Geology Reference
In-Depth Information
Figure 3.7
PROFILE: Contour cuts at 100-nT intervals on uncorrected
profile 1700S, by diurnal curve and parallel curves. The reference base
standard value (SV) is 32 100 nT, and the points at which the diurnal curve
intersects the profile therefore correspond to points on the ground where the
corrected value of the magnetic field is also 32 100 nT. Curves parallel to
the diurnal identify points at which the SVs differ from those at the diurnal
base by integer multiples of 100 nT, which is an appropriate contour interval
for mapping the data shown. MAP: Contour cut plot of Line 1700S and two
adjacent lines. Only the 'cuts' need be plotted on the map and some may be
omitted if they are very close together.
data. The profiles that are so easily produced from the semi-continuous data
provided by alkali-vapour instruments should be carefully examined for
artefacts of the sorts described in Section 1.7.4.
3.4.6 Noise in ground magnetic surveys
Magnetic readings in populated areas are often affected by
cultural noise
,
i.e. stray fields from pieces of iron and steel that are irrelevant to the survey
objectives. Since these are often quite small and likely to be buried within a
metre of the ground surface, the effects are very variable. Even if no obvious
sources are visible, profiles obtained along roads are usually very distorted
