Pendulum motion in airborne HEM systems

Helicopter electromagnetic systems typically have a transmitter-receiver coil pair in a towed bird that, in flight, behaves as a complex compound pendulum. Pendulum motions of the towed bird create not only a geometric and inductive effect in the measured signal, but they may also contribute to altitude error. The oscillation of the towed-bird pendulum can be observed from both video recordings and GPS positions and is most usefully broken down into two modes: one in the direction of travel (in-line) and one perpendicular to the direction of travel (cross-line). We analyse the electromagnetic and altitude data for a RESOLVE survey on the Chowilla flood plain, near Renmark, South Australia. The in-line and cross-line motions of the towed bird are shown to have different frequencies of oscillation. In-line motions, which cause pitching of the towed bird, and cross-line motions, which generate roll, create a systematic error in data easily evident in the dimensionless in-phase divided by inductive limit (R/G) domain. We show that the bird swing due to both modes of pendulum motion is responsible for altitude error, and that a filter designed to use the observed period of oscillation can largely correct the R/G domain data. The filtering process does not require any more measurements than are usually provided from a typical electromagnetic survey.