A Comparison of Electromagnetic Induction and Electrical Resistivity Tomography Techniques for Monitoring of Shallow Soil Moisture Dynamics

Abstract:

While the capacity of both electromagnetic induction (EMI) and electrical resistivity tomography (ERT) to monitor shallow soil moisture dynamics has been extensively examined, there have been few studies comparing the results of these two techniques. These comparative studies have primarily treated ERT as the more reliable method and focussed on the potential shortcoming (i.e., calibration and stability) of EMI devices. Further, these studies have been very limited in terms of their duration and the range of soil moisture conditions observed.

Concurrent EMI and ERT surveys we acquired during a 36-month period to monitor changes in shallow moisture conditions at a clayey vineyard located in Vineland, Ontario, Canada is an excellent data set for such a comparison between these geoelectrical techniques. A wide range of soil moisture conditions were encountered during this monitoring program including wet spring and fall, dry summer, and frozen winter periods. Also, the effects of variations in hydrological processes between contrasting annual cycles (e.g., wet versus dry summer conditions) can be seen in these hydrogeophysical data. In addition, procedures were followed during the EMI data acquisition to minimize calibration and stability issues.

We found very good agreement between these two geoelectrical techniques during the relatively wet conditions during fall to spring period. However, there is significant deviation between these methods during the summer period with the EMI data indicating less conductive (i.e., drier) conditions than the ERT data. This deviation is larger for the EMI horizontal dipole data, indicating that the source of this discrepancy is located in shallow near-surface. In addition, the comparison of gravimetric soil moisture measurements with both geoelectrical data sets shows a substantially better correlation between soil moisture and the EMI data, implying that there are significant limitations with the ERT technique in this application. The results of modelling and additional ERT surveys indicate that very short electrode spacings are required to adequately characterize the thin, highly resistive surface layer where desiccation cracking was observed. Hence, we have concluded that EMI methods are better suited for monitoring soil moisture dynamics in this system.