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## field3d.hlp

The FIELD3D program uses the modes calculated by KRAKEN and produces a shade file which contains a sequence of 2-D slices of the acoustic field. It is commonly used to compute a field in plan view, i.e. as a function of horizontal coordinates (x,y). It can also be used to compute the field on a vertical slice along any fixed bearing through the 3-D environment.

FIELD3D uses a tiling of the ocean environment based on triangles. The terminology is taken from finite-elements. To define the triangles you must do the following:

(1) Lay out a grid of points (nodes) where you will construct environmental files for KRAKEN and solve for the modes. A coarse rule-of-thumb is to pick points every 10 km but obviously a coarser spacing can be used in sites with less environmental change.

(2) Assign a number to each of the nodes.

(3) Form a triangulation of the nodes. That is, connect the nodes with lines such that the grid is divided into a number of triangles. This should be done with an eye towards keeping the area of the individual triangles uniform. All nodes should be a corner of at least one triangle. Each triangle is referred to as an element.

There are algorithms for performing this step automatically and if you write one I would be glad to receive it. If instead you do this by hand you will rapidly discover the merits of using a regular grid.

(4) Assign a number to each of the elements.

You now have the information required by FIELD3D to describe your triangulation. In the input file you first tell FIELD3D the coordinates of each node and the name of the file containing the modes at each node. You then tell FIELD3D how you connected the nodes to form a triangulation. This is done by specifying the node numbers which define the corners of each successive element (triangle).

```Files:

Name           Unit         Description
Input
*.FLP            5       FieLd Parameters
*.MOD          30-99     MODe files

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

---------------------------------------------------------

EXAMPLE AND DESCRIPTION OF FLP FILE:

'MUNK3D'                       ! TITLE
'STDFM'                        ! OPT
9999                           ! M (number of modes)
0.001  0.001                   ! XS  YS (source position) (km)
1  1000.0                      ! NSD  SD(1:NSD) (m)
1   800.0                      ! NRD  RD(1:NRD) (m)
0.0  100.0  501                ! RMIN  RMAX  NR (km)
19  0.0  360.0 /               ! NTHETA  THETA(1:NTHETA) (degrees)
5                              ! Number of SSP's (NSSP)
100.0     0.0  'SCR:MUNKT0'   ! (x, y) i=1, NSSP (km)
0.0   100.0  'SCR:MUNKT90'
-100.0     0.0  'SCR:MUNKT0'
0.0  -100.0  'SCR:MUNKT270'
0.0     0.0  'SCR:MUNKT0'
4                              ! NELTS
5  1  2                        ! Nodes of corners
5  2  3
5  3  4
5  4  1
4.0  360.0  90                 ! ALPHA1  ALPHA2  NALPHA
500.0  160                     ! STEP  NSTEPS
0.3                            ! EPMULT

(1) - OPTIONS

Syntax:
OPT
Description:
OPT(3:3): Type of caculation.
'STD' (Standard) for an Nx2D run.
'GBT' (Gaussian beam trace) for a 3D run.
'PDQ' For a fast preview run.
The 'STD' option neglects horizontal
refraction but runs a lot faster.
Avoid using the 'GBT' option:
it requires some care to use
properly. Option 'PDQ' runs about 3x as fast
as 'STD' but is less accurate.
OPT(4:4): TESCHECK (tesselation check) flag.
'T'  Perform the tesselation check.
'F'  omit    the tesselation check.
For all but the simplest setups the user will
INVARIABLY make an error in setting up the
triangulation.  The first step to avoid this
is to run PLOTTRI to get a plot of the
triangulation.  Even after that one should
invoke this 'TESCHECK' option however for
large problems some time can be saved by
turning off this feature after the
triangulation has been checked once.
OPT(5:5): Type of beams.
There are several types of Gaussian beams
available.  I suggest using 'M'. This option
is ignored unless the Gaussian beam
calculation has been selected.
OPT(6:6): Ray file flag.
Use 'R' to have a file of ray path
trajectories (in the horizontal plane) written
to disk for subsequent plotting using the
PLOTRAYXY program.  These rays show the
horizontal refraction of individual modes.
This option is ignored if you select a 'STD'
or 'PDQ' run for then the ray paths are just
straight lines.

(2) - NUMBER OF MODES

Syntax:
M
Description:
M:    Number of modes to use in the field computation.
If the number of modes specified exceeds the
number computed then the program uses all the
computed modes.

(3) - SOURCE COORDINATES

Syntax:
XS  YS
Description:
XS:  X-coordinate of source (km).
YS:  Y-coordinate of source (km).

(4) - SOURCE/RECEIVER DEPTHS

Syntax:
NSD  SD(1:NSD)
NRD  RD(1:NRD)
Description:
NSD:  The number of source depths. (<3)
SD(): The source   depths (m).
NRD:  The number of receiver depths.
(<51 and NR*NRD < 54000)
RD(): The receiver depths (m).

(5) - RECEIVER RANGES

Syntax:
RMIN  RMAX  NR
Description:
RMIN: First receiver range (km). MUST BE ZERO!
RMAX: Last  receiver range (km).
NR:   Number of receiver ranges.
(NR<4094 and NR*NRD <= 210000)

Syntax:
NTHETA  THETA(1:NTHETA)
Description:
NTHETA:  Number of radials. (<101)
THETA(): Angles for each radial (degrees).

For full circle (or disc) coverage our plotting program
likes to have a repeated radial, say 0 and 360 degrees.

You cannot have multiple sources, receivers and bearings
in a single run: at least one of NSD, NRD or NTHETA must
be 1.

(7) - NODES

Syntax:
NNODES
X(1)       Y(1)       FILNAM(1)
X(2)       Y(2)       FILNAM(2)
.
.
.
X(NNODES)  Y(NNODES)  FILNAM(NNODES)
Description:
NNODES:    Number of nodes. (<1000).
X():       X-coordinate of node (km).
Y():       Y-coordinate of node (km).
FILNAM():  Name of the mode file for that node.
Use the name 'DUMMY' to produce an acoustic
absorber.

(8) - ELEMENTS

Syntax:
NELTS
NODE1(1)       NODE2(1)       NODE3(1)
NODE1(2)       NODE2(2)       NODE3(2)
.
.
.
NODE1(NELTS)  NODE2(NELTS)  NODE3(NELTS)
Description:
NELTS:    Number of elements. (<1500)
NODE1():  Number of node at first corner of the triangle.
NODE2():     "   "   "    " second   "   "    "    "
NODE3():     "   "   "    " third    "   "    "    "

In this fashion we define a tiling of triangular
elements. The ordering of the elements is arbitrary.

(9) - GAUSSIAN BEAM INFO

Syntax:
ALPHA1  ALPHA2  NALPHA
STEP  NSTEPS
EPMULT
Description:
ALPHA1: First angle for beam fan (degrees).
ALPHA2: Last    "    "    "   "     "
NALPHA: Number of beams in fan.
STEP:   Step size (m).
NSTEPS: Number of steps.
EPMULT: Epsilon multipler for beam initial conditions.

This Gaussian beam info can be omitted if the 'STD' option
in block (1) is used.

---------------------------------------------------------------

To get a rough idea of run time, consider a 50 Hz deep water problem
with 60 modes (waterborne modes only) and for 37 radials with 501
range points per radial.  On a 1 megaflop workstation, this required
about 3 minutes with option 'STD' and 6 hours when including horizontal
refraction via option 'GBT'.

Run time is roughly proportional to M * NTHETA * NR.

Dimensional constraints:
Number of modes at a single node <= 1500
Number of distinct sets of modes <= 250
```

Next: Plotting routines Up: Acoustic Field Calculations Previous: field.hlp

Michael B. Porter
Tue Oct 28 13:27:38 PST 1997