Technical Notes - Helicopter Electromagnetics

Effects of Bird Height

The EM field due to small (dipolar) sources such as the helicopter electromagnetic (HEM) system transmitters falls off in strength proportional to the distance from the source cubed. Thus, the response from a layered earth or conductor also falls off proportional to the distance cubed, plus a factor due to the fall-off of the secondary field from the earth and/or conductor. (The secondary field fall-off depends on the shape and size of the conductive source.) However, the apparent resistivity and conductivity-thickness determined using the algorithms employed by Fugro are calculated independent of altitude, so variations in the system height do not affect the calculated apparent resistivity or conductivity-thickness.

The low altitude and slow flying speed of helicopter EM systems provide 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 that would be measured over a large, shallow, vertical conductor when surveyed with an HEM system at 30 m, 60 m and 100 m altitude. Note the fast fall-off in the anomaly amplitude, and the widening of the anomaly peak (reducing the accuracy of the interpretation of target location).

Resolution of closely spaced conductors by a vertical coaxial 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. The resolution of an EM system depends strongly on the height of the system above the conductor(s)

Figures 2 and 3 compare the changing response 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 two conductors at 40 m separation, and possibly at 20 m separation. The high altitude data (100 m) would not reliably distinguish the two conductors at either separation in real data conditions. (The scale in Figure 3 is amplified to show the anomalies in detail.)

The single peak anomaly of the vertical coaxial coil configuration provides the sharpest anomaly possible for defining target location.

Greg Hodges  August 2004