Monitoring Yearly Changes and Their Influence on Electrical Properties of the Shallow Subsurface at Two Sites Near the Rio Grande, West Texas

We present the results of a case study that monitored changes in the electrical properties of soils on a bi-monthly basis between August 1999 and November 2000 at two sites located along the banks of the Rio Grande, west Texas. One site was located within an abandoned channel of the Rio Grande and showed relatively homogeneous layering, while the second site exhibited complex interfingerings of crevasse splay (sands and silts) and floodplain muds. Repeated EM-31 ground conductivity measurements at both sites showed that conductivities within the upper 3m of soil could change by up to a factor of 2 within a year’s time. DC resistivity studies suggested up to a factor of 5 change in resistivity values within the upper 1m of soil. These changes appear to be related to remobilization of salt in the soil following rainfall or irrigation events. These dramatic fluctuations make it difficult to image deeper (>5m depth) conductivity changes in the subsurface.