikely

Source: Korosi dan Penanggulangannya, Puslitbang Metalurgi-LIPI, 1987

Based on the table above, the anticipated corrosivity of soil at each measured point is:

Site : (…….)

Position: (coordinate )

Layer Resistivity (Ohm)

Thickness (m)

Depth (m)

Anticipated corosivity Descriptions

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APPENDIX

Result of Soil Electric Resistivity Measurements

D-1. Soil Resistivity Measurement Data Sheet

Point: (contoh data sheet dilapangan)

electrode spacing Electrode spacingCurrent 1st Record 2nd Record 3th Record

AB/2 AB MN A B M N I (mA)R

(ohm)e

(%)R

(ohm)e

(%)R

(ohm)e

(%)0.75 2 0.5 5.25 6.75 5.75 6.251.5 3 1 4.5 7.5 5.5 6.5

2.25 5 1.5 3.75 8.25 5.25 6.753 6 2 3 9 5 75 10 3.3 1.05 11 4.35 7.656 12 4 0 12 4 8

D-2. Soil Resistivity Measurement Data Interpretation

Point: (contoh hasil pengukuran)

0 .1 1 1 0

1

1 0

1 0 0

B H - 0 2

Appa

rent

Res

istiv

ity (

ohm

-m)

S p aci n g ( m )

B al a i T ek n o l o g i P erm u k i m an , I n d o n esi a

1 1 0 1 0 0

0 .1

1

1 0

Dep

th (

m)

R esi st i v i ty ( o h m - m )

D-3. Daily Soil Resistivity Measurement

Point Number: (contoh dokumentasi)

Date / Time: Weather: Fine

Soil type:

Supervisor :