Borehole ground penetrating radar (BGPR) operated in zero offset profiling (ZOP) mode has promise for monitoring rapidly changing water contents within the subsurface. However, the coexistence of multiple travel paths through the subsurface can give rise to measurement errors. Specifically, in layered systems with sharp changes in water content with depth, critically refracted waves may arrive before direct waves at some depths. Velocity profiles are determined based on analyses of the travel time of the first-arriving energy at each depth. Therefore, correct velocity analysis requires that these travel times be classified according to the path followed by the first-arriving energy. We establish criteria that can be used to identify first-arriving critically refracted waves from travel time profiles. Through hypothetical examples and a field experiment, we demonstrate that these criteria allow for more accurate determination of the water content profile. However, these corrections are limited if thin, high water content layers are present in the subsurface.

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