Geology 5660/6660Applied Geophysics
14 Apr 2014
© A.R. Lowry 2014For Wed 16 Apr: Burger 338-340 (§5.11)
Last Time: DC Electrical Resistivity • An Apparent Resistivity Pseudosection is a visual representation of “weighted averages” of resistivity structure. A “zero-order” approximation of true structure (but with artefacts introduced by “negative sensitivity zones” of the electrode current/voltage pairs)• Inversion of resistivity data (i.e., optimal parameterization of a forward model) uses the physics of the voltage response to give more detail and confidence.• Applications including permeability mapping, lithologic mapping, karst & tunnel detection• Successfully used to inform past field studies for this class!
Thursday’s magnetometer measurements on the West Cache fault:
-100
-80
-60
-40
-20
0
20
40
60
80
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Time between measurements (hours)
Difference in measurements (Gammas)
Week 1 RMS: 34.8
Week 2 RMS: 35.2
-150
-140
-130
-120
-110
-100
-90
-300 -250 -200 -150 -100 -50 0 50 100 150 200 250 300 350 400Profile Distance (m)
Vertical Magnetic Field (Gammas)
Averaged Data RMS: 14.0
Induced Polarization (Induced Potential)IP refers to the transient voltage measured after an electrical current is turned off.
Physical process is probably similar to charging a battery.
At contacts between the two, this results in a buildup of charge (sometimes called overvoltage).
One mechanism:current flow in most rocksis dominated by ionic conduction in pore fluidelectrolytes, but in some minerals (e.g., sulfides, graphite, magnetite, native metals), flow occurs by electronic conduction.
Another mechanism: In clays, can get electrodialysis or “membrane polarization”: A membrane effect in which one ion in the pore fluids is more mobile than the other (cation mobility > anion in this example)
Two approaches to measurement:
• IP in the time domain: Measured as “chargeability”, area of the voltage decay curve divided by voltage measured before current was turned off (hence units of s).
• IP in the frequency domain: Apply an alternating current instead of DC, and measure amplitude and phase as a function of frequency of the AC.
Applied Current
Measured Voltage
Applied Current ResultingVoltage
1 10 100 1000 10000
Alluvium
Gravels
Limestone/Dolomite
Granite/Granodiorite
Clean Sandstone
Shale
Volcanic Tuffs
Siltstones
2-8% Sulfide
8-20% Sulfide
20+% Sulfide
Chargeabilities of Various Rocks
Example:Two samples of montmorillonite (clay)soil at a waste contamination site.Apparent resistivitydecreases withfrequency because ofinduced polarization;the lower-app sampleis contaminated (organic NAPL +electrolytic APL)
Phase peaks are different for differentfluids…
(From a gold mining prospectin Durango,Mexico)
For a mediumcomprised ofn different materials, apparent chargeability ais related to a
by … So contains information about resistivity
structure and is especially sensitive to changes in resistivity…Attractive for mining applications.
€
a = η i∂ logρ a∂ logρ ii=1
n
∑
Apparent Chargeability
Apparent Resistivity
Another gold prospect:mineralized boulderfield in east-central
British Columbia
Spontaneous Potential (SP)SP is an electrical potential that arises “without” an applied current (i.e., electrical current flow arising from natural processes or disturbances that are not initially electrical)
Three most common applications:
(i) Well-logging: Electrochemical potential arises from differential diffusion of ions into drilling mudcake from permeable formations electrical current flow