Taylor, T.

British Geological Survey, Edinburgh,



A geomagnetic observatory regularly requires

the fabrication of bespoke parts, brackets or equipment installed

close to, or even touching instrument sensors. Metals and components

used for this can sometimes be contaminated (cheap brass), disguised

(plated steel), or changed by their fabrication (for example welded or

cold-worked austenitic stainless steel). Materials expected to be

non-magnetic’ can sometimes be slightly tainted, affecting

observatory data if installed.

Passing a test sample over a compass is not very responsive, and doing

the same over a zeroed fluxgate theodolite sensor is very sensitive to

background interference and changing field. Therefore, a simple means

of testing small components in a noisy lab environment was sought.

The Tri-Mag arranges three miniature fluxgate magnetometers about a

tube; this compensates for external fields and creates a sensitive

gradiometer inside. The outputs from each sensor then pass through a

series of analogue filters to remove noise and boost sensitivity,

producing an audible/visible output on detection of a magnetic