TR-52 : Geoelectrical techniques for study of soil moisture

Abstract

A systematic investigation of streaming potentials (S.P.) shows that they vary considerably with time. To study Influence of water movements on S.P. anomaly-patterns, repeated measurements were conducted during May-September, 1987 at site comprising of alluvium formation. An electrode array having six-electrodes at regular intervals of one foot was permanently installed in a borehole and potential gradients as well as potential differences were measured on daily basis. An inverse correlation was observed between the S.P. anomalies and the rainfall events suggesting that S.P. measurements may become a valuable and standard technique In hydrology.

Studies of moisture migration in the vadose zone were conducted at two field sites using a Wenner resistivity array. Gravimetric moisture measurements on soil samples taken at each of the field sites were made to a maximum depth of 1 .50 meters, and these results were compared with resistivity values. Resistivity was measured using a multi electrode, switch-controlled Wenner-array, enabling repeated measurements at various electrode positions without disturbing physical location of the electrodes.

Effect of rainwater infiltration was studied by monitoring resistivity before and after rainfall events. Archie's empirical relationship was utilised to correlate moisture and resistivity measurements. Results indicate that (i) moisture is retained for long periods of time in clay/silt rich soils, (ii) moisture migration is slow below a moist soil zone and is not readily detected by surface resistivity measurements, and (iii) near-surface moisture variation can be defined by surface resistivity measurements. Small anomalies encountered in monitoring resistivity for study of moisture-migration caution a high precision field measurements and interpretation of resistivity data.