INTRODUCTION - Field Geophysics - page 37

Figure 1.18 Distortion in automated contouring of linear anomalies.
(a) Introduction of closures in the peak of a linear aeromagnetic anomaly
caused by the contouring program seeking (as most do) to equalise gradients
in all directions. A similar effect can be seen in the 'bubbling' of the very
closely spaced contours on the south side of the anomaly in (b). In neither
case are the features required by the actual data, which exist only along the
traverse lines indicated by lines in (a) and by discrete points in (b). (b) 'Her-
ringbone' pattern due to a consistent difference in background levels on lines
measured in opposite directions (see discussion in text). The effect is barely
visible on the large main anomaly (thick contours at 100-nT intervals) but
very obvious in the low-gradient areas where contours are at 10-nT intervals.
(c) 'Herringbone' pattern due to parallax error. In this case there is a consis-
tent offset between contour 'cuts' along lines recorded in opposite directions,
regardless of anomaly magnitude.
will inevitably be a tendency for a sensor carried in front of the observer to be
closer to the ground when going uphill than when going downhill. How this
effect will appear on the final maps will vary with the nature of the terrain,
but in an area with constant slope there will be a tendency for background
levels to be different on parallel lines traversed in opposite directions. This
can produce herringbone effects on individual contour lines in low gradient
areas (Figure 1.18).
Heading errors occurred in airborne (especially aeromagnetic) surveys
because the effect of the aircraft on the sensor depended on aircraft orienta-
tion. A similar effect can occur in a ground magnetic survey if the observer
is carrying any iron or steel material. The induced magnetisation in these
objects will vary according to the facing direction, producing effects similar
to those described above as being produced by constant slopes.