GRAVITY METHOD - Field Geophysics - page 44

precision. Calibration factors vary slightly over G-meter ranges, being tab-
ulated by the manufacturer for 100-mGal intervals.
Because steel is a good conductor of heat, LaCoste G and D meters cannot
be adequately insulated and thermostatic control is essential. Their weight,
of about 5 kg, is effectively doubled by the rechargeable battery required
by the heater. A battery charger is needed in the field, since a single charge
lasts only one or two days, depending on thermostat setting and external
temperature. Batteries must be changed before they are exhausted because
the drift for two or three hours after reaching operating temperature is so
high that the instrument is unusable. Drift may then be very low, in which
case it can be extrapolated linearly across intervals during which the meter
has been off-heat. However, the main form of drift is by discontinuous tares ,
of perhaps as much as 1 mGal, which can occur at any time. An instrument
that suffers tares more than about once a month requires servicing.
Even quite inexperienced observers have little difficulty in achieving
accuracies of 0.01 mGal with LaCoste meters, particularly if an electronic
readout has been installed.
2.2.4 Setting up a manual gravity meter
Manual gravity meters are normally read on concave dishes supported by
three short stubs, which should be pressed firmly but not too deeply into
the ground. The undersurface of the dish must not touch the ground since
a fourth support point allows 'rocking' back and forth. Thick grass under
the dish may have to be removed before a reading can be taken. Extension
legs may be attached to the stubs but readings will then take longer, the dish
itself may have to be levelled (some incorporate a bull's-eye bubble) and the
height of the meter above the ground will have to be measured.
The meters themselves rest on three adjustable, screw-threaded feet and
are levelled using two horizontal spirit-levels (Figure 2.3), initially by being
moved around the dish until both level bubbles are 'floating'. The temptation
to hurry this stage and use the foot-screws almost immediately should be
resisted.
Usually one of the levels (probably the cross-level , at right angles to the
plane of movement of the balance arm) is set parallel to a line joining two
of the feet. Adjustments to the third foot then scarcely affect this level. The
quickest method of levelling is to centre the cross-level bubble, using one or
both of the two foot-screws that control it, and then use the third screw to set
the long-level. Experienced observers often use two screws simultaneously
but the ability to do this efficiently comes only with practice.
Once the meter is level, the reading is obtained by rotating a calibrated
dial to bring a pointer linked to the spring system to a fixed point on a
graduated scale viewed through a telescopic eye-piece. The alignment is