Adjustment distance in TM mode electromagnetic induction

SUMMARY The concept of adjustment distance‐the range over which induced currents in 2‐ or 3‐D structures re‐adjust themselves to their ‘normal’ 1‐D configurations outside the anomalous domain‐first introduced into the subject of magnetotellurics a decade ago by Ranganayaki & Madden, is re‐examined in the light of an analytical solution obtained previously by the authors using a generalized thin sheet model of the TM mode ‘coast effect’. Ranganayaki & Madden defined the adjustment distance to be r: = (τλ) 1/2 where τ and λ are respectively the integrated conductivity and integrated resistivity of the two layers in the generalized thin sheet. While on physical grounds this parameter is a reasonable estimate for many practical applications, its independence of the frequency of the inducing field is less than satisfactory. The parameter d given by the analytic solution is frequency dependent, but reduces to r for long periods. A more detailed comparison of the two parameters shows that r is always an upper bound of d under conditions likely to occur in nature and may even represent a significant overestimate of adjustment distance when the conductance of the surface layer is high, such as for an ocean. An alternative parameter for adjustment distance developed by Fainberg & Singer is found to be in good agreement with d , however, and for the model investigated varies with period in a similar manner over its range of validity. For more moderate values of surface conductance such as those found over continental regions, Ranganayaki & Madden's parameter serves as an accurate measure of adjustment distance over all periods of interest.