INTRODUCTION

We have seen that magnetic anomalies of simple bodies are more complex than gravity anomalies. There are several reasons for this. One is that the source of magnetic anomalies is magnetization (a vector) whereas for gravity anomalies the source is density (a scalar). The magnetization vector can have any direction. The background magnetic field and the direction of induced magnetization vary with latitude. To further confuse matters, magnetometers may measure total field, vertical field component or horizontal field components or, with some modern instruments, all three orthogonal components. The basic result is that the very same body may have very different magnetic signatures in different situations although the gravity signature, in contrast, is always the same.

Let’s review these ideas by looking at various simple cases for a "blob' (sphere, dipole). At high latitudes the vertical component anomalies are favored because they are symmetrical (peak value over source). For the same reason horizontal component anomalies are easiest to interpret at low latitudes. We can easily sketch expected anomalies by drawing lines of force for a dipole of desired orientation and then taking the desired component. We can do the same for total field anomalies.

TOTAL FIELD INSTRUMENTS

Many magnetometers measure the magnitude of the field vector. Airborne flux-gate instruments use auxiliary devices to continually orient these analog sensors parallel to the field. These instruments were in wide use from the 1940’s onward. In the 1960’s proton precession instruments became common. They sense only the field magnitude and do not require orientation. The only drawbacks are that they yield digital output at intervals of a few seconds and do not work in high gradient areas. Alkali vapor (optically pumped, cesium) total field magnetometers are more precise and produce essentially continuous output (?).

TOTAL FIELD ANOMALIES

In deriving formulae for total field anomalies of simple bodies we’ll really be finding the component of the total field (that is, vector sum of background field plus field of body) taken along the background field direction. This approximation is very good in the usual case where the field of the body is much less that the background field. The approximation would be very poor in places like Kursk in Russia where the anomalous field is bigger that the background field.

EFFECT OF LATITUDE OR INCLINATION

Let’s sketch south-to-north profiles over a dipole source as a function of latitude or background field inclination. As before, we do this by drawing the dipole lines of force and then estimating the components parallel to Bo. Remember that the field strength is inversely related to the spacing of the lines of force.

At high latitudes the total field anomaly over a dipole (sphere) with induced magnetization looks like the vertical field, vertical component case. There is a positive peak over the dipole and broad, weak negatives around it. On a map the contours are circles.

At a mid-latitude site we see that the positive peak is south of the body and that there is a weak negative to the north. The map pattern is not circular.

At a very low latitude the total field profile is the same as that of the horizontal field, horizontal component case. There are symmetrical positive anomalies south of the sphere and north of it. Over the sphere we see a big negative anomaly. The map pattern is not circular in this case either.

EFFECT OF REMANENT MAGNETIZATION

Here the number of possible patterns becomes outrageous. The NRM can be any direction and have any magnitude with respect to the induced magnetization. We’ll investigate several cases in the same way as before: sketch the lines of force and take the appropriate components.

ANOMALY FORMULAE

These can be found in texts such as Telford et al., 1990, and in professional journal articles. Many modern computer packages include options for selecting body shape, remanent and induced magnetization vectors, profile and map view output and so on. Be sure to test the program with some simple cases because coding errors may be present.

REFERENCES AND READINGS

Butler and Kean, 1993, Geophysics, 58, 434-440.

Doyle, 1990, Geophysics, 55, 134-146.

Gay and Hawley, 1991, Geophysics, 56, 902-913.

McCaffrey et al., 1995, Geophysics, 60, 408-412.

Telford, W. M., L. P. Geldart and R. E. Sheriff, 1990, "Applied Geophysics" (second edition), Cambridge University Press, 770 pp.