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Applied Geophysics 4 credits SP07 call#84362/#87487 Instructor Dr. John Louie, LME 217, ph 784-4219 louie@seismo.unr.edu and GEOL 493/693 Mining Exploration Geophysics 4 credits SP07 call #89118/88005 Instructor Dr. Gary Oppliger, LME 304, ph 784-7056 oppliger@mines.unr.edu Office hours: 12:00-1 pm M,T,W GEOL 492/692

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Electrical Properties of Rocks and Electrical Resistivity Methods

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Resistance Definition of an OHM An ohm is a resistance in a conductor that produces a potential difference of one volt when a current of one ampere is flowing through it.volt ampere R =

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Ohm’s Law http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmlaw.html#c1

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Resistance vs Resistivity Resistance is relevant only to a particular measurement circuit. Units: Ohms Resistivity is an intrinsic property of all physical materials Units: Ohm-Meters Apparent Resistivity is a resistivity estimate based on a assuming a half-space geometry. Units: Ohm-meters

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Electrical Resistivity vs Electrical Conductivity Conductivity = σ = 1/ρ (mho/meters) Resistance = ρ (ohm-meters)

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Calculating Resistance from Resistivity http://www.cflhd.gov:80/agm/index.htm

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Factors Influencing Electrical Conductivity in Rocks Metallic Sulfide Mineral Content Porosity (connected/effective - fractures or pores) Clay Content Pore saturation (% air or gas) Water salinity (TDS) Hydrocarbon Fluid Saturation Rock Matrix intrinsic resistivity Fluid temperature

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Archie’s Law

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Formation Factor

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The conductivity of most geological formations can be fit to Archie’s Law

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Influence of Permeability A rock with a non-conducting matrix must be permeable (connected pores) as well as porous to conduct electricity. Darcy's Law: Ohm's Law: where Despite the similarity between Darcy’s and Ohm’s Laws, electric currents have zero viscosity so even a narrow crack can provide an effective electrical connection between pores that not contribute to hydraulic permeability.

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Comparison of electric and hydraulic properties. ElectricalHydraulic Transverse resistance: T = h i i = H l Transmissivity: T h = h i k i = K l H Longitudinal conductance: S= h i i = H/ l Leakance: L h = k i /h i = K t /H Average aquifer resistivities: l, t Average hydraulic conductivities: K l, K t http://www.cflhd.gov:80/agm/index.htm

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Electrical resistivity of rocks with various wt % of sulfide. Metallic Sulfide Mineral Content http://www.cflhd.gov:80/agm/index.htm

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Effect of Water Temperature http://appliedgeophysics.berkeley.edu:7057/dc/figures/fig43_7.jpg

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Conductivity Ranges of Various Materials http://www.cflhd.gov:80/agm/index.htm

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Resistance vs Resistivity Resistance is relevant only to a particular measurement circuit. Units: Ohms Resistivity is an intrinsic property of all physical materials Units: Ohm-Meters Apparent Resistivity is a resistivity estimate based on a assuming a half-space geometry. Units: Ohm-meters

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Calculating Resistance from Resistivity http://www.cflhd.gov:80/agm/index.htm

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Four Electrode Resistivity Measurement on rock sample … are used to avoid electrode contact resistance effects seen in two electrode measurements. C1 C2 current I P1P2

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Four electrode resistivity arrays http://www.cflhd.gov/agm/images/fig90.jpg

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The Basic Concept of an Earth Resistivity Measurement http://www.cflhd.gov/agm/images/fig91.jpg

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Fig. 5.4g Electrode Contact Resistance is typically much higher than the intrinsic earth resistivity

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Fig. 5.4g Electrode Contact Resistance is concentrated around each electrode

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Fig. 5.4g If a standard two electrode resistivity meter were used to measure the earth’s “resistance” we only obtain information on the quality of the electrode contacts – not the earth’s resistivity

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http://appliedgeophysics.berkeley.edu:7057/dc/em44.pdf Pole-Pole Array

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Pole-Dipole Array http://appliedgeophysics.berkeley.edu:7057/dc/em44.pdf

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Pole-Dipole Array http://appliedgeophysics.berkeley.edu:7057/dc/em44.pdf

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Wenner Array http://appliedgeophysics.berkeley.edu:7057/dc/em44.pdf

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Schlumberger http://appliedgeophysics.berkeley.edu:7057/dc/em44.pdf

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Dipole-Dipole Array

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Fig. 5.4g The electric potential varies as 1/r around a single current electrode on a homogeneous half-space

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Fig. 5.5g

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Fig. 5.9g

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Fig. 5.6g Equal potential voltage surfaces between the electrodes

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Depth of current flow between two current electrodes

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Fig. 5.14g

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Fig. 5.13g (a)

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Fig. 5.13g (b)

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Fig. 5.13g (c)

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Fig. 5.12g (a)

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Fig. 5.12g (b)

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Fig. 5.12g (c)

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Fig. 5.14g

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Fig. 5.15g (a)

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Fig. 5.15g (b)

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Fig. 5.13g (b)

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Fig. 5.13g (c)

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Fig. 5.14g

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Fig. 5.15g (a)

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Fig. 5.15g (b)

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Fig. 5.18g (a)

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Fig. 5.18g (b)

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Fig. 5.18g (c)

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Fig. 5.19g

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Fig. 5.21g (a)

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Fig. 5.20g

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Fig. 5.21g (b)

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Fig. 5.21g (c)

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Fig. 5.22g

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Fig. 5.23g (a)

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Fig. 5.23g (c)

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Fig. 5.24g (a)

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Fig. 5.24g (c)

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Fig. 5.25g (a)

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Fig. 5.25g (b)

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Fig. 5.26g

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Fig. 5.27g (a)

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Fig. 5.27g (b)

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Fig. 5.28g

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Fig. 5.29g

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Fig. 5.30g (a,b)

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Fig. 5.30g (c,d)

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Fig. 5.31g

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Fig. 5.33g (a)

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Fig. 5.33g (b)

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Fig. 5.33g (c)

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Fig. 5.39g

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One point removed

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C:\Shortcut to Geo-CD-ROM.exe

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Airborne EM Resistivity

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http://ece.uprm.edu/~pol/waves_review.pdf

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http://appliedgeophysics.berkeley.edu:7057/dc/archie/index.html

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