department of mining engineering, it bhu

Anshu Kumar

ABSTRACT

The project deals with geophysical considerations in determining a site suitable for setting electrical power establishments in any area inside or outside the mine. There is also a focus on the layout of power cables in the sub-surface foundations. Electrical resistivity tests were conducted for determining the specific electrical properties of different types of rock strata. The study also considers the effect of other parameters like porosity, confinement, magnetic behavior, thermal resistivity and geological anomalies. Each and every electrical establishment takes into consideration the Earthing requirement for the safety of human beings, animals, consumer property and utilities equipment. Primary Rocks that have low resistivity free fron joints and cracks are normally most suitable for making the foundation of any establishment. Presence of water reduces the earthing effect and also weakens the strata and wearing down the performance of underground power cable network by corrosion .

INTRODUCTION

Our earth is made up of numerous layers of rocks which may be metallic or non metallic. Metallic minerals are relatively good conductors of electricity, whereas most of the common rock forming minerals are generally poor conductors. This fact is the basis for geophysical testing methods which measure resistivity to evaluate the properties of subsurface mineral deposits. These methods are most useful at shallow depths (<500 feet), but are occasionally used for applications at greater depths. Electrical methods can be used either on the surface or down drill holes. Some electrical methods, called “active” methods, introduce electricity into the ground, creating an artificial electrical field in which charges in the electrical current between electrode can be measured. “Passive” methods measure current flow related to naturally occurring electrical currents. These methods measure the electric potentials, which develop due to the electrochemical action between minerals and pore fluids.

Metallic sulfide minerals or graphite are the most efficient mineral conductors. Pore waters contained in underground formations also conduct electricity very well, and it is the very presence of these waters which makes electrical prospecting methods possible. In most rock materials, the amount of porosity and the chemistry of pore waters have a greater influence on conductivity than do metallic mineral grains. Where the pore waters contain salts (such as sodium chloride) in solution, the methods work especially well. Clay minerals containing only a slight amount of moisture are also easily ionized.

When two electrodes are placed in the ground and voltage is applied across them, current flows from one electrode to the other. In a homogenous conductor, the electron “flow lines” are perpendicular to the lines along which the potential is constant. Zones of abnormally high or abnormally low conductivity cause the current flow lines to become distorted, causing variations from the predicted values. These variations, or anomalies, can then be mapped out to try to locate buried ore deposits.

Geometry of current flow lines and equipotential lines in a vertical section below the surface for voltage generated at stations A and B

Conductivity and resistivity are inversely related: high conductivity equates to low resistivity. At a constant voltage, the relationship between resistance and current are expressed mathematically by Ohm’s Law:

V = I R

where V is the voltage ( in volts), I is the current (in amps), and R is the resistance (in ohms).

The amount of resistance is a function of the composition and physical condition of the rock. In order to measure the amount of resistance of the rock we must specify two other factors, including the length and

cross-sectional area of the region where the electrical current is being conducted, specifically the region of a cylinder where the current is passed. When these factors are specified, the amount of resistance is referred to as the “resistivity”. The formula for resistivity is:

r = (R)(S) / l

where r is the resistivity (in ohm-meters), S is the unit area of the cylinder cross-section, and l is the unit length of the cylinder.

In practice, several different pairs of electrodes are set up at different spacings. As the spacing between the electrode pairs increases, the detection depth increases. In this manner, changes in resistivity with depth can be plotted. This sequential testing technique also detects lateral changes in resistivity along the survey line. This information can be used to pinpoint zones which have strong resistivity contrasts.

Earthing System in a Sub Station comprises of Earth Mat or Grid, Earth Electrode, Earthing Conductor and Earth Connectors.

Earth mat or GridPrimary requirement of Earthing is to have a low earth resistance. Substation involves many Earthings through individual electrodes, which will have fairly high resistance. But if these individual electrodes are inter linked inside the soil, it increases the area in contact with soil and creates number of parallel paths. Hence the value of the earth resistance in the interlinked state which is called combined earth value which will be much lower than the individual value.

The inter link is made through flat or rod conductor which is called as Earth Mat or Grid. It keeps the surface of substation equipment as nearly as absolute earth potential as possible.To achieve the primary requirement of Earthing system, the Earth Mat should be design properly by considering the safe limit of Step Potential, Touch Potential and Transfer Potential.Step potential - which is the potential difference available between the legs while standing on the ground.

Touch potential - which is the potential difference between the leg and the hand touching the equipment in operation.

The factors which influence the Earth Mat design area. Magnitude of Fault Current b. Duration of Fault

c. Soil Resistivityd. Resistivitiy of Surface Material e. Shock Durationf. Material of Conductorg. Earthing Mat Geometry

OBJECTIVE

We perform tests on several core lengths of rock samples under different moisture and joint conditions. Rocks of different petrology are tested to develop a database of igneous, sedimentary and metamorphic rocks. The data is studied along with data obtained from electrical imaging and other geophysical techniques.

Although each sounding is interpreted assuming the subsurface layers are horizontal, the results are combined to produce a geo electrical section showing the variation of bedrock along the profile line. Dips of 30* or more can be accommodated with little loss of precision if the azimuth of the electrode expansion is parallel to the strike.

Contours of data obtained are plotted on plan of the area to chalk out the layout of cables and the best possible site for setting up sub stations and transmission or distribution towers.

In locations where the water table i s gradually falling, we may have dry earth with high resistance. These factors emphasize the importance of a continuous, periodic program of earth resistance testing to find out long term suitability of any site.

We also aim to find the suitable rock or soil mixture that can be mixed with synthetics to provide the adequate resistance as per our needs and use in foundation of electrical establishments.

LITERATURE SURVEY

METHODS TO CARRY OUT RESISTIVITY SURVEY

Electrical resistivity methods are the most important of the geo- physical methods that can be used for the mapping of water-filled fracture zones, because the measurements are sensitive to changes in moisture content, and the field procedures are relatively straight- forward and inexpensive. There is, however, a wide variety of equipment and field procedures, and the correct choice may be critical to the success of the survey.

In the laboratory, we have the 3 pole and the 4 pole tests to measure the voltage and current readings which are used to calculate the resistance of the core samples.

Three survey techniques have been developed for different applications.

1. Constant separation traversing, in which the electrode spacing is kept constant and all the electrodes are moved laterally between measurements, is used to examine lateral changes in the geological structure. These traversing techniques are popular in archaeological surveys but have largely been superseded by electromagnetic traversing methods for deeper investigations.

2. Vertical Electrical Sounding (VES, electrical depth probing or electrical drilling) is a technique used to examine the vertical change in resistivity. In this technique, the spacing between electrodes is

progressively increased between measurements, while the centre of the whole array is kept constant. As the electrode spacing increases, the current penetrates to greater depths and so a plot of apparent resistivity against electrode spacing provides a picture of the variation of resistivity with depth. In this case the data may be interpreted quantitatively to provide resistivities and thicknesses of subsurface layers.

3. Electrical imaging is a recent development, which involves a combination of both traversing and sounding, to produce an image along a section through the subsurface.

TEST LOCATIONS

Test locations are dictated by engineering objectives, however an attempt should be made to measure the variation in material quality or condition throughout the largest possible volume of the structure – typically that with a face area of3m x 3m minimum. From such a large map of the conductivity distribution in the sub surface, it should be possible to identify the area of interest. Since no coupling or contact with the surface of the structure is required, the surface of the structure remains unmarked. As a result of the portability of the instrument, the non-harmful nature of the radiation and the continuous emission and receptivity of electromagnetic fields, the structure can be tested rapidly, safely and without disruption of other activities. As the in-situ calibration is of great importance in the interpretation of the readings obtained, it is recommended that, if conductivity surveys have to be repeated over a period of time, calibration settings should be recorded so that they can be exactly reproduced. Thus measurements taken on different dates can be compared for structural condition

monitoring. It is normal procedure to test the structure along survey lines either longitudinally or vertically, thus a series of traverse or reading stations should be marked out for investigation.

The number of readings required is dependent upon:

- the accuracy and resolution required in the evaluation,

- upon the instrument used,

- the operating mode of use of the equipment itself.

PRINCIPLE OF TEST

The application of this electromagnetic technique for measuring conductivity involves the use of a transmitter coil energised with an alternating current and a receiver coil located a short distance away. The time-varying magnetic field arising from the current induces very small currents in the structure. These currents generate a secondary magnetic field which is sensed, together with the primary field, by the receiver coil. The conductivity equipment permits the measurement of near surface average conductivity. It should be noted that the results are averaged over the depth of penetration. This secondary field is a function of the inter coil spacing, the operating frequency and the conductivity of the materials, and reveals the presence of a conductor and provides information on its geometry and electrical properties.

PROCEDURE

The procedure will involve marking out the masonry wall in traverses either horizontally or vertically. If the meter is to be operated in automatic mode, readings will be collected continuously along the survey lines. If the meter is operated in manual mode, stations will have to be marked on the traverses, at regular intervals related to the intercoil spacing. Minimum recommended spacing between reading stations is equal to the spacing between transmitter and receiver on the meter. A denser grid of reading points will give a better resolution in the final contour map. The lateral extent of the volume whose conductivity is sensed by the meter is approximately the same as the vertical depth.

3-D RESISTIVITY MEASUREMENTS CHARACTERISING TAR CONTAMINATE WASTE DEPOSITS

STUDY OF GEOPHYSICAL MAPPING IN MOCHUDI, BOTSWANA

A feasibility study for improvement of the water supply to villages in the Republic of Botswana incorporated geophysical surveys in the Mochudi area. Mochudi village is approximately fifty-eight kilometres to the north-west of Gaborone in southern Botswana. The available geological maps for this area indicate an escarpment of sandstones. grits and conglomerates of the Lower Waterberg Series, overlying unconformably the granites and granitic gneiss of the Basement Complex. (Fig. 1). Major faults, with a N.W.-S.E. trend are shown through both series.

Geophysical investigation was proposed for an area of low ground to the east of Mochudi village, (Fig. 1), which was expected to consist almost entirely of Basement Complex rocks overlain by alluvial deposits. Dykes of basic composition, up to several hundred metres in thickness, are known to occur within the Basement rocks, but only isolated outcrops in a highly decomposed state have been mapped in this area. Aerial photographs and topographic maps of the Mochudi, area were studied for evidence of linear structural features, such as fault zones and major joint systems. The 5 ft contour interval of the Mochudi 1 : 6000 plan was particularly useful in this respect and clearly indicated a series of N.W.-S.E. trending valleys through the Waterberg escarpment. Major linear structural features, with the same trend, were traced, from aerial photographs, through the adjacent upland areas.

The importance of careful location of boreholes, in this type of geological environment, is supported by the low water yields of existing

boreholes in this area. Out of six holes bored in the Mochudi area in 1956/57, three were regarded as failures and three yielded 45 I/m, or more. The successful boreholes, such as B.H. 38 at Mabodisa, were apparently sited on, or near, magnetic anomalies. Many more holes were bored between 1957 and 1974, and the yields were again very variable. If geophysical methods were used to site these holes, the success rate was far less than is usual for geophysically located holes in this type of environment.

A geophysical survey, utilising electrical resistivity and magnetic methods, was carried out in 1974 to establish a strategy for the siting of new boreholes.

ELECTRICAL RESISTIVITY MAPPING

The first two days of the geophysical survey were spent in carrying out electrical resistivity depth soundings at seven existing borehole site; in the Central and Eastern Mochudi areas. In each case the Schlumberger configuration of electrodes was used because of its advantages over alternative configurations, particularly the speed and simplicity of the field procedures. A low frequency alternating current resistivity meter was used for all resistance measurements, and this equipment proved to be satisfactory for current electrode separations up to the maximum required distance of 400 m. The azimuth of the survey line, in each case, was close to the expected trend of geological structures, in order to minimise the effect of lateral variations in ground resistivity. The field curves were matched with theoretical curves to provide the thickness and actual resistivity of each layer. A summary of the results obtained is shown in Table 1 along with the water yields from the boreholes.

TABLE 1

Borehole no.

Water yield (l/m)

Thickness of surface layer (m)

Resistivity of surface layer (ohm-m)

Thickness of second layer(m)

Resistivity of second layer (ohm-m)

Resistivity of third layer (ohm-m)

805 227 0.4 40 3.2 4 10002720 189 0.4 300 40 22 4101036 91 0.54 101 4.3 4 400+1668 57 0.3 45 7.5 21 2251632 25 0.2 60 13 40 2002108 19 0.5 100 5 70 210-4002098 15 0.13 2500 10 0-100 200+

Basically a three layer resistivity model was obtained in each case. The first and third layers have relatively high resistivity values, related respectively to the superficial deposits and Basement Complex rocks. The intermediate layer of low resistivity represents decomposed crystalline rocks with a high moisture content. Some of the curves were very irregular for the larger current electrode separations, because of the presence of water-filled fractures within the unweathered crystalline rocks. High water yields were obtained from areas of both shallow and deep weathering; in the former case water being obtained from fractures in otherwise unweathered rocks. The low yield boreholes are characterised by a generally higher resistivity level for the intermediate layer. The large scale electrical resistivity mapping procedures had, therfore, to be designed to locate zones of deep weathering, as well as water-filled fracture zones. Continuous electrical resistivity traversing along parallel N-S lines across the area of interest was adopted rather than a large number of electrical resistivity depth-soundings on a grid pattern. The Wenner configuratrion was used for all

traverses, primarily to simplify the field procedures, although it was recognized that narrow vertical fracture zones might be missed. Fortunately, the current electrode spacing of 30 m, and the station interval of 30 m, which were chosen to investigate variations in the depth of weathering, also proved to be satisfactory for the location of the major fracture zones. An isoresistivity map has been prepared from these resistivity traversing data (Fig. 2). The contour interval is 10 ohm-m for most of the map but, where the resistivity gradients and anomalies are large, only the 100 ohm-m contours have been drawn. Apparent resistivity values have been plotted for each resistivity station to enable the amplitude of each anomaly to be determined, particularly where, for clarity, contour lines have been omitted. Significant areas of low or high resistivity have been indicated on this map by appropriate symbols. Although there is belt of low resistivity associated with the Notwani River, there are several obvious elongated anomalies across the area with a N.W.-S.E. trend. It is the zones of low resistivity that are of particular interest, although high gradients may indicate faulted margins between rock types of different resistivity. The high yielding borehole in the north of the area (B.H.38) appears to be located on a fault between Waterberg and Basement Complex rocks, whereas other high yielding boreholes, such as B.H.850 and B.H. 720, are located in areas of anomalously low resistivity. The low yield boreholes, such as B.H. 1668 and B.H. 1632, are in areas of relatively high resistivity, as indicated by the electrical resistivity depth sounding. Several other areas of anomalously low electrical resistivity were identified, that are well away from existing boreholes, and these were further investigated.

Fig 2 Isoresistivity map of area

CONCLUSIONS

The effect of water in fractured zones can be effectively studied using electrical resistivity tests. At Mochudi, several sites were selected and subsequently drilled with considerable success. Water yields of 420 l/m and 250 I/m at Sites A and B respectively justify, this approach. Lower yields were recorded for the borehole at Site C. but this is a reflection of the lack of recharge to the water-filled fractures at this particular location.

LOCATION OF FAULTS BY RESISTIVITY TESTS

Recent application of electrical imaging has proved successful in the location of faults where there is a good resistivity contrast (Figure 3). Resistivity soundings can also be employed to measure the throw of a fault, if care is taken to orientate the soundings parallel to the fault, although less accurately. Magnetic and gravity methods are usually restricted to the investigation of major faults, particularly where basic igneous rocks or basement rocks are involved. If it is only the location of the fault line that is required, electromagnetic (particularly ground conductivity) surveys are a cost-effective means of mapping the fault, especially when it occurs near the surface. Where the fault cuts basic igneous dykes, it can usually be traced through mapping the positions of the dykes magnetically. Probably the most cost-effective means of locating near-vertical fracture and fissure zones is by electromagnetic profiling. The fractures and associated weathering reduce the resistivity of the host rock, and it is this, which is identified on the traverse.

Figure 3: Electrical image across a near-vertical fault between low resistivity Mercia mudstones and high resistivity Sherwood Sandstone

STUDY OF EFFECT OF CONFINING PRESSURE ON ELECTRICAL RESISTIVITIES OF DIFFERENT TYPES OF

SANDSTONE CORE SAMPLES

Electrical measurements of sandstone cores were conducted under both laboratory simulating overburden conditions. Measurements under laboratory conditions revealed a mean Archie cementation factor of 1.619 and a mean saturation exponent of 1.343. The laboratory conditions cementation factors and saturation exponents were lower than the general values cited in the literature. Measurements under overburden conditions revealed higher cementation factors and saturation exponents. The mean cementation factor was 1.851 and the mean saturation exponent was 2.144. Results indicated that confining pressure significantly affect the electrical parameters. Lower cementation factors were obtained for the well cemented and consolidated sideritic samples than for poorly consolidated samples. However, higher saturation exponents were obtained for the sideritic samples. The Archie cementation factor was found to be independent of the degree of cementation for the studied samples. Laboratory conditions measurements revealed a non-linear trend of data points on resistivity index vs. water saturation log-log plots. Data points fell on a concave downward path, probably due to surface conductance of clay minerals. Resistivities of five sandstone samples, fully saturated with brine, were measured while increasing confining pressure stepwise from 250 to 2300 psi and then decreased back to 250 psi. The formation resistivity factor increased as confining pressure was increased, and decreased as confining pressure was decreased. Results

showed hysteresis between loading and unloading stages. The increase in relative formation factor was higher for low porosity samples than for highly porous samples. The formation factors and the cementation exponents at laboratory conditions and overburden conditions were tested using different equipment. The changes in mean formation and cementation factors were 45% and 13.9%, respectively. These changes are between the parameters obtained from two different methods. The mean formation factor increased 9.1% when confining pressure was increased from 250 to 2300 psi in the same measurement system.

The confining pressure was most effective on resistivity between 0 and 500 psi. Comparison of water saturations obtained using laboratory and overburden conditions electrical parameters indicated that a fully water saturated sample appears to be oil bearing if laboratory conditions electrical parameters are used instead of overburden electrical parameters. The error is between 18.8% and 61% for a 30% porosity sample, depending on the water content of the sample. For a 15% porosity sample, the error is between 27.9% and 71.8%. The electrical parameters obtained under laboratory conditions underestimate water saturation calculations. The error is more significant in oil zones as water saturation decreases. The error also increases as the sample porosity decreases

.

RESULTS AND DISCUSSIONThe resistivities of various rock as obtained from the resistivity test are as under. Although the range of resistivities for rocks and minerals is quite large, normal values for common rocks and minerals are fairly well established (Table below). Normally, resistivity does not vary as the frequency is varied. However, if native metals or other metallic minerals are present, the resistivity changes dramatically as the frequency varies.

Common Rocks/Materials

Resistivity

(ohm meters)

Ore Minerals Resistivity

(ohm meters)

Clay 1 – 100 Pyrrhotite 0.001 – 0.01Graphitic Schist 10 – 500 Galena 0.001 – 100Topsoil 50 – 100 Cassiterite 0.001 – 10,000Gravel 100 – 600 Chalcopyrite 0.005 – 0.1Weathered Bedrock

100 – 1000 Pyrite 0.01 – 100

Gabbro 100 – 500,000

Magnetite 0.01 – 1,000

Sandstone 200 – 8,000 Hematite 0.01 – 1,000,000

Granite 200 – 100,000

Sphalerite 1000 – 1,000,000

Basalt 200 – 100,000

   

Limestone 500 – 10,000     Slate 500 –

500,000   

Quartzite 500 – 800,000

   

Greenstone 500 – 200,000

   

.

Most soil and rock minerals forming building materials are insulators and conduction through the rock matrix only takes place when certain clay materials, native metals and graphite are present. The minerals in the sand and silt fractions of the soil are electrically neutral and are generally excellent insulators. The electrical conductivity of the material is thus primarily controlled by the particle size, the amount of water present in the pores and by the conductivity of the pore fluid. The general trend is that conductivity will increase with reducing particle size, increasing moisture content and increasing salt content.

Measurements made on material as a function of the moisture content by weight, show a conductivity that increases approximately as the square of the moisture content.

The solutions of salts in pore water will substantiallyincrease the material conductivity.

The temperature dependence of the electrical conductivity of the electrolyte is almost entirely due to the temperature dependence of the viscosity of the liquid and a change in conductivity of 2.2% per degree may be expected. This phenomenon implies that for high seasonal changes of

temperature, the conductivity over the normal range of ambient temperature may double.

The following are the methods to lower the earth resistance:1. Treatment of soil using bentonite powder,fly

ash2. Lengthening the earth electrode used in

establishment3. Using multiple rods for earthing

In general,we must go with hard igneous rock foundation of granite rocks as they are impervious and hard in strength.Use of suitable additive can be done to suitably modify the resistivity. Porous rocks containing water may cause hindrance in earthing and thus safe workings.The sandstone beds can be used after a certain depth below a hard non porous rock.

REFERENCES

McDowell P W, Barker R D, Butcher A P, Culshaw M G , ‘Geophysics in engineering investigations (2002) Geological Society- Engineering Geology special publications, London

Materials and Structures/Materiaux et Constructions, Vol. 34, April 2001,In-situ and non-destructive test proposed test method.

IEEE (1992) Guide for soil thermal resistivity measurements. Inst. of Electrical and Electronics Engineers, Inc. New York.

Bulletin of the International Association of Engineering Geology de I'Association Internationale de Geologie de L'Ingenieur N*19 258 --264 Krefeld 1979, Geophysical Mapping of water filled fracture zones in rocks.

Keller, G. V. and Frischknecht, F. C., 'Electrical methods in geophysical prospecting', Ch. 1. Pergamon Press, N.Y., 1966.

Heiland, C. A., 'Geophysical exploration', New York, Hafner Publishing Co. 1968

Lab exercise manual of DMTC, Alaska

Kearey, P. and Brooks, M., 'An Introduction to Geophysical Exploration', Oxford, 1991

Earthing Practices in sub stations, T & SS Training Centre,Madurai

Salih Saner , Maclean Amabeoku, Mimoune Kissami,1996, Formation resistivity response to loading and unloading confining pressure