Geophysics

EM Induction
Surface geophysics is a valuable reconnaissance tool for vadose zone hydrology studies. Electromagnetic induction measures the apparent electrical conductivity of the subsurface, which varies with clay content, water content, and salinity. Apparent electrical conductivity (ECa) can be measured with above ground, noninvasive EM meters such as the EM31, EM34, and EM38 meters (Geonics Inc., Mississauga, Ontario). A downhole EM meter (EM39) can also be used to monitor the vertical variation in EC. The various frequency domain conductivity meters manufactured by Geonics differ in the distances between the transmitter and receiver coils, and the frequency at which they operate resulting in different exploration depths. The exploration depth of the instruments increases with increased intercoil spacing and decreased transmission frequency and also varies with the orientation of the coils relative to land surface. Variations in ECa measured with the above-ground EM31 meter were affected by variations in clay content in a playa/interplaya setting, water content in a fissure, and chloride content adjacent to a drainage system. These factors affecting ECa were confirmed by comparing ECa measured with the downhole EM39 meter and clay, water, and chloride content of soil samples from boreholes. The hydrologic significance of parameters controlling ECa was evaluated. EM induction was very useful in interpolating and extrapolating from borehole data.

Reference
Scanlon, B. R., Paine, J. G., and Goldsmith, R. G., 1999, Evaluation of electromagnetic induction as a reconnaissance technique to characterize unsaturated flow in an arid setting: Ground Water, v. 37, pp. 296-304. [PDF]

 

Spatial and Temporal Variability of Soil Water Content
EM induction has also been used to monitor spatial and temporal variability in water content and water storage in the shallow subsurface (1.5 m) in a field-scale prototype engineered cover (17 m × 34 m) designed for waste containment. Water content was monitored with a neutron probe, and bulk soil electrical conductivity was monitored with a Geonics EM38 ground conductivity meter at 10 locations at approximately monthly intervals over a 3-yr period. A simple linear regression model was developed to predict average water content in the upper 1.50 m of the soil profile at any location at any time (R2 0.80, average standard deviation = 0.009 m3/m3) and spatially averaged water content over the entire area at any time (R2 0.99, average standard deviation = 0.003 m3/m3). The model residual water content values did not appear to drift significantly with time, indicating that once the model is calibrated over a sufficient range of water content values with the neutron probe, further neutron probe measurements may not be necessary. EM induction has several advantages over traditional techniques for monitoring water content, including non-radioactivity, speed and ease of use over large areas, and noninvasive character, which is important for avoiding development of preferred pathways.

Reference
Reedy, R. C., and Scanlon, B. R., 2003, Soil water content monitoring using electromagnetic induction: Journal of Geotechnical and Geoenvironmental Engineering. [PDF]

March 2006