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Exploration Geophysics Exploration Geophysics Society
Journal of the Australian Society of Exploration Geophysicists
RESEARCH ARTICLE

Pendulum motion in airborne HEM systems

A.C. Davis, J. Macnae and T. Robb

Exploration Geophysics 37(4) 355 - 362
Published: 2006

Abstract

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.

https://doi.org/10.1071/EG06355

© ASEG 2006

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