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. (However, the present version requires the user write a subroutine to supply the sound speed in cases where the profile is both range and depth dependent.)
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
ANGLES Given the source and reciever sound speeds, computes
the angle of the limiting ray.
PLOTSSP Plots the 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
---------------------------------------------------------
EXAMPLE AND DESCRIPTION OF ENV 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:
(1) - TITLE
Syntax: TITLE
Description:
TITLE: Title of run enclosed in sinqle quotes
(2) - FREQUENCY
Syntax: FREQ
Description:
FREQ: Frequency in Hz
(3) - NUMBER OF MEDIA
Syntax: NMEDIA (<20)
Description:
Dummy parameter for compatibility with KRAKEN.
(4) - OPTIONS
Syntax: OPTION
Description:
OPTION(1:1): Type of interpolation to be used for the SSP
'S' for cubic Spline
'C' for C-linear (recommended)
'N' for N2-linear
'A' for Analytic. The user must modify the
analytic formulas in ANALYT.FOR and re-link.
Use PLOTSSP to check that
the SSP looks the way you thought it should.
Apart from potential typos, this will also
show up fit-problems which might occur with
the spline option. Splines yield a
poor fit to certain kinds of curves, e.g.
curves with sharp bends.
Splines were previously recommended because the code did
not have a careful treatment of discontinuities in
the derivative of the SSP. The latest version does.
OPTION(2:2): Type of top boundary condition
'V' VACUUM above top
'A' ACOUSTO-ELASTIC half-space.
Requires another line as described in block (4a).
'R' Perfectly RIGID
'F' Reflection coefficient from a FILE
Requires additional lines as described in block (4b).
(tip: you can use the program BOUNCE to calculate a reflection
coefficient for a multilayered medium.)
For open ocean problems option 'V' should
usually be used for the surface BC.
OPTION(3:3): Attenuation option
'N' Nepers/m.
'F' dB/(kmHz) (F as in Frequency dependent)
'M' dB/m (M as in per Meter)
'W' dB/wavelength (W as in per Wavelength)
'Q' quality factor.
OPTION(4:4): Added volume attenuation
'T' Thorp attenuation formula.
OPTION(5:5): '*' read in a *.ati file containing the
top altimetry in range/depth pairs. The ranges are given in
km and depths in meters. The first line specifies the number of
pairs following.
(4a) - TOP HALFSPACE PROPERTIES
Syntax:
ZT CPT CST RHOT APT AST
Description:
ZT: Depth (m).
CPT: Top P-wave speed (m/s).
CST: Top S-wave speed (m/s).
RHOT: Top density (g/cm3).
APT: Top P-wave attenuation. (units as given by option(3:3) above)
AST: Top S-wave attenuation. ( " " " " " " )
This line should only be included if OPT(2:2)='A', i.e.
if the user has specified a homogeneous halfspace for
the top BC.
(4b) - TOP REFLECTION COEFFICIENT
Syntax:
NTHETA
THETA(1) RMAG(1) RPHASE(1)
THETA(2) RMAG(2) RPHASE(2)
.
.
.
THETA(NTHETA) RMAG(NTHETA) RPHASE(NTHETA)
Description:
NTHETA: Number of angles.
THETA(): Angle.
RMAG(): Magnitude of reflection coefficient.
RPHASE(): Phase of reflection coefficient (degrees).
Example:
3
0.0 1.00 180.0
45.0 0.95 175.0
90.0 0.90 170.0
These lines should be contained in a separate '.TRC' file.
This file is only required if OPT(2:2)='F', i.e. if the
user has specified that the top BC is read from a '.TRC'
(Top Reflection Coefficient) file.
(5) - SOUND SPEED PROFILE
Syntax:
NMESH SIGMA Z(NSSP)
Z(1) CP(1) /
Z(2) CP(2) /
.
.
.
Z(NSSP) CP(NSSP) /
Description:
NMESH: Dummy parameter for KRAKEN compatibility
SIGMA: Dummy parameter for KRAKEN compatibility
Z(NSSP): Depth at bottom of medium (m).
This value is used to detect the last
SSP point when reading in the
profile which follows.
The following should be omitted when the 'A' option
is used (indicating that an analytic profile is
supplied by a user written subroutine).
Z(): Depth (m). Note that the surface starts at the first
depth point specified. Thus if you have say, XBT
data which starts at 50 m below the surface, then
you'll need to put in some SSP point at 0 m,
otherwise the free-surface would be placed at 50 m
giving erroneous results. Try to keep the number of
depth points to the minimum necessary to describe the
physics: a fine SSP sampling can force a fine step-size
for integrating the rays.
CP(): P-wave speed (m/s) (Must be followed by a '/'
for compatibility with the KRAKEN program.)
(6) - OPTIONS
Syntax: OPTION SIGMA
Description:
OPTION(1:1): Type of bottom boundary condition
'V' VACUUM below bottom
'R' Perfectly RIGID
'A' Acoustic halfspace
Note that shear is not implemented; however,
the input format allows it to be specified
for compatibility with KRAKEN.
'F' Reflection coefficient from a FILE.
Requires a Bottom Reflection Coefficient file with
extension '.BRC'. The format is the same as
that used for a Top Reflection coefficient.
(tip: you can use the program BOUNCE to calculate a reflection
coefficient for a multilayered medium.)
OPTION(2:2): '*' read in a *.bty file containing the
bottom bathymetry in range/depth pairs. The ranges are given in
km and depths in meters. The first line specifies the number of
pairs following.
SIGMA: Bottom roughness (currently ignored)
(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.
(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 beams (default)
'B' Gaussian beams
OPTION(3:3): '*' read in a source beam pattern file (sbpfil)
' ' 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 fortran code, The ray file and eigenray files have the same simple ascii format and can be plotted using the fortran code plotray.f, or the Matlab script plotray.m.
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.
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 amp1
angle2 amp2
...
with angle in degrees and amplitude in dB.
(9) - BEAM FAN
Syntax:
NBEAMS
ALPHA(1:NBEAMS)
Description:
NBEAMS: Number of beams (use 0 to have the program
calculate a value automatically, but conservatively).
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).
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 rcvrs, 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.