Research On The Multibeam Detecting Technology Of Shallow Subbottom Profile

Subbottom profilers (SBPs) are nowadays common acoustical devices that aim at exploring the first layers of sediment below the seafloor. With the improvement of the performances of SBPs and new applications of these systems, multibeam shallow subbottom profiling especially parametric multibeam shallow subbottom profiling becomes the tendency of substrate exploration. But the large aperture of the receiving array is the biggest drawback in the development of this technique. In this dissertation, the fractional fourier transform (FRFT) is introduced into the parametric multibeam shallow subbottom profiling for reducing the requirement of the receiving array's aperture and removing the superposition of the specular return, the methods of shallow subbottom profiling are researched at the same time.Linear frequency modulation signal has different focused feature on different fractional fourier domains. For the superiority of FRFT in processing multiple linear chirp signals and to achieve the goal of removing the superposition of specular returns, the principles and applications of FRFT and the source encoding of multibeam shallow subbottom profiling are researched. The benefits of the analytic signal with the FRFT are illustrated, and the theoretical formula of chirp signal distribution position in the fractional fourier domain is expressed. The rules of source encoding are proposed using this theoretical formula to achieve the goal of removing the superposition of the specular returns. All the procedures are illuminated by computer simulation.A combination of matched filtering and autoregressive (AR) predictive filtering is used to calculate the seismic impulse response and the detection resolution which is limited by the band-limited seismic wavelet is improved. Weighted least squares fitting method is used to estimate the accurate location and amplitude of seismic impulse response, and then the seismic impedance profile is estimated with the detection resolution improved at the same time. Because the high frequency attenuation significantly alters the impulse response inversion of the shallow subbottom, hence, the effection of attenuation is considered in the estimation to achieve appropriate impendance estimation. The procedures of pridictive filtering and weighted least squares fitting are simulated in details and the errors of estimation are given at the same time.