MEASUREMENT OF INTERFACE BETWEEN FRESH WATER AND SALT WATER USING ENERGY ABSORPTION CHARACTERISTICS IN MICROWAVE

To examine the effectiveness of time domain reflectometry (TDR) for finding the interface between fresh and salt water layers, the possibility and accuracy in measuring the interface level were investigated with a two-rod TDR probe of 48cm. In the TDR waveform, reflection coefficient values indicate the magnitude of electrical energy of a microwave step-pulse given by TDR cable tester. The level was estimated by analyzing the waveform feature that the coefficient value will abruptly reduce at the interface due to energy absorption by salt water. In the experiment, to reproduce various groundwater qualities in a coastal aquifer, the electrical conductivity of fresh water was arranged from 0.0005 to 0.4 S m super(-1). The fresh water was set to the upper layer than salt water of 5.2 S m super(-1) in the experimental column. In the situations where the conductivity or layer thickness of fresh water was large, the energy loss into the water prevented us from measuring the level. However, when the thickness is thin for the high conductivity condition, the interface could be more detectable due to reduction of the loss. The root mean square errors for the measured levels were less than 1cm under static hydraulic conditions even if the measured levels fluctuated. These results may suggest that the proposed method for detecting the interface is effective.