Methods of detecting an interface between materials having different dielectric loss factors using an apparatus having a transmission line including an inner conductor surrounded by dielectric and a shielding conductor are disclosed. Sublengths of inner conductor are exposed along the line, with adjacent sublengths being separated by shielded sublengths. In one embodiment the apparatus is excited by a signal and inserted into the materials to a predetermined depth. Signal attenuation is measured. The interface between strata is identified based upon signal attenuation. In another embodiment the apparatus is excited by a signal and progressively inserted into the materials. Apparatus position is tracked and t-he attenuation of the signal is monitored as a function of insertion distance to detect ranges having a substantial rate of change of attenuation and ranges having substantially no change of attenuation. The interface is identified based upon differences in the rate of change in attenuation.