Page 148 - Introduction to Mineral Exploration

7: GEOPHYSICAL METHODS 131

80° 80°
60°
70° 60,000
60,000
50,000 60°
50° 30°
40° 30° 40,000
40,000
20° 10°
0° 10° 0°
20° 30°
40° 50°

60° 30°
70 o
30,000 80°

60°
70°
50,000 60,000
50,000
60,000
40,000
80°
180° 120° 60° 0° 60° 120°
FIG. 7.3 The Earth’s magnetic field. Solid lines are contours of constant dip, dashed lines are contours of
constant total field strength (in nT).

intervals of 5 nT or less. During such surveys, Magnetized bodies produce very different
fixed ground magnetometers provide digital anomalies at different magnetic latitudes
records of readings taken every few minutes. (Fig. 7.4), the regional changes in dip being far
Corrections are made either by using these more important from an interpretation point
records directly or by adjustments based on of view than the regional changes in absolute
systems of tie lines flown at right angles to magnitude. Because of the bipolar nature of
the main lines of the survey, or (preferably) by magnetic sources, all magnetic anomalies have
a combination of the two. Tie lines can be both positive and negative parts and the net
misleading if they cross flight lines in regions total magnetic flux involved is always zero.
of steep magnetic gradients, and especially if This is true even of vertically magnetized
the terrain is rugged and the aircraft was at sources at a magnetic pole, but the negative
different heights on the two passes over an flux is then widely dispersed and not obvious
intersection point. on contour maps. Magnetic maps obtained near
During periods of sunspot activity, streams the poles are therefore more easily interpreted
of high energy particles strike the upper atmos- than those obtained elsewhere, and reduction-
phere and enormously increase the ionospheric to-pole (RTP) processing has been developed
currents, causing changes of sometimes hun- to convert low and middle latitude maps to
dreds of nanotesla over times as short as 20 their high latitude equivalents. If, however (as
minutes. Corrections cannot be satisfactorily is usually the case with strongly magnetized
made for these “magnetic storms” (which are bodies), there are significant permanent mag-
not related to meteorological storms and may netizations in directions different from the
occur when the weather is fine and clear) and present Earth field, RTP processing will distort
work must be delayed until they are over. and displace anomalies. It is far better that