A Study for Estimating the Reflection Coefficients of the Layering Sediments Using Chirp Sonar
Date Issued
2015
Date
2015
Author(s)
Yang, Chiao
Abstract
In order to know the physical properties of seabed sediments, people traditionally adopted to practical grabbing the samples of seabed sediments and analyzed it directly. But there are dozens of drawbacks like inefficient for vast area and unrepresentative for region as a whole. This study aimed at looking for a method that is more efficient to acquire the reflection coefficients of seabed sediments and further to each interface of sediment layers beneath. The tool used is the geological chirp sonar called which the Xstar system composed of a transducer vehicle SB-216S produced by the EdgeTech in 1990. The survey was conducted in Chu-Nai offshore area and operated bandwidth in 2-10kHz. Draw upon acoustic reflection power to calculate the reflection coefficients that would contemplate on the factors of energy spreading and attenuation when acoustic waves transport the downwards in different mediums. In order to estimate the amount of energy loss, to precisely find the values of reflection coefficient, the spreading effect on beamwidth as well as the Fresnel zone effect on reflected energy have been considered. The calibration ratio is therefore obtained, and the value is compatible with the study adopted local seabed sediment sample to amend the reflection coefficient according to conclusion of Hamiltion(1970). The method we study is feasible to calculate reflection coefficient on seabed layers further beneath, and the result would illustrate more deviated in the deep when the singal/noise ratio is significantly decreased. If we utilized acoustic power to calculate reflection coefficient, we shall first considering the representative of Near-field pressure measurement is adequate or not. Or we have to set up a rationally calibration process of acoustic field. At last, we compare the reflection coefficients of 3 acoustic interfaces in shallow layers beneath seabed with the core drilling data nearby to attest what we calculate is in keeping with reality.
Subjects
chirp sonar
reflection coefficient
sediment classification
transmission loss
Type
thesis
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