Primary Field Compensation in Airborne TEM – Refining a Proven Technique through the Integration of High and Low Moment Information Projected onto a Common Exponential Basis

Abstract:

Near-surface resolution in airborne TEM measurements depends on the ability to measure the earth response very close to the end of the current turn-off. Unfortunately, the current turn-off interval is typically also the time interval with the highest primary field signal level. The finite bandwidth of the recording system results in a superposition of the latest part of the on-time primary field signal and the earliest parts of the off-time earth response. Such distortion of the recorded earth response makes the early-time data unfit for interpretation, and the severity of the effect depends on the bandwidth of the recording system and the magnitude of the primary field signal. Consequently, many different methods have been developed to reduce the primary field signal, such as field bucking, prediction and subtraction, in-phase removal and various geometrical arrangements.

The SkyTEM system uses the zero-positioning method. A major advantage of this method is that its efficacy is basically frequency independent, meaning that it also works during rapid changes in the transmitted current. A further adaptation for early time measurement is the use of high (HM) and low (LM) magnetic moments, where early time information is obtained primarily from the low moment. The small current enables more precise timing and better waveform stability. Due to these properties, and a specially designed waveform shape, a method for further reduction of primary field influence has proven possible – the primary field compensation technique.

At high altitude we derive primary field intensity levels from the LM on-time signal and determine a gate-dependent prediction function based on a correlation analysis between these primary field intensity levels and the observed signal levels in each gate. This prediction function is used at production level to remove the primary field influence at each sounding position.

Essential for this method is the ability to extract an unbiased primary field intensity level at each sounding position, where the on-time signal may be influenced by the earth. In this talk we will describe how the use of off-time data for both HM and LM measurements, decomposed onto an exponential basis, allows us to improve the estimate of the primary field intensity level and thereby the primary field compensation technique.