Geology Reference
In-Depth Information
2.4.5 Drift corrections
Absolute observed gravities are obtained by adding the absolute value at the
drift base to the drift-corrected gravity differences.
To correct manually for instrument drift, readings are first tidally corrected
and the corrected initial reading at the drift base is then subtracted from every
other corrected reading in turn. The result of doing this to the final reading
at the drift base gives the total drift. The
pro rata
corrections to the other
stations can then be calculated or estimated graphically to the necessary
accuracy. The drift is assumed to be linear and the sign of the correction is
dictated by the requirement that, after correction, the relative values for all
occupations of the drift-base should be zero.
Short-term drift is dependent mainly on temperature, and the assumption
that it has been linear in the time between two base readings is unlikely to
be true if large changes in temperature have occurred and then been wholly
or partly reversed during that time. In the CG-5, the internal temperature is
recorded and compensated automatically.
2.4.6 Barometric pressure and water-level corrections
Gravity meters will 'drift' in response to changes in atmospheric pressure,
but magnitudes have been steadily reduced by improved instrument design.
For the CG-5, the quoted value of pressure sensitivity is now only 0.015 µGal
per millibar (0.15 µGal kPa
−
1
), so that typical diurnal changes of several
millibars have negligible effects. On the other hand, an elevation change of
only 10 metres produces a change in pressure of about a millibar, so that
successive readings at points differing in elevation by hundreds or thousands
of metres can be measurably affected by pressure-induced apparent drifts.
It is hard to imagine such surveys requiring microGal accuracies.
Large pressure changes imply changes in atmospheric loading, and a mil-
libar increase in pressure will reduce the real gravity field by 0.3-0.4 µGal.
The lack of a universally applicable formula for calculating this effect is
another reason why, in very-high-accuracy work, frequent visits to base
stations are advisable. Altitude-dependent pressure changes are taken into
account in the 1980 version of the International Gravity Formula (see
Section 2.1.1).
Readings taken on openwork piers or jetties are affected by tidal changes
in sea level. If the sea actually comes in under a reading point, the effect
could be as much as 0.04 mGal for a metre change in sea level. Occasionally
base stations have been established on such structures for use by research
vessels, but there can be few other reasons for reading a gravity meter on
them. A reading taken on a mole (rock-fill jetty) or on a low cliff dropping
steeply to the sea could also be affected, perhaps by as much as 0.02 mGal
per metre of sea-level change.
