Page 194 - Acquisition and Processing of Marine Seismic Data
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3.6 INLINE WAVES                              185

           close to the bird locations are obviously three or  filters, the most important contribution to elim-
           four times higher than those at channels off the  inate the bird noise is supplied by the stacking
           birds. Schoenberger and Mifsud (1974) manufac-  process, which can remove almost all of the rem-
           tured a special streamer section with 20 hydro-  nant bird noise from the data.
           phones located 2.4 m apart and mounted a bird
           at the center of the section in order to observe
           how the bird noise spreads along the streamer.         3.6 INLINE WAVES
           The result indicates that the bird noise is apparent
           only at nearby channels and is almost symmetri-
                                                           The noise appearing as linear events with
           cal. While close hydrophones (1.2 m apart) are
                                                        trace-by-trace consistency on the raw shot
           quite noisy, they observed almost no noise ampli-
                                                        records is classified as inline waves. Raw seismic
           tudes at the hydrophones located farther than
                                                        data may contain several linear events from dif-
           3.6 m from the bird location. Their analysis also
                                                        ferent sources, such as
           provided information about the mechanism of
           the bird noise: Symmetry and rapid decay are  • Tail buoy noise
           notthecharacteristicsoftheturbulenceeffectfrom  • Mechanical cable noise (tug and strum)
           the water flow around the bird fins, which sug-  • Direct waves
           gests that the bird noise is transferred onto the  • Refracted waves
           streamer and hydrophones by mechanicalmeans.
              Fig. 3.15A shows an example shot record with  3.6.1 Tail Buoy Noise
           distinct bird noise appearing as vertical high-
           amplitude bands at the channels close to the bird  The far end of the streamers is determined by
           locations indicated by arrows. Amplitude resto-  a tail buoy, generally with an rGPS transmitter
           ration processes, especially AGC, may signifi-  (Section 2.1.8). Depending on the weather condi-
           cantly increase the amplitudes of the bird   tions, movement of the tail buoy may generate
           noise. Fig. 3.15B illustrates a shot gather filtered  low-amplitude linear noise around the far off-
           with an f-k filter to make the bird noise much  sets of the shot gathers. Its amplitude depends
           more prominent. Bird noise can easily be recog-  on the ocean surface conditions, and if the sea
           nized on f-k spectrum: The amplitudes come up  conditions are rough, the tail buoy may act as
           as horizontal bands on the 2D Fourier spectra  a seismic source, such that the generated signal
           because they are seen as vertical bands on time  propagates from far to near offsets along the
           sections (B in Fig. 3.15B).                  streamer. Therefore, tail buoy noise is observed
              Fig. 3.16A shows a shot gather with bird noise  at far offsets as events with linear but reverse-
           as vertical high-amplitude stripes. The f-k spec-  dip continuous amplitudes on the shot records
           trum of the shot indicates that the bird noise has  (Fig. 3.17). Its form does not change with the
           approximately 20-Hz frequency and it is seen as  tow speed for 4–8 knots range, and its frequency
           a horizontal band along the wavenumber axis.  band is rather low and is around 20 Hz.
           The f-x spectrum, which illustrates the fre-    A reasonable way to avoid tail buoy noise
           quency content of each trace separately, shows  during the acquisition is to tow the buoy as far
           high amplitude vertical bands indicated by   as possible from the last active section of the
           arrows around the dominant bird noise fre-   streamer. The tail buoy noise has reverse dip
           quency. A suitable f-k polygon that accepts only  on the shots, hence its amplitudes appear in neg-
           reflection amplitudes in the f-k panel removes  ative panel on the f-k spectrum, and therefore it
           most of the noise arising from the birds     is generally easy to remove this noise using a
           (Fig. 3.16B). In addition to the band-pass or f-k  suitable f-k filter.
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