Geology Reference
In-Depth Information
The design and implementation of a geophysical survey requires careful
consideration of the following main factors:
(a)
Target discrimination
The nature and degree of the contrast in physical properties between a
target and its surroundings is of primary importance in the feasibility
assessment and choice of techniques. However, information may be
limited or non-existent, and in these cases the geophysicist should
recommend a trial survey or the application of multiple techniques.
Trials are recommended wherever the assumptions made in designing
the survey are suspect. Usually a day is all that is required to determine
whether the chosen methods can detect the presence of a target in actual
field conditions. This is an often neglected stage in the execution of
a geophysical survey but is one that could save much geophysicist's
pride and client's money were it more routinely used.
Once it has been decided, on the basis of observation, modelling
and/or experience, what the geophysical response of a buried target is
likely to be, the sensitivity of the equipment and the distribution of the
survey stations needed to meet the survey objectives can be specified.
(b)
Detection distance
In addition to the composition of the target and its surroundings, geo-
physical methods are sensitive to the relationship between target size
and detection distance. In general, the greater the depth of the target,
the larger its volume and/or cross-sectional area must be for it to be
detectable.
(c)
Survey resolution
The choice of sampling interval (frequency or spacing of sampling
points) is critical to the success of a survey and its cost-effectiveness.
The appropriate interval is dictated by the geophysical 'footprint' of
the target, which may be tens of centimetres for small-diameter shallow
pipes, a few metres for narrow fault zones, and kilometres for ore bodies
at depth. An anomaly must be adequately sampled to meet the survey
objectives. Although it is almost equally important that resources are
not wasted in collecting more data than are required, it has to be
remembered that under-sampling can produce completely fictitious
anomalies (Figure 1.7).
In some cases, particularly on brownfield sites, surface obstructions
can prevent the collection of regularly spaced data. The obstructions
may be removable, but unless their impact on the survey outcome is
fully understood by the field observer, they may not be dealt with at
the appropriate time.
