Magnetic susceptibility is the degree of magnetization of a material in response to an applied magnetic field. If magnetic susceptibility is positive then the material can be paramagnetic, ferromagnetic, ferrimagnetic, or antiferromagnetic. In this case the magnetic field is strengthened by the presence of the material. Alternatively, if magnetic susceptibility is negative the material is diamagnetic. As a result, the magnetic field is weakened in the presence of the material.
An oscillator circuit in the sensor produces a low intensity (approx. 80 A/m RMS) non-saturating, alternating magnetic field (0.565 kHz for the MS2C sensor and 2 kHz for the MS2E sensor). Any material in the near vicinity of the sensor, that has a magnetic susceptibility, will cause a change in the oscillator frequency. The electronics convert this pulsed frequency information into magnetic susceptibility values.
The Bartington loop sensor (MS2C), ruggedised for Geotek applications, is used for magnetic susceptibility measurements on whole cores. It is available in a range of internal diameters and is mounted between the two main box sections in such a way that no magnetic or metallic components come close to the sensor. For maximum resolution of magnetic susceptibility the loop-diameter/core-diameter ratio should be as small as possible. Hence for small diameter cores some data degradation may occur and a smaller magnetic susceptibility loop may be desirable. A wide range of internal diameters can be substituted to optimise the core diameter to loop size; please contact Geotek if you wish to obtain additional loops.
The Bartington point sensor (MS2E) is mounted on an arm that allows the sensor to be placed on the core surface for each measurement. It uses the same electronics as the loop sensor. Note that the original point sensor (MS2F) required an interface box that is placed in the line from the sensor to the MS3 electronics. The point sensor gives much higher spatial resolution that the loop sensor but is less sensitive. Its field of influence is about 1 cm in diameter and so it cannot be used on whole cores.
Calibration and Processing:
The magnetic susceptibility sensor is electronically set to measure a single standard sample of a stable iron oxide which has been tested and analysed by the manufacturer (Bartington Instruments Ltd). Therefore, all magnetic susceptibility sensors supplied should record exactly the same value for any given sample, and that value should be the same as a measurement made on a different measuring system. In that sense the magnetic susceptibility system is calibrated absolutely. Since the calibration has been set electronically it should not alter. A calibration sample is provided which can be used to check the long term consistency of the calibration. The data obtained from the magnetic susceptibility system provides uncorrected, volume specific magnetic susceptibility, which can be converted to either corrected volume specific magnetic susceptibility or mass specific magnetic susceptibility automatically in the Geotek MSCL software.
Changes in magnetic susceptibility correlate with changes in sedimentary provenance and/or diagenetic environment. Magnetic susceptibility records are frequently used for inter-core correlation.
Bartington Instruments (http://www.bartington.com/)
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