Geology 5660/6660Applied Geophysics

16 Apr 2014

© A.R. Lowry 2014For Fri 18 Apr: Burger 338-340 (§5.11)

Last Time: IP (Induced Polarization) • Induced Polarization (IP) measures the chargeability of transient voltage after DC current is shut off (time-domain IP) or the phase of voltage relative to an AC current (frequency-domain IP)• Results from overvoltage (charge buildup) ~ boundaries between ionic & electronic conduction by electrodialysis (membrane effect) in clays• Chargeability ~ anticorrelates with resistivity; highest for sulfide ores• Heavily used in mining industry; sometimes in well logging and environmental applications• Spontaneous Potential (SP): A “natural” voltage, i.e., occurs without artificially injecting electrical current

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0 0.1 0.2 0.3 0.4 0.5 0.6

~Time between repeat measurements (hours)

Difference (m)

East (m)North (m)Up (m) Difference in

repeated GPSmeasurementson the resistivityline (week 1)…

Difference on thegrav/mag line(week 2).

-36

-26

-16

-6

4

14

0.0 0.2 0.4 0.6 0.8 1.0Difference in Measurement Time (hours)

Difference in Position (m)

However we’re working on this,and in the meantime our positionsare good enough to do (at least)preliminary analyses of theseismic, resistivity and magneticdata sets.

We’ll discuss data analysis andgoals for the project write-uptomorrow.

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

Note that more permeable formationsexpress as more negative spontaneouspotential…

(ii) Mining: A second process occurs in ore bodies that are partly above the water table (oxidizing conditions) and partly below (reducing). Resulting differences in charge concentration produce electrical current.

(iii) Karst investigation: Water flowing through a cavity can build up a charge as well (“streaming” or electrokinetic potential)

Magnetotelluric (MT) MethodSimultaneously record 3D magnetic field and horizontal electric field… Passive measurement, so uses natural sources.

For a sine wavesin(t),

where sd is an averagedresistivity overthe “skin depth”(depth of significantpenetration of the electric fieldE and magneticfield H)

€

E sin ωt( )∝ ρ sd H sin ωt( )

Magnetotelluric Method

Three highest-energy sources of electrical current in Earth: (1) Global lightning strikes (predominates in the 10,000 Hz to ~1 Hz frequency range) Gives from 10s of m to a few km

Magnetotelluric Method

Three highest-energy sources of electrical current in Earth: (2) Ionospheric resonances (predominate periods of seconds to minutes: here ~20 s period near sunset) Gives from ~2 to 10 km

(The small,high frequency spikes correspond to distant lightning strikes. Note here they denote magnetic field as B instead of H…)

Magnetotelluric MethodThree highest-energy sources of electrical current in Earth: (3) Ionospheric disturbances caused by interactions of high energy particles and electrons emitted by the sun (predominates periods of days and longer) Gives from 10s to 100s of km (& to the core if observed for long enough!!!)

… Note this is the same interaction responsible for aurora borealis… Note also that ALL of these sources are extremely high energy (much higher than we could reasonably generate!)

(Real-time ionospheric total electron content)

“Space weather”, or flux of high-energy charged particles &electrons, is also ofpractical interestbecause of effects on satellites (electronflux greater than some threshold can induce charge onelectroniccomponentry ofsatellites; dischargehas fried acommunicationssatellite or two…)

NOAA GOES Electron Flux

USU GAIM Total Electron Content