# FIELD3D

#

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 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)

1 ! Nsx
0.001 ! Sx coordinates of source (km)
1 ! Nsy
0.001 ! Sy coordinates of source (km)

1 ! NSD

1000.0 ! SD(1:NSD) (m)

1 ! NRD

800.0 ! RD(1:NRD) (m)

501 ! NR

0.0 100.0 501 ! RMIN RMAX (km)

19 ! NTHETA

0.0 360.0 / ! 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.

OPT(7:7): Selects a source beam pattern

'*' Read in a source beam pattern file.

'O' Omnidirectional (default).

(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 (xy)

Syntax:

NSx

Sx(1:NSx)

NSy

Sy(1:NSy)

Description:

NSx: The number of source coordinates in x.

Sx(): x-coordinates of the sources (km).

NSy: The number of source coordinates in y.

Sy(): y-coordinates of the sources (km).

(4) - SOURCE/RECEIVER DEPTHS

Syntax:

NSD

SD(1:NSD)

NRD

RD(1:NRD)

Description:

NSD: The number of source depths.

SD(): The source depths (m).

NRD: The number of receiver depths.

RD(): The receiver depths (m).

(5) - RECEIVER RANGES

Syntax:

NR

RMIN RMAX

Description:

NR: Number of receiver ranges.

RMIN: First receiver range (km). MUST BE ZERO!

RMAX: Last receiver range (km).

(6) - RADIALS

Syntax:

NTHETA

THETA(1:NTHETA)

Description:

NTHETA: Number of radials.

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.

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.

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