Practical considerations: making measurements of susceptibility, remanence and Q in the field

Here we consider how measurements of magnetic susceptibility, magnetic remanence and Königsberger ratios (Q) can be made in the field. A basic refresher is given on how induced magnetisation differs from remanent magnetisation and what distinguishes multidomain from single domain behaviour of magnetite particles. The approximation of an infinite half-space, which is the usual assumption for using most handheld susceptibility meters, is experimentally investigated and it is found that a block 100 × 100 × 60 mm is the minimum requirement for the meters tested here. The susceptibilities of chips of a dacite, an andesite and a spilite (altered basalt) are also experimentally investigated for a range of chip sizes from a few mm down to 200 μm. The relationship is quite flat until very small grain sizes are reached where the susceptibility either decreases or increases, which is interpreted as an indication of the grain-size fraction where the magnetite resides. Making susceptibility measurements on bags of rock chips is investigated and guidelines given. The temperature of susceptibility meters is also found to be a factor and five meters have been tested for temperatures from 0°C to 50°C, the stated operating range of most meters. Finally Breiner’s method to separate induced magnetisation from remanent magnetisation using a field magnetometer is discussed. A new fluxgate based pendulum instrument to allow a more controlled implementation of Breiner’s method is also described.

The measurement of magnetic susceptibility, magnetic remanence and Königsberger ratios can be made in the field, either using a total field magnetometer or a new portable fluxgate device that is described. Various problems of using a magnetic susceptibility meter on non-ideal rock, core and chip samples can be avoided.