The external magnetic field

The changes in geomagnetic field caused by external sources have lower amplitude than changes stemming from the internal changes, however, their period is much shorter and thus could seriously affect magnetic survey. The periodical variations are the Solar diurnal variations with period of 24 hours and amplitude of several tens of nT and Lunar variations with period of 25 hours and amplitude of 2 nT. Next, there are emphshort period variations with periods of tenth of seconds up to tu tens of minutes with amplitudes from tenth of nT up to tens of nT. These could be periodical or random and are mostly effects of the Solar activity. The most important are magnetic storms. They are effect of increased Solar activity, could appear several times per month and last even for several days. The amplitude of the storms could be several thousands of nT and have a random fluctuation. Effects of these variations could be easily removed from the measured magnetic data in a similar way as in the gravity prospection – using a base station and subtracting the base-station data from the measured ones. However, the magnetic storm has such a high amplitudes and random course that it is best to avoid measurements during the storm.

Spatial variations of the main field

Local variations of the main field originates from the local change in concentration of magnetic minerals in the near surface rocks. The anomalies could have very different amplitudes, exceptionally they could even double the Earth’s main field. They are usually localised and hence the

magnetic maps are often hard to read (Fig. 3.10, compare with the Bouguer anomaly map in Fig. 2.20). The sources of magnetic anomalies could not be very deep since temperatures below ∼ 40 km should be above the Curie point, the temperature at which rocks lost their magnetic properties (≈ 550 ◦C). Thus local anomalies must be associated with features in the upper crust.