Geophysical quantification of a moisture content profile in the near surface

Abstract

Geophysicists commonly have a simplistic view of the water table as a sharp interface between the vadose zone (unsaturated region) and the phreatic zone (saturated region). In reality, this boundary is a transition zone where moisture content varies continuously with depth. Since geophysical methods respond to depth-related variations in water content, the use of this simplistic model could lead to significant errors in the interpretation of surface geophysical data. An improved model for the moisture content profile that incorporates different soil structures, soil type and water qualities would allow better interpretation of near-surface geophysical surveys. In this study, an analysis of the relationship between time-varying moisture content and the response of commonly used near-surface geophysical methods has been performed using the soil-moisture simulation program LEACHM. Model studies reveal that the interpreted depth to water table will be less than the true depth to 100% saturation, and that the non-uniqueness of interpretation generates a wide range of possible vadose models that fit data to within 5%. Heavy summer rainfall events significantly change surface geophysical responses over a two-week period; lighter winter rain will introduce smaller changes, but with greater frequency. Additionally, temperature variations in the top one metre are also important for pore-water electrical conductivity. Such variability has important consequences for the repeatability of measurements over time, and in comparing data from different areas.