ELECTROMAGNETIC STUDY OF TEMPORAL VARIATIONS IN SOIL PROPERTIES IN TERRY COUNTY, TEXAS

The purpose of this study is to determine if non-invasive electromagnetic methods can be calibrated to detect variations in soil moisture. We collected Electromagnetic (EM34) conductivity, D.C. resistivity, spontaneous potential (SP) and ground penetrating radar (GPR) measurements several times in a small region over the period of eight month. The GPR data proved to be of no use because there is a surface layer that is too conductive to penetrate. For the offsets possible with our equipment the D.C. resistivity data indicated a very thin surface layer that made deep penetration impossible and explained to us why the GPR failed. The EM34 was however able to penetrate this thin layer. The only measurements to be repeated every trip to the field were the EM34 measurements, taken in the vertical dipole (VD) and horizontal dipole (HD) orientations using 10, 20 and 40 meter coil spacing, and the SP readings. To study the effect of precipitation we made three consecutive sets of measurement following a rainstorm by 1, 6 and 10 days. These measurements were compared with one another and compared to a different set of measurements collected after a long drought (assumed to be the reference "dry soil" set of reading). While the SP measurements showed spatial anomalies (possibly related to vegetation) it showed no obvious temporal changes that could be correlated with the rain event. Temporal changes in the EM34 conductivity measurements appear to correlate with the drying of the surface layer. There also appears to be a weak correlation in the spatial anomalies of the EM34 and the SP measurements. We are constructing a monitoring station that will automatically measure spatial and temporal variation in SP and conductivity (including induced polarization measurements). This station will also monitor temporal changes in the depth to water table, soil moisture and soil temperature to enable comparison of subsurface properties with surface observations.