Breaking through the 25/30 Hz barrier: Lowering the base frequency of the HELITEM airborne EM system

Abstract

Performance of airborne electromagnetic (AEM) systems has changed dramatically since the first helicopter time-domain systems were employed in the early 2000s. These systems have experienced dramatic improvements in transmitter power and system noise reduction, such that signal-to-noise has improved to a level that inductively induced polarization (airborne IP) and superparamagnetic effects are detectable. Lowering base frequency below 25 Hz has been a goal to improve discrimination of very conductive targets and to explore under conductive cover. Although easy to achieve in ground EM surveys, noise caused by receiver motion on airborne platforms has prevented this barrier from being surpassed. Here we discuss the technological innovations that led to and survey results from an AEM system capable of operating at a base frequency of 15 Hz. The noise level increase we observe using 15 Hz over operating the same system at 30 Hz is largely identical to the theoretically expected amount caused by decrease in data available for stacking. We show results from an area with very conductive and thick cover to highlight the extended measurement time of the 15 Hz base frequency.

CGG’S New Helitem-C AEM Systems

“…The transmitter diameter is 35 m generating a MultiPulse waveform (4 ms to 10 ms, 1ms square pulse with turn-off time as low as 35 s) with dipole moment from 0.6 MAm 2 to 1.3 MAm 2 . To improve the low-frequency bandwidth, CGG has developed a new receiver architecture for the Helitem35C to dramatically reduce coil motion noise and allow operation at lower base frequencies (Konieczny et al, 2016). Electronically, low base frequency operation on an AEM platform is not difficult; receiver coil motion at the lower base frequency is the limiting factor, resulting in significantly higher noise levels.…”