The Null Method Of Measurement; Resolution - Bartington Mag-01H Operation Manual

4. The noise level, and therefore resolution, of the magnetometer. Two sensitivity settings
are provided: x1, which produces a resolution of 1nT, and x10, which produces a resolution of
0.1nT. The x10 sensitivity setting would normally only be used under ideal conditions.
Note: The system is usually operated with the assigned magnetometer and theodolite
combination specified in the test documentation. Should a replacement magnetometer
be used, offset errors up to 5nT may be encountered. The accuracy of the system will not
be compromised provided that the four-measurement method described in this manual is
used.

3.1. The Null Method of Measurement

Measurements are obtained by operating the probe mounted on the theodolite in the null mode.
In this mode observations are performed with the probe orientated perpendicular to the direction
of the terrestrial magnetic field F. At an angle of 90° to the field the response of the probe
produces the greatest sensitivity to small changes in the field direction.
Note: Ultimate accuracy will depend on the prevailing magnetic conditions. The high
flexibility cable connecting the probe to the magnetometer does not interfere with the
measurement procedure and will operate down to very low temperatures.
Note: The use of low magnetic signature components within the Mag-01H instrument
enables this unit to be used close to the probe head. Errors will not be significant with
the probe and magnetometer as close as 0.5 metres, although the magnetometer is, in
practice, usually placed on the ground.
Variation in either D or I is indicated by d. Variation in Declination (dD) or Inclination (dI) around
a fixed null point can be continuously monitored via the analogue output. For small angular
changes, the relationship between magnetometer output and angle will be linear.

3.2. Resolution

The theoretical resolution R to small changes in field direction is proportional to the resolution
of the magnetometer and the strength of the perpendicular magnetic field. When measuring
Inclination, the magnetic axis of the probe is parallel to the vertical plane of investigation.
Therefore the full field strength F is available and the resolution becomes RdI = F sine dI.
When measuring Declination the magnetic axis of the probe is parallel to the horizontal plane
of investigation. As a result, only the horizontal field component H = F cosine I is available.
Therefore, for Declination measurements, the resolution becomes RdD = H sine dD.
BARTINGTON INSTRUMENTS
Page 7 of 32 OM2020/5