Monitoring of soil moisture and groundwater levels using ultrasonic waves to predict slope failures

We investigated a novel method of monitoring soil moisture and groundwater levels using ultrasonic waves to predict slope failures due to heavy rainfall. The detector is composed of an ultrasonic transducer and a metallic pipe whose lower end is inserted vertically into the ground. An ultrasonic wave generated by the transducer at the upper end of the pipe travels through the pipe to the ground, and is reflected at the soil's surface inside the pipe. The soil's moisture and groundwater level are monitored by using the intensity and propagation time of the reflected ultrasonic wave. The intensity depends on the temperature of the detector and the propagation time depends on the air in the pipe, and we developed methods of calibrating these. The intensity of the reflected wave in the indoor experiments increased with the soil's water content and changed dramatically depending on whether the soil was fully dry, wet, or saturated. The propagation time decreased with rising groundwater levels. This method has the main advantage that both the soil's moisture and water levels can be monitored with one detector.