1. Geology 5660/6660 Applied Geophysics 15 Apr 2016
Last Time: Self-Potential (SP); Magnetotelluric (MT)
• Spontaneous Potential (SP): “Natural” amperage (i.e.,
without our injecting current) can arise from:
 Brine infiltration of drilling mudcake in permeable
formations (Petroleum industry wireline application)
 Oxidation/reduction reactions at the water table (Mining)
 Streaming potential in cavities (Karst investigations)
• Magnetotelluric (MT) measures 3D magnetic field H,
horizontal electrical field E in time series to get electrical
resistivity structure: where sd is
averaged resistivity over an -dependent skin depth.
• Passive method  sources include lightning strikes
(10,000–1 Hz; ionospheric resonances (1–.001 Hz);
ionospheric disturbances (.001–very low Hz)
Read for Mon 21 Apr: We’re Done!
© A.R. Lowry 2016

2. Maple Grove Hot Springs Position Data
James kindly made this useful map…

3. Maple Grove Hot Springs Magnetic Data
• First (grey diamonds) & last (blue diamonds) profile
measurements are consistent outliers…
Magnetic drift, or other problem?

4. Maple Grove Hot Springs Magnetic Data
All Data
When plotted on a
realistic distance
scale, these look
like they might
contain real signal
(especially if we
leave out first and
last measurement
profiles, which
approximately
cancel each other
anyway).
Leaving out first and last
measurement profiles

5. Maple Grove Hot Springs Resistivity Raw (a = 3) Looks like some
interesting stuff here
too! But not exactly
what I would have
expected...
Positioned

6. “Space weather”, or
flux of high-energy
charged particles &
electrons, is also of
practical interest
because of effects
on satellites (electron
flux greater than
some threshold can
induce charge on
electronic
componentry of
satellites; discharge
has fried a
communications
satellite or two…)
NOAA GOES Electron Flux
USU GAIM Total Electron Content

7. Example effect of a sunspot. The repeating signal (strongest
in Ex and Hy) is the diurnal variation as Earth rotates within
the solar wind. The large departure in Hx ~ day 12 is a sunspot.
This
example
gave
resistivity
constraint
to depths
of a few
100 km.

8. MT array deployment for EarthScope;
plans include eventual deployment to
the east coast.

9. Yellowstone-Snake River Plain System: Seismic Vs (Rayleigh wave tomo)
Electrical Resistivity 
Kelbert et al., 2011 EarthScope Science
Meeting
Wagner et al., EPSL 2010

10. Meqbel et al., 2014 Earth Planet. Sci. Lett.

11. Resistivity structure beneath an advancing subduction zone…
(Marlborough region, northern South Island, New Zealand)
Wannamaker et al., Nature, 2009

12. Resistivity structure a bit south of here… From older data
Wannamaker et al., UGA/AAPG Guidebook 30, 2001.
Resistivity structure a bit south of here… From older data
& inversion. Newer techniques are drawing interest from
industry!

13. Resistivity structure a bit south of here… From newer data
Wannamaker et al., Geochem. Geophys. Geosys., 2008.
Resistivity structure a bit south of here… From newer data
& inversion...