Geology 228/378 Applied and Environmental Geophysics Lecture 6. DC resistivity Surveys

Transcription

1 Geology 228/378 Applied and Environmental Geophysics Lecture 6 DC resistivity Surveys

2 Direct current (DC) Resistivity. Introduction 2. Current flow in the ground 3. Schlumberger, Wenner, dipole-dipole, pole-dipole arrays 4. Field methods and instrumentation 5. Data interpretation 6. Field Examples

3 Ohm s Law (discovered in 827) V = IR Georg Simon Ohm ( )

4 It's Resistivity, NOT Resistance R = ρ = ρ L A RA L So the unit for resistivity is ohm-meter

5 For a point source in an infinity medium, we have the resistance R and potential V expressed as = = = = = = = S jds I r I r I IR V r r r A L R σ π π ρ π ρ π ρ ρ

6 Furthermore ρ = 4 π rv I or σ = 4 I π rv This resistivity is called the apparent resistivity. Only when the material is uniform, the apparent resisitivity is equal to the constant, real resistivity. ρ 4πrV 4πr V 4πr V = = = = a I ri I r 2 2 V jr

7 Most geophysical resistivity surveys have the measurements occurred at the surface of the earth. The air above the ground is literally an insulator (zero conductivity) and the current only flows in the ground. Thus, for calculating the current density on an equal-potential sphere, the surface area becomes from the closed spherical surface to the surface of the lower-hemisphere, and the potential changes to Electric field and current V = ρ 2 I π r I E = gradv = V J = σe = σ V r V

8 In practice, the field surveys usually measure the Voltage V, other than the potential itself. This voltage V is the difference of potential between 2 points. IN DC resistivity surveys the voltage is usually measured by two electrodes planted on the surface. ρ I ρ I V = V V 3 = I 2πr 2πr 3 r r 3 V V 3

9 For the current can be physically flowing through the ground, we have to have 2 poles: one for current injected in (source) and one for the current flow out (sink). Thus, both the source and sink will generate an electric potential, but with opposite Dipole polarity. V V 2 = and 2 = ρ I π r 2 ρi πr 2 r r 2

10 And the total potential for the two poles is ) ( r r I r I r I V V V = = + = π ρ π ρ π ρ And the total voltage between two points generated by the two poles is ) ( 2 ) 2 2 ( ) 2 2 ( r r r r I r I r I r I r I V V V + = = + = π ρ π ρ π ρ π ρ π ρ

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12 APPARENT RESISTIVITY because ) ( r r r r I V a + = π ρ I V k I V r r r r a = + = ) 2π ( ρ then

13 GEOMETRIC FACTOR K is the geometric factor that describes the geometry of the electrode configuration being used: A B M N V I = r r r r K π 2 + = NB AN MB AM K π

14 rr3 V ρ a = 2π ( ) r3 r I Pole-Dipole array

15 Schlumberger Array K = 2π CP C P 2 C P 2 + C P 2 2

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18 2l 2l(n-) 2l ρ a = 2πl ( n )( n + ) n V I

19 Data plotting

20 Dipole-Dipole Array

21 R, S : received, source signals G : geometrical spreading P S, P R : radiation pattern, receiver coupling L : propagating path

22 Pole pole Pole - dipole Dipole - dipole Wenner Schlumberger ELECTRODE ARRAYS

23 CHOICE OF THE BEST ARRAY Depends on: ) type of structure to be mapped 2) sensitivity of the resistivity meter 3) background noise level Things to be considered: ) depth of investigation 2) sensitivity of the array to vertical and horizontal structures 3) horizontal data coverage 4) signal strength.

24 DIPOLE-DIPOLE ADVANTAGES Low EM coupling between current and potential circuits Good for depth penetration High resolution and is sensitive to vertical resistivity boundaries (e.g. dykes and cavities)

25 DIPOLE-DIPOLE DISADVANTAGES Poor for vertical resolution of horizontal structures (e.g. sills or sedimentary layers) Data collected from dipole-dipole array are easily affected by near-surface resistivity variations and therefore can produce noisy data at sites with cultural relics Small signal strength for large values of n

26 Apparent Resistivity Pseudo-section for a Block model

27 Wenner Pole-pole Dipole - dipole Pole-dipole Block model response

28 Near Surface Layer

29 Near Surface Layer Response, Plan View

30 Near Surface Layer Response, Pseudosection

31 Buried Vertical Contact

32 Buried Vertical Contact Response, Plan View

33 Buried Vertical Contact Response, Pseudosection

34 3D Prism

35 3D Prism Response, Plan View

36 3D Prism Response, Pseudosection

37 Pole-pole array sensitivity

38 Pole-dipole array sensitivity

39 Dipole-dipole array sensitivity

40 Wenner array sensitivity

41 Schlumberger array sensitivity

42 Resistivity Surveys

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44 AGI Sting R- and the Swift automotive switchbox

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46 DC Resistivity Interpretation R, S : received, source signals G : geometrical spreading P S, P R : radiation pattern, receiver coupling L : propagating path

47 Electric current in layered media The current flow in the layered media deviates from that observed in the homogeneous media. In particular, notice that in the layered media the current flow lines are distorted in such a way that current preferentially seems to be attracted to the lower-resistivity portion of the layered media. In the model on the left, current appears to be pulled downward into the 50 ohm-m layer. In the model on the right, current appears to be bent upward, trying to remain within the lower resistivity layer at the top of the model. This shouldn't be surprising. What we are observing is the current's preference toward flowing through the path of least resistance. For the model on the left, that path is through the deep layer. For the model on the right, that path is through the shallow layer.

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52 Sting/Swift prg: DIP-DIP title 2 unit electrode spacing 3 array No. dip-dip=3 93 No. of data points -middle point used 0 0-no IP st: apparent rho-location 2nd: PP2 spacing 3rd: dipole separation factor n 4th: apparent resistivity

53 DC Resistivity Interpretation R, S : received, source signals G : geometrical spreading P S, P R : radiation pattern, receiver coupling L : propagating path

54 CURRENT CONDUCTION IN ROCKS Electrolytic conduction occurs by the relatively slow movement of ions within an electrolyte Electronic conduction is the process by which metals, for example, allow electrons to move rapidly, so carrying the charge This is applicable in zero and low frequency case

55 crystalline rock can lead to low resistivities if they are filled with fluids. The resistivities of various earth materials are shown below. Material Resistivity (Ohm-meter) Air Pyrite 3 x 0^- Galena 2 x 0^-3 Quartz 4 x 0^0-2 x 0^4 Calcite x 0^2 - x 0^3 Rock Salt 30 - x 0^3 Mica 9 x 0^2 - x 0^4 Granite 00 - x 0^6 Gabbro x 0^3 - x 0^6 Basalt 0 - x 0^7 Limestones 50 - x 0^7 Sandstones - x 0^8 Shales 20-2 x 0^3 Dolomite 00-0,000 Sand -,000 Clay - 00 Ground Water Sea Water 0.2

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57 Archie s law: In the ground, and in low frequencies, electricity is essentially conducted through the interstitial water in pores by ionic transport ρ = a φ m S n ρ w ρ effective formation resistivity; ρw pore water resistivity; φ porosity; S saturation; a ; m.3-2.5; n ~2.

58 EC Variation with Depth EC (micr omhos / cm)

59 EC Variation with Depth Cr Variation with Depth EC vs Cr EC (micromhos/cm) Cr (mg/l) y = x R 2 = Depth (ft) Depth (ft) C r ( m g /l) EC ( micromhos/cm) 25 25

60 DC Resistivity Case Studies = L ds R S e P P G S R ) ( ) ( ) ( ω α ω ω : propagating path receiver coupling radiation pattern, :, spreading : geometrical : received, source signals, L P P G S R R S

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67 Detection of Saltwater Intrusion along the Noyo River, California

68 Resistivity and Seismic Survey Results

69 Piezocone GeoVIS Demonstration Plume Control and Containment System ESTCP LTM Test Cell NVBC Port Hueneme In-Situ BioBarrier (Mid Plume) NVBC Port Hueneme In-Situ BioBarrier (Leading Edge) GeoVIS/Piezocone Facility Patterson Rd. 23 rd Ave. N Pleasant Valley Rd. Pacific Ave. ESTCP NFESC/ASU In-Situ BioBarrier NEX Gas Station Site

70 Electrode locations for DC resistivity Surveys East Lot of Building W3 7 W AGI Personnel Gate N Wells from previous extraction system Hydraulic Test Wells Proposed electrode location Utility Pole 20 Vehicle Gate 20 Vehicle Gate (Inject) W Water Storage Tanks Utility Shed Note: Layout displayed with 0 x 0 grid

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74 2/28/2005

75 4//2005

76 4/ - 2/28, 2005

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79 Day37 4 Day

80 .5 Z=2 ft DEC (S/m) Y distance (ft) Z=5 ft DEC (S/m) Y distance (ft)

81 Homework: The averaged electric conductivity of the groundwater found at the National Chromium site is about 00 microsiemens per centimeter.it is equivalent to 0 millisiemens per meter. The averaged formation conductivity found by the DC resistivity tomography is millisiemens per meter. By assuming the porosity is 35%, 00 % saturation, and a =.0, m=2.0 and by Archie s law, what is the estimated formation conductivity?