Sensitivity and dents of hydrophone pipes with different thickness

Yuiko AMANO, Kazuo YOSHIDA, Michinobu NONAKA, Takahiro ITOH and Takahisa MIZUYAMA

Pipe hydrophones have been used widely to monitor the mountain streams bed load in Japan. They consist of a microphone in a steel pipe and measure bed load indirectly using the sound of collisions. Pipe hydrophones can measure small particle bed loads more sensitively than the plate geophones used widely in Switzerland. However, in several places where hydrophones have been installed, the measurements are negatively affected because the pipes are dented by collisions with large and many sediment particles. One way to solve this problem is to increase the thickness of the pipes. However, this changing might reduce the sensitivity of the hydrophone. In this study, we conducted experiments to examine effects of the pipe thickness on measurements by hydrophone. The amount of dents due to stone collisions were also calculated using the Ellinas equation. The experiments and calculations showed that the sensitivity of thicker pipes dulled slightly, although the dents were much smaller compared with‚Rmm thickness pipe, adopted generally in Japan. Therefore, hydrophones with thicker pipes are recommended to measure in streams where large particles or high sediment discharge is expected.