BELLHOP


Overview

Sample Input

Description of Inputs

Source/Receiver Depths and Ranges

Run Type

Beam Fan

Numerical Integrator Info

Running BELLHOP



Overview

BELLHOP computes acoustic fields in oceanic environments via beam tracing. The environment treated consists of an acoustic medium with a sound speed that may depend on range and depth.

A theoretical description may be found in:

Michael B. Porter and Homer P. Bucker, ``Gaussian beam tracing for computing ocean acoustic fields,'' J. Acoust. Soc. Amer. 82, 1349--1359 (1987).

Michael B. Porter and Yong-Chun Liu, “Finite-Element Ray Tracing'', Proceedings of the International Conference on Theoretical and Computational Acoustics, Eds. D. Lee and M. H. Schultz, pp. 947-956, World Scientific (1994).

The following programs are used with BELLHOP :

BELLHOP     Main program for doing Gaussian beam tracing

PLOTRAY     Produces plots of central rays of beams

PLOTATI     Produces plots of the altimetry

PLOTBTY     Produces plots of the bathymetry

ANGLES      Given the source and receiver sound speeds, computes the angle of the limiting ray.

PLOTSSP     Plots the sound speed profile

PLOTSSP2D   Plots the range-dependent sound speed profile

BELLHOP produces pressure fields in the NRL standard format and can therefore be plotted using the MATLAB script, plotshd.m.

The steps in running the program are as follows:

   1. Set up your environmental file and run PLOTSSP to make sure the SSP looks reasonable.

   2. Do a ray trace.  That is,

      A. Run BELLHOP with the ray trace option to calculate about 50 rays.

      B. Run PLOTRAY to make sure you have the angular coverage you expect.  Do the rays behave irregularly? If so reduce the step-size and try again.

   3. Re-run BELLHOP using the coherent, incoherent or semicoherent option for transmission loss. (Use the default number of beams.)

   4. Run plotshd.m to plot a full range-depth field plot.

   5. Double the number of beams and check convergence.

Files:

        Name           Unit         Description

Input
        *.ENV            1       ENVironmental data

Output
        *.PRT            6       PRinT file
        *.RAY           21       RAY   file
        *.SHD           25       SHaDe file

Sample Input (Environmental) File:

'Munk profile'        ! TITLE
50.0                  ! FREQ (Hz)
1                     ! NMEDIA
'SVN'                 ! SSPOPT (Analytic or C-linear interpolation)
51  0.0  5000.0       ! DEPTH of bottom (m)
    0.0  1548.52  /
  200.0  1530.29  /
  250.0  1526.69  /
  400.0  1517.78  /
  600.0  1509.49  /
  800.0  1504.30  /
 1000.0  1501.38  /
 1200.0  1500.14  /
 1400.0  1500.12  /
 1600.0  1501.02  /
 1800.0  1502.57  /
 2000.0  1504.62  /
 2200.0  1507.02  /
 2400.0  1509.69  /
 2600.0  1512.55  /
 2800.0  1515.56  /
 3000.0  1518.67  /
 3200.0  1521.85  /
 3400.0  1525.10  /
 3600.0  1528.38  /
 3800.0  1531.70  /
 4000.0  1535.04  /
 4200.0  1538.39  /
 4400.0  1541.76  /
 4600.0  1545.14  /
 4800.0  1548.52  /
 5000.0  1551.91  /
'V'  0.0
1                       ! NSD
1000.0 /                ! SD(1:NSD) (m)
2                       ! NRD
0.0 5000.0 /            ! RD(1:NRD) (m)
501                     ! NRR
0.0  100.0 /            ! RR(1:NR ) (km)
'R'                     ! Run-type: 'R/C/I/S'
51                      ! NBEAMS
-11.0 11.0 /            ! ALPHA(1:NBEAMS) (degrees)
200.0  5500.0  101.0    ! STEP (m)  ZBOX (m)  RBOX (km)

Description of Inputs

The first 6 blocks in the ENVFIL are common to all the programs in the Acoustics Toolbox. The following blocks should be appended for BELLHOP:       

 (7) - SOURCE/RECEIVER DEPTHS AND RANGES

       Syntax:

          NSD
          SD(1:NSD)
          NRD
          RD(1:NRD)
          NR
          R(1:NR )

       Description:

          NSD:  The number of source depths
          SD(): The source depths (m)
          NRD:  The number of receiver depths
          RD(): The receiver depths (m)
          NR:   The number of receiver ranges
          R():  The receiver ranges (km)

This data is read in using list-directed I/O you can type it just about any way you want, e.g. on one line or split onto several lines.  Also if the depths or ranges are equally spaced then you can type just the first and last depths followed by a '/' and the intermediate depths will be generated automatically.

You can specify a receiver at zero range; however, the BELLHOP field is singular there--- the pressure is returned as zero.

 (8) - RUN TYPE

       Syntax:

          OPTION

       Description:

          OPTION(1:1): 'R' generates a ray file
                       'E' generates an eigenray file
                       'A' generates an amplitude-delay file (ascii)
                       'a' generate  an amplitude-delay file (binary)
                       'C' Coherent     TL calculation
                       'I  Incoherent   TL calculation
                       'S' Semicoherent TL calculation
                            (Lloyd mirror source pattern)

          OPTION(2:2): 'G' Geometric hat beams in Cartesian coordinates (default)
                       'g' Geometric hat beams in ray-centered coordinates
                       'B' Geometric Gaussian beams

          OPTION(3:3): '*' read in a source beam pattern file
                       'O' don't (default)

          OPTION(4:4): 'R' point source (cylindrical coordinates) (default)
                       'X' line  source (cartesian coordinates)

          OPTION(5:5): 'R' rectilinear grid (default)
                       'I' irregular grid

The ray file and eigenray files have the same simple ascii format and can be plotted using the Matlab script plotray.m.

The eigenray option seems to generate a lot of questions. The way this works is that BELLHOP simply writes the trajectories for all the beams that contribute at a given receiver location. To get a useful picture you normally want to use a very fine fan, only one receiver location, and the geometric beam option. See the examples in at/tests.

The amplitude-delay file can be used with the Matlab script stackarr.m to 'stack the arrivals', i.e. to convolve them with the source spectrum and plot the channel response. stackarr.m can also be used to simple plot the impulse response.

For TL calculations, the output is in the shdfil format used by all the codes in the Acoustics Toolbox and can be plotted using the Matlab script, plotshd.m. (Use toasc.f to convert the binary shade files to ascii format for use by plotshd.m or whatever plot package you're using.)

The pressure field is normally calculated on a rectilinear grid formed by the receiver ranges and depths. If an irregular grid is selected, then the receiver ranges and depths are interpreted as a coordinate pair for the receivers. This option is useful for reverberation calculations where the receivers need to follow the bottom terrain. This option has not been used much. The plot routines (plotarr) have not been modified to accomodate it. There may be some other limitations.

There are actually several different types of Gaussian beam options (OPTION(2:2)) implemented in the code. Only the two described above are fully maintained.

The source beam pattern file has the format

       NSBPPts
       angle1  power1
       angle2  power2
        ...

with angle following the BELLHOP convention, i.e. declination angle in degrees (so that 90 degrees points to the bottom). The power is in dB. To match a standard point source calculation one would used anisotropic source with 0 dB for all angles. (See at/tests/BeamPattern for an example.)

 (9) - BEAM FAN

       Syntax:

          NBEAMS ISINGLE
          ALPHA(1:NBEAMS)

       Description:

                  NBEAMS: Number of beams
              (use 0 to have the program calculate a value automatically, but conservatively).

          ISINGLE: If the option to compute a single beam in the fan is selected (top option)
              then this selects the index of the beam that is traced.

          ALPHA(): Beam angles (negative angles toward surface)

For a ray trace you can type in a sequence of angles or you can type the first and last angles followed by a '/'.  For a TL calculation, the rays must be equally spaced otherwise the results will be incorrect.

 (10) - NUMERICAL INTEGRATOR INFO

       Syntax:

          STEP ZBOX RBOX

       Description:

          STEP:  The step size used for tracing the rays (m). (Use 0 to let BELLHOP choose the step size.)
          ZBOX:  The maximum depth to trace a ray        (m).
          RBOX:  The maximum range to trace a ray       (km).

The required step size depends on many factors.  This includes frequency, size of features in the SSP (such as surface ducts), range of receivers, and whether a coherent or incoherent TL calculation is performed.  If you use STEP=0.0 BELLHOP will use a default step-size and tell you what it picked.  You should then halve the step size until the results are convergent to your required accuracy.  To obtain a smooth ray trace you should use the spline SSP interpolation and a step-size less than the smallest distance between SSP data points. Rays are traced until they exit the box ( ZBOX, RBOX ).  By setting ZBOX less than the water depth you can eliminate bottom reflections. Make ZBOX, RBOX a bit (say 1%) roomy too make sure rays are not killed the moment they hit the bottom or are just reaching your furthest receiver.


Running BELLHOP

The main issue to be aware of is that ray tracing is very sensitive to environmental interpolation (both boundary and volume). The Gaussian beam options reduce that sensitivity significantly; however, one should still be attentive to this issue. The spline interpolation option to the SSP should be used with particular caution. In some cases, the spline fit is very smooth as desired; in other cases, the spline introduces large wiggles between ssp points, in its effort to produce a smooth curve. Use PLOTSSP to see how your fit looks.

BELLHOP numerically integrates the ray equations to trace a ray through the ocean. To avoid artifacts at discontinuties in the SSP, the step size is dynamically adjusted to make sure a step always lands on an SSP point, rather than stepping over it. (The beam curvature needs to be adjusted at each such point.) It's better to not use more points to describe the SSP than necessary to capture the physics because BELLHOP will end up using lots of small steps to have each ray land on the SSP points. Similarly, BELLHOP uses the altimetry and bathymetry points to define segments in range, and adjusts the step size so that the rays land on each segment boundary.

BELLHOP has no direct capability for modeling elastic wave propagation; however, elastic boundaries can be treated using BOUNCE to generate an equivalent reflection coefficient.

You can have BELLHOP use a range-dependent SSP by creating a separate SSPFIL containing that SSP data in a matrix form. (See Range-Dependent SSP File). The range-dependent SSPFIL is read if you select 'Q' (quadrilateral) for the SSP interpolation. The depths for the SSP points are read from the ENVFIL; the ranges are specified in the SSPFIL. See the example in at/tests/Gulf.

BELLHOP will produce some artifacts for receivers very close the the surface or bottom, because a beam is essentially folded onto itself upon reflection. The zone of overlap (which depends on the fatness of the beam) is not treated with a lot of care. You can minimize such artifacts by making the beams narrow, which in turn can often be done by using lots of rays. If you want to explore some behavior of the field for a receiver on the bottom, you generally should offset it a little bit. Alternatively, you can use reciprocity and interchange the role of the source and the receiver; sources near the bottom are not a problem.