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

摘要

We investigate the properties of EM signals in several different response-pararneter domains to identify calibration errors in helicopter electromagnetic (H EM) data. In particular, we define a dimension less response parameter a, 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 alpha is the phase phi of the secondary field. We further define a dimensionless amplitude response parameter, 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 phi beta domain.