Electrical prospecting involves detection of surface effects produced by electrical current flow in the ground.

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1 Electrical Surveys in Geophysics Electrical prospecting involves detection of surface effects produced by electrical current flow in the ground. Electrical resistivity method Induced polarization (IP) method Self potential (SP) method Electromagnetic (EM) method

2 Electrical Resistivity Survey To investigate the subsurface resistivity distribution beneath an area of interest. In general, an electric current is injected into the ground, and the resulting voltage differences are measured at the surface ofthe Earth True resistivity of the subsurface can be estimated from this voltage measurement. Anomalous conditions or inhomogeneities within the ground, such as relatively conducting or resistive zones, are inferred from the fact thatthey they deflect the current and distort surface potential readings. The ground resistivity is related to various geological g parameters such as the mineral and fluid content, porosity and degree of water saturation in the rock. Have been used in hd hydrogeological, lmining, i and geotechnical investigations, and for environmental surveys.

3 Fundamental Resistivity Theory Electrical resistivity is a measure of how a material resists a steady electrical current flow. The electrical resistivity of a cylindrical sample of length L (m) and uniform cross section area A (m 2 ), having resistance R between the end faces, is given by ρ = RA / L The unit of resistivity ρ is ohm meter (Ωm). The resistance R is given in terms of the electric potential V applied across the ends of the cylinder and the resultant current I flowing through it, by Ohm s law R=V/I. The units of R, V, and I are ohms (Ω), volts (V), and amperes (A) respectively.

4 Fundamental Resistivity Theory The fundamental physical llaw upon which h geophysical resistivity itiit surveys are based is Ohm s law which governs the current flow in the ground. Ohm s law can be written in vector form as J = σe, where J is the current density, E is the electric field intensity, and σ is the electrical conductivity with it unit of siemens/m or mho/m, the reciprocalof of resistivity ρ (σ = 1/ ρ). In practice the electric potential V is measured. The relationship between V and E is given by E = grad V. Combining above two equations we get J = σ grad V.

5 Fundamental Resistivity Theory The simplest approach to the theoretical study of the current flow in the ground is to consider first the case of a homogeneous isotropicsubsurface subsurface and a single point current source on the ground surface. In this case, the current flows radially away from the source. The equipotential surfaces develop into a hemispherical shape, p, with the current flow perpendicular to the equipotential surface. At some distance r from the current source, the hemispherical shell has surface area 2πr 2, so the current density J is J = I/2πr 2.

6 Fundamental Resistivity Theory Since ρ = 1/ σ and using J = I/2πr 2 then equation J = σ grad V I πr 1 V = ρ 2 2 r V rr = ρi / 2πr can be written as The potential V at distance r from the current source is given by integrating g the above equation, the result of which is 2 V r 2 = ( ρ I / 2 π r ) dr = I ρ / 2 π r This equation provides the fundamental relationship for electrical prospecting performed at the surface of a uniform isotropic earth.

7 Fundamental Resistivity Theory In reality, a single electrode, by itself, cannot inject current into half space; a return electrode is required such that the current flows into the ground via one (source) and exits via the other (sink) electrode. I B A M N The potential measured at passive electrode P1 due to current entering and exiting via active electrodes C1 and C2 is V = Iρ 1 2π rc 1 r P 1 2 P C P The minus sign in the second term of this equation recognizes the change in sign of the current at the source and sink electrodes C 1 and C 2, and where r C1 P 1 is the distance between P 1 and C 1 while r C2 P 1 is the distance between P 1 and C 2.

8 Fundamental Resistivity Theory I The potential measured at passive electrode P2 due to current entering and exiting via active electrodes C1 and C2 is V = Iρ 1 2π rc 1 r P 2 2 P C P The minus sign in the second term of this equation recognizes the change in sign of the current at the source and sink electrodes C 1 and C 2, and where r C1 P 2 is the distance between P 1 and C 1 while r C2 P 2 is the distance between P 2 and C 2.

9 Fundamental Resistivity Theory In practice a potential difference between two points, rather than an absolute potential, is measured. The potential difference for a four electrode array is given by V V V g y + Δ = Δ P P I V V V V ρ + = Δ P C P C P C P C r r r r V π ρ ρ The resistivity of a half space is then given by solving above equation for ρ, that is, I + π V r r r r V P C P C P C P C Δ + Δ = ρ I = k ΔV ρ k is called the geometric factor which depends on the specific configuration k is called the geometric factor which depends on the specific configuration of current and potential electrodes.

10 Electrode Configurations/Arrays Common arrays used in resistivity surveys and their geometric factors. Note that the dipole dipole, pole dipole and Schlumberger arrays have two parameters, the dipole length a and the dipole separation factor n. While the n factor is commonly an integer value, non integer values can also be used. k is the geometric factor.

11 ρ = k ΔV I gives the true resistivity that would be calculated from potential measurement over a homogeneous half space with the 4 electrodes configuration. The resistivity so obtained is constant and independent of both the electrode configuration and the surface location of the electrodes. For an inhomogeneous earth the resistivity ρ, computed from the equation will vary according to the geometric arrangement of the electrodes oronon the horizontallocationof location of the array. The resistivity obtained, for an inhomogeneous subsurface is, therefore, properly viewed as an apparent resistivity, written as ρ ΔV a = k I

12 The apparent resistivity should not be considered as some kind of a spatially averaged resistivity of the homogeneous subsurface formation. Itis the resistivity that the potential readings would assign to the ground if it were homogeneous. The relationship between the apparent resistivity and the true resistivity is a complex relationship. To determine the true subsurface resistivity from the measured apparent resistivity values isthe inversion problem or inverse modeling.

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15 Electrical Resistivity of Earth Materials Electric current flows in earth materials at shallow depths through two main methods; electronic conduction current current flow via free electrons, such asin metals, importantwhen conductive minerals are present, such metal sulfides and graphite in mineral exploration electrolytic conduction current flow via the movement of ions in groundwater, common mechanism for environmental and engineering surveys

16 Electrical Resistivity of Rocks, Soils, and Minerals

17 Electrical Resistivity of Earth Materials Igneous and metamorphic rocks typically have high resistivity values. The resistivity of these rocks is greatly dependent on the degree of fracturing, andthe percentageof the fracturesfilled filled with ground water. Thus a given rock type can have a large range of resistivity, from about 1000 to 10 million ohm m, depending on whether it is wet or dry. This characteristic is useful in the detection of fracture zones and other weathering features, such as in engineering and groundwater surveys. Sedimentary rocks, which are usually more porous and have higher water content, normally have lower resistivity values compared to igneous and metamorphic rocks. The resistivity values range from 10 to about ohm m, with most values below 1000 ohm m. m. The resistivity values are largely dependent on the porosity of the rocks, and the salinity of the contained water.

18 Electrical Resistivity of Earth Materials Unconsolidated sediments generally haveeveneven lower resistivity values than sedimentary rocks, with values ranging from about 10 to less than 1000 ohm m. The resistivity value is dependent on the porosity (assuming all the pores are saturated) as well as the clay content. Clayey soil normally has a lower resistivity value than sandy soil. Note the overlap in the resistivity values of the different classes of rocks and soils. This is because the resistivity of a particular rock or soil sample depends on a numberof factorssuchasthe such as the porosity, the degree of water saturationandtheand the concentration of dissolved salts. Groundwater has resistivity values vary from 10 to 100 ohm m depending on the concentration of dissolved salts. Note the low resistivity (about 0.2 ohm m) of seawater due to the relatively high salt content. This makes the resistivity method an ideal technique for mapping the saline andfresh water interface in coastal areas.

19 Electrical Resistivity of Earth Materials Metallic sulfides (such as pyrrhotite, galena and pyrite) have typically low resistivity values of less than 1 Ωm. Note that the resistivity value of a particular ore body can differ greatly from the resistivity of the individual crystals. Other factors, such as the nature of the ore body (massive or disseminated) have a significant effect. Most oxides, such as hematite, do not have a significantly low resistivity value, except magnetite. Industrial contaminants Metals, such as iron, have extremely low resistivity values. Chemicals that are strong electrolytes, such as potassium chloride and sodium chloride, can greatly reduce the resistivity of ground water to less than 1Ωm even at fairly low concentrations. Hydrocarbons, such as xylene, typically have very high resistivity values. However, in practice the percentage of hydrocarbons in a rock or soil is usually quite small, and might not have a significant ifi effect on the bulk resistivity.

20 Electrical Resistivity of Earth Materials Archie s law gives the relationship between the resistivity of a porous rock and the fluid saturation factor. ρ = a φ m n Sw ρ = resistivity of the rock ρ = resistivity of the pore water w S = (volume of water in pores)/(total volume of pores) w m = cementation factor, ~ 2 for well-cemented formations, ~ 1.5 for moderate to poorly cemented formations n = saturation exponent, normally = 2 ρ a = coefficient of saturation, = between φ = fractional porosity It is applicable for certain types of rocks and sediments, particularly those that have a low clay content. The electrical conduction is assumed to be through the fluids filling the pores of the rock. w

21 Electrical resistivity field procedures 1-D Vertical electrical sounding and horizontal profiling surveys 2-D Electrical resistivity surveys 3-D Electrical resistivity surveys

22 Electrode Configurations/Arrays

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25 1 D Vertical Resistivity Sounding

26 1 D Vertical Resistivity Sounding

27 h1 =12 k = -0.45

28 RES1D inversion program

29 1 D Horizontal Resistivity Profiling

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31 2 D Resistivity survey The arrangement of electrodes for a 2 D resistivity survey and the sequence of measurements used to build up a pseudosection.

32 2 D Resistivity survey The use of the roll-along method to extend the area covered by a 2-D survey.

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38 Pseudosection Data Plotting Method Pseudosections are normally used to display apparent resistivity data from a 2 D resistivity survey. A horizontal location is defined as the mid point of the electrode array used to make a given apparent resistivity measurement. A vertical location is defined to be some distance that is proportional to the separation between the electrodes or estimate depth (pseudo depth) of electrode array used. For the dipole dipole array, for example, apparent resistivity data are plotted at the intersection of the two lines drawn at a 45 o angle to the horizon from the center of the current (C 1 C 2 ) and the potential (P 1 P 2 ) dipole pairs.

39 Pseudosection Data Plotting Method

40 Pseudosection Data Plotting Method Pseudosections give very approximate pictures of the subsurface resistivity distribution beneath the survey lines; however they provide only a distorted picture of the subsurface because the shape of the contours depends on the type of array. These data have to be modeled or inverted to convert the pseudosections into a 2 D resistivity section that is ready for geological interpretation. The main use today of the pseudosection is for data quality analysis. Poor quality apparent resistivity measurements, which normallystand out as extreme values onthe pseudosections, are readily identified and removed.

41 The apparent resistivity pseudosections from 2 D imaging surveys with different arrays over a rectangular prism.

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43 Advantages and Disadvantages of Different Electrode Arrays Choices of electrode arrays depend on Sensitivity y of arrays to vertical or horizontal changes in subsurface resistivity Depth of investigation Signal strength (Signal/Noise Ratio S/N) Horizontalcoverage (duration time ofinvestigation) If survey in a noisy area and need good vertical resolution and have limited survey time, use Wenner array. If good horizontal resolution and data coverage is important, and your resistivity meter is sufficiently sensitive and there is good groundcontact contact, use the dipole dipole array. If not sure, or need both reasonably good horizontal and vertical resolution, use the Schlumberger array. If have a system with a limited number of electrodes, the pole dipole array with measurementsin both the forward and reverse directions might be a viable choice.

44 Wenner Array High S/N (highest among others) Good for noisy area Good vertical resolution == good for horizontal structure detection (layers of subsurface) Dipole Dipole Array Low o S/N (Lowest) (o es) Good data coverage Good horizontal resolution == good for vertical structure detection (cavity dike, ore body) Schlumberger Array S/N > dipole dipole but < Wenner Good choice if both horizontal & vertical resolutions required Pole Dipole l Array S/N > dipole dipole but < Schlumberger Good horizontal resolution Good horizontal coverage Good if both forward & reverse surveys acquired

45 Estimated Depth of Investigation for Different Arrays. Example: dipole-dipole n=6, a =10 (L=80) max. depth = 10x1.73 = 17 m or 80x0.216=17 m

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49 3 D Resistivity survey Array Types for 3 D Dipole dipole Pole dipole Pole pole The arrangement of the electrodes for a conventional 3 D survey

50 3 D Resistivity survey

51 3 D Resistivity survey Using roll along method to s r e Using roll along method to survey 10x10 grid with a resistivity meter system with 50 electrodes. a) 10x5 grid in x direction b) 10x5 grid in y direction

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