Improvement in bottom detection for research of demersal fish using hydroacoustics

            DooJin Hwang, Kohji Iida, Yoshimi Takao, Yoichi Miyanohana,
                        Kouichi Sawada and Tsuyoshi Okumura

                     Ebidai, Hasaki, Kashima, Ibaraki 314-04, Japan

     The bottom  detection is very  important for defining  an integration layer  with 
the bottom as  a reference basis during  acoustic survey of demersal  fish. Therefore, 
we  have considered  theoretically  elements  which form  bottom  echoes, and  have 
devised  a  stable  bottom detection  method  namely  maximum  differential  voltage 
method, which  was not influenced  by variable levels  and waveform transformation.   
The method  has been shown  to be effective  using in-situ bottom  echo waveforms 
and computer simulation data.
     A  comparison  between  near-bottom   SV  profiles  acquired  in  Funka  Bay 
Hokkaido of  Japan and  East China Sea,  with the  threshold method and  maximum 
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differential voltage method is 4!-6 dB higher within 2 m from the bottom.