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

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

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

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

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

“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

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.

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

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

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

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

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!

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...