Helicopter electromagnetic data can be used to map the magnetic susceptibility of the geology surveyed. Three natural rock parameters affect the EM response measured: conductivity, magnetic permeability (susceptibility) and dielectric permittivity. If certain assumptions are made about the geological model, then the conductivity and susceptibility can be simultaneously measured. The EM-derived susceptibility measurement complements the magnetic survey-derived susceptibility, and will be different in several aspects:

Figure 1 shows the total magnetic field measured over a section of the Garden Obonga survey just west of Lake Nipigon, flown for the Ontario Geological Survey. The region is mostly granitic intrusives, with some mafic metavolcanic and metasedimentary rocks, and numerous diabase sills. While the sills have high magnetic susceptibility, they are generally flat lying, and therefore perpendicular to the earth's magnetic field, so they do not show a strong response on the magnetic survey. A thick diabase sill covers the area outlined in white in figures 1 and 2. The presence of the sill is hard to detect in the magnetic data - only a change in the "texture" gives an indication of something different relative to the surrounding data.

The magnetic susceptibility derived from the EM data is shown in figure 2. The area of the diabase sill is immediately obvious by the high susceptibility values. Because the magnetic field of the HEM system is much smaller, it is much more sensitive to the near-surface geology.

Figure 1 - Total Magnetic Field	Figure 2 - EM-derived Magnetic Susceptibility

Note also that the magnetic field inside the diabase sill is generally lower than outside - this would lead an interpreter to first assume that the rocks outside the white line have a higher magnetic susceptibility.

The feature labeled as 'A' is a bifurcating diabase dike. Comparison of the EM-derived susceptibility and the magnetic field map show the higher resolution that results from the smaller magnetic field of the EM system. In the EM-susceptibility map the two separate dikes are much more obvious.

The diabase dike shown at 'B' appears as a strong low magnetic anomaly - presumably due to strong remanent magnetisation. Other dikes on the property are magnetic highs. On the EM-derived susceptibility, all the dikes will have the same shape, making them easier to interpret.

It is important to note that the shape of the magnetic anomalies of the dikes also depends on the strike relative to magnetic north, and the dip of the magnetic field (vertical at the magnetic pole, horizontal at the magnetic equator). In any environment, at any strike, the shape EM-derived magnetic susceptibility anomaly will depend only on the dip of the dike itself.

The EM-derived susceptibility is an additional product available from any HEM survey. Of course, the mapped apparent conductivity is also measured, across a range of frequencies and effective depths. Figure 3 shows the apparent resistivity of the low frequency, which effectively mapped the conductive iron formations ("IF") and Lac des Iles intrusives (S.W. corner). Higher frequency EM apparent resistivity maps the conductivity of the near-surface geology, and weak conductors.

Figure 3. Low Frequency Apparent Resistivity

 

Greg Hodges, Chief Geophysicist, May, 2003