Transient electromagnetic method (TEM) analysis and apparent resistivity interpretation: the importance of early-time sounding curve distortion

Late-time apparent-resistivity expressions are widely used to transform transient electromagnetic (TEM) diffusion-field responses into resistivity sounding curves. At short delay times, however, the assumptions underlying the late-time approximation are not necessarily satisfied and the resulting apparent resistivities may be systematically distorted. This study quantifies that distortion for loop-source TEM configurations using analytical calculations and forward modelling over homogeneous and layered geoelectrical structures. The effects of transmitter-loop dimension, time delay, and subsurface resistivity are examined, and the transition from early- to late-time behaviour is expressed through a dimensionless diffusion-distance criterion. For the formulation tested, errors in late-time apparent resistivity fall below 5% when τ/a is approximately 13 or greater, and below 10% when τ/a exceeds approximately 10, where τ is the electromagnetic diffusion distance and a is the equivalent circular-loop radius related to the transmitter-loop side L through a = L/√π. Layered-model tests show that early-time deviations can reach approximately 44% and are most pronounced for larger loops and conductive near-surface conditions. Tabulated critical times and a worked example are provided for survey design, data quality control, and applications in which TEM apparent resistivity is used alongside plane-wave electromagnetic soundings or for magnetotelluric static-shift correction.