Technical Notes - Helicopter Electromagnetics

HEM Advantages and Disadvantages

The low altitude and slow flying speed of helicopter EM systems provide for maximum anomaly resolution and sensitivity to weak conductors. The narrow bandwidth of frequency domain systems provides a much higher signal-to-noise (S/N) ratio than can be achieved with the wide bandwidth necessary for a time-domain EM system. This compensates for the lower transmitted energy of a helicopter system relative to the larger fixed wing systems.

Figure 1 shows the effect of survey altitude on the sensitivity of an HEM system. The graph shows the EM anomalies which would be measured over a large, near-surface, vertical conductor when surveyed at 30 m, 60 m and 100 m altitude. Note the fast fall off in the anomaly strength. Figures 5 and 6 show the loss in resolution, as described in the section on Applicability below.

The Dighem^V system has an 8m coil separation on most frequencies. This inherently provides for a higher S/N ratio than systems with shorter coil separations, since moving the receiver coils further from the transmitters allows them to operate in lower primary fields.

Figure 2 shows models calculated over a large conductive plate at 10m depth, calculated with coil separations of 5 m, 6.5 m, and 8 m. Other than the coil separation, the systems are identical. The response in the 8 m system is approximately four times stronger than the anomaly from a 5 m system. The ability to resolve this anomaly is also a function of the noise envelope of the system, which is generally comparable in relative ppms for each system. The geologic noise envelope depends on the signal measured from changing geology, and will scale up and down by the same proportion as the conductor anomaly. The system noise is largely dependent on the ability of the system to remove the strong primary field at the receiver, which is dominated by the distance between the receiver and the transmitter.

The Dighem^V system has 3 coplanar coils at (nominally) 900 Hz, 7200 Hz, and 56 kHz to provide resistivity calculations over a wide range of frequencies. The lower frequencies penetrate deeper into the earth to map deeper zones and to see through conductive layers at surface. The higher frequencies are more sensitive to higher resistivities and to thin, weak conductors near surface.

There are two vertical coaxial coil pairs at (nominally) 900 Hz and 5500 Hz. These coils are oriented to be most sensitive to steeply dipping conductors, and the range of frequencies provides sensitivity to weak conductors as well as strong or deep conductors.

The difference in sensitivity of the two coaxial frequencies to target conductance and host resistivity is shown in Figure 3. The conductor to the west is weak, and near surface, and has a much stronger response from the higher frequency of 5500Hz. The conductor to the east, number 2, is much more conductive, but is buried 40m deep in a conductive host rock. The response of the 900 Hz coaxial system is proportionately higher, and would be much more obvious in real data, where the higher response of the 5500 Hz data to the host would be varying as the host resistivity varied.

The dual coil geometry arrangement of the DighemV provides maximum sensitivity to conductors at any dip, and comparison of the response from any two coils at the same frequency provides the information necessary for interpretation of the conductor geometry. Figure 4 shows data over modeled conductors of varying dips of 90, 60 and 30 degrees. The vertical conductor shows a dual peak on the coplanar data and a single peak on the coaxial data. As the conductor has progressively shallower dip one shoulder of the coplanar anomaly disappears, and the coaxial anomaly shifts up-dip as a second, down-dip shoulder to the anomaly starts to appear.

Greg Hodges, Chief Geophysicist, 1999

REFERENCES:

Fraser, D.C, 1978, Resistivity mapping with an airborne multi-coil electromagnetic system: Geophysics, 43, 144-172

Huang, H, Fraser, D.C, 1998, Magnetic permeability and electric resistivity mapping with a multi-frequency airborne EM system: Exploration Geophysics, 29, 249-253