Geology Reference
In-Depth Information
levels often take a few days to settle into their new positions, during which
time they must be rechecked with special care.
2.2.6 Meter calibration
The readings from LaCoste manual meters are combinations of values read
from a dial and numbers displayed on a mechanical counter. The sensitivity
of G-meters is such that the final figure read from the dial corresponds to
approximately 0.01 mGal.
Readings are converted to gravity units using calibration factors specific to
the individual instrument. The factors quoted by the manufacturers require
the insertion of a decimal point somewhere in the reading, its position
depending on whether results are desired in mGal or g.u. They are not
affected by changes in reading sensitivity but may alter slowly with time
and should be checked regularly. This can be done by the manufacturers
or by using calibration ranges of known gravity interval. Calibration ranges
usually involve gravity changes of about 50 mGal, which is within the range
of even the most limited-range meters, and almost always make use of
the rapid change of gravity field with elevation. An elevation difference of
about 250 m is generally necessary, although in some cases local gravity
gradients can also play a part. Travel times between top and bottom stations
should normally be less than 15 minutes, and the two stations should be
well marked and described. A
run
should consist of at least an ABAB tie
(Section 1.6.2), giving two estimates of gravity difference. If these differ by
more than 0.02 mGal, more links should be added.
D-meters, which have separate fine and coarse adjustment dials, can be
checked over different sections of their fine ranges by slightly altering the
coarse setting. Most meters need a little time to stabilise after coarse adjust-
ment, but if this is allowed it may be possible to identify minor irregularities
in the calibration curve. With G-meters, only one part of the curve can be
monitored on any one calibration range and, because of slight irregularities
in the pitch of the adjustment screws, different meters used on the same
range may give results that differ consistently by a few hundredths of a
milliGal.
Commercial manufacture of manual gravity meters has now ceased. They
are still in use in large numbers and may be available second-hand, but are
steadily being replaced by automatic models.
2.2.7 Automatic gravity meters - the CG-5
In the automatic CG-5 (Figure 2.4), the reading system, batteries and
vacuum-insulated quartz sensor are all contained within a single housing
and carrying case, which considerably simplifies field operations. The elim-
ination of the cable, which in LaCoste meters links the instrument to the
