Feb 2, 2001 - East-South HVDC Transmission Project in India ... south India, because the owner (Power Grid of India) modified the central location.
Report of Earth Physical Parameters Test on Electrode Site of East-South HVDC Transmission Project in India
Institute of Applied Geophysics, Central South University February 2 nd , 2001
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Earth Physical Parameters Test on Electrode Site of East-South HVDC Transmission Project in India
Survey Party: Institute of Applied Geophysics, CSU Charge of Project：Bai Yicheng Survey Member：Bai Yicheng Chen Rujun Yan Jiabin Author of Report：Bai Yicheng Chen Rujun Yan Jiabin
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Contents 1. 2. 3. 4. 5. 6. 7.
Preface………………………………………………………1 O u t l i n e o f t o p o g r a p h y, l a n d f o r m a n d g e o l o g y … … … … . . 3 Field working method and technology……………...3 Quality evaluation………………..………………………18 Data Processing and interpretation method……………. 20 Electrical stratifcation in electrode site………….…….26 The testing of thermal conductivity and capacity of soil…………………………………………………………..27 8. Investigation of soil temperature, underground water temperature, resistivity of rainwater and electroosmosis in elec trode s i t e . . … … … … … … … … … … … … … … … … … 2 9 9. C o n c l u s i o n … … … … … … … … … … … … … … … . … … … . . 3 1 Appendix A Data table of WENNER electrical sounding in each electrode site………………………..…………………..32 Appendix B Curves of Apparent resistivity and phase of MT measurement in each electrode site …………………………45 Appendix C Curves of average apparent resistivity data in each electrode site…………………………………..………60 Ap p e n d i x D D i a g r a ms o f e l e c t r i c a l l a ye r s … … … … … … … 6 6 Ap p e n d i x E Hi s t o g r a ms o f e l e c t r i c a l l a ye r s ..… … … … … … .6 9
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I. Preface In September 27 th , 2001, Design Research Institute of Central South University of China and Central South Electric Power Design Institute of China National Power Co. (CSEPDI) signed a contract as “Earth Physical Parameter Testing of Electrode Site for Project of East-South DC Transmission of India”. Based on the above contract, DRICSU carry out the measurement of electrical property and heat parameter of the Earth of the two electrode sites near the two alternative stations in east and south part of India for the project of East-South HVDC transmission of India. The working group of above project arrived at India in Nov 9 th , 2000, arrived at field in Dec 2, 2000, and began to carry out the work of field data acquisition. We finished the work of field data acquisition in Feb 5 th , 2001, and departed India in Feb 17 th , 2001. The total days we stayed in India are 81 days. Before the working group came back to China, three English working summaries of our work for the three sites were offered to the Siemens. In the period of carrying out the above project, based on the requirement of Siemens, we finished the testing work of three sites as site Talcher #2 and Talcher 7(Rohila) in east India, and Kola site in south of India. At this point, we finished more work than the requirement of contract that required the testing work of two sites. In the working period of Earth electrical properties testing in the Kolar site of south India, because the owner (Power Grid of India) modified the central location of electrode site for several times, the testing area changed for several time, and increased the working content. To the Wenner electrical sounding method, because the restriction of field situation, topography and the field of vision, the large electrode space sounding of a=1000m can not be carried out, We use the MT method as replacement after we got the agreement of first party (CSEPDI). Although the topography in the testing area has the character of small fluctuation, but the largest altitude difference is about 10m, it has no large adverse affect to resistivity sounding, and it has no requirement for topography correct. But the testing area is rich in shrub, and the field of vision is very limited, above reason leads to great trouble for working. We use portable GPS to locate each 4
sounding points, the measurement error is about 3 -5m. We found no industrial equipment in testing area, industrial interference is small, and it is suitable for MT investigation. The Wenner sounding apply alternative current resistivity method, and the instrument applied is dual-frequency digital IP in frequency domain, the working frequency is 4/13 0.308 Hz. The magnetotelluric (MT) sounding uses MT-1 magnetotelluric system, which was imported from EMI of California of USA in early 90s. We carried out quality check during the period of data acquisition, the data quality satisfy the requirement of standard. The testing result reflect strong regularity, which reflect the homogeneousness of underground layered rock, and each parameter in this report satisfy the requirement of design task, can be used in electrode site design during the period of electrode site selection. The reports of English and Chinese version are the same in law. During the working period of the project, we enjoy the great support and help from Siemens, Power Grid, Transmission department of CSEPDI, Investigation department of CSEPDI, and we’d like to thank them sincerely.
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II. Outline of topography, landform and geology 2.1 Outline of topography, landform and geology in Kolar electrode site The name of Kolar electrode site is Chikkadasarahalli. It locates in the southeast of Karnataka state, about 100 km in the east of Bangalore, which is the capital of Karnataka state. The topography of the site is high, beca use the site is in the west of Decan plateau. The most area of the site is farmland. There is man -made forest in east, forest in the south, and a hill in the west. There is a temple in the west of the above hill. North of site is flat with forest. The surface of electrode site is covered with clay. Based on the result of electrical sounding carried out this time, the thickness of clay range from 50 to 60 meters with resistivity as 40 to 50 .m. The rock under clay is Archean regolith and basement with resistivity as 1500 to 2000.m. 2.2 Outline of topography, landform and geology in Talcher electrode site The Talcher Converter Station locates in the center of Orrisa state, and is about 80 km in the northwest about 180 km of Bhubneshawar, which is the capital of Orissa state. The distance between Talcher #2 electrode site and Convert Station is about 22Km. Talcher #2 electrode site locates in the northern of a small hill. There is a small river in the east of electrode site and the distance is about 700 meters . There is a small village with the distance as several hundred meters. The northern part of electrode site is forest, and the southern part of electrode site is shrubbery. The surface is covered with clay, and there is rock found in some part of electrode site. The distance between Talcher #2 electrode site and Convert Station is about 40Km. There is shrubbery found in most part of the electrode site. The topography is fat, and the site area inclines from south to north slightly. The surface of the site is covered with clay
III. Field working method and technology We use Wenner electrical sounding method to investigate the electrical properties of shallow depth soil of electrode site, and MT sounding method to investigate the electrical properties of deep soil and rock of electrode site.
3.1 Field working method and technology of Wenner electrical 6
sounding 3.1.1 Introduction to the theory of Wenner electrical sounding Resistivity method is a branch method of electrical investigation method. Resistivity method uses conductivity difference of rock and ore in the crust as material base, to find mine or solve other geological problem though observe and study distribution of static or alternative electrical field established by manpower. Practice has proved that resistivity method has achieved excellent geological effect and played an important role in metal and nonmetal mine exploration, the study of geological structure, hydrological and engineering investigation, and energy exploration. Resistivity sounding method is one method of resistivity me thod. It enlarge the electrode space between the two sides of a same sounding point, and this enlarge the depth of investigation, thus get the apparent resistivity changing information of sounding point with vertical sounding depth changed from small to large. The sounding curve of a reflects the changing of vertical geological situation of a sounding point. The factors that cause the variation of a are the thickness of each electrical strata, the value of resistivity, number of electrical strata, and the electrode space. When investigation target is horizontal electrical strata or electrical strata with tilt angle less than 20 degree, we can interpret the true resistivity and thickness of each electrical stratum quantitative, based on sounding curve plotted by apparent resistivity via different electrode space.
X
A(I) H2
2
Hn-1 Hi
H1
h1 h2
... ...
1
hi
... ...
i
n
Z
Figure.1 horizontal strata We have assumption that surface is horizontal, there are n horizontal strata underground, and resistivity for each stratum is 1 , 2 , 3 , ……, n-1 , n respectively, and the thickness of each stratum is h 1 , h 2 , h 3 , ……, h n-1 , h n respectively, the distance between surface and the bottom of each stratum is H 1 , 7
H 2 , H 3 , ……, H n-1 , H n =. A point source supply current at point A, and the current intensity is I. (See Figure 1) Apply cylindrical coordinate, set the origin in point A, axis Z is vertical down. Because of solution of problem shows symmetry to axis Z, are independent of , the potential distribution satisfies the Laplace’s equation as follow:
2U 1 U 2U (3.1.1) 0 r 2 r r z 2 Solute the equation (3.1.1), we can find the potential expression of surface under the condition of horizontal strata applied by point electric source: I (3.1.2) T1 (m) J 0 (mr)dm 2 0 m is the variable of integration. J 0 mr is a specialized function known as Bessel U r ,0
function of order zero whose behavior is known completely. T1 (m) is known as kernel function and is controlled by the thickness and resistivities of the underlying layers. The kernel function can be built up relatively simply for any number of layers using recurrence relationships which progressively add the effects of successive layers in the sequence. T 1 (m) can be found by following recurrence expressions: Tn (m) n 1 e 2 mhi Ti 1 (m) 1 e 2 mhi Ti (m) i i i 1 e 2 mhi Ti 1 (m) 1 e 2 mhi
(3.1.3)
in the above expression, i=n-1, n-2, ……, 2, 1. To the Wenner electrode configuration, we can have U w = U a – U 2a from equation (3.1.2). Based on U w I We have the apparent resistivity of Wenner configuration as follow:
aw 2a
(3.1.4)
aw 2a T1 (m)J 0 (ma) J 0 (2ma)dm 0
8
(3.1.5)
AM
NB
a
A
a
a M
N
B
Figure.2 The Wenner configuration The Wenner electrical sounding electrode configuration is shown in figure 2. During the process of electrical sounding, it always keeps the relationship as a= MN=AB/3. Compared with Schlumberger electrode configuration, its potential electrode space is relative large, and the received signal intensity (potential difference) is large. The Wenner electrode configuration can inject small electric current and receive strong signal, and it can improve observation precision, and so make the current supply system portable to some degree. 3.1.2 Field working method and actual work completed We use dual-frequency digital IP (DFIP) instrument to carry out Wenner sounding work. Professor He Jisan of Central South University of Technology, to overcome the disadvantage direct current resistivity instrument that is weak to remove the effect of interference, develops the above instrument. The DFIP owns great anti-interference ability in the area affected by strong industrial interference, and its supply current is less than 1/20 of that for time domain IP instrument, as a result, it can use dry battery as power supply. It is not necessary to use generator as power supply. The name of instrument is SQ-1 portable dual-frequency IP instrument. Its working frequency is 4 Hz and 4/13 Hz. When it is used for alternative current resistivity method, only one frequency is needed, and the working frequency is 4/13 Hz for the soil testing of electrode site. According to the requirement of first party, the electrical sounding points of a=1100m are uniformly scattered in the area of electrode site. Other sounding points are uniformly scattered in the 1km 2 area with its center as the center of electrode site. Power Grid specifies the area of electrode site, and then we use Global Position System (GPS) measure the coordinate of four corner points that confine the area of electrode site. At last, we calculate central point of electrode site according to the coordinate of above 4 corner points. Based on the above results, 9
we plot a plane map that represents 1.2km 2 area with electrode site in center, and then we design the location of each sounding point on above map. We input the design coordinate of each sounding point into GPS, and then use GPS as navigator to find the position of sounding point in the field. After we arrive the design position of point, decide the actual position of each sounding point according to the actual observation requirement and field situation. The actual scatter maps of Wenner electrical sounding points of Kolar site, Talcher #2 site and Talcher #7 site are shown in figure 3, figure 4 and figure 5 respectively. The direction of electrode spread is decided b y field condition, and considering factors that benefit field-testing. Most of directions are north to south and east to west. During the period of data acquisition, the magnitude of injected current is decided by guaranteeing precision of measurement. 1. The Wenner sounding points completed in Kolar electrode site are shown as follow: a = 1100m, 12 sounding points, a = 1300m, 20 sounding points, a = 1500m, 5 sounding points, a = 1700m, 8 sounding points. The total sounding points are 45 in Kolar electrode site. 2. Wenner sounding points completed in Talcher #7 site: a = 1100m, 13 sounding points, a = 1300m, 17 sounding points, a = 1500m, 4 sounding points, a = 1700m, 5 sounding points. The total sounding points are 39 in Talcher #7 electrode s ite. 3. Wenner sounding points completed in Talcher #2 site: a = 1100m, 8 sounding points, a = 1300m, 5 sounding points, a = 1700m, 3 sounding points. The total sounding points are 16 in Talcher #2 site.
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Figure 3 Actual Positon Map of Electrode Site and Electrical Sounding Points in Chikkadasarahalli Site of Kolar Terminal 289800 L12-01
L12-17 L11-07
L11-03
289600 L10-02
L10-03
L09-03
289400
IV
N
L11-11
L10-07
L10-11
L10-13
L09-11
L09-07
I L08-02
289200
L08-03 L08-05 L08-07 L08-09 L08-11 TEST
L07-03 L07-05
L06-01 289000
L06-02
L08-13
L07-11
L06-03 L06-05 L06-07 L06-09 L06-11
L06-13
L06-17
L05-03 L05-05 L05-07L05-09
L05-11
Legend 1-100m 1-300m 1-500m 1-700m
L03-03
III
L03-05
288800 L02-01
288600 2513900
2514100
L03-09 L03-11
II
L02-07
2514300
2514500
I - Land planned by PG III- Land proposed by Siemens
2514700
L02-17
2514900
2515100
II - Land proposed by CSEPDI IV- Land proposed finally by PG
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Figure 4 The Actual Position Map of Electrode Site and Electrical Souning Points in #7 Site of Talcher Terminal P26-10 1167000
P26-14
P26-18
1166900 P24-11
P24-14
P24-17
1166800
P22-12
1166700
P22-13
P22-14
TC41 1166600
1166500 P18-10
P20-12
P18-11
P22-15
P22-16
TC31 P20-13
P18-13
P20-14
P18-14
P20-15
P20-16
P18-15
P18-17 P18-18
1166400 P16-12
P16-13
P14-12
P14-13
1166300
P16-14
P14-14 TC11
1166200 P12-11
P16-15
P14-15
P16-16
P14-16 TC21
P12-14
1166100
P12-18 Souning Point
P10-14 1166000 P10-10
a = 100m a = 300m a = 500m a = 700m
P10-18
1165900 3294500 3294600 3294700 3294800 3294900 3295000 3295100 3295200 3295300 3295400 3295500
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Figure 5 The Actual Position Map of Electrode Site and Electrical Souning Points in #2 site of Talcher Terminal PT11 1170300
Sounding Point
TB1
a = 100m a = 300m a = 700m
PT12 PT13
1170200
PT14 TB4 PT15
1170100
PT06 PT07 PT08
1170000
PT09 PT10
PT17 1169900
TB2 PT01
1169800
PT02
PT03 PT04
TB3 PT05
1169700
3284100 3284200 3284300 3284400 3284500 3284600 3284700
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3.2 Field working method and technology of MT electromagnetic sounding 3.2.1 Introduction to method and theory of MT sounding Magnetotelluric method was developed by Tikhonow (Soviet, 1950) and Cagniard (France, 1953) independently. MT method is an electromagnetic sounding method based on the principle of electromagnetic induction in the Earth. The field source is natural electromagnetic filed. When the MT sounding method is carried out, we record two horizontal electrical field components of orthogonal Ex and Ey, and three magnetic field components of orthogonal Hx, Hy, and Hz continually at the same at the sounding point. We find the impedance Z of the sounding point by processing the above logged time series. The impedance Z=E/H when the layers of Earth is horizontal and isotropic (1 dimens ion electrical earth model). The apparent resistivity is defined as
a 0.2T Z
2
(3.2.1)
From the expression (3.2.1), we can interpret the underground electric properties of the Earth. The depth that the field decades to 1/e of that on the surface is known as the skin depth, which is also known as the exploration depth:
H 503
f
（3.2.2）
where is resistivity, f is frequency in Hz, H is depth. According to the above equation, it could be realized that the exploration depth increases with the decrease of the frequency. Because it is based on the electromagnetic field, MT method can penetrate high resistivity layers. Also because of the plan wave properties of natural source, the horizontal effects of MT method are less. The natural EM field has wide frequency band from 0.0001 Hz to 10000 Hz. So it can be used to investigate underground electric property with depth from several hundred meters to several hundred kilometers. Because of the above advantage of MT method, MT method offers a effective and powerful way to oil exploration, the study of earth structure, and the study of electric properties of crust and up mantle. 3.2.2 Field working method and exact work completed According to requirement of first party and the theory of MT method, we have 6 sounding points in each electrode site (Talcher #2 has only 3 sounding point 14
because of the working arrangement of Siemens). Those 6 sounding points are scattered uniformly in site area of 1 square kilomet er. We use the following principle to decide the actual position of each sounding point: 1. Each sounding point should be scattered uniformly in site area of 1 square kilometer by all means, 2. Each sounding point should be far away area that interferes the MT measurement by all means, such as all kind of roads, 3. Sounding point should be located on area that is flat and wide. The deployment of sounding station follows the standard of magnetotelluric sounding strictly, and we try our best to reduce all kin ds of interference, which effects the measurement. The position of each sounding point is measured by GPS. L type configuration is used for station setup. Electrical field sensor (electrode) is Pb-PbCl electrode, which is buried underground at the depth as 30-50cm. The contact resistance is reduced by watering. The length of electrode space is about 100 meters. Magnetic field sensor is BF -4 sensor, which is specialized by American marine force. X direction points to North, and Y direction points to East. The orientation is measured by forest compass, and the error of above measurement is less than 0.5 degree. Because of the restriction of topography, five components method is used in the investigation. The five components are two electric channels (Ex, Ey) a nd three magnetic channels (Hx, Hy, Hz). The survey frequency ranges from 0.001 Hz to 320 Hz. The above frequency range is divided to 5 sample bands, the corresponding sample frequency are 1000 Hz, 100 Hz, 10 Hz, 1 Hz and 0.125 Hz respectively. We record time series in each sounding points up to 5-6 hours, and the number of frequency recorded is great than 40. The plane position of MT sounding points in Kolar site, Talcher #2 site and Talcher #7 site are shown is figure 6, figure 7 and figure 8 respectively.
3.3 Acquisition of soil sample 13 soil samples are collected in Kolar electrode site, their positions are shown in figure 9. 12 soil samples are collected in Talcher #7 electrode site, their positions are shown in figure 10.
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Figure 6 Actual Positon Map of Electrode Site and MT Sounding Points in Chikkadasarahalli Site of Kolar Terminal 289800 KMT-04
N KMT-03
289600 KMT-02
289400
IV
I
KMT-05 289200
KMT-01
289000
III
288800
KMT-06
288600 2513900
2514100
2514300
2514500
I - Land planned by PG III- Land proposed by Siemens
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II 2514700
2514900
2515100
II - Land proposed by CSEPDI IV- Land proposed finally by PG
Figure 7 The Actual Position Map of Electrode Site and MT Souning Points in #7 Site of Talcher Terminal 1167000
RMT-5 RMT-6
1166900
1166800
1166700 TC41
TC31
1166600
RMT-4
1166500
1166400 RMT-2 1166300 TC11
TC21
1166200
1166100 RMT-3 1166000
RMT-1
1165900 3294500 3294600 3294700 3294800 3294900 3295000 3295100 3295200 3295300 3295400 3295500
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Figure 8 The Actual Position Map of Electrode Site and MT Souning Points in #2 site of Talcher Terminal MT-3 1170300
TB1
1170200 TB4 1170100 MT-1 1170000
1169900
MT-2
TB2
1169800 TB3 1169700 3284100 3284200 3284300 3284400 3284500 3284600 3284700
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Figure 9 Actual Positon Map of Electrode Site and Soil Samples in Chikkadasarahalli Site of Kolar Terminal 289800 KS-05
N KS-03
289600
KS-04
IV
289400
I
KS-13 KS-01 KS-02
289200
KS-07 KS-12
KS-06
KS-10
KS-11
289000
III
KS-09
288800
II
KS-08
288600 2513900
2514100
2514300
2514500
I - Land planned by PG III- Land proposed by Siemens
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2514700
2514900
2515100
II - Land proposed by CSEPDI IV- Land proposed finally by PG
Figure 10 The Actual Position Map of Electrode Site and Soil Samples in #7 Site of Talcher Terminal 1167000 RS-03 1166900
1166800
1166700
RS-12 RS-09
RS-11 TC41
RS-08 TC31
RS-10 1166600 RS-04
RS-02 1166500
RS-07
1166400 RS-01 1166300
RS-05
RS-06
TC11
TC21
1166200
1166100
1166000
1165900 3294500 3294600 3294700 3294800 3294900 3295000 3295100 3295200 3295300 3295400 3295500
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IV. Quality evaluation 4.1 Methods to ensure the data quality of Wenner sounding 1. Ensure the potential difference measured by receiver great than 5mV. 2. Record the potential difference measured by receiver when t he relative error of two continuous measurements of potential difference is less than 0.5%. 3. Use excessive electrodes and water electrode to reduce the contact resistance. 4. When the electrode space increases to some degree, we apply methods as adding power supply voltage, using excessive electrodes, and watering electrodes to ensure precision. 5. Apparent resistivity corresponding to each current electrode space is real time calculated at the field. According to theory of electric sounding, we decide the current measurement of apparent resistivity correct or not based on the relation between different apparent resistivity measurements corresponding to different electrode space. And then remove error measurement caused by electrode space error and other unknown er ror. 6. We select wide-field direction as electrode spread direction based on topography and the surface of earth. During the period of moving electrodes, we use compass direct right way, and try our best to reduce and orientation error and electrode space error caused by moving electrodes. 7. Before the start of working for each time, we calibrate instrument completely, to ensure the instrument in very good condition.
4.2 Result of quality check for Wenner electric sounding The precision of apparent resistivity is estimated by mean square relative error. The expression is follow: 2
2( xi xi' ) ' i 1 xi xi 100% 2m m
(4.2.1)
Where xi is original measurement, xi' is measurement for check, m is the n umber of total points checked. According to the requirement of criterion, the absolute value of should be less than 5%, The results of data quality check for each site are shown in table 1. Table 1: The results of quality check for Wenner electric sounding
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Site name
Original sounding points
Sounding points checked
Percent of check (%)
Check precision (%)
Criterion requirement (%)
Kolar site 45 4 8.9 3.4 5 Talcher #2 16 2 12.5 4.1 5 Site Talcher #7 39 4 10.3 3.7 5 site We can find from above table that the data quality reach re quirement。
4.3 Methods to ensure the data quality of MT sounding 1. Monitor waveform of time domain signal of MT signal at the field, to distinguish signal from noise. 2. A saturation switch was set in sampling duration. When a signal is exceed 10V, the control computer will discard the samples automatically and redo the sampling processing. 3. We apply data quality control during the process of indoor MT data processing, and reject data with error great than 5%. 4. In order to avoid the interference of wind, the magnetic senso rs were buried in depth of 30 to 40cm, vertical magnetic sensor is buried great than 2/3 part of sensor, and the part above surface is buried by soil tightly. All cable is hold down tightly by clay or stone. 5. The non-polarization electrodes were buried in depth of 20 to 30cm. Water was used to reduce the contact resistance.
4.4 Result of quality check for MT sounding Two sounding points are checked, about 13.3% percent of total MT sounding points. It is great than the requirement of oil and natural gas indu stry that require 3% percent of total sounding points should be checked. The data quality is measured by mean square relative error, and the expression as:
2( Ai Ai' ) ' i 1 Ai Ai n
2n
2
100%
(4.4.1)
In formula (4.4.1), i is frequency number (i = 1, 2, ……, n) , A i is apparent resistivity or phase value of original point, A ’ i is apparent resistivity of phase value of check point. MT check point is KMT-1 and RMT-5 in Kolar site and Talcher site respectively. The result of check is shown is table 2.
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Table 2: Result of data check for MT sounding Sounding Mean square relative error (%) point xy xy yx yx KMT-5 4.43 4.21 4.90 1.46 RMT-1 4.86 3.54 4.52 2.02 From the above result, error for each item is less than 5%, and data quality reaches the requirement.
V. Data processing and interpretation method 5.1 Data processing and interpretation method for Wenner sounding 5.1.1 Main process of data processing and interpretation 1. Carry out system analysis to electric sounding curves of each electrode sit e, and find the shape of electric sounding curves of each site are basically consistent with H type. The above situation indicates that the electric layers in each site are one dimension structure. 2. We calculate geometric average of electric sounding appare nt resistivity corresponding to same electrode space of sounding points in scope of each electrode site. 3. Based on electric reflect coefficient method and actual geological situation of each site, the number of electric layers at the range of exploration de pth in each site is found out, and initial parameters of electric layers are calculated. 4. Use damping least square method to carry out inversion, to produce final parameter of each electric layer. 5.1.2 Introduction to electric reflect coefficient method The interpretation of electric sounding data is a import part of electric sounding work, and the quality of interpretation has direct affection to the achievement of electric sounding method. From 30s to 70s of 20 th century, the quantitative interpretation method of electric sounding is mainly the method of master curves. The widely used master curves are master curves developed mainly by Schlumberger Co. and “ Schlumberger sounding master curves for two and three electric layers” developed by fourth railway engineering department of ministry of communications of China. Above master curves played important way in promoting the application of electric sounding method. But the interpretation using master curve method is complicated, and the precision of interpr etation changed with different person and often out of control, and the error of above interpretation 23
is big. The interpretation efficiency is also low. The digital interpretation method, which is developed since 70s of 20th century, apply computer software to process and interpret electric sounding data. Compared with master curve method, its error caused by human being is small, and its efficiency is high. The digital interpretation method improves the interpretation of electric sounding data, and enlarges the application area of electric sounding method. But digital interpretation method has some disadvantages also. The apparent resistivity curve of electric layers which with layer’s number great than three exists the phenomena of equivalence. This makes object function exist many extremes. On the other hand, if we don’t know the resistivity of middle layer, there exist many solutions to a given electric sounding curve. The more the number of layers, the phenomena of equivalence is more complicated. Mr. Sun Jingrong of China first develops electric reflect coefficient method in 80s. The above method interpret the electric sounding curve of apparent resistivity based on apparent electric reflect coefficient, apparent true resistivity and the gradient of apparent reflect coefficient. The above method has the characters of high resolution, finding depth of middle layer without the information of resistivity of middle layer, the depth range of interpretation almost as AB/2, fast and simple operation. Electric reflect coefficient method has been applied in China widely. Principle of electric reflect coefficient method (1) Reflect coefficient of medium Electric current will reflect and transmit when electric current encounters interface of resistivity in direct current field. The magnitudes of reflect current and transmission current is determined by resistivity of medium in both sides of interface. The capacity of interface that reflect current is called reflect coefficient K, and the ability of interface that transmits current is called transmission coefficient D. The magnitude of above two parameters is determined by following expressions: 1 12 1 (5.2.1) K12 2 2 1 12 1
D12
2 1 2 1 K12 2 1 12 1
(5.2.2)
where μ 12 = ρ 2 / ρ 1 . It is clear that K 12 =0 when ρ 1 = ρ 2 , and it indicates the medium is uniform and no interface exist, and no reflect exist; When ρ 2 ＞ ρ 1 , K 12 ＞0，the resistivity of medium in no-power source side is greater, the ability to reflect current is higher. When ρ 2 ＜ ρ 1 , K 12 ＜0，the resistivity of medium in no-power source side is smaller, the ability to transmit current is higher. Reflect coefficient and transmission coefficient are two correlated physical conception. It 24
is two different expressions to same problem in essential, and one can be converted to another. Now we use reflect coefficient to discuss problem. (2) Apparent reflect coefficient In the exploration of resistivity method, the conception of apparent resistivity is put forward based on resistivity of medium. The conception of apparent reflect resistivity is put forward based on reflect coefficient of medium in similar way. Because first order derivative of the logarithm of apparent resistivity ratio ( ρ S / ρ 1 ) divided by the logarithm of electrode space ratio (r/h 1 ) is corresponding to reflect coefficient of the interface of medium, so the apparent reflect coefficient K S is given as d lg S 1 (5.2.3) KS d lg r h1 or
KS
r h1
S 1
d S 1 d r h1
(5.2.4)
In above two expressions, the former is logarithm expression, and the last is arithmetic expression, both are same. In practical application, the above two can be converted to: lg S n / S n 1 (5.2.5) K S n lgr n / r n 1 or K S n
r n r n 1 S n S n 1 r n r n 1 S n s n 1
(5.2.6)
in which electrode distance related K S is geometric average of original electrode distance, and given as
rK n r n r n 1 Obviously, apparent reflect coefficient K S comes from apparent resistivity curve ρ S , it is slope of double logarithm of apparent resistivity curve ρ S . At the condition of uniform medium, K =0, and K S =0, is the true reflect coefficient of uniform medium. There two cases to the value of K when resistivity interface exists in medium. One case is 0K 1, and 0
