GRAVITY METHOD - Field Geophysics - page 46

Other precautions have to be taken. It is vital that the level bubbles are
checked whilst the dial is being adjusted, and especially immediately after
a supposedly satisfactory reading has been taken, since natural surfaces can
subside slowly under the weight of an observer, off-levelling the meter. On
snow or ice, the levels have to be adjusted almost continuously as the dish
melts its way down, unless it is insulated from the surface by being placed
on a small piece of plywood.
All mechanical measuring systems suffer from whiplash , and two readings
will differ, even if taken within seconds of each other, if the final adjustments
are made by opposite rotations of the reading dial. The only remedy is total
consistency in the direction of the final adjustment.
Severe continuous vibration, as from nearby machinery or the roots of
trees moved by the wind, can make reading difficult and may even displace
the reading point. Earthquakes can make the pointer swing slowly from side
to side across the field of view and survey work must be stopped until the
disturbance is over. Fortunately, this effect is rare in most parts of the world,
although very large earthquakes can affect gravity meters at distances of
more than 10 000 km.
2.2.5 Meter checks
A series of checks should be made each day before beginning routine survey
work. The meter should first be shaken down by tapping the dish gently with
a pencil between readings until a constant value is recorded (this can also
be done if, as sometimes happens, the pointer 'sticks' on one or other of the
stops).
The levelling system should then be checked. Because astatic systems
are asymmetric, the effect of a levelling error depends on the direction of
tilt. A slight error on the cross-level, which is at right angles to the balance
arm, produces a reading of gravity field multiplied by the cosine of the
tilt angle (an error of about 0.015 mGal for a tilt of 0.01 ◦ ). Off-levelling a
correctly adjusted cross-level will therefore reduce the reading, regardless
of the direction of offset. To check that this actually happens, the meter
should be set up and read normally and the cross-level should then be offset
by equal amounts in both directions. The pointer should move roughly
the same distance in the same direction in each case. Meters are usually
considered usable if the movements are at least in the same direction, but
otherwise the level must be adjusted.
If the long-level, in the plane of the balance arm, is in error, reading
sensitivity (the amount the pointer moves for a given rotation of the dial) is
affected. The recommended sensitivity and instructions for resetting will be
found in the handbook. The actual sensitivity can be estimated by moving
the dial by a set amount and noting the pointer movement. After adjustment,