An AEM system calibration

Abstract

VTEM data was collected over a ground loop laid out near Cape Naturaliste, WA. The system calibration was determined to be correct within experimental error. The current induced in the ground loop and the airborne VTEM response agree within experimental error to theoretical predictions of amplitude, shape and decay characteristics from channel 6 onwards. Because the background response of the ground dominated the early time channels, it was not possible to exactly quantify the early time calibration for Channels 1 to 5. With GPS systems fitted to the airborne loop and the helicopter, 10 bathymetry survey lines were analysed to determine the statistical variations to be expected in system geometry in ?straight and level balanced flight?. Approximately Gaussian variances found for geometry parameters imply that the VTEM system geometry for this specific system could be specified as a mean value with 95% confidence limits at 2 standard deviations. The mean geometry and 95% limits for VTEM-24 Tx-Rx assembly were: 35 (±3) m below and 22 (±4) m behind the GPS sensor on the tail of the helicopter, displaced 3 (±3) m to port. Tilted nose down at 5.5 (±6)º and tilted to starboard 1 (±6)º The VTEM geometry has been historically quoted as a nominal 40 m below the helicopter altimeter, although some reports have used a figure of 37 m. This measurement suggests that true VTEM loop altitudes are in fact somewhat higher than the numbers delivered in historic data. Correct vertical offset is critical in quantitative modelling. Further, previously modelled dips of discrete conductors may need to be adjusted by 5.5 º to be consistent with the systematic loop tilt. The loop swing in-line (distance behind helicopter) shows a small energy peak at the 12 sec pendulum frequency, plus additional effects at longer periods attributed to gradual speed variations. The loop swing from side-to-side exhibits a 20-30 sec period, identical to the sideways helicopter accelerations, and is hypothesised to be due to controlled pilot steering adjustments to keep on-line.