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Summary
Exploration for targets at depth or targets obscured by conductive overburden have historically been a challenge with airborne EM methods. Although modern systems have been improved with greater primary transmitter moments, noise from receiver coil motion in the Earth’s ambient field limits detection of secondary target signals, especially in late time. The new Helitem² system uses a patented low-noise receiver and a 50% duty cycle square pulse transmitter waveform to achieve increased signal detectability for deep and covered targets.
A series of demonstration surveys were conducted by surveying a known target to compare several helicopterborne time-domain system configurations. Despite having a larger dipole moment, a half sine pulse at standard 30 Hz base frequency was predicted to have lower responses than low base frequency (15 Hz and 7.5 Hz) square pulse operation in a thin-plate nomogram over a wide range of target conductances. At early times, the sharper (quicker) turn off of the square wave results in much more highfrequency energy and therefore better signal for weakly conductive targets and better near-surface resolution. At the other extreme, the response from very conductive targets is determined by the area under the transmitter curve, so the low frequency square waves with 16 and 33 ms widths should produce more than twice the signal as the half sine. Survey line profiles and decay curves over the target and background locations confirmed these predictions for a 400 m deep target and variable overburden.
The combination of pulse width, power, and low noise enabled the system to be effective at low base frequencies, where very late time data is beneficial for detecting strong and deep targets.