Applied Geophysics

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GEOL 492/692
Applied Geophysics 4 credits SP07
call84362/87487Instructor Dr. John Louie, LME
217, ph 784-4219 louie_at_seismo.unr.edu and GEOL
493/693 Mining Exploration Geophysics 4 credits
SP07 call 89118/88005Instructor Dr. Gary
Oppliger, LME 304, ph 784-7056 oppliger_at_mines.unr.
edu Office hours 1200-1 pm M,T,W
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Electrical Properties of RocksandElectrical
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.                      
R
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Ohms Law
http//hyperphysics.phy-astr.gsu.edu/hbase/electri
c/ohmlaw.htmlc1
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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
Resistance ? (ohm-meters)
Conductivity s 1/? (mho/meters)
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Calculating Resistance from Resistivity

• http//www.cflhd.gov80/agm/index.htm

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Factors Influencing Electrical Conductivity in
Rocks
Porosity (connected/effective - fractures or
pores)
Pore saturation ( air or gas)
Hydrocarbon Fluid Saturation
Water salinity (TDS)
Clay Content
Metallic Sulfide Mineral Content
Fluid temperature
Rock Matrix intrinsic resistivity
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Archies Law
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Formation Factor
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The conductivity of most geological formations
can be fit to Archies 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 Darcys and Ohms
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 S hiri Hrl Transmissivity   Th Shiki KlH
Longitudinal conductance S Shi/ri H/rl Leakance LhSki/hi Kt/H
Average aquifer resistivities rl, rt Average hydraulic conductivities Kl, Kt

• http//www.cflhd.gov80/agm/index.htm

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Metallic Sulfide Mineral Content
Electrical resistivity of rocks with various wt
of sulfide. 

• http//www.cflhd.gov80/agm/index.htm

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Effect of Water Temperature
http//appliedgeophysics.berkeley.edu7057/dc/figu
res/fig43_7.jpg
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Conductivity Ranges of Various Materials
http//www.cflhd.gov80/agm/index.htm
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http//www.cflhd.gov80/agm/index.htm
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http//www.cflhd.gov80/agm/index.htm
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http//www.cflhd.gov80/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.gov80/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.
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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 earths resistance we
only obtain information on the quality of the
electrode contacts not the earths resistivity
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Pole-Pole Array
http//appliedgeophysics.berkeley.edu7057/dc/em44
.pdf
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Pole-Dipole Array
http//appliedgeophysics.berkeley.edu7057/dc/em44
.pdf
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Pole-Dipole Array
http//appliedgeophysics.berkeley.edu7057/dc/em44
.pdf
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Wenner Array
http//appliedgeophysics.berkeley.edu7057/dc/em44
.pdf
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Schlumberger
http//appliedgeophysics.berkeley.edu7057/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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Airborne EM Resistivity
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http//ece.uprm.edu/pol/waves_review.pdf
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http//appliedgeophysics.berkeley.edu7057/dc/arch
ie/index.html
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