Browsing by Author "Bagley, Thomas"

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Estimating overburden thickness in resistive areas from on-time airborne em data
(2019-09-27) Bagley, Thomas
We propose a method to invert two-component time-domain EM data to a thick-sheet over half-space model, as a solution representing an overburden on top of bedrock. We first estimated the conductivity of the lower half-space using a combination of inversion for half-space (if appropriate) or a thin sheet over half-space (also when appropriate). This yielded a number of estimates, which could be combined to give a reasonable estimate of the lower half-space conductivity for the survey area. With this estimation an equation solver was used to solve the thick-sheet over half-space model for sheet thickness and sheet conductivity. The output of the algorithm was generally stable when applied to GEOTEM data in an area of moderately resistive overburden over a generally more resistive half-space. Although it did not reliably reproduce the overburden thicknesses as measured in the reference drill holes, it did give an estimate that was reasonable in the conductive areas.
Estimating the parameters of simple models from two-component on-time airborne electromagnetic data
(2022-01-01) Bagley, Thomas; Smith, Richard S.
The horizontal and vertical components of an on-time electromagnetic (EM) response can be used to estimate the parameters of simple models such as thin sheets, half-spaces, thin sheets over a lower half-space, and a two-layer model. The formulas used in these methods are valid in areas where the on-time response is essentially proportional to the conductivity or conductance, the so-called “resistive limit.”The half-space and thin sheet over lower half-space models can be combined to give an estimate of the conductivity for a lower half-space below a thick sheet that might be reasonable for the entire survey area. With this estimation, an equation solver can be used to estimate the thickness and conductivity of the overlying thick sheet over the whole sur- vey area. This latter approach seemed most appropriate for the Russell South area in the Athabasca Basin, Canada, where GEOTEM data have been collected. The output of the algorithm was generally stable. Although it did not always reliably reproduce the overburden thicknesses as measured in a set of reference drillholes, it did give an estimate that was reasonable in the relatively conductive areas.