Geology Reference
In-Depth Information
If transmitters, receivers and/or electrodes are laid out in straight lines and
the whole array can be reversed without changing the reading, the mid-point
should be considered as the reading point. Special notations are needed for
asymmetric arrays, and the increased probability of positioning error is in
itself a reason for avoiding asymmetry. Great care must be taken in recording
the positions of sources and detectors in seismic work.
1.5.1 Station numbering
Station numbering should be logical and consistent. Where data are collected
along traverses, numbers should define positions in relation to the traverse
grid. Infilling between traverse stations 3 and 4 with stations 3
1
/
4
,3
1
/
2
and
3
3
/
4
is clumsy and may create typing problems, whereas defining as 325E a
station halfway between stations 300E and 350E, which are 50 metres apart,
is easy and unambiguous. The fashion for labelling such a station 300
+
25E
has no discernible advantages and uses a plus sign that may be needed, with
digital field systems or in subsequent processing, to stand for N or E. It is
good practice to define the grid origin in such a way that S or W stations do
not occur, and this may be essential with data loggers that cannot cope with
either negatives or directions.
Stations scattered randomly through an area are best numbered sequen-
tially, as read. Positions can be recorded in the field by pricking through
the field maps or air-photos and labelling the reverse sides. Estimating co-
ordinates from maps in the field may seem desirable but mistakes are easily
made and valuable time is lost. Station coordinates are now often obtained
from GPS receivers (see Section 15.2), but differential or RTK (
real-time
kinetic
) GPS may be needed to provide enough accuracy in detailed surveys.
If several observers are involved in a single survey, numbers can easily be
accidentally duplicated. All field topics and sheets should record the name
of the observer. The interpreter or data processor will need to know who to
look for when things go wrong.
1.5.2 Recording results
Geophysical results are primarily numerical and must be recorded even more
carefully than the qualitative observations of field geology. Words, although
sometimes difficult to read, can usually be deciphered eventually, but a set of
numbers may be wholly illegible or, even worse, may be misread. The need
for extra care has to be reconciled with the fact that geophysical observers
are usually in more of a hurry than are geologists, since their work may
involve instruments that are subject to drift, draw power from batteries at
frightening speed or are on hire at high daily rates.
Numbers may, of course, not only be misread but also miswritten. The
circumstances under which data are recorded in the field are varied but
