Description of Environmental File

1. Title

2. Frequency

3. Number of Media

4. Top Option

Top Halfspace Properties

Top Reflection Coefficient

Twersky Scatter Parameters

5. Sound Speed Profile

6. Bottom Option

Bottom Halfspace Properties

Bottom Reflection Coefficient

 

The following can be repeated as many times as wanted in a single ENVFIL. KRAKEN and KRAKENC will generate a separate MODFIL for each case stopping when it detects an end-of-file.


(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

Description:

NMEDIA: Number of media.

The problem is divided into media within which it is assumed that the material properties vary smoothly. A new medium should be used at fluid/elastic interfaces or at interfaces where the density changes discontinuously. The number of media in the problem is defined excluding the upper and lower half-space.

BELLHOP is limited to one medium (NMEDIA=1) and actually ignores this parameter.


(4) - Top Option

Syntax:

TOPOPT

Description:

TOPOPT(1:1): Type of interpolation to be used for the SSP.

'C' for C-linear,
'N' for N2-linear (n the index of refraction),
'S' for cubic Spline,
'Q' for Quadrilatteral 2D SSP (BELLHOP   only; reads the SSP from a file)
'H' for Hexahedral     3D SSP (BELLHOP3D only; reads the SSP from a file)
'A' for Analytic.
    The user must modify the analytic formulas in ANALYT.FOR then re-compile and link.

If your not sure which option to take, I'd suggest you use 'C'.  The 'N' option is virtually identical to 'C'. It's provided to facilitate precise intermodel comparisons with codes that use a certain numerical technique that requires that type of interpolation.

Option 'S' is a little dangerous because splines yield a poor fit to certain kinds of curves, e.g. curves with sharp bends.  If you insist on splines, you can fix a bad fit by dividing the water column into two 'media' at the bend.

Run PLOTSSP to check that the SSP looks the way you thought it should. Apart from potential typos, this will also show up fit-problems.


TOPOPT(2:2): Type of top boundary condition

'V' VACUUM above top
'A' ACOUSTO-ELASTIC half-space.
 Requires another line with the halfspace parameters as described in block (4a).
'R' Perfectly RIGID
'F' Reflection coefficient from a FILE

These files for complicated, multi-layered media can be generated using BOUNCE. (Available in KRAKENC, SCOOTER, and BELLHOP. Not available in KRAKEN and SPARC.) For a Top Reflection Coefficient, the file should have the extension '.TRC'. For KRAKEN/KRAKENC this option for tabulated reflection coefficients is somewhat experimental at this time. A complicated reflection coefficient may well cause problems for the mode-finder. Finally, a reflection coefficient tabulated only for real angles does not provide a good result for complex angles of incidence. This happens when the sediment sound speed is less than the water sound speed. In that case, the modes are evanescent in the upper part of the water column and therefore have a complex angle of incidence.

'S' for Soft-boss Twersky scatter.
'H' for Hard-boss Twersky scatter.
'T' for Soft-boss Twersky scatter, amplitude only.
'I' for Hard-boss Twersky scatter, amplitude only.

The Twersky scatter options require another line as described in block(4b). Mnemonically, T, I options are one letter after S, H in the alphabet. Current wisdom is that option T is most appropriate for ice scatter.

For open ocean problems option 'V' should be used for the top BC.  The Twersky options are intended for under-ice modeling.


TOPOPT(3:3): Attenuation units.

'N' Nepers/m.
'F' dB/(kmHz)       (F as in Freq. dependent)
'M' dB/m            (M as in per Meter)
'W' dB/wavelength   (W as in per Wavelength)
'Q' quality factor
'L' Loss parameter (a.k.a. loss tangent)

 KRAKEN ignores material attenuation in elastic media. (KRAKENC treats it properly).

TOPOPT(4:4): Added volume attenuation.

'T' Thorp attenuation formula.

If you invoke this option, Thorp attenuation is added to the explicitly specified attenuation (ap, as).

 

TOPOPT(5:5): Altimetry option; Slow/robust root-finder.

'~' Read in a *.ati file containing the top altimetry. (BELLHOP only; '*' was used earlier and still works)
'_' Flat surface, i.e. no altimetry file. (BELLHOP only; this is the default if blank)
 '.' As in: I want all the modes and I don't care how long it takes. Period.

The '.' option works only with KRAKENC, which sometimes has trouble finding a complete set of modes in the complex plane.



TOPOPT(6:6): Single beam (BELLHOP only)

'I' Calculate only a single beam from the specified fan.
    The actual beam number is specified after NBEAMS, under
    the section describing the beam fan.
' ' Default: trace all beams


(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) )
AST:  Top S-wave attenuation. (  "   "    "    "   "   "     )


This line should only be included if TOPOPT(2:2)='A', i.e. if the user has specified a homogeneous halfspace for the top BC.

(4b) - Twersky Scatter Parameters

Syntax:

BUMDEN  ETA  XI

Description:

BUMDEN: Bump density (ridges/km).
ETA:    Principal radius 1 (m).
XI:     Principal radius 2 (m).

      This line should only be included when one of the Twersky-scatter options is selected.

 

(5) - Sound Speed Profile

Syntax:

NMESH  SIGMA  Z(NSSP)
Z(1)     CP(1)     CS(1)     RHO(1)     AP(1)     AS(1)
Z(2)     CP(2)     CS(2)     RHO(2)     AP(2)     AS(2)
.
.
.
Z(NSSP)  CP(NSSP)  CS(NSSP)  RHO(NSSP)  AP(NSSP)  AS(NSSP)

Description:

NMESH:   Number of mesh points used in the internal discretization.

The number of mesh points should be about 10 per vertical wavelength in acoustic media. In elastic media, the number needed can vary quite a bit; 20 per wavelength is a reasonable starting point.

The number of mesh points used depends on the initial mesh and the number of times it is refined (doubled).  The number of mesh doublings can vary from 1 to 5 depending on the parameter RMAX described below.

If you type 0 for the number of mesh points, the code will calculated NMESH automatically.

BELLHOP ignores this parameter as it's not relevant to its numerical technique.

SIGMA:   RMS roughness at the interface (ignored by BELLHOP and SPARC)

Z(NSSP): Depth at bottom of medium (m).
   This value is used to detect the last SSP point when reading in the profile that follows.

Z():     Depth (m).
   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. The points Z(1) and Z(NSSP) MUST correspond to the depths of interfaces between media.

CP():    P-wave speed (m/s).
CS():    S-wave speed (m/s).
RHO():   Density (g/cm3).
   Density variations within an acoustic medium are at present ignored.

AP():    P-wave attenuation (units as given in Block 2)
AS():    S-wave attenuation (  "   "    "    "   "   ")

These lines giving the SSP should be omitted when the 'A' option is used (indicating that an analytic profile is supplied by a user written subroutine).

The '/' character signals that the remaining data on the line is the same as in the previous line of SSP data. For the very first line the default or 'previous' line is:

0.0 1500.0 0.0 1.0 0.0 0.0

This block should be repeated for each subsequent medium.

Only KRAKEN, SCOOTER, and BOUNCE can use the shear wave information. SPARC and BELLHOP ignore it.


 

(6) - Bottom Option

      Syntax:

         BOTOPT  SIGMA

      Description:

         BOTOPT(1:1): Type of bottom boundary condition.

'V' VACUUM below bottom.
'A' ACOUSTO-ELASTIC half-space.
    Requires another line with the half-space parameters as described in Block (6a).

'R' Perfectly RIGID.
'F' Reflection coefficient from a FILE

These files for complicated, multi-layered media can be generated using BOUNCE. (Available in KRAKENC, SCOOTER, and BELLHOP. Not available in KRAKEN and SPARC.) For a Bottom Reflection Coefficient, the file should have the extension '.BRC'. For KRAKEN/KRAKENC this option for tabulated reflection coefficients is somewhat experimental at this time. A complicated reflection coefficient may well cause problems for the mode-finder. Finally, a reflection coefficient tabulated only for real angles does not provide a good result for complex angles of incidence. This happens when the sediment sound speed is less than the water sound speed. In that case, the modes are evanescent in the upper part of the water column and therefore have a complex angle of incidence.


'P' Precaculated internal reflection coefficient from a FILE (available in KRAKENC and SCOOTER, not KRAKEN).
    These files are generated using BOUNCE.

Option 'A' is generally used for ocean bottom modeling.

BOTOPT(2:2):
    '~' Read in a *.bty file containing the bottom bathymetry. (BELLHOP only; '*' was used earlier and still works)
    '_' Flat surface, i.e. no bathymetry file. (BELLHOP only; this is the default if blank)

SIGMA:  Interfacial roughness (m).

 

(6a) - Bottom Halfspace Properties

Syntax:

ZB  CPB  CSB  RHOB  APB  ASB

Description:

ZB:   Depth (m).
CPB:  Bottom P-wave speed (m/s).
CSB:  Bottom S-wave speed (m/s).
RHOB: Bottom density (g/cm3).
APB:  Bottom P-wave attenuation. (units as given by TOPOPT(3:3) )
ASB:  Bottom S-wave attenuation. (  "   "    "    "   "   "     )


This line should only be included if BOTOPT(1:1)='A', i.e. if the user has specified a homogeneous halfspace for the bottom BC.
 

These lines should be contained in a separate '.BRC' file. This file is only required if BOTOPT(2:2)='F', i.e. if the user has specified that the bottom BC is read from a '.BRC' (Bottom Reflection Coefficient) file.

For KRAKEN/KRAKENC this option for tabulated reflection coefficients is somewhat experimental at this time. A complicated reflection coefficient may well cause problems for the mode-finder. Finally, a reflection coefficient tabulated only for real angles does not provide a good result for complex angles of incidence. This happens when the sediment sound speed is less than the water sound speed. In that case, the modes are evanescent in the upper part of the water column and therefore have a complex angle of incidence.