Geology 5660/6660 Applied Geophysics 30 Mar 2016 Last Time: Magnetic Methods • Magnetic field strength decays as 1/r3 • Total intensity of magnetization: where remanent magnetization IR is in the direction of HE at the time of magnetization… i.e., when the rocks cooled through their Curie temperature • Must know strength & direction of the Earth’s main field! For Fri 1 Apr: Burger 499-520 (§8.1-8.2) © A.R. Lowry 2016

Geology 5660/6660 Applied Geophysics 30 Mar 2016 Last Time (Cont’d): Earth’s Main Magnetic Field • Earth’s Main Field derives from “core dynamo”… convective flow of Ni-Fe outer core + coriolis forces ( + feedback)  electrical current flow  magnetic field ( + feedback) + single-crystal Fe inner core ( + feedback) • Mostly dipolar, oriented ~ (and precesses around) rotation axis; varies nonlinearly through time… • Express the vector field at a point as either intensity HE, inclination i, declination or local Hx, Hy, Hz components: For Fri 1 Apr: Burger 499-520 (§8.1-8.2) © A.R. Lowry 2016

Problem: If both Earth’s main field and crustal field are determined from measurements, how do we separate them? Core field dominates long wavelengths (small n of spherical harmonics). We describe core field only out to n = 14–15 where it dominates the total field.

Measurement: Fluxgate magnetometer: Wire coils wound in opposite directions; these cancel & produce zero current in secondary coil in absence of external magnetic field, but if aligned with a field, one core reinforces, other counteracts external field resulting in a current. Gives intensity in the orientation of the coils. (Direction?)

Measurement: Proton precession magnetometer: Bottle containing a hydrogen-rich fluid (distilled water or hydrocarbon) is surrounded by a wire coil. Current through the coil produces a strong magnetic field; protons align with field… Current is shut off & as protons realign with ambient magnetic field, they precess at a frequency determined by magnetic field strength (0.042576 Hz/nT). So, measure frequency of the induced AC current and convert to a total field strength. (Lots of other types but these two are most commonly used for terrestrial geophysics!)

Remove/avoid all metal objects when collecting data!!! Data Reduction: Remove/avoid all metal objects when collecting data!!! Keep magnetometer high off the ground to reduce “noise” Sensitive to variations in ionosphere, magnetosphere: Perform looping and correct for drift; don’t bother measuring during solar storms! Correct for elevation if > a few hundred m (~0.03 nT/m) Horizontal position correction: Use WMM if latitude change is > a few hundred m (correction here ~6 nT/km) From http://www.ngdc.noaa.gov/geomag-web/#igrfwmm Component Field Value Secular Variation Declination 11.724 degrees -0.1072 °/year Inclination 66.7027 degrees -0.0219 °/year Horizontal Intensity 20706.8 nT -25.8 nT/year North Component (x) 20274.8 nT -17.4 nT/year East Component (y) 4207.6 nT -43.2 nT/year Vertical Intensity (z) 48086.8 nT -110.7 nT/year Total Intensity 52355.6 nT -111.9 nT/year

Aeromag measurements of magnetic anomalies over Nevada… Most magnetic anomaly maps on scales < 500 km use magnetometer data collected by flying an airplane in a grid pattern over the target region

Commonly use in combination with gravity, e.g. this kimberlite prospect in Botswana

Satellite measurements of induced + remanent magnetization of the Earth’s crust (scalar total field anomaly relative to Earth’s main, i.e. core, field)

Satellite data are more self-consistent and useful for regional-scale tectonics studies– Here depth-integrated magnetic susceptibility shows strong relationships to Proterozoic accretion history of the mid-continent.

Combination of global aeromag and CHAMP satellite crustal magnetic anomalies… Note merging problems!

Conterminous US satellite/aeromag merge is better… (albeit badly aliased in this image)

Modeling Magnetic Anomalies: Laplace’s equation for magnetism (analogous to gravity) is: where is magnetic field, a is magnetic potential,  is magnetic permeability, is dipole moment per unit volume If we assume a magnetic “monopole” (i.e., -m part of the dipole is so far away we can ignore it) at the origin, then the potential (A is cross-sectional area) and