electrical properties of rocks and electrical resistivity methods
TRANSCRIPT
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Electrical Properties of Rocksand
Electrical Resistivity Methods
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ResistanceDefinition of an OHMAn 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.
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/meter) (siemens/meter, S/m)
Resistivity = ρ (ohm-meter)
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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 PermeabilityA 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.
Electrical Hydraulic
Transverse resistance: T = hii = Hl
Transmissivity: Th = hiki= KlH
Longitudinal conductance: S= hii = H/l
Leakance: Lh=ki/hi = Kt/H
Average aquifer resistivities: l, t
Avg. hydraulic conductivities: Kl, Kt
• 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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http://www.cflhd.gov:80/agm/index.htm
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http://www.cflhd.gov:80/agm/index.htm
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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 or Ω
Resistivity is an intrinsic property of all physical materials Units: ohm-meters or Ω-m
Apparent Resistivity is a resistivity estimate based on a assuming a half-space geometry. Units: ohm-meters or Ω-m
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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
P1 P2
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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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Electrode Contact Resistance is typically much higher than the intrinsic earth resistivity
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Electrode Contact Resistance is concentrated around each electrode
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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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The electric potential varies as 1/r around a single current electrode on a homogeneous half-space
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Equal potential voltage surfaces between the electrodes
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Depth of current flow between two current electrodes
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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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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