STUDY OF BIRD ATTITUDE EFFECT AND CORRECTIONS ON FREQUENCY-DOMAIN HELICOPTER EM SYSTEM

Abstract

For the helicopter electromagnetic (HEM) system, the receiver and transmitter coils are housed in an instrument pod called the bird, which is typically slung about 30m below the helicopter. In actual fact, due to the flight speed, topographic relief, flight velocity, etc., the bird may exhibit dynamically some yaw, pitch, and roll rotation when in flight. Such attitude changes in the bird yield in-phase and quadrature responses that are different from the responses that would have occurred if the bird is flying straight and levelly. It would affect the electromagnetic coupling and bring error to measured data. It is called as bird attitude effect. In this study, we use the 3D finite difference method to calculate EM response of the bird rotation model and analyze the effect in different frequencies, different angles and different coils (vertical coaxial (VCX) and horizontal coplanar (HCP)). The results demonstrate that the yaw rotation does not have obvious influence on the measured data, the VCX coil data is mainly polluted by roll rotation but both roll and pitch rotation are affect the HCP coil. The pitch would bring greater error than the roll on the HCP coil with the same rotation angle. In additional, the bird attitude has greater effect at higher frequencies. Our research can help determine the parameters for the HEM transmitting system design and provide theoretical basis for the analysis and correction of bird attitude effect in HEM data.