Geology 5660/6660Applied Geophysics

28 Mar 2014

© A.R. Lowry 2014For Mon 31 Mar: Burger 429-449 (§7.4–7.6)

Last Time: 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:

€

HE =r

H E = XE2 +YE

2 + ZE2

€

i = tan−1 ZE

XE2 +YE

2

⎛

⎝

⎜ ⎜

⎞

⎠

⎟ ⎟

€

d = tan−1 YE

XE

⎛

⎝ ⎜

⎞

⎠ ⎟

500 yrs beforereversal

mid-reversal 500 yrs afterreversal

Glatzmeier modeling:• Solid inner core magnetized opposite main field; forced to rotate by applied torque precession (~0.2°/yr for real Earth)• Inner core stabilizes field dipole; long time required to diffuse outer core field to inner core controls reversal timescale

Problem: If both Earth’s main field and crustal field are determined from surface measurements, how do we separate?

Core field dominates long wavelengths (small n of sphericalharmonics). We describe core field only out to n = 14–15where it dominates the total field.

Measurement: Fluxgate magnetometer:Wire coils wound inopposite directions;these cancel &produce zero currentin secondary coilin absence of externalmagnetic field, but if aligned with a field,one core reinforces,other counteractsexternal field resultingin a current.

Gives intensity in theorientation of the coils.(Direction?)

Proton precession magnetometer:Bottle containing a hydrogen-rich fluid (distilled water or hydrocarbon)is surrounded by a wire coil. Currentthrough the coil produces a strong magnetic field; protons align with field… Current is shut off & as protons realign with Earth’s 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 usedfor terrestrial geophysics!)

Measurement:

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.geomag.bgs.ac.uk/data_service/models_compass/wmm_calc.html

Component Field Value Secular VariationDeclination 12.177 degrees -8.1 arcmin/yearInclination 66.809 degrees -1.6 arcmin/yearHorizontal Intensity 10887 nT -15.8 nT/yearNorth Component (x) 20351 nT -5.1 nT/yearEast Component (y) 4392 nT -51.4 nT/yearVertical Intensity (z) 48598 nT -100.7 nT/yearTotal Intensity 52870 nT -98.7 nT/year