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 can be due to stretching of the spring over time or to changes in spring properties related to temperature changes. To help minimize the latter, gravimeters are either temperature controlled or constructed out of materials that are relatively insensitive to temperature changes. Even still, gravimeters can drift as much as 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 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.

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*Definition from the Encyclopedic Dictionary of Exploration Geophysics by R. E. Sheriff, published by the Society of Exploration Geophysics.

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