Identification of calibration errors in helicopter electromagnetic (HEM) data through transform to the altitude-corrected phase-amplitude domain

Abstract

We investigate the properties of EM signals in several different response-parameter domains to identify calibration errors in helicopter electromagnetic (HEM) data. In particular, we define a dimensionless response parameter α, derived from frequency-domain data, that is numerically identical to the historic wire-loop response parameter, and is closely related to the thin-sheet and half-space response parameters. The arctangent of α is the phase ϕ of the secondary field. We further define a dimensionless amplitude response parameter β, calculated as the ratio between inductive limits estimated from the data and from system geometry. The inductive limit calculated from geometry provides an initial altitude correction to the data amplitudes. Additional data corrections further correct phase effects and altimeter variations. The amplitude and phase errors in calibration become independent differences between the data and the fitted model in the ϕβ domain. This investigation was undertaken in the response-parameter domain rather than in the data domain, allowing the analysis to be independent of absolute values of conductivity and system frequencies. Statistical analysis in the ϕβ domain of recently acquired data suggests that amplitude calibration errors in HEM data may cause fitted models to have systematic depth errors of 1 to 2 m for near-surface conductors; variable altitude measurement errors are about 1.5 m over seawater.

Sea ice as an evaluation target for HEM modelling and inversion

“…Fitterman, 1998;Ley-Cooper et al, 2006). Calibration values for gain and phase are commonly derived from ground calibration with coil rigs and phase bars or from built in calibration coils, which are activated at high altitude.…”

HEM calibration and bird‐swing correction: An insular example

“…The data (dark blue) is poorly fit by the (light blue) layered earth model. In Figure 4, the 34 kHz (yellow cross) data over seawater have been transformed to the φβ φβ φβ φβ domain and are contoured to form a vertically elongated group, with the vertical elongation due to height changes with pendulum motion (Ley-Cooper et al, 2006). The spread of data is shown with vertical and horizontal bars, which bars are repeated at lower frequencies in different colours.…”

Quantifying AEM system characteristics using a ground loop

“…Various calibration processes have been applied, for example, flying over regions of known geologic structure ͑Ley-Cooper and Macnae, 2007͒, water bodies of known conductivity ͑Nelson, 1973; Kovacs and Holladay, 1990;Ley-Cooper et al, 2006͒, and areas of high-resistivity or high-altitude base-level adjustments ͑Vrbancich et al, 2000͒. Until now, no consistent calibration procedure has yet been designed that can be applied to all AEM systems.…”