Technical Notes - Helicopter Electromagnetics

HEM Applicability

The Dighem^v system has been the de facto standard for HEM systems used in mining exploration for over 20 years.

Resolution of closely-spaced conductors by an HEM system cannot be surpassed by other configurations of airborne electromagnetic systems. The low flying height of the receiver (30m), the high sampling rate (10 samples/second), the low flying speed (approx 30m/s), and the fixed geometry between the transmitter and receiver all contribute to making the measured anomalies as sharp, and as narrow as physically possible. Figures 5 and 6 compare the changing response with altitude of the system over three models: a single conductor, and two conductors separated by 20m, and by 40m. The lower altitude data clearly distinguishes the conductors at 40m separation, and possibly at 20m separation. The high altitude data (100m) would not reliably distinguish the three models under real data conditions.

Magnetite-bearing conductors have a very high magnetic permeability relative to average crustal rocks, and so produce a distinctive effect on frequency domain system. The high permeability of these conductors has the effect of lowering, or even reversing the anomaly measured in the in-phase component (Huang and Fraser, 1998). At lower frequencies, which have lower positive responses due to the conductivity of the target, the effect of permeability can reverse the anomaly, making a distinctive negative peak. The higher frequencies are proportionately less affected, and so still provide an accurate measure of the conductivity-thickness.

Overburden (flat-lying) responses are easily discriminated from bedrock (steeply dipping) responses by the geometry of their anomalies, as detected by the different orientation of the coaxial and coplanar systems in the Dighem^v. In the case of shallow-dipping-to-horizontal bedrock conductors, they are often distinguishable by their conductance, or by analysis of the source depth through inverse data modeling. Figure 4 shows the variation in response from different coil orientations to vertical and shallow dipping conductors.

The depth of exploration of an electromagnetic system is a function of the system frequency, geometry and sensitivity of the host resistivity and of the target size, geometry and conductance. Conductors have been detected by Dighem^v surveys at depths of 150m in resistive host rocks.

Greg Hodges, Chief Geophysicist, 1999