ABSTRACT
Currently, the geoelectric survey is developed
so rapidly. It is caused development of electronic and computer so fast with
high precision and accurate performance.
The development of resistivity meter for multy channels has been created for
resistivity 2-D. Those equipments are so expensive, however handy to use in the
field. The explanation and using the one channel resistivity meter could be use
for the resistivity 1-D and 2-D. The sample will be taken with for the Wenner
electrode arrangement only and it will be applied to other array.
1.
INTRODUCTION
The meaning of resistivity one channel is the
equipment has been existed before appeared the resistivity 2-D. Those equipments
are OYO OHM Type 2115, Yokogawa Resistivity meter type 3244, ABEM SAS 300, and
NANIURA NRD 300 HF. The equipment for survey geoelictlric 2 D (2 Dimensi) had been development such us ABEM SAS 4000, GEORES,
OYO PROFILER 4 in 10 years ago.
The resistivity meter could be used for
resistivity 1-D and 2-D. The sample of the survey will be applied to Wenner
Electrode Arrangement. The procedure for
exc cution is explain in the following items.
2. THE BASIC THEORY OF
ELECTRICITY
The principle of resistivity survey is to inject
electric current trough 2 ( two ) electrode current ( ∆ I ) , so there is
influence the differences of a pair inner potensial electrode ( ∆ V). If we knows the diferences current and potensial
, so we can get the resistance ( R ) from OHM LAW :
R = (∆ V)/(∆ I) in ohm. ( 1 )
If the electric current trought
the homogeneus sylinder of bar , so the value R depend on the lenght of bar ( L
) and the sylinder area ( A ).
R = L / A (ohm-m) (
2 )
The equation above has the fixed
value in unit of ohm-m. To know the
resistivity of material the equation became :
r = AV/ L I ( ohm-m) or r = K R ( 3 )
where K= A./L is the geometric factor, which is depend on the
position of the current electrode and potential elctrode. The geometric factor are different from the
each of electrodes arrangement as shown in Figure 1.
Figure 1. The
geometric factor from various electrode arrangements.
The geometric factor for
electrode arrangement Of WENNER is K = 2pa
and r = 2pa R in unit ohm-m, where a or L
is the distance of electrode WENNER.
In electrode arrangement of
SCHLUMBERGER m the geometric factor as follows :
K = 2 p/(1/AM – 1/AN) – (1/BM –
1/BN) or
K = p{(AB)2 – (MN)2 } , where AB=current electrode and MN=electrode pot.
4 MN
ρ = KR = p{(AB)2 – (MN)2 }
R (
4 )
4 MN
Figure
2. The electrode arranggement of SCHLUMBERGER
In the SCHLUMBERGER methode the
electrode potensial is fixed and will be change at certained distances. The
maximum distance of AB/2 is not more than
5 x MN/2.
3.
RESISTIVITY 1 D FOR WENNER ELECTRODE ARRANGEMENT
The first
attempt to measure electrical resistivity of soils was made at the end
nineteenth century with the two-electrode technique. Whitney et.al.(1887),
Gardner (1898), and Briggs (1899 ) developed relationships between soil method
measures the sum of both electrical resistivity and soil water content,
temperature, and salt content, The two-electrode method measures the sum of both soil
resistivity and the contact resistivity between the electrode and soil. The
latter is very eratic and unpredicted.
Wenner (1915 ) based on the work of Schlumberger
suggested that a linear array of four equally spaced electrode would minimize
are separated contact problems if potential measuring and current – induced electrodes
are separated in space. Since then all electrical resistivity methods applied
in geophysics and soil are based on the standards four electrodes principles.
The Wenner Electrode Arrangement devided into Wenner α , Wenner β , and Wenner γ shown on Figure 1. The
procedure for theWenner Electrode Arrangement is as follows :
1. Determine the point O on the
ground surface.
2. The four electrode arrangement
set on the ground and measure the deferent of current ( ∆ I ) and different of
potential ( ∆ V ). The data is write on
the field data sheet
3. The first Wenner Electrode Arrangement set on C1 P1
= P1 P2 =C2 P2 = a = 1.0
meter.
4. The first Wenner Electrode Arrangement set on C1 P1
= P1 P2 =C2 P2 = a = 1.5
meter.
5. The first Wenner Electrode Arrangement set on C1 P1
= P1 P2 =C2 P2 = a = 2 meter
6. The first Wenner Electrode Arrangement set on C1 P1
= P1 P2 =C2 P2 = a = 3 meter.
7. The first Wenner Electrode Arrangement set on C1 P1
= P1 P2 =C2 P2 = a = 5 meter
8. The first Wenner Electrode Arrangement set on C1 P1
= P1 P2 =C2 P2 = a = 7 meter.
9. The first Wenner Electrode Arrangement set on C1 P1
= P1 P2 =C2 P2 = a = 10 meter.
10. The first Wenner Electrode Arrangement set on C1 P1
= P1 P2 =C2 P2 = a = 12 meter.
11. The first Wenner Electrode Arrangement set on C1 P1
= P1 P2 =C2 P2 = a = 15 meter.
12. The first Wenner Electrode Arrangement set on C1 P1
= P1 P2 =C2 P2 = a = 20 meter.
13. The first Wenner Electrode Arrangement set on C1 P1
= P1 P2 =C2 P2 = a = 25 meter.
14. The first Wenner Electrode Arrangement set on C1 P1
= P1 P2 =C2 P2 = a = 30 meter.
15. The first Wenner Electrode Arrangement set on C1 P1
= P1 P2 =C2 P2 = a = 40 meter.
16. The first Wenner Electrode Arrangement set on C1 P1
= P1 P2 =C2 P2 = a = 50 meter
17. The first Wenner Electrode Arrangement set on C1 P1
= P1 P2 =C2 P2 = a = 60 meter
18. The first Wenner Electrode Arrangement set on C1 P1
= P1 P2 =C2 P2 = a = 75 meter
19. The first Wenner Electrode Arrangement set on C1 P1
= P1 P2 =C2 P2 = a = 100
meter
20. In the field plotted data on
double log paper.
21. The resistivity curve could be
intrepreted using RES 1D, VESPC, RESINT 53, IP2Win and GRIVEL.
Figure 3. Three Types Wenner Electrod Arrangement
Figure 4. The equipotential lines
, current line and datum points on the four electrod array ( Wenner and
Schlumberger ).
Figure 6. Interpretation
resistivity curve with software IP2Win
Figure 7. The Geological Profile
based on Resistivity Values
3.
RESISTIVITY 2 D FOR WENNER ELECTRODE ARRANGEMENT
An example of the electrodes arrangement and
measurement sequence that can be used for a 2-D electrical imaging survey shown
in Figure 6 . The procedures to get data resistivity 2-D with Wenner α
electrode arrangement with 20 electrodes and electrode spacing a with level 6.
Firstly, mark the peg on the ground with numbering from 1 to 20.
The first step is to make all the possible
measurement with the Wenner array with electrode spacing “1a”. For the first measurement electrodes number
1,2,3. and 4 are used. Notice the electrode 1 is used as the first current electrode
C1 , electrode 2 as the first potential electrode P1 ,
electrode 3 as the second potential electrode P2 and electrode 4 as
the second current electrode C2. for the second measurement,
electrode number 2, 3, 4 and 5 are used for C1 , P1 , P2
, C2 respectively. This
repeated down the line of electrodes until electrodes 17, 18, 19 and 20 are
used for the last measurement with “1a” spacing. For a system with 20
electrodes , note that there are 17 ( 20-3) possible measurements with “1a”
spacing for the Wenner array.
After completing the sequence of measurements
with “1a” spacing, the next sequence of measurement with “2a” electrode spacing
is made. First electrodes 1.3.5 and 7 are used for the first measurement. The
electrodes are chosen so that the spacing between adjacent electrodes is “2a”.
For a system with 20 electrodes , note that there are 14 ( 20-2x3) possible
measurement with “2a” spacing.
The same process is repeated for measurements
with “3a”, “4a”, “5a” and “6a” spacings. To get the best results with the
equipment one channel, the measurement in the field survey should be
measurements one step by one step and record data on the field sheet. The field
sheet is shown on Appendix 2 and the program resistivity 2-D as shown on
Fig .The sample of Wenner Electrode
Arrangement is shown on the RES2DINVx32
program on the folder of LANDFILL.DAT. The LANDFILL.DAT is an example
field data for the Wenner array obtained by survey with 50 electrode.
Figure 8. Multy – Channel Digital Resistivity Meter
Figure 9. Program RES2DINV for LANDFILL SURVEY
Figure 10. Sequence of measurement to build up
a pseudosection in resistivity 2-D
3.
CONCLUSSION AND RECOMMENDATION
1.
The
resistivity meter one channel can be used for measuring survey on resistivity
1-D and resistivity 2-D.
2.
The
measurement of resistivity 1-D is very fast and for 1 (one) day works could be
finish 5 or 6 sounding points.
3.
The
measurement of resistivity 2-D is take time then the equipment 1 (one )
channels. It is caused to many datum points ( 334 datums ) and required 1 (one ) or 2 (two days ) measure
REFERENCES :
1. Geoelectrical Imaging 2-D 3-D GEOTOMO SOFTWARE,
Agust 2004, RES2DINV ver.3.54 for Windows 98/Me/2000/NT/XP. Rapid 2-D Resistivity
& IP inversion using the least – squares method.
2. Loke,
M.H. Dr. Electrical imaging surveys for environment and engineering studies. A
practical guide 2 D and 3-D surveys. Copyright (1997,1999,2000) email:mhloke@pc.jaring.my.
3. University of Moscow. 2000 IP2Win. A program
for resistivity interpretation.
4. Wenner F. 1955. A method of measuring earth
resistivity. US Dept. Com.Bur. Standard Sci.Paper 258.
5. Briggs,L.J. 1899. Electrical instrument for
determining the moisture , temperature, and soluble salt content of soils. U.S.
Dept, Agr. Bul,15,
6. Gadners, F.D., 1898 The electrical metod of
moisture determination in soils: results and modifications in 1897, U.S. Dept.
Agr. Bul,12,
7. Whitney, M,F.F.D. Garned. And L,J. Briggs,
1897., An Electrical Methodof determining the moisture content of arable
soil.U.S. Depth. Agr.Bul.6.
Appendix 1
Single Channel Resistivity Meter
Naniura NRD 300 HF
Appendix 2
Field Data Sheet For Resistivity 2D
