Geology Reference
In-Depth Information
compared to those along parallel traverses through open fields only 10 or
20 m away.
One approach to the noise problem is to try to take all readings well
away from likely sources, noting in the field topics where this has not been
possible. Alternatively, the almost universal presence of ferrous noise can
be accepted and the data can be filtered. For this method to be successful,
many more readings must be taken than would be needed to define purely
geological anomalies. The technique is becoming more popular with in-
creasing use of data loggers, which discourage note-taking but allow vast
numbers of readings to be taken and processed with little extra effort, and
is most easily used with alkali vapour and fluxgate instruments, which read
virtually continuously. It is, however, only safe to dispense with notebooks
for comments on individual stations if the survey grid as a whole is well
surveyed, well described and accurately located.
High-cut filters cannot be used in archaeological surveys, since they are
likely to remove the signals along with the noise. Data in such surveys are
usually displayed as images in which each pixel corresponds to an individual
reading point and is coloured or grey-shaded according to the value (see
Figure 1.12). Low-cut filters may be used prior to display to emphasise the
short-wavelength features.
3.5 Simple Magnetic Interpretation
Field interpretation of magnetic data allows areas needing infill or checking
to be identified and then revisited immediately and at little cost. Good inter-
pretation requires profiles, which preserve all the detail of the original read-
ings, and contour maps, which allow trends and patterns to be identified. For-
tunately, the now almost ubiquitous laptop computer has reduced the work
involved in contouring (providing the necessary programs have been loaded).
3.5.1 Forms of magnetic anomaly
The shapes of magnetic anomalies vary dramatically with the dip of the
Earth's field, as well as with variations in the shapes of the source bodies
and their directions of magnetisation. Simple sketches can be used to obtain
rough visual estimates of the anomaly produced by any magnetised body.
Figure 3.8a shows an irregular mass magnetised by induction in a field
dipping at about 45
◦
. Since the field direction defines the direction in which
a positive pole would move, the effect of the external field is to produce
the distribution of poles shown. The secondary field due to these poles is
indicated by the dashed lines of force. Field direction is determined by the
simple rule that like poles repel.
If the secondary field is small, the directions of the total and background
fields will be similar and the anomalous field will not be detected near C
