Thick conductive bed

All curves are symmetrical with respect to the bed middle (Figs. 5.39-5.46). With an increase of parameter ri2, for example due to an increase of the frequency, the width of an intermediate zone where apparent conductivity, Oa, differs from that corresponding to a uniform medium with conductivity 02 becomes narrower. At the same time differentiation of curves to some extent decreases, i.e. the ratio of Oa/oi at the bed middle to that within the surrounding medium becomes smaller. For instance if H/L = 4 and oi/o2 = 4, the ratio Oa/oi calculated by Doll s theory is equal to 3.6 while for parameter U2 = 0.32 it is equal to 2.8. In another case when H/L = 4 and oi/o2 = 16, values of this ratio are equal to 11.9 and 9.5, correspondingly. 

With an increase of parameter n the vertical response of the induction probe becomes better, i.e. apparent conductivity o approaches closer to the apparent conductivity in a medium with conductivity oi. 

Let us consider some simple ways to determine the bed thickness by using a two-coil induction probe. In accord with Doll s theory the ratio of o /oi corresponding to the bed interface and its middle is  

As is seen from this relation that only for relatively large ratios of H/L parameter r is close to 0.5. For example if a = 4, 7 = 0.54, while if a = 10, 77 = 0.51. 

If determination of the bed thickness is based on the use of points of the profiling curve corresponding to half of the maximal value, an error does not exceed 2.5% if a = 4.0 

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