Relative Performances of Central and Dipole Frequency Soundings over a Layered Earth

Abstract

Variable frequency soundings in the audio-range replaces shallow conventional direct current methods for determination of layer parameters when surface layer resistivity is high. Central frequency soundings (CFS) is one such method that involves measurement of the existing vertical magnetic field component induced at the centre of a horizontal circular or square loop. Dipole method of frequency sounding using small horizontal coplanar loops (abbreviated DFS) measuring the same field component is also considered. Theoretical studies on CFS and DFS over two- and three-layer horizontally stratified earth are carried out and the response characteristics computed and analysed.

Theoretical response curves for CFS and DFS over two- and three-layer earth models are presented in convenient forms. Response curves under similar geological-physical conditions are compared. The study indicates that the relative superiority of a method is controlled largely by the nature of the conductivity contrast and the ratio of the first layer thickness to loop radius or dipole separation. While CFS shows a better resolution of conductivity contrast, DFS works better at high frequencies. For resistive substratum, however, both lack proper resolution.