Exploration: Applications
Magnetotellurics (MT)
MT methods jointly measure electric and magnetic field variations. This enables calculation of the resistivity structure of underlying rocks, the depth being related to the period of the signal. Shallow exploration requires short period (high frequency), while deeper exploration needs low frequency. Bartington’s fluxgate sensors have the advantage over other sensors in low frequency, offering higher sensitivity and lower noise. This method will be used for a range of mineral and oil exploration to determine either the location of ore bodies or the shape of potential oil traps, using the Mag-03MSL or MSESL sensor.
Time Domain Electromagnetics (TDEM)
In the TDEM method, an AC magnetic field is created by injecting an AC current in loop. The response of underlying rocks to this AC field is then examined. If conductive material is present, the AC magnetic field will generate eddy currents which will in turn generate a secondary field that varies in intensity with the conductivity of rocks. Fluxgate sensors detect this secondary field. A fluxgate sensor is superior to coils for highly conductive targets because induction coils work on decay of the magnetic field rather than intensity: the higher the conductivity, the slower this decay. Bartington Mag-03 low noise sensors can be used – the Mag-03MC or MS for ground work, or unpackaged or in two-parts for borehole work.
Controlled Source Electromagnetics (CSEM)
The CSEM method – similar to TDEM but applied at sea – requires very low noise because the strength of the variations is very small. An array of Bartington Mag-03MSSL sensors is ideal for this application.
Borehole Magnetic Susceptibility
Using the Bartington BSS-02B Sonde, measuring borehole magnetic susceptibility determines the variation in susceptibility within a borehole at high spatial resolution. In diamond exploration, ore bearing rocks are kimberlites (primitive volcanic rocks) with very high susceptibility that is easily detectable with the BSS-02B Sonde. Iron ore bodies will also have an increased susceptibility, the intensity of the susceptibility helping to characterise the percentage of iron in the rocks.
Core Logging Correlation
Magnetic susceptibility measurement provides extra information in petrophysical studies. For both mineral and oil exploration, if cores are extracted, the Bartington MS2/MS3 Meter combined with the MS2C probe will permit fine stratigraphical work on the cores. Over several cores, distribution of layers enables the user to determine the geometry of a structure (oil trap or ore body).
