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
'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)
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:
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.
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.)
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.
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.
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.