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Measurements and modeling of mid to high frequency bottom backscattering strengths

Title
Measurements and modeling of mid to high frequency bottom backscattering strengths
Author
손수욱
Alternative Author(s)
Son, Su-Uk
Advisor(s)
Choi, Jee Woong
Issue Date
2015-08
Publisher
한양대학교
Degree
Doctor
Abstract
Mid- to high-frequency bottom scattering in shallow water is greatly influenced by the roughness of the bottom surface in case of hard sediment conditions such as sand and gravel. In this case, the backscattering from the bottom interface roughness predominates over that from the volume inhomogeneities within the sediment. Thus, it is important to measure the bottom roughness spectrum to predict the bottom scattering strengths precisely. To investigate the effect of roughness on bottom backscattering, in the laboratory experiment we measured the bottom backscattering strength at 50 kHz for two types of roughness. Bottom roughness was measured using the 5-MHz ultrasonic roughness measurement system developed at Hanyang University, and the measured backscattering strengths were compared with model predictions to understand how different bottom roughnesses affect the backscattering strengths. In conclusion, the overall measured bottom backscattering strengths were significantly different for the two types of bottom roughness studied, even though the geoacoustic parameters of both bottoms were the same except for the interface roughness. In addition, the measurements were in good agreement with the model predictions obtained by using the measured roughness parameters for both bottoms. The experimental investigation showed that the bottom roughness spectrum is an important factor in bottom backscattering in hard bottom conditions. Subsequently, bottom backscattering experiments at 614 kHz were performed off the southern coast of Gyeonggi Bay in the Yellow Sea, where the bottom is sandy sediment. Bottom roughness spectra were estimated using the bathymetry data obtained with a multibeam echo sounder. However, there was a problem in applying the backscattering model to these data because of the lack of spatial resolution. Assuming that the bottom roughness spectra of the ocean floor follow a power-law spectral form, we expanded the estimated power-law spectral curve to the high-wavenumber region. To investigate the validity of this assumption, we performed a geoacoustic parameter inversion using grid search methods and compared the results with the estimated roughness parameters obtained from the multibeam echo sounder data and found them to be in reasonable agreement. The results showed that extrapolation of the power law to a high-wavenumber region that corresponds to mid- to high frequency is reasonable. The bathymetry data obtained using the multibeam echo sounder can be used to estimate the roughness parameters for applying the high-frequency backscattering model.
URI
https://repository.hanyang.ac.kr/handle/20.500.11754/127744http://hanyang.dcollection.net/common/orgView/200000427045
Appears in Collections:
GRADUATE SCHOOL[S](대학원) > ENVIRONMENTAL MARINE SCIENCES(해양환경과학과) > Theses (Ph.D.)
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