Amplitude and phase correction of helicopter EM data

Abstract

We aim to develop a quantitative method for recalibration of historic helicopter electromagnetic data sets. Recent research has shown that frequency-domain helicopter electromagnetic data collected over a conductive half-space such as calm seawater can be used to correct system calibration errors. However, most historic surveys consist only of data collected over land, where the conductive half-space assumption is rarely justified. We estimate the required recalibration parameters by analyzing systematic misfits in the inversion of statistically chosen measures of historic data. Our method requires the identification, within the survey area, of a zone of conductive responses that are reasonably uniform. From this zone, a set of altitude-corrected median responses are estimated. These are inverted using geologically specifiedconstraints to obtain a best-fit layered earth model. Systematic inconsistencies between the median measured altitude and the inverted depth to surface are attributed to altitude error. Remaining frequency-dependent fitting errors are assumed to be the calibration errors. We tested the method with partial success on helicopter electromagnetic data sets collected over uniform deep sediments where seawater data were also available and two different inland surveys over multiple lithologies in one general area. At high frequencies, our method works reliably. Recalibration of low-frequency data is not possible if the area used as a reference consists of moderate or poor conductors. In this case, data amplitudes are small and are greatly affected by imperfect drift and magnetic susceptibility corrections. Historic helicopter electromagnetic data may require amplitude rescaling up to 20%–30%, with phase shifts of up to 3°.

Metalliferous mining geophysics — State of the art after a decade in the new millennium

“…Accurate measurements of system geometry are not required for measuring the in-phase response with the frequency-domain systems because such systems are rigid. Efforts to improve the calibration of helicopter frequency-domain systems were shown to result in better data being derived from AEM systems (Ley- Cooper and Macnae, 2007;Macnae et al, 2008). A procedure involving a ground loop was devised by Davis and Macnae (2008a, b) to improve understanding of the time-domain waveform shape and of timing and altimeter geometry errors; that procedure also results in derivation of better fidelity information from the data.…”

HEM calibration and bird‐swing correction: An insular example

“…Recent work (Ley-Cooper and Macnae, 2007) has shown that historic helicopter frequency-domain electromagnetic (HFEM) data is often miscalibrated. One very important calibration problem tends to be an altitude error.…”

Accurate 1D forward and inverse modeling of high-frequency helicopter-borne electromagnetic data

“…Full solutions are applied by Yin and Hodges (2005) and Siemon et al (2009b). The usage of the full solution gains increasing importance as accurate highfrequency HEM data achieved by improvements in hardware developments (Fountain, 2008) and data processing (e.g., Fitterman, 1998;Fitterman and Yin, 2004;Ley-Cooper and Macnae, 2007;Siemon, 2009) enable new and challenging applications in nearsurface surveying (e.g., de Louw et al, 2011).…”