3.6. Data Quality Control

Variometer drift should be monitored regularly through frequent absolute observations. Chapter 4 describes absolute measurements and Chapter 5 describes the derivation of continuous variometer baselines from spot measurements and the calculation of baseline corrected data.

It is mandatory that continual recordings of the variometer temperature are also made as temperature recordings can be of benefit when determining the source of baseline variations. It is important to monitor both the variometer electronics and sensor separately as the electronics and the sensor typically have differing temperature coefficients.

The use of a continually recording scalar magnetometer is mandatory for one-second IMO. Since proton precession magnetometers and Overhauser effect magnetometers generally have a very low temperature coefficient and output near-absolute values without the need for baseline measurements, they provide a high quality reference data set against which baseline-corrected vector variometer data can be compared, albeit only the scalar component. Such comparisons can identify baseline drift in the variometer or changes in the local field gradient. In addition, as the variometer and the scalar magnetometer are spatially separated, such comparisons can also serve to identify local magnetic contamination.

The first derivative of the variometer and scalar data can be a good indicator for spikes and noise. Where there is another observatory in relatively close proximity, comparisons of the two sets of vector data can frequently highlight scaling or alignment errors and also steps, spikes or drifting in a variometer data set that are not necessarily evident when observing a single data set.

It is good practice to maintain good written records of instrument changes, site changes or changes in personnel. Where there is a major change of instrument that differs significantly from the original IMO application, this should be communicated to the INTERMAGNET secretary.