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:

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-80

-60

-40

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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

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-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