Geology Reference
In-Depth Information
Figure 3.6
Diurnal control, with variations monitored both by a memory
instrument at the diurnal base and by repeat readings at various sub-bases.
The time periods during which individual survey lines were being read are
also shown. The shifts needed to make the sets of sub-base values fall on
the diurnal curve provide estimates of the differences between the standard
values (SVs) at the diurnal base and at the sub-bases. The greatest error
introduced by using straight-line interpolation between the diurnal values
derived from measurements at the sub-bases would have been about 5 nT
and would have affected Line 1700S over the thickened part of the curve.
Interpolation using a smooth curve instead of straight lines would have
significantly reduced this error.
be processed by hand each evening. If only a contour map is required, this
can be based on profiles of uncorrected readings, as shown in Figure 3.7.
Fewer calculations are needed, and errors and peculiarities in the data are
immediately obvious.
Even if no computer is available to do the hard work, plotting magnetic
profiles should be a field priority since these provide the best way of assessing
the significance, or otherwise, of diurnal effects and noise. For example, the
profile in Figure 3.7 shows very clearly that, with 100 nT contours, the 5 nT
discrepancy shown in Figure 3.6 between the diurnal curves based on direct
observation and base reoccupations is unimportant. It also shows the extent
to which the contours leave significant magnetic features undefined.
If a computer is used to calculate corrected values at each field point,
profiles should still be produced, for quality control, but using the corrected
