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Field monitoring of soil moisture and groundwater levels is one of the countermeasures against slope failure caused by heavy rainfall. We developed a new monitoring method for soil moisture and groundwater level using ultrasonic waves. An ultrasonic detector in this measuring method is easily installed underground and at low cost, and therefore, this method is useful for multipoint monitoring in a wide field area. However, since the measurement data are greatly influenced by temperature, this temperature-dependent effect must be corrected. Previously, the dummy pair system using two detectors had solved this problem. In this paper, we propose another improved temperature correction method using one detector and evaluate the usefulness of both methods in detail.

Field monitoring of soil moisture and groundwater levels is important to predict slope failures due to heavy rainfall. We have investigated a novel monitoring method using ultrasonic waves. The detector is composed of an ultrasonic transducer and a brass pipe whose lower end is inserted vertically into the ground. Ultrasonic waves generated by the transducer at the upper end of the pipe travel through it underground, and are reflected from the soil's surface at the bottom of the pipe. The soil's moisture is monitored by the intensity of the reflected ultrasonic waves and the groundwater level is monitored by the propagation time of the same waves. Indoor experiments were carried out using an artificial rainfall simulator and a soil tank. Shigaraki soil and Toyoura sand were used as soil samples in the experiments. The intensity of the reflected waves increased with the soil's water content and changed dramatically depending on whether the soil was fully dry, wet, or fully saturated. The propagation time decreased as the groundwater level rose. This method has advantages in that the detector can easily be installed on the ground surface and can simultaneously monitor both the soil moisture and groundwater level.

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.

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