GRAVITY METHOD - Field Geophysics - page 49

deteriorate with age more quickly than other rechargeable batteries, and
overheating or overcharging can destroy them, or even make them explode.
They can also be damaged if they are allowed to discharge completely. The
CG-5 gives an audible warning as full discharge is approached, and the
charger supplied is designed to prevent overcharging.
Surveying with the CG-5 is not always just a matter of switching on and
starting work. The manufacturer recommends that the instrument should be
stored 'on-heat', but if it has been powered down, it requires 4 hours to reach
operating temperature and a further 48 hours to fully stabilise. Clearly, it
is better to bring it fully operational to the survey area, but this may not
be possible if the travel time exceeds half a day. Transport by air presents
additional problems. The regulations covering lithium-ion batteries make it
illegal for the meter to be in the hold baggage, and this can be useful when
dealing with ground staff who want to see the meter handed over to the
tender mercies of their baggage handlers. However, although the capacity
of the batteries is in principle low enough for them to be accepted as cabin
baggage, their mere presence means that the meter can be rejected simply
'at the discretion of the airline'.
All the techniques used, in surveys with manual meters, to control drift
and convert meter readings to absolute values by the use of base stations
must be used also in surveys with automatic meters.

2.3 Gravity Reductions
In gravity work, more than in any other branch of geophysics, large and (in
principle) calculable effects are produced by sources that are not of direct
geological interest. These effects are removed by reductions that involve the
sequential calculation of a number of recognised quantities. In each case a
positive effect is one that increases the magnitude of the measured field and
the sign of the reduction is opposite to that of the effect it is designed to
remove.

2.3.1 Latitude correction
Latitude corrections are usually made by subtracting the normal gravity,
calculated from the International Gravity Formula, from the observed or
absolute gravity. For surveys not tied to the absolute reference system, local
latitude corrections may be made by selecting an arbitrary base and using
the theoretical north-south gradient of about 0.812 sin 2 λ mGal km − 1 .

2.3.2 Free-air correction
The remainder left after subtracting the normal from the observed gravity
will be due in part to the height of the gravity station above the sea-level
reference surface. An increase in height implies an increase in distance from