Streamed data - a source of insight and improvement for time domain airborne EM

Abstract

The capacity to record streamed data, a time series of raw data from the transmitter current monitor and the electromagnetic sensors, was introduced to the QUESTEM 450 airborne EM system by the partners of the Airborne EM Systems Program at the CRC AMET during 1997. This has provided the opportunity to replay, analyse and reprocess data from any survey flown with the system. Much has been learnt about the various elements that contribute to the recorded response, and improvements have been made to the way in which the ground response is isolated from other unwanted influences. QUESTEM 450 is a time domain airborne EM system. A waveform with half-sine current pulses of alternating polarity is fed into a large horizontal transmitter loop slung around an aircraft. A three-component receiver coil set is towed in a bird behind the aircraft. Signals are sent from the bird to a receiver unit in the aircraft, where they are processed and recorded. Streamed data represents the most fundamental level of recorded data. The strongest contribution to the overall response is a combination of the voltage induced in the receiver coils from changes in the magnetic field associated with currents flowing in the transmitter loop and signals related to secondary currents induced in the metal frame of the aircraft. These two signals are never separated, and together make up the response termed the "primary". The amplitude and shape of the primary varies as the transmitter to receiver loop geometry changes. Streamed data can reveal the effects of these changes in great detail. Another element of the response is related to the movement of the coil in the Earth's magnetic field and is termed "coil motion noise". This voltage will be induced under two circumstances; when the coil rotates with respect to the Earth's field, and when the strength of the field varies at a rate within the pass band of the system. Computer-intensive processing methods applied post-flight to streamed data achieve higher levels of suppression of this form of noise than do methods traditionally applied on-board in real time. "Sferic" signals are the result of lightning strokes. Streamed data reveal that only the high amplitude, high frequency components of these signals are significant in the context of the present QUESTEM 450 system. A study of the character of these signals in streamed data resulted in an improvement to the method of suppression applied in real time on-board the aircraft. Powerlines radiate strong, time-varying magnetic fields which are visible in streamed data over very broad areas. The principal means of suppression for this element of the measured signal is a combination of careful choice of base frequency and stacking of the response. In the vicinity of a powerline, the amplitude of the response and/or the spatial variation in the field can significantly impair the ability to extract the ground response from the total response.