Simultaneous one-dimensional inversion of loop-loop electromagnetic data for both magnetic susceptibility and electrical conductivity

Colin G. Farquharson, Douglas W. Oldenburg & Partha S. Routh.

[Submitted to GEOPHYSICS.]


Magnetic susceptibility affects electromagnetic loop-loop observations in ways that cannot be replicated by conductive, non-susceptible Earth models. The most distinctive effects are negative inphase values at low frequencies. Inverting data ``contaminated'' by susceptibility effects for conductivity alone can give misleading models: the observations strongly influenced by susceptibility will be under-fit, and those less strongly influenced will be over-fit to compensate, leading to artifacts in the model. Simultaneous inversion for both conductivity and susceptibility enables reliable conductivity models to be constructed, and can give useful information about the distribution of susceptibility in the Earth. Such information complements that obtained from the inversion of static magnetic data because electromagnetic measurements are insensitive to remanent magnetization. We present the capabilities of an algorithm that simultaneously inverts susceptibility-affected data for one-dimensional conductivity and susceptibility models. The solution is obtained by minimizing an objective function comprising a sum-of-squares measure of data misfit and sum-of-squares measures of the amounts of structure in the conductivity and susceptibility parts of the model. Positivity of the susceptibilities is enforced by including a logarithmic barrier term in the objective function. The trade-off parameter is automatically estimated during the course of an inversion using the GCV criterion. This enables an appropriate fit to the observations to be achieved even if good noise estimates are not available. As well as synthetic examples, we show the results of inverting airborne data-sets from Australia and Heath Steele Stratmat, New Brunswick.

Return to my home page ...
Return to GIF publications page ...
Last update: 26 October 2001.
Colin Farquharson (