Motion-induced noise from vibration of a moving TEM detector coil: characterization and suppression

Abstract

Motion-induced noise results from the movement of a TEM detector coil in the Earth's magnetic field. The motion-induced noise is low-frequency noise whose amplitude may be orders of magnitude higher than the other noise sources. It is a potential problem to any kind of moving transmitter-receiver TEM system, such as those used in airborne EM or any ground-based pulled-array transient electromagnetic system. The exact amplitude distribution and frequency content of the motion-induced noise depends on the system construction. In this paper a method for suppression of motion-induced noise is devised based on the availability of three-component measurements. The three-component data form the basis of an analysis of the deterioration caused by motion-induced noise of the signal quality from moving receiver systems. In the direction of the main field the coupling to the motion-induced noise is minimal. Thus, by summation of the three components projected onto an estimated direction of the main field a noise-suppressed signal is obtained. The relative direction of the main field is obtained as the direction of minimal coupling in a least squares sense. Because the coil orientation changes along the profile, it must be re-estimated regularly. The noise suppression factor of an individual component depends on its proximity to the main field direction, so the further the direction of a component departs from that of the main field, the more disturbance by motion-induced noise, and the higher noise suppression ability of the algorithm. Interpretation of projected TEM data requires corresponding projections of the forward modelled responses to be incorporated into an inversion scheme.