Geology Reference
In-Depth Information
seldom ideal. Observers are usually either too hot, too cold, too wet or too
thirsty. They may, under such conditions, delete correct results and replace
them with incorrect ones, and data recorded on geophysical field sheets
should therefore never be erased. Corrections should be made by crossing
out the incorrect items, preserving their legibility, and writing the corrected
values alongside. Something may then be salvaged even if the correction
is wrong. Precise reporting standards must be enforced and strict routines
must be followed if errors are to be minimised. Reading the instrument
twice at each occupation of a station, and recording both values, reduces the
incidence of major errors.
Loss of geophysical data tends to be final. Some of the qualitative obser-
vations in a geological notebook might be remembered and re-recorded, but
not strings of numbers. Copies are therefore essential and should be made
in the field, using duplicating sheets or carbon paper, or by transcribing the
results each evening. Whichever method is used, originals and duplicates
must be separated immediately and stored separately thereafter. Duplication
is useless if copies are stored, and lost, together with the originals. This,
of course, applies equally to data stored in data loggers incorporated in, or
linked to, field instruments. Such data should be downloaded, checked and
backed-up each evening.
Digital data loggers can greatly simplify field operations but are often
poorly adapted to storing non-numeric metadata. This design feature ig-
nores the fact that observers are uniquely placed to note and comment on
a multitude of topographic, geological, man-made (
cultural
) and climatic
factors that may affect the geophysical results. If they fail to do so, the
data they have gathered may be interpreted incorrectly. If data loggers are
not being used, comments should normally be recorded in the notebooks,
alongside the readings concerned. If they are being used, adequate supple-
mentary positional data must be stored elsewhere. In archaeological and site
investigation surveys, where large numbers of readings are taken in very
small areas, annotated sketches are always useful and may be essential.
Sketch maps should be made wherever the distances of survey points or
lines from features in the environment are important. Field observers also
have a responsibility to pass on to their geological or geophysical colleagues
information of interest about places that only they may visit. Where these
would be useful, they should be prepared to record dips and strikes, and
perhaps to return with rock samples.
1.5.3 Accuracy, sensitivity, precision
Accuracy must be distinguished from sensitivity. A modern gravity meter, for
example, may be sensitive to field changes of 1 microGal but an equivalent
level of accuracy will be achieved only if readings are carefully made and
