ANALYSIS OF SOUND SPEED MEASUREMENTS FROM ACOUSTIC SOURCES

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					                                                                                                                                                        OS 72B-0355 Analysis of Acoustic Signals from Ship Traffic at Pioneer Seamount
                                                                                                                                                         Carl O. Vuosalo,1 Craig Huber,1 Michael D. Hoffman,1 Newell Garfield,2 and Roger Bland1
                                                                                                                                          1. Physics and Astronomy Department and Romberg Tiburon Center for Environmental Studies, San Francisco State University
                                                                                                                                              2. Department of Geosciences and Romberg Tiburon Center for Environmental Studies, San Francisco State University
                                                                                                                                                                                                         http://www.physics.sfsu.edu/~seamount

                                                                                                                                                                    In August 2001 a vertical linear array (VLA) of four hydrophones was installed by NOAA-PMEL at Pioneer Seamount, 95 km off                                                                                                 cable
                                                                                                                                                                    the California coast and 930 m below the surface. The four channels, digitized at 1000 Hz with 16-bit precision, are available in                                                                         cable   BREAK

                                                                                                                                                                    near real time. In this data set the loudest and most obvious signals are those created by passing ships. We present a method for
                                                                                                                                                                    analyzing the ship signals to determine the speed of each ship and its distance of closest approach to the array. The observatory is
                                                                                                                                                                    now offline due to a broken cable, and due to uncertain funding, no repairs have yet been planned.


                                                                                                                                                                                      Ship Detection and Speed Calculation
                                                                                                                                                               Software was developed in the IDL programming language to automatically
                                                                                                                                                                                                                                                                                                                                    ROMBERG TIBURON CENTER
                                                                                                                                                               find ship signals in the data set and then calculate the speed and distance of closest
                                                                                                                                                               approach of each ship. The first step in this process is to make a Fourier transform
                                                                                                                                                               spectrogram of the raw acoustic data. This transformed data is scanned for the
                             Topography in the vicinity of the array, which is shown                                                                           distinctive interference pattern created by ships. These signatures are considered
                             as three vertical dots.                                                                                                           to be two sharp peaks that are integer multiples of each other and that occur below
                                                                                                                                                               21 Hz. The second step is to perform a Fourier transform on the Fourier transform
                                                                                                                                                               spectrograms. At those locations previously found to contain ship signatures, the
                                                                                                                                                               double Fourier transforms are scanned to find a sequence of sharp peaks. This set
                                                                                                                                                               of peaks is then fitted with a calculated distance and time curve, and if a
                                                                                                                                                               reasonable fit is achieved, the presence of a ship is confirmed, and its speed and
                                                                                                                                                               distance can be obtained from the parameters of the curve.                                                                                  Fourier transforms of two ship signals to give frequency vs. time spectrograms. The top panels
                                                                                                                                                                                                                                                                                                           show data summed from all four hydrophones. The middle panels show data from only the top
                                                                                                                                                                                                                                                                                                           hydrophone of the array. The bottom panels show Fourier transforms of the middle panels.
                                                                                                                                                                                                                                                                                                     500
                                                                                                                                                                                                                                                                                                     Hz                                                                                            FOUR HYDROPHONES
                                                                                                                                                                                                                                                                                                                                    FOUR HYDROPHONES                                          COMPUTER SIMULATION
                                                                                                                                                                                                                                                                                                                                                                                                   • dmon = 3600 m
                                                                                                                                                                         Fourier
                                                                                                                                                                       transform
                                                                                                                                                                                                                                       R                                                                                                                                                           • speed 15.8 m/s (30 knots)
                                                                                                                                                                                                                                                                                                                                                                                                   • flat sea bottom
                                                                                                                                                                      acoustic data
                                                                                                                                                                                                                                       q                                                                                                                                                           • white noise
                                                                                                                                                                                                                                                                    d
                                                                                                                                                                         Scan for
                                                                                                                                                                           ship
                                                                                                                                                                        signatures
                                                                                                                                                                                                                        R  d / tan q
                                                                                                                                                                                                                                   c
                                                                                                                                                                                                                       a sin q  n
                                                                                                                                                                                                                                                                             a                       0
                                                                                                                                                                                                                                                                                                     Hz
              Underwater topography near Pioneer Seamount. (Graphics courtesy of PMEL/NOAA)
                                                                                                                                                                          Fourier                                                  f                                                                 500
                                                                                                                                                                                                                                                                                                     Hz                                                                                                  ONE HYDROPHONE
                                                                                                                                                                                                                                                                                                                                      ONE HYDROPHONE
                                                                                                                                                                        transform
         0
         Hz
                             500
                              Hz Hz
                                   0                       500
                                                           Hz
                                                                 0
                                                                 Hz
                                                                                       500 0
                                                                                       Hz Hz
                                                                                                                      500
                                                                                                                      Hz
                                                                                                                            0
                                                                                                                            Hz
                                                                                                                                                 500
                                                                                                                                                  Hz
                                                                                                                                                                      spectrograms                               Rd         af / c     2
                                                                                                                                                                                                                                               1
15 min
                                                                                                                                                                     Scan result for                                                                              Faculty Talks, September 9, 2002
                                                                                                                                                                                                                                                                                                                                                                                                          ONE HYDROPHONE
                                                                                                                                                                         peaks
  time




                                                                                                                                                                                                        One method of calculating ship range R, based upon separation
                                                                                                                                                             Fit set of peaks to distance vs.           between maxima in the Fourier transform f, separation between
                                                                                                                                                            time curve to confirm ship and              hydrophones a, and the speed of sound in water c.
                                                                                                                                                             determine speed and distance.                                                                                                           0
                                                                                                                                                                                                                                                                                                     Hz

  0                                                                                                                                                                                                     The computer simulation shown to the right, for the faster ship,                                                                 “FFT OF FFT”
                                                                                                                                                                                                                                                                                                                                                                                                               “FFT OF FFT”
                 9 m/s                 14 m/s (28 knots)                 7 m/s                          9 m/s                        6 m/s                                                              is based on this geometry, using an expression for the intensity,
                                                                                                                                                                                                                                           2
                                                                                                                                                                                                                                     p0
              THE LOUDEST SHIP SIGNALS. Signals like this arrive about once every three days.                6:00-
                                                                                                             8:00
                                                                                                                                                                                                                                  I  2 cos 2 
              The are the loudest sounds observed at Pioneer Seamount.                                        UT                                                                                                                      r
                                                                                                             8:00-
                                                                                                             10:00
                                                                                                              UT
                                                                                                                                                                                                        where  is the difference in phase for paths from the ship to a
                                                                                                             10:00-                                                                                     single hydrophone and to its image beneath the sea floor. For                                      Fitted speed curves for two ships. Plot of surface range in meters from the array vs. time in seconds.
                                                                                                             12:00
                                                                                                              UT                                                                                        the simulation shown and for the spectrogram to which it is                                        Green curves based upon blue points. Red points were rejected.
                                                                                                             12:00-
                                                                                                             14:00                                                                                      compared, only the signal from the upper of the four
                                                                                                              UT
                                                                                                             14:00-
                                                                                                                                                                                                        hydrophones was used.
                                                                                                             16:00
                                                                                                              UT
                                                                                                             16:00-
                                                                                                             18:00
                                                                                                              UT
                                                                                                             18:00-
                                                                                                             20:00
                                                                                                              UT
               13 m/s                                                                  9 m/s                 20:00-
                                            16 m/s (32 knots)                                                22:00
                                                                                                              UT
                                                                                                             22:00-
                                                                                                             24:00
   SOME FASTER SHIPS. The speed of these ships has been estimated from the slope of the                       UT

   interference maxima, since the fitting procedure described to the right is disrupted by the presence of
   the constant-frequency lines on these spectrograms (vertical lines). These lines are probably due to                Spectrograms for an 18-hour period (23:00 July 20 to 17:00 July 21, PDT)
   engine noise, while the smoother part of the pattern is due to prop noise.                                          showing numerous ship signals. The patterns gating on and off probably
                                                                                                                       represent boats stopping or idling while fishing. (All spectrograms 0-500
                                                                                                                       Hz, frequency increasing upwards.)
                                                                                                                                                                                                                                                                                                       Ship at 08:15 on Sept. 8, 2001.                             Ship at 21:15 on Sept. 10, 2001.
                                                                                                Help and advice is gratefully acknowledged from:Chris Fox, Jonathan Klay, Andy Lau, and Haru Matsumoto, NOAA-PMEL; Jim Mercer                                                                          Speed: 6.1 m/s. Closest approach: 645 m.                    Speed: 15.8 m/s. Closest approach: 3656 m.
                                                                                                         and Lyle Gullings, APL, University of Washington; and John Bourg and Jim Lockhart, San Francisco State University                                                                             This calculated closest approach may be
                                                                                                                                                                                                                                                                                                       too low.

				
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