- GeoRef, Copyright 2005, American Geological Institute. Reference includes data supplied by Society of Exploration Geophysicists, Tulsa, OK, United States
In two-loop electromagnetic sounding, the electromagnetic coupling between two vertical-axis loops of wire is measured as a function of frequency, for frequencies ranging from 20 Hz to 20 kHz. If the electrical structure of the earth beneath the loops is horizontally stratified, these data may then be interpreted in terms of a sequence of layer resistivities and thicknesses. This interpretation is accomplished by computing a series of curves for various resistivity profiles and comparing them with the field data to determine which matches best. Calculation of the theoretical models is carried out by applying a linear filter to solve the appropriate integral expression. Interpretation is aided by using an interactive non-linear least-squares algorithm iteratively to adjust the model parameters. This procedure was used to interpret two-loop induction soundings made along the Arctic Slope of Alaska during 1969 to determine permafrost thickness and character. The results indicate that two-loop induction sounding is an effective method for mapping permafrost characteristics.