igu dam earthmat design revised

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moderately high lightning prone area (figure 1). The earthing system, fundamentally

provides the lowest resistance path to a faulty current flow and protect humans as well as

equipment from lightning, short circuit etc. The earthing system comprise of an earthing

mat i.e., mesh of current conducting MS stripe, buried horizontally at a depth of about

half-a meter below the surface and ground rods at suitable points. All non-current

carrying parts contribute little towards lowering the ground resistance.

Figure1. World lightning map. Average yearly counts of lightning flashes per square

kilometer based on data collected by NASA satellites between 1995 and 2002

(http://geology.com/articles/lightning-map).

Several variable factors are involved in the design of earthing mat conductor. The

earthing mat has to be designed for the site conditions to have low overall impedance and

a current carrying capacity consistent with the fault current magnitude. The following

parameters are most controlling factors for the design of earthing mat (Mousa, 1998;

Zipse, 1994; Sen, 2001 and Choudhary, 2008):

• Magnitude of fault current

• Duration of fault

• Soil resistivity

• Resistivity of surface material

• Shock duration

2

Projectsite



• Material of earthing mat conductor

• Earthing mat geometry

However, in the present study Geophysical Resistivity Imaging survey has been carried

out for estimation of earth resistivity of surface/ sub-surface earthmaterial/ rock whichhave a great role for proper design of earthmat. The Earth resistivity measurements of

Power house site and HRT Intake-I & II foundations for earthmat designing, have been

conducted by Kapil and Ramanaiah (2004) and Pal (2009) respectively. Present scope of

study is to carryout Resistivity Imaging survey at Dam foundation area using state-of-the-

art instrument, Terrameter SAS 4000 and ES464, Resistivity Imaging System. Figure-2

shows location of Resistivity Imaging survey for earthmat design of Dam Foundation,

Subansiri lower HE Project.

2. Field setup

Conducting Resistivity Imaging survey over a rugged terrain with completely rock

exposed, is a challenging task and therefore, need special arrangements at the site. As per

protocol of Resistivity Imaging survey, forty one electrodes are to be properly inserted in

the ground along a line at a constant interval, for transmission of sufficient current during

data acquisition. Inserting forty one electrodes in the exposed rock is very difficult task.

Further, if it is inserted in the rock, then also transmission of current is impossible. Toovercome these problems, forty one holes of 1ft depth have been drilled using Jack

hammer (45mm diameter) at five meter interval along the straight line at El 90m, which

runs along block joint of S-4 and S-5 of the Dam foundation. The line has been shown in

Figure-2. A mixture of clay-mud and salt in 1:5 ratio has been prepared. All the drilled

holes are filled up with the clay-mud and salt mixture. Then steel electrodes are inserted

in these holes. Two numbers 21-takeout 5meter spacing Lund Imaging cable have been

spread in continuous order. The 1st Lund Imaging cable has been spread starting from 1st

electrode at upstream and ending at 20th electrode. The 2nd cable has been spread from

20th electrode and ending at 41st electrode in the down stream. The electrodes have been

connected to the corresponding takeouts of Lund Cables through connecting clips. Last

takeout of the 1st Lund cable and 1st takeout of the 2nd Lund cable have been connected to

the electrode selector, ES464 and further, connected with the main resistivity meter,

Terrameter SAS4000. Figure-3 shows the field set up for Resistivity Imaging survey.

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3. Instrument and software used

A state-of-art microprocessor based resistivity meter, Signal Averaging System

Terrameter, SAS 4000 and Electrode Selector ES464 manufactured by ABEM, Sweeden

has been used for conducting the resistivity imaging survey. It has three main units viz.,

Transmitter, Receiver and Microprocessor. The resistance of layered earth is

automatically calculated using inbuilt microprocessor and displayed and also written in

digital form. The Signal Averaging System takes a numbers of consecutive readings

automatically and displays the averages of the consecutive readings. Hence, the obtained

apparent resistivities are highly authentic (Kapil, et, al. 2006).

RES2DINV 3.58 software has been used for construction of 2D cross-section of

resistivity distribution. Figure-4 is a schematic diagram, representing an example of

the electrodes arrangement and measurement sequence which is used for a 2D resistivity

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imaging survey. The 2D model used by the inversion program in RES2DINV, which

consists of a number of rectangular blocks (Griffiths and Barker 1993), as shown in

Figure-5. The arrangement of the blocks is loosely tied to the distribution of the data

points in the pseudo section. The distribution and size of the blocks is automatically

generated by the program using the distribution of the data points as a rough guide. The

depth of the bottom row of blocks is to be set approximately equal to the equivalent depth

of investigation (Edwards 1977) of the data points with the largest electrode spacing. The

survey is usually carried out with a system where the electrodes are arranged along a line

with a constant spacing between adjacent electrodes. A forward modelling subroutine is

used to calculate the apparent resistivity values, and a non-linear least-squares

optimization technique is used for the inversion routine (deGroot-Hedlin and Constable

1990, Loke and Barker 1996). The program supports both the finite-difference and finite-

elementforward modelling techniques. This program is useful for surveys using the

Wenner, pole-pole, dipole-dipole, pole-dipole, Wenner-Schlumberger and equatorial

dipole-dipole (rectangular) arrays (Loke, 2001).

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Figure 2. Field set up for Resistivity Imaging survey and data acquisition at Dam

foundation.

Field setup of

SAS4000 and

ES464

Electrode connected

to the Lund cable

through connector

DT InletU/S Coffer Dam

Dam Excavation and

concreting on progress



Figure 4. Schematic diagram representing resistivity imaging data acquisition procedures

using Terrameter SAS-4000 and ES464, a computer controlled multi-electrodesurvey setup.

Figure 5. Arrangement of the blocks used in a model together with the apparentresistivity datum points for generation 2D cross section

4. Geology of the area

6

ES464Terrameter

SAS-4000



The rocks exposed in the Subansiri Lower HE Project are sandstone of Middle Siwalik

formation. These are soft/ weakly cemented rocks with variation in grain size. Generally,

sandstones are grey in colour, medium to fine grained and have characteristic of salt and

pepper texture. At dam site rocks are steeply dipping with southern and southeasterly

dips. The rock masses are mostly massive to moderately jointed in nature. Thesesandstones occasionally contain quartzite pebbles, of older geological formation,

deposited with sandy sediments.

5. Data Processing

The measured resistivities are stored in a raw data binary format with the file extension

“.S4k”, in the ABEM Terrameter SAS 4000 during data acquisition. At the end of the

survey, SAS 4000 has been connected to the PC through RS232 data transfer cable. Theraw data collected at site has transferred to the PC using Import tool of SAS4000 Utility

software (the baud rates of both the Terrameter and PC have been kept same for data

transfer). The raw data then converted to “.dat” format using Data conversion tool of the

Utility software for further processing in RES2DINV 3.58. The converted “.dat” file can

be opened in a notepad to visualize the nature (high/ low/ moderate resistivity) of the

recorded data and could also be edited. In the notepad, the converted data file has been

arranged so that the topography data are placed immediately after apparent resistivity

data points with starting of flag 2 followed by the number of topographical data points.

At the end of the topography data points, four Zeros have been placed to indicate the end

of the file. Noise could be removed using “Exterminate bad datum points” tool of

RES2DINV. The apparent resistivity data values are displayed in the form of profiles for

each data level. The bad data points usually have comparatively too large or too small

apparent resistivity, which could obviously be identified and also could be removed

through selection. Optimized damping factor has been utilized for data inversion. The

results of inversion process have been displayed through Display tool of RES2DINV. In

the Display section original data has been examined and edited/ filtered out the bad data

points on the basis of distribution of the percentage difference between logarithms of the

observed and calculated apparent resistivity values. Further, the inversion of the filtered

data has been carried out using least-squares-inversion method through finite-element

forward modeling technique. At the end of the inversion process, three numbers 2D

model cross sections viz., Measured Apparent Resistivity Pseudosection, Calculated

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Apparent Resistivity Pseudosection and Inverse Model Resistivity section are

constructed. Finally, 2D cross section of resistivity distribution has obtained by including

topography in the model resistivity section using logarithmic contour intervals.

6. Results

2D model of resistivity distribution along the profile at Dam foundation as shown in

figure-2, has been estimated using Wenner Profiling technique and presented in figure-6.

Vertical exaggeration of this cross section is 3. The profile length is 200m with 5m

electrode spacing. The resistivity distribution in this section represents about 8m in depth

from the excavated surface. The lowest resistivity observed in this section is 40 Ohm-m,

and maximum resistivity found is 200 ohm-m with an average of 78 ohm-m.


figure-2, has been estimated using Wenner Sounding technique and presented in figure-7.


electrode spacing. The resistivity distribution in this section represents about 30m in

depth from the excavated surface. The resistivity distribution in this section varies from

minimum 25 Ohm-m to maximum 250 ohm-m with an average of 76 ohm-m.

Figure 6. 2D model of resistivity distribution of Dam foundation area using Wenner Profiling technique. The profile length is 200m with 5m electrode spacing. The resistivity

distribution in this section represents about 8m in depth from the excavated surface

(minimum 40 Ohm-m, average 78 ohm-m and maximum 200 ohm-m).

8



Figure 7. 2D model of resistivity distribution of Dam foundation area using Wenner

Sounding technique. The profile length is 200m with 5m electrode spacing. The

resistivity distribution in this section represents about 30m in depth from the excavated

surface (minimum 25 Ohm-m, average 76 ohm-m and maximum 250 ohm-m).


figure-2, has been generated using Dipole Dipole technique and presented in figure-8.


electrode spacing. The resistivity distribution in this section represents about 35 m in

depth from the excavated surface. The resistivity distribution in this section varies from

minimum 15 Ohm-m to maximum 400 ohm-m with an average of 76 ohm-m.

Figure8. 2D model of resistivity distribution of Dam foundation area using Dipole

Dipole technique. The profile length is 200m with 5m electrode spacing. The resistivitydistribution in this section represents about 35 m in depth from the excavated surface

(minimum 15 Ohm-m, average 76 ohm-m and maximum 400 ohm-m).


figure-2, has been generated using Schlumberger technique and presented in figure-9.

9



The profile length is 200m with 5m electrode spacing. The resistivity distribution in this

section represents about 40m in depth from the excavated surface. The lowest resistivity

observed in this section is 15 Ohm-m, and maximum resistivity found is 350 ohm-m with

an average 75 ohm-m.

7. Discussion

The results obtained from different 2D resistivity sections viz. figure-6 to figure-9 of

Dam foundation area reveal that the estimated minimum, maximum and average

resistivities have been observed to be varied with different resistivity measurement

protocols. The different resistivity imaging techniques utilize different geometric

configurations (flow path) for current transmission/ current flow. Due to different flow

paths (depth of current penetration) utilized in different techniques (Wenner profiling,Wenner sounding, Dipole-

Figure9. 2D model of resistivity distribution of Dam foundation area usingSchlumberger technique. The profile length is 200m with 5m electrode spacing. The

resistivity distribution in this section represents about 40m in depth from the excavated

surface (minimum 15 Ohm-m, average 75 ohm-m and maximum 350 ohm-m).

Dipole and Schlumberger) at a particular profile, the minimum, maximum and average

resistivity values have been varied in different sections. Different high and low resistive

10

Location

ongoing Drill

hole



pockets of dry rock/ moist weathered zones have been encountered during the survey

through different resistivity measurement techniques and consequently values of

resistivity lows and highs have also been changed. The real earth is not homogeneous, it

is typically heterogeneous in nature. The estimation of 2D true resistivity section is done

through forward modeling and least-squares optimized inversion technique consideringhemispherical layered-earth model. A comparatively low resistive vertical zone has been

found near RD-80 of the 2D resistivity sections (figure-6 to figure-9), and is inferred to

be due to mud accumulation consequent to drilling activity. The drilled hole and

surroundings is saturated with this mud-water.

Present study reveals that average resistivity value at Dam foundation varies in a range of

75 ohm-m to 78 ohm-m as obtained using different resistivity measurement protocols.

The average resistivity value at Intake-I foundation area varies in a range of 80 ohm-m to

85 ohm-m while, the average resistivity value at Intake-II foundation area varies in a

range of 74 ohm-m to 76 ohm-m as obtained using different protocols (Pal, 2009).

Similarly, the resistivity value varies in a range of 40 ohm-m to 80 ohm-m at Powerhouse

area (Kapil and Ramanaiah, 2004). These comparative studies indicates that the

resistivity value of sandstone of Middle Siwalik formation around the project site is about

80 ohm-m.

.8 Summary

The present study reveals that the technique applied for conducting Resistivity Imaging

Surevey in the rugged terrain is very efficient and could be effectively utilized to provide

Earth resistivity for proper design of Earthmat. The average resistivity value at Dam

foundation varies in the range 75 ohm-m to 78 ohm-m as obtained using different

resistivity measurement protocols viz., Wenner, Dipole Dipole, Schlumberger etc. of

Resistivity Imaging surveys. As such, resistivity approximation of 78 ohm-m can be

utilized for suitable design of Earthmat and calculation of Touch and Step Potential

voltages. On the basis of observed resistivity distribution, the mesh of earthmat

conductors could be augmented in the high resistive areas and accordingly, earthmat

could be reduced in the comparatively low resistive areas.

Acknowledgement

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The authors are thankful to Dr. Gopal Dhawan, Executive Director, Engineering Geology

and Geotechnical Division, Faridabad; Sh.Om Prakash, General Manager(SLP);Sh.N.K.Mathur, Chief(Geology); Sh.S.L.Kapil, Chief(Geophysics); for their keen interest

in this study. The authors are also thankful to Sh. Suman Hazra, AM(Geology); Sh.Vinod

Kumar, Geologist and Sh.Manoj Kumar Brahma, Supervisor(Survey), NHPC

Gerukamukh, Assam for their help and cooperation during this study.

References

ABEM Instrument AB, S-172 66 Sundbyberg, Sweden, 2007. Terrameter SAS 1000 /

4000 Manual, 2007, ABEM Printed Matter No 93109.

Choudhary, M.K., 2008. Earthinhg practice-additional. Best Practices in Distribution

Systems Operation and Maintenance (O&M), Distribution Reform, Upgrades and

Management (DRUM) Training Program, 2008. obtained from following website,

on January, 2009. www.scribd.com/doc/3417042/OM-65B- Earthing - Practice s-

Additional.

deGroot-Hedlin, C. and Constable, S., 1990. Occam's inversion to generate smooth, two-

dimensional models form magnetotelluric data. Geophysics, 55, 1613-1624.

Edwards, L.S., 1977. A modified pseudosection for resistivity and inducedpolarization.

Geophysics, 42, 1020-1036.

Griffiths D.H. and Barker R.D., 1993. Two-dimensional resistivity imaging and

modelling in areas of complex geology. Journal of Applied Geophysics, 29, 211-

226.

Loke, M.H. and Barker, R.D., 1996. Rapid least-squares inversion of apparent resistivity

pseudosections by a quasi-Newton method. Geophysical Prospecting, 44, 131-152

Loke, M.H., 2001. Tutorial : 2-D and 3-D electrical imaging surveys. Geotomo Software,

Malaysia.

Kapil, S.L. and Ramanaiah, D.V., 2004. Report on dry season earth resistivity

measurements at powerhouse site, Subansiri Lower Project, Arunachal Pradesh.

NHPC LTD Internal Report.

Kapil, S.L.; Jyotirmoy and Pal, S.K., 2006. Report on geophysical survey involving

seismic tomography, seismic refraction and resistivity imaging, Siang Lower HE

Project, Arunachal Pradesh. NHPC LTD Internal Report.

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http://www.scribd.com/doc/3417042/OM-65B-Earthing-Practices-Additional






Mousa, A. M., 1998. The Applicability of Lightning Elimination Devices to Substations

and Power Lines, IEEE Trans. on Power Delivery, Vol. 13, No. 4, October 1998,

pp. 1120-1127.

Pal, S. K., 2009. Final Report on earth resistivity measurements for design of earthmat at

Intake-I and Intake-II foundations, Subansiri Lower H. E. Project. NHPC LTDInternal Report.

RES2DINV 3.58, 2009. Geoelectrical Imaging 2D & 3D Geotomo software, 115 Cangkat

Minden Jalan 5, Minden Heights, 11700 Gelugor, Penang, Malaysia.

Sen, P. K., 2001. Understanding Direct Lightning Stroke Shielding of Substations.

PSERC Seminar Golden, Colorado, November 6, 2001.

Zipse, D. W., 1994. Lightning Protection Systems: Advantages and Disadvantages, IEEE

Trans. On Industry Applications, Vol. 30, No. 5, Sept/Oct. 1994, pp. 1351-1361.

List of Figures

Figure 1. World lightning map.Figure2. Location of Resistivity Imaging survey for earthmat design of Dam Foundation

Figure3. Field set up for Resistivity Imaging survey and data acquisition at Dam

foundation.

Figure 4. Schematic diagram representing resistivity imaging data acquisition proceduresusing Terrameter SAS-4000 and ES464, a computer controlled multi-electrode

survey setup.

Figure5. Arrangement of the blocks used in a model together with the apparent resistivitydatum points for generation 2D cross section

Figure6. 2D model of resistivity distribution of Dam foundation area using Wenner

Profiling technique.Figure7. 2D model of resistivity distribution of Dam foundation area using Wenner

Sounding technique

Figure8. 2D model of resistivity distribution of Dam foundation area using Dipole Dipoletechnique.

Figure9. 2D model of resistivity distribution of Dam foundation area using Schlumberger

technique.

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igu dam earthmat design revised

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