The longitudinal conductivity is one of main parameter in TEM method, but it rarely be used in base metal geophysical prospecting due to many causes.In this paper,a new deduced parameter— is defined and its algorism is proposed. Parameter is a first relative value in TEM.A lot of results of modle caculations and measured data processing are as same direct 1D inversion as apparent resistivity's. But the caculation is so easy and the solution is unique. ΔS_τ^rt could be used in center-near zone for in-loop and coincident loop configuration data processing.