PRECISION DANS LES ETUDES MAGNETIQUES AERIENNES SON INCIDENCE SUR LA QUALITE DES RESULTATS*

The accuracy of an airborne magnetic survey is limited by the following factors:

• 1  Slow (diurnal) variations of the earth's field.
• 2  Fast variations of the earth's field (earth field “noise”).
• 3  Drift in measuring equipment.
• 4  Magnetometer “noise” (electronics, orientors …).
• 5  External disturbances, miscompensation.
• 6  Irregularities in flight altitude.
• 7  Location errors.

The relative importance of these different causes of errors is analyzed from two viewpoints:

• a)  Direct interpretation of anomalies on actual records,
• b)  Compilation of isogram maps and their quantitative interpretation (second derivative, reduction to the pole, …).

In conclusion, these errors do not affect the direct interpretation of profiles of standard quality. For instance, if the average altitude of the plane above the magnetic bodies is 1000 m, a variation of its flight level of about 100 m does not introduce serious errors in the interpretation.

On the other hand, compilation of contoured isogam maps is often difficult, and compensation of the previous errors is not always possible except by approximate, rule‐of‐thumb methods. Furthermore, when the interval between lines is greater than the depth of magnetic masses below the plane of flight, the contours are interpolated and do not represent the magnetic field. Sometimes the contouring can offer different solutions, and the map is therefore not objective; this could then result in errors larger than those from the other causes.

To evaluate the quality of the map and its interpretation we must know the location of flight lines and the compilation process employed. It is also wise to avoid quantitative interpretation of small anomalies and limit oneself to a qualitative interpretation.