Development of a sonar system to estimate the seafloor subsurface burial depth of a towed gamma-ray spectrometer

Hirao, Yoshihiro, Ohnishi, Seiki, Thornton, Blair, Yano, Yusuke, Odano, Naoteru and Ura, Tamaki (2014) Development of a sonar system to estimate the seafloor subsurface burial depth of a towed gamma-ray spectrometer. Marine Technology Society Journal, 48 (3), 155-166. (doi:10.4031/MTSJ.48.3.13).

Abstract

A sonar system and image processing method have been developed to continuously estimate the seafloor subsurface burial depth of a gamma-ray spectrometer that is towed along the seafloor by a ship. The towed gamma-ray spectrometer, called the RESQ hose, is an instrument designed to measure radiation levels in seafloor sediments that were contaminated as a result of the accident at the Fukushima-Daiichi nuclear power plant. In situ measurements of subsurface burial depth can increase the accuracy of the radionuclide concentration levels estimated by the RESQ hose by allowing for more accurate modeling of the measurement conditions. The reliability of the system is verified through sea trials performed near the outlet of the Abukuma River. Continuous measurements of burial depth are achieved with an outlier ratio of less than 5%. The burial depths measured by the device show a strong correlation with the acoustic backscatter intensity of side-scan sonar images obtained along the surveyed transect, indicating that burial depth is dependent on sediment type. We examine the influence of burial depth on the conversion factors between detected gamma-ray count rates and the concentrations of 134Cs and 137Cs. For both nuclides, the calculated factors increase by nearly 80% as burial depth increases from 0 to 11 cm. The converted results from the measured 137Cs count rates are compared with a core sample obtained near the transect, with the nearest point within a factor of two of the sampling result.

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