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• Background informa3on on Magne3c Anomalies
• Deriva3on of a Model
• Fi:ng the Model to Spreading Ridge Data
How are Magne3c Anomalies formed?
What are we measuring?
What are we measuring?
Magne3c Anomalies give a 3mescale for the seafloor going away from the ridge.
Half magne3c anomaly
10my full magne3c anomaly
How do we measure the magne3c field at sea?
total magne3c field = magne3c field (earth) + magne3c field (anomaly)
We measure the magnitude of the total field.
measured scalar anomaly
What causes the differences between the observed anomaly and the magne3c polarity
3mescale?
• Earth filter -‐ smoothed by depth
• Skewness -‐ due to magne3c field inclina3on
• Superposi3on -‐ due to slow spreading rate
Effect of depth (earth filter):
Arbitrary horizontal units
200 2000
4000
Magne3c Anomaly (nT)
Effect of inclina3on (skewness): De
grees o
f skew/
Magne
3c Ano
maly
Arbitrary offset
Limits of model resolu3on (superposi3on):
Modeling the Magne3c Anomalies
Modeling the Magne3c Anomalies
Modeling the Magne3c Anomalies
No change in the y direc3on.
Modeling the Magne3c Anomalies
Assume that this spreading ridge is located on the magne3c pole.
Modeling the Magne3c Anomalies
Only the z component of the Earth’s magne3c field is non-‐zero.
reversal paZern x earth filter
Modeling the Magne3c Anomalies The spreading ridge is not at the magne3c pole; this introduces a phase shi] θ.
Modeling the Magne3c Anomalies
• A-‐ scalar magne3c anomaly (T) • k-‐ wave number 1/λ (m-‐1) • zo-‐average seafloor depth (m) • θ-‐ skewness • C-‐unknown constant • μo-‐magne3c permeability=4π10-‐7 • p(k)-‐ fourier transform of magne3c 3mescale
Modeling spreading rate
Distance from spreading center
Mid Atlan3c Ridge
MAR spreading rate and skew Spreading rate:
13500m/my Skew: -‐15 degrees
Pacific-‐Antarc3c Ridge
Pacific-‐Antarc3c Ridge Spreading rate:
46500 m/my Skew: 20 degrees
Ques3ons?
