Resolution analyses for selecting an appropriate airborne electromagnetic (AEM) system
Niels B. Christensen 1 3 Ken C. Lawrie 21 Department of Geosciences, University of Aarhus, Aarhus, DK-8000 Aarhus C., Denmark.
2 Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia.
3 Corresponding author. Email: nbc@geo.au.dk
Exploration Geophysics 43(4) 213-227 https://doi.org/10.1071/EG12005
Submitted: 13 January 2012 Accepted: 8 June 2012 Published: 5 July 2012
Abstract
The choice of an appropriate airborne electromagnetic system for a given task should be based on a comparative analysis of candidate systems, consisting of both theoretical considerations and field studies including test lines.
It has become common practice to quantify the system resolution for a series of models relevant to the survey area by comparing the sum over the data of squares of noise-normalised derivatives. We compare this analysis method with a resolution analysis based on the posterior covariance matrix of an inversion formulation. Both of the above analyses depend critically on the noise models of the systems being compared. A reasonable estimate of data noise and other sources of error is therefore of primary importance. However, data processing and noise reduction procedures, as well as other system parameters important for the modelling, are commonly proprietary, and generally it is not possible to verify whether noise figures have been arrived at by reasonable means. Consequently, it is difficult – sometimes impossible – to know if a comparative analysis has a sound basis. Nevertheless, in the real world choices have to be made, a comparative system analysis is necessary and has to be approached in a pragmatic way involving a range of different aspects.
In this paper, we concentrate on the resolution analysis perspective and demonstrate that the inversion analysis must be preferred over the derivative analysis because it takes parameter coupling into account, and, furthermore, that the derivative analysis generally overestimates the resolution capability. Finally we show that impulse response data are to be preferred over step response data for near-surface resolution.
Key words: electromagnetic, resolution analysis, system comparison, time domain.
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