The magnetic susceptibility (MS) of a volume of rock is a function of the amount of magnetic minerals, (mainly magnetite and pyrrhotite), contained within the rock. MS measurements can provide a rapid estimate of the ferromagnetism of the rock. These measurements can be interpreted to reflect lithological changes, degree of homogeneity and the presence of alteration zones in the rock mass. During the process of hydrothermal alteration, primary magnetic minerals (e.g. magnetite) may be altered (or oxidized) to weakly- or non-magnetic minerals (e.g. hematite). Anomalously low susceptibilities within an otherwise homogeneous high susceptibility (ferromagnetic) rock unit may be an indication of altered zones.

Basic flows and diabase dikes containing higher concentrations of magnetic minerals can be easily outlined with magnetic susceptibility measurements when they occur within a sedimentary sequence that normally contains little or no magnetic minerals.

• The Geoinstruments TH-3C Probe

  The magnetic susceptibility tool is a Geoinstruments model TH-3C probe which uses a signal processing unit developed at the GSC (Bristow and Bernius, 1984; Bristow, 1985). The probe contains a coil, 42 mm in diameter by 0.5 m in length, in an electrical bridge circuit energized at a frequency of 1400 Hz. When the probe passes through magnetically susceptible material, the coil inductance changes causing the bridge to become unbalanced. The bridge is balanced automatically by changing the energizing frequency. This change in frequency is proportional to magnetic susceptibility. Since the measurements are made inductively (i.e., with EM coils not contact electrodes), the tool can be used inside plastic casing and in dry holes. Susceptibilities in the range of 0 to 2.0 SI can be measured with this tool. The volume of investigation or `sample volume' is roughly a sphere of 30 cm radius, surrounding the sensing coil in the probe. Logging is normally carried out at 6 m/minute and a measurement is taken every second or each 10 cm along the hole.

• The BRGM 'ROMULUS' probe

  The Romulus probe is a low-frequency (4 KHz), two-coil electromagnetic induction probe. It consists of coaxial transmitting and receiving coils spaced 85 cm apart. It compensates for the primary field and measures in-phase and quadrature components of the secondary field. These two quantities are approximately proportional to magnetic susceptibility and electrical conductivity of the rock around the borehole.

  For the magnetic susceptibility (in-phase) measurement, the sensitivity is 3.14x10^-3 SI/volt and the measuring range is 10^-5 to 3.5x10^-2 SI.

• Geonics EM-39

  The magnetic susceptibility sonde is similar in design to that of the induction electrical conductivity sonde; the so-called . in-phase. response of the instrument is a measure of the formation magnetic susceptibility and hence has a similar (but slightly less) depth of penetration into the formation as well as limitations in vertical resolution. Where earth materials exhibit high electrical conductivity(>200 mS/m) a correction for conductive effects must be made to the raw magnetic susceptibility field data. A detailed description of the tool is given by McNeil et al. (1996).

• The Geoinstruments TH-3C probe

  The Maxwell-bridge circuit which is used in the TH-3 probe also allows conductivity of material close to the coil to be measured simultaneously with susceptibility. This is accomplished by resolving the change in complex impedance seen by the bridge into its inductive and resistive vector components. (Resistive material around the coil causes the coil to behave as a transformer with the resistive material acting as a combined and distributed "secondary winding" and "load"). Resistivity measurements using this technique are limited to a range of 10^-1 ohm-m to 103 ohm-m (conductivity = 10 mho/m to 10^-3 mho/m). In practice only a few sedimentary formations would normally have resistivities low enough to fall within this range, while in igneous rocks only graphitic conductors or mineralized zones such as massive sulphides would be included (Bristow and Bernius, 1984).

• The BRGM 'ROMULUS' probe

  As mentioned above in the section on magnetic susceptibility, the secondary field at the receiver coil is measured and the out of phase component is proportional to electrical conductivity (quadrature). The sensitivity is 0.46 mho/m/volt and the measuring range is 10^-3 to 5.5 mho/m.