Geology Reference
In-Depth Information
by exact reoccupations. Concrete or steel markers can be quickly destroyed,
either deliberately or accidentally, and it is usually better to describe station
locations in terms of existing features that are likely to be permanent. In
any survey area there will be such points that are distinctive because of the
presence of man-made or natural features. Written descriptions and sketches
are the best way to preserve the information for the future. Sketches, such
as those shown in Figure 2.7, are usually better than photographs, because
they can emphasise salient points.
Permanence can be a problem, and maintaining gravity bases at interna-
tional airports is almost impossible because building work is almost always
underway (and, these days, because attempting to read a geophysical instru-
ment anywhere near an airport is likely to trigger a security alert). Geodetic
survey markers are usually reliable but may be in isolated and exposed lo-
cations. Statues, memorials and historic or religious buildings often provide
sites that are not only quiet and permanent but also offer some shelter from
sun, wind and rain.
1.7 Real-Time Profiling
During the past 20 years, automation of the geophysical equipment used in
small-scale surveys has progressed from a rarity to a fact of life. Although
many of the older types of instrument are still in use, and giving valuable
service, they now compete with variants containing the sort of computer
power employed, 40 years ago, to put a man on the Moon.
1.7.1 Data loggers
The integration of data loggers into geophysical instruments has its draw-
backs. At least one manufacturer proudly boasted 'no notebook', even
though the instrument in question was equipped with only a numerical key
pad so that there was no way of entering text comments (
metadata
) into the
(more than ample) memory. Other automated instruments have data displays
that are so small and so poorly positioned that the possibility that the ob-
server might actually want to look at, and even think about, the observations
as they are being collected has clearly not been considered. Unfortunately,
pessimism in this respect is often justified, partly because of the speed with
which readings, even when essentially discontinuous, can now be taken and
logged. Quality control thus often depends on the subsequent playback and
display of whole sets of data, and it is absolutely essential that this is done
at least once every day. As Oscar Wilde might have said (had he opted for a
career in field geophysics), to spend a few hours recording rubbish might be
accounted a misfortune. To spend anything more than a day doing so looks
suspiciously like carelessness.
