Instrument Drift
Definition
Drift - A gradual and unintentional change in the reference value with respect to which
measurements are made*.
Although constructed to high-precision standards and capable of measuring changes in gravitational
acceleration to 0.01 mgal, problems do exist when trying to use a delicate instrument such
as a gravimeter.
Even if the instrument is handled with great care (as it always should be - new gravimeters
cost ~$30,000), the properties of the materials used to construct the spring can change with
time. These variations in spring properties with time can be due to stretching of the spring
over time, or to changes in spring properties related to temperature changes. To help
minimize the later, gravimeters are either temperature controlled or constructed out of
materials that are relativel insensitive to temperature changes. Even still, gravimeters can
drift as much as a 0.1 mgal per day.
Shown above is an example of a gravity data set** collected at the same site over a
two day period. There are two things to notice from this set of observations. First, notice the oscillatory
behavior of the observed gravitational acceleration.
This is related to variations in gravitational acceleration caused by the tidal attraction
tidal attraction of the
sun and the moon.
Second,
notice the general increase in the gravitational acceleration with time. This is
highlighted by the green line. This line represents a least-squares, best-fit straight line to the
data. This trend is caused by instrument drift. In this particular example, the instrument
drifted approximately 0.12 mgal in 48 hours.
*Definition from the Encyclopedic Dictionary of Exploration Geophysics by R. E. Sheriff,
published by the Society of Exploration
Geophysics.
**Data are from: Wolf, A. Tidal Force Observations, Geophysics, V, 317-320, 1940.