The 2LAYINV program is used to interpret geophysical, electromagnetic VLF-R data (apparent resistivity and phase) measured along a single profile at a single frequency. The inversion is made separately for each data point using a one-dimensional two-layer earth model. As such the program suits well the interpretation of the thickness and resistivity of relatively thin resistive (soil and till) layers and the resistivity variations of the basement rocks.
Starting from an automatic initial model linearized inversion with adaptive damping is used to optimize the thickness and the resistivity of the overburden layer and the resistivity of the basement so that the model minimizes the error between the measured and the computed VLF-R data. Laterally constrained inversion is obtained by minimizing the roughness of the model, that is the variation of the model parameters between neighboring points, together with the data error. As a result, a smoothly varying (Occam inversion) model will be obtained. The roughness and/or the fix/free status the parameters can be set manually to allow discontinuities and to incorporate a priori data into the model. Note that the inversion model is actually one-dimensional and that the interpretation results are shown as a 2-D resistivity pseudosection.