ธรณ ฟ ส กส อ.ดร. ส ว มล อ ดพ วย

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1 Geophysics ธรณ ฟ ส กส อ.ดร. ส ว มล อ ดพ วย Office: GB111-4 ภาคว ชาธรณ ว ทยา คณะว ทยาศาสตร มหาว ทยาล ยเช ยงใหม มหาวทยาลยเชยงใหม

2 GEOPHYSICS Physics of the earth = applies principles and methods hdof physics to study the properties of the Earth -Global geophysics --> earthquakes, deep structure of the Earth, plate tectonics, geodynamics, Earth s magnetic field, etc. -Applied/exploration l geophysics--> physical measurements to locate mineral deposits, petroleum, etc.

3 Lecture Outline Resistivity, Induced polarization & Self-potential methods การส ารวจทางด านไฟฟ า Electromagnetic method การส ารวจทางด านแม เหล กไฟฟ การสารวจทางดานแมเหลกไฟฟาา Ground penetrating radar method การส ารวจทางด านเรดาร ผ านทะล พ นด น Gravity Method การส ารวจทางด านความโน มถ วง Magnetic Method การส ารวจทางด านแม เหล ก Seismic method การส ารวจทางด านคล นไหวสะเท อน 9 Lectures 3 Lectures 3 Lectures 3 Lectures 3 Lectures 9 Lectures

4 Week1 (7, 9 Nov) Week2 (14, 16 Nov) Week3 (21, 23 Nov) Week4 (28, 30 Nov) Week5 (5, 7 Nov) Week6 (12, 14 Nov) Lab Outline Field 1D Resistivity Survey (Schlumberger)/(Wenner) Field 2D Resistivity & IP surveys (Dipole - Dipole) Resistivity curve matching Resist87/RES1D Week7 (19, 21 Dec) Res2DInv Week8 (26, 28 Dec) Week9 (2, 4 Jan) Week10 (9, 12 Jan) Week11 (16, 18 Jan) Week12 (23, 25 Jan) Week13 (30 Jan, 1 Feb) Week14 (6, 8 Feb) Week15 (13, 15 Feb) Week16 ( 20, 23 Feb) No lab (หย ดว นพ อแห งชาต ) No lab (หย ดชดเชยว นร ฐธรรมน ญ) ฐ No lab (ส ปดาห สอบกลางภาค) No lab (หยดชดเชยว นส นป และว นข นป ใหม ) (หย ดชดเชยวนสนปและวนขนปใหม) Field Gravity & Magnetic Surveys Surfer & Gravity Correction Surfer & Magnetic Correction Field Refraction & Reflection Seismic Surveys Grapher & Refraction Seismic Interpretation Reflection Seismic Interpretation RockWorks

5 Electrical Resistivity Survey To investigate the subsurface resistivity distribution beneath an area of interest. In general, an electric current (direct current, dc) is injected into the ground, and the resulting voltage differences are measured at the surface of the Earth. True resistivity of the subsurface canbe estimatedfrom this voltage measurement. Anomalous conditions or inhomogeneities within the ground, such as relatively lti l conducting or resistive iti zones, are inferred from the fact that they deflect the current and distort surface potential readings. The ground resistivity is related to various geological parameters such as the mineral and fluid content, porosity and degree of water saturation in the rock. Have been used in hydrogeological, mining, and geotechnical investigations, and for environmental surveys.

6 Fundamental Resistivity Theory Electrical resistivity is a measure of how a material resists a steady electrical lcurrent 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.

7 Fundamental Resistivity Theory The fundamental physical lawupon which geophysical resistivity 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 = the current density E = the electric field intensity σ = the electrical conductivity [ siemens/m] or [mho/m] = reciprocal 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) Combiningabove two equations we get

8 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 isotropic 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, 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.

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

10 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 P 1 due to current entering and exiting via active electrodes C 1 and C 2 is V = I ρ 1 1 π rc 1P r 1 C2 P1 2 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. r C1 P 1 = the distance between P 1 and C 1 r C2 P 1 = the distance between P 1 and C 2. 1

11 Fundamental Resistivity Theory I The potential ti measured at passive electrode P 2 due to current entering and exiting via active electrodes C 1 and C 2 is V P 2 = Iρρ 1 1 π rc 1P r 2 C2 2 P 2 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. r C1 P 2 = the distance between P 1 and C 1 r C2 P 2 = the distance between P 2 and C 2

12 I ti t ti l diff b t t i t th th b l t 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 = Δ 2 1 P V P V V + = Δ P C P C P C P C P P r r r r I V π ρ P C P C P C P C r r r r π The resistivity of a half space is then given by solving above equation for ρ, that is, 1 that is, r r r r I V P C P C P C P C + Δ = π ρ I = k ΔV ρ k is called the geometric factor which depends on the specific configuration of current and potential electrodes.

13 Electrode Configurations/Arrays การบ านข อท 1 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.

14 ρ = k ΔV I Fundamental Resistivity Theory gives the true resistivity values 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 electrode configuration and surface location. For inhomogeneous earth the resistivity ρ, computed from the equation will vary according to the geometric arrangement of the electrodes or on the horizontal location of the array. The resistivity obtained, for aninhomogeneous subsurface is, therefore, properly viewed as an apparent resistivity, written as V ρ = k Δ a I

15 Fundamental Resistivity Theory The apparent resistivity i i average resistivities i i i of the inhomogeneous subsurface formation. The apparent resistivity = 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 is the inversion problem or inverse modeling.

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18 Electrical Resistivity of Earth Materials Electric current flows in earth materials at shallow depths through two main methods; electronic conduction current flow via free electrons, such as in metals, important when 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

19 Electrical Resistivity of Rocks, Soils, and Minerals

20 Electrical Resistivity of Earth Materials Igneous and metamorphic rocks >high resistivity values >resistivity greatly depends on degree of fracturing, and percentage ofthe fractures filled withgroundwater >large range of resistivity, ~ million ohm m, depending on whether wet or dry >this characteristic i = useful lin detection of fracture zones and other weathering features,e.g. in engineering and groundwater surveys Sedimentary rocks >usually more porous and have higher water content >normally lower resistivity values compared to ig. & met. rocks >resistivity values range from 10 ~ ohm m most values < 1000 ohm m >resistivity values largely depends on porosity of rocks, and the salinity of the contained water.

21 Electrical Resistivity of Earth Materials Unconsolidated sediments >generally have even lower resistivity i i values than sedimentary rocks >resistivity values ranging from ~ 10 <1000 ohm m >resistivity value depends on porosity (assuming all the pores are saturated) t as well as the clay content. t >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 because the resistivity of a particular rock or soil sampledepends on a number of factors such as the porosity, the degree of water saturation and the concentration of dissolved salts. Groundwater resistivity values vary from ohm m depending on concentration of dissolved salts Seawater low resistivity (~ 0.2 ohm m) due to the relatively high salt content. This makes the resistivity method an ideal technique for mapping the saline and fresh water interface in coastal areas.

22 Electrical Resistivity of Earth Materials Metallic sulfides (such as pyrrhotite, galena and pyrite) > typically low resistivity i i values < 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) i d) have a significant effect. Most oxides,, such as hematite,, do not have a significantly low resistivity value, except magnetite. Metals, such as iron, have extremely low resistivity values. Industrial contaminants >Chemicals that are strong electrolytes, such as potassium chloride and sodium chloride, can greatly reduce the resistivity of ground water to less than1ωm 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, andmightnot not haveaa significant effect on the bulk resistivity.

23 Electrical Resistivity of Earth Materials Archie s lawgives the relationship between the resistivity of a porous rock and the fluid saturation factor. ρ = aφ m n Sw ρ = resistivity of the rock ρ w = resistivity of the pore water S w = (volume of water in pores)/(total volume of pores) 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 >applicable for certain types of rocks and sediments, particularly those that havea a low clay content. >electrical conduction is assumed to be through the fluids filling the pores of the rock. w

24 Electrical resistivity field procedures 1 D Vertical electrical sounding and horizontal profiling surveys 2 D Electrical resistivity surveys 3 D Electrical l resistivity i i surveys

25 Electrode Configurations/Arrays

26 Depth of Current Flow in the Ground Current flow tends to occur close to the surface. Current C tpenetration ti can be increased dby increasing i separation of current electrodes.

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29 1 D Vertical Resistivity Sounding

30 1 D Vertical Resistivity Sounding

31 h1 =12 k =-0.45

32 RES1D inversion program

33 1 D Horizontal Resistivity Profiling

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35 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.

36 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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42 Pseudosection Data Plotting Method PseudosectionsP d are normally used to display apparent resistivity it 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 th( (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.

43 Pseudosection Data Plotting Method C2 C1 C2 C1 P1 C2 P2 C1 P1 P2 P1 P2 P1 P2 P1 P2

44 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 ofthe 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 qualityapparent apparent resistivity measurements, which normally stand out as extreme values on the pseudosections, are readily identified and removed.

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

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47 Advantages and Disadvantages of Different Electrode Arrays Choices of electrode arrays depend on Sensitivity of arrays to vertical or horizontal changes in subsurface resistivity Depth of investigation Signal strength (Signal/Noise Ratio S/N) Horizontal coverage (duration time of investigation) 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 dt coverage is important, t and your resistivity meter is sufficiently sensitive and there is good ground 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 measurements in both the forward and reverse directions might be a viable choice.

48 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 S/N (Lowest) 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 Array S/N > dipole dipole but < Schlumberger Good horizontal resolution Good horizontal coverage Good if both forward & reverse surveys acquired

49 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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53 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

54 3 D Resistivity survey

55 3 D Resistivity survey Using roll along method to survey 10x10grid with a resistivity meter system with 50 electrodes. a) 10x5 grid in x direction b) 10x5 grid in y direction

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61 Z=0 m Z=5 m Z=10 m Z=15 m Z=20 m Z=25 m

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