Magnetic Anomalies

A magnetic anomaly is the change in magnitude of the earth's magnetic field with respect to the expected value for that location.   Large volumes of magnetic materials will change the intensity of the earth's field.

The units of magnetic anomalies are nanoTesla (nT) or the equivalent gamma.

The magnitude of the earth's magnetic field varies dramatically with latitude,  from 25,000 gammas at the equator to 70,000 gammas at the magnetic poles.  Variations due to geology typically have a magnitude of a few hundred gammas, which is a very small fraction of the total field.  In addition, for practical matters using compasses for navigation we really only care about the orientation of the magnetic vector (and hence in which direction a compass will point).  There is also temporal change in the value of the field, which must also be corrected for.

Magnetite crystals in mid ocean ridge basalt orient with the earth's field until they cool below the Curie temperature at about 600°C (their melting temperature is about 1000°C). Below the Curie temperature the grains keep their permanent magnetization.  If we measure their  orientation, we can work out how the sea floor has moved since it cooled.  The Curie temperature is well below the melting temperature, and while the rock is solid the magnetic crystals can still reorient themselves.  Large man made iron features like a cannon will also acquire a permanent magnetization when they are built, but when the cannon is moved, we really don't care about how it was oriented when it cooled.

Because the earth's field intensity changes significantly on a daily basis and on longer time scales, readings must be corrected so they can be compared, although local relative changes can be detected immediately by looking at the observed readings.

Like gravity, the magnetic field obeys an inverse power law, and anomalies drop off significantly with distance.  To resolve fine features, the magnetometer must be close to the anomalous magnetic material; as the survey height increases, the anomaly becomes more diffuse because the distance to the magnetic material is larger, the effect is less, and the change with position becomes much less.  This means we want to fly the aircraft low, or the underwater instrument close to the bottom.

The marine magnetic anomalies result from the remnant (or permanent) magnetization acquired when the material cooled below its Curie temperature.  With a substantial thickness of the ocean crust having magnetic properties aligned in the direction of the earth's field when the crust cooled, they will make a measureable change in the intensity of the field.  They could have cooled when the earth's field was reversed, in which case they would subtract from the earth's field; and they could have cooled at a different latitude, having a different magnitude.

In looking for manmade objects like shipwrecks or pipelines, the remnant magnetization would have been acquired when the magnetic material cooled.  If the material were a number of iron cannons, or a large ballast mound, the remnant magnetization of each individual piece of iron would essentially be random and they would cancel each other out.  In this case the induced magnetization of the entire mass of ferrous material could create a recognizable anomaly.  Being exposed to the earth's field causes some electrons to align with the earth's field, and increase the intensity.

When we do want to look at the magnitude of the field, we normally look at the anomalies, the deviation of the measured intensity from that calculated for the location based a model of the earth's field.  The intensity can be measured by towing a magnetometer behind a survey ship, or by using an aircraft. The best results will be from deep tows.  Measurement from the sea surface or aircraft or spacecraft will record the general features, but smoothing means the results are much worse that from towed ship measurements.


last revision 4/9/2016