Geology Reference
In-Depth Information
Figure 1.7
Aliasing. The dashed curve shows a magnetic profile as it should
have been recorded, the solid line shows the spurious anomaly that would be
deduced by using only data from the widely spaced reading points indicated
by vertical lines on the distance axis. Aliasing can occur in time as well as
in space, if time-varying signals are sampled too infrequently.
(d)
Site conditions
The suitability of a site for collecting good quality geophysical data is
often overlooked in survey design. The issues affecting data quality that
could be of concern are often specific to the method or methods being
proposed. For example, signal degradation may occur or geophysical
'noise' may be introduced in electromagnetic and magnetic surveys
by the presence of surface metallic structures and overhead power
lines. In microgravity or seismic surveys, noise may result from traffic
movements or wind and waves. If the noise exceeds the amplitude of
the anomaly due to the target and cannot be successfully removed, the
target will not be detectable. The best way to assess the likely influence
of site conditions is to visit the site at the design stage and/or carry out
a trial survey.
Field observers should be fully briefed on the objectives of the survey
and mindful of the design aspects, so that departures of the field conditions
from any assumptions made can be reported in good time, allowing the
design to be modified where possible. They should immediately report any
unexpected conditions, and any geological information provided by drillers
to which the geophysicist who designed the survey may not have been privy.
They may also obtain useful information relating to previous land-use in
conversations with the client or casual passers-by, and this also should be
passed on.
1.3.2 Preparing for a survey
The design of a regional or even a local geophysical survey can be greatly
assisted by using the geographic data now freely available on the internet.
