INTRODUCTION - Field Geophysics - page 29

1.6 Bases and Base Networks
Bases or base stations are important in gravity and magnetic surveys, and
in some electrical and radiometric work. They may be:

1. Drift bases - Repeat stations that mark the starts and ends of sequences

of readings and are used to control drift.

2. Reference bases - Points where the value of the field being measured

has already been established.

3. Diurnal bases - Points where regular measurements of background are

made whilst field readings are taken elsewhere.

A single base may fulfil more than one of these functions. The reliability
of a survey, and the ease with which later work can be tied to it, will often
depend on the quality of the base stations. Base-station requirements for
individual geophysical methods are considered in the appropriate chapters,
but procedures common to more than one type of survey are discussed below.

1.6.1 Base station principles
There is no absolute reason why any of the three types of base should
coincide, but surveys tend to be simpler and fewer errors are made if every
drift base is also a reference base . If, as is usually the case, there are too
few existing reference points for this to be done efficiently, the first step in
a survey should be to establish an adequate base network.
It is not essential that the diurnal base be part of this network and, because
two instruments cannot occupy exactly the same point at the same time, it
may actually be inconvenient for it to be so. However, if a diurnal monitor
has to be used, work will normally start each day by setting it up and end with
its removal. It is good practice to read the field instruments at a drift base
at or near the monitor position on these occasions, noting any differences
between the simultaneous readings of the base and field instruments.

1.6.2 ABAB ties
Bases are normally linked together using ABAB ties (Figure 1.13). A read-
ing is made at Base A and the instrument is then taken as quickly as possible
to Base B. Repeat readings are then made at A and again at B. The times
between readings should be short so that drift, and sometimes also back-
ground variation, can be assumed linear. The second reading at B may also
be the first in a similar set linking B to a Base C, in a process known as
forward-looping .
Each set of four readings provides two estimates of the difference in field
strength between the two bases, and if these do not agree within the limits
of instrument accuracy ( ± 1 nT in Figure 1.13), further ties should be made.