Geoelectrical Monitoring of Shallow Vadose Zone Moisture Dynamics over Three Annual Cycles:Electromagnetic Induction Results

Abstract

Since late summer 2010, high-resolution electromagnetic induction (EMI) surveys have been performed as a part of a larger hydrogeophysical investigation to monitor changes in shallow moisture conditions at a clayey vineyard located in Vineland, Ontario, Canada. Our data set consists of 66 acquisition days during a 36-month period ending in August 2013. A wide range of soil conditions are witnessed at the site including wet spring and fall, dry summer, and frozen winter periods. Also, we were able to observe variation in hydrological processes between contrasting annual cycles (e.g., wet versus dry summer conditions). During the initial two-years of the study, EMI surveys were performed approximately bi-weekly along 25- metre lines at three monitoring sites within the vineyard. Data was collected using Geonics EM38 and EM31 ground conductivity metres in both vertical and horizontal dipole orientations. The final year of the study focused on EM38 surveys at only one of these sites, with data acquisition occurring weekly during a significantly wet summer period. Analysis of the data shows significant temporal variation in the EMI response due to seasonal moisture and provides insight towards the distribution of soil moisture in the subsurface. Geophysical observations are supported by nearby weather station data and gravimetric water content measurements. EM38 observations have been compared to predicted EM38 results calculated from data obtained by electrical resistivity tomography surveys at the site. This analysis shows reasonable agreement between observed and predicted values during conductive periods, with results displaying increasing deviation during progressively more resistive periods. Furthermore, this deviation is strongest in the horizontal dipole, suggesting that this discrepancy is caused by very shallow near surface conditions. Additional EMI studies at the site included seasonal field-scale EM38 surveys during the first year of data acquisition, two years of data acquired by EM38 vertical profiling, observing the seasonal functioning of tile drains, and investigating the response to storm events as observed by the EMI technique. The complete geophysical data set collected at this site is unique in the scientific community, and will provide the groundwork for significant future research in the field of hydrogeophysics