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R D-A128 514 ACOUSTIC RAY TRACING AND THREE-DIMENSIONL PROPAGATION i/i
LOSS REPRESENTATIO..(U) NAVAL OCEANOGRAPHIC OFFICE NSTL
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, without a front , it was found that the Gulf Stream Nort!, '! 7 -ouLd etY* increase or decrease propagation loss depending on distance ot the
receiver from the front; the maximum difference was l-2 di;. ITe -outh I :of the Stream had no significant effect on sound intensity.
Three-dimensional propagation loss diagrams show that e~ffcts of th,North Wall on propagation loss persist at all receiver depths from ,asurface to 1000 meters.
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NAVOCEANO IN 3700 - 55 -76
ACOUSTIC RAY TRACINGAND
THREE-DIMENSIONAL PROPAGATION LOSSREPRESENTATION IN THE GULF STREAM REGION
by
E. KHEDOURI
and
P. GABORSKI
SEPTEMBER 1977
U. S. NAVAL OCEANOGRAPHIC OFFICEWASHINGTON, D. C. 20373
Approved for pusblic r,,ise;distribution unlimited
l~8
NAVOCEANO TN 3700-55-76
TABLE OF CONTENTS
. ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . .
I. INTRODUCTION ...... ..................... ..
II. METHOD ........... ........................ 1
III. SOUND VELOCITY PROFILES, RAY TRACES AND PROPAGATION
LOSS . . . . . . . . . . . . . . . . . . . . . . . . 3
IV. 3-D PROPAGATION LOSS REPRESENTATION .... ......... 5
V. CONCLUSIONS .......... .. ................... 7
ACKNOWLEDGEMENTS ........... ...................... 9
REFERENCES ............ ......................... 10
FIGURES
Figure 1 - Surface Thermal Structure and Location of AXBTProfiles ...... .................... . .11
Figure 2 - Sound Velocity Profiles for Section A ..... . 12
Figure 3 - Sound Velocity Profiles for Section B ..... 12
Figure 4 - Sound Velocity Profiles for Section C ..... . 13
Figure 5 - Ray Trace and Propagation Loss for Section A 14
Figure 6 - Ray Trace and Propagation Loss for Section B 15
Figure 7 - Ray Trace and Propagation Loss for Section C. 6 ISlope to Sargasso Water ... ............ . 16
IFigure 8 - Ray Trace and Propagation Loss for Section C. A
Sargasso to Slope Water. .............. 17 F-e.~,.
%1 Figure 9 - 3-D Propagation Loss for Section A ......... .. 18 D-i c -
Figure 10 - 3-D Propagation Loss for Section B. .. ....... 19 st. to
Figure 11 - 3-D) Propagation Loss for Section C. Slope to BSargasso Water ..... ................. .. 20
Figure 12- 3-D Propagation Loss for Section C.Sargasso toA -al._,bljt, e.
Slope Water ...... .................. . 21 • *, an-
NAVOCE.ANO TIN 3700-55-76
ABSTRACT
Acoustic ray traces and intelisities for lixed and variable rect,iver
depths were calculated for three cross sections in the Gulf Stream region
using AXBT temperature profiles and historical .ound velocity data. Two
sections were in the Sargasso Sea and one crossed the Gulf Stream. it is
shown that for shallow source and receiver depths, a 4 dB difference in
intensity is found in the Sargasso Sea due to sonic layer depth differences.
When sound intensities across the Gulf Stream were compared to intensities
for similar situation without a front, it was found that the Gulf Stream
North Wall could either increase or decrease propagation loss depending on
distance of the receiver from the front; the maximum difference was 18 dB.
The South Wall of the Stream had no significant effect on sound int-.isity.
Three-dimensional propagation loss diagrams show that effects of the
North Wall on propagation loss persist at all receiver depths from sea
surface to 1000 meters.
..
- ---'- ,--
I. INTRODUCTION
Effects of oceanic fronts and eddies on acoustic sound propagation based
on measured or calculated values have been documented in several publications
(Gemmill, 1974; Gemmill and Khedouri, 1974; Levenson and Doblar, 1976).
There is little doubt that a strong front such as the Gulf Stream has a
significant effect on sound propagation. A front can either increase or
decrease transmission loss depending on the distance of the source or receiver
from the front, and the magnitude of transmission loss depends on factors
such as bottom topography, bearing angle of sound transmission to the front
and the depth of the source or receiver. Direct measurement of sound intensity
across a front at various range and depth intervals is impractical. However,
computer simulation and three-dimensional representation of propagation loss
can be used to determine intensity variation for a given cross section as a
function of range and depth.
The objective of this paper is to show theoretical sound paths and
propagation loss values for three cross sections in the Gulf Stream region
using methods which are adaptable to operational use. Figure 1 shows the
location of the cross sections and the surface thermal structure of the area.
Sections A and B are in Sargasso water and section C crosses the Gulf Stream.
The data for this report consist of airborne expendable bathythermograph
(AXBT) temperature profiles taken during February 1976, combined with deep
historical sound velocity data.
II. METHOD
In frontal regions where horizontal temperature gradients exist along a
given cross section, a multiple profile model (range dependent environment)
must be used for ray tracing and propagation loss calculations in order to
I
reflect the sound velocity changes along the cross section. NRL GRASS model
(Cornyn, 1973) was used for all the acoustic calculations in this report.
This is a multiple profile and variable bottom depth experimental model
used primarily for research purposes. The model was significantly modified
* I by the authors to make it more suitable for operational use.
The NRI. version of GRASS requires either sound velocity vs depth, or
salinity and temperature vs depth as input. The program automatically
extrapolates sound velocity profiles to the ocean floor. This extrapolation
can result in erroneous sound velocity values when the bottom is considerably
deeper than the deepest available data point and consequently, in deep
waters, relatively deep sound velocity data are required. This type of
data, however, is not readily available operationally. One of the
modifications made to GRASS was to combine it with the ICAPS merge program
(Ranesen and Tucker, 1975). This program merges temperatures from AXBT
traces or similar near-surface temperature data, with mean seasonal temperature
and salinity data from historical files for a given geographic area (5 degree
square), to create sound velocity profiles from sea surface to the ocean floor.
This modification made it possible to use AXBTs or similar readily available
temperature data as input to the program. A variation of the above technique
is to merge the AXET temperatures with historical temperatures and salinities
for a particular water mass (Fisher, 1976). This method uses the near-surface
temperature characteristics to select the proper water mass. The method gives
superior results in frontal regions because oceanic fronts are not constrained
* by fixed geographical boundaries. The water mass concept was used in this
report for determining sound velocity profiles from AXBT temperature profiles.
2
-'- -r 'V.-v - -vj. '- - - - S - -r-. -
Another modification made to GRASS was the use of three-dimensional plots
to display propagation loss vs range vs receiver depth. This modification
allows the propagation loss at all receiver depths (1-1000 mn) to be displayed
in one diagram which shows the changes in propagation loss as receiver depth
is varied.
III. SOUND VELOCITY PROFILES, RAY TRACES AND PROPAGATION LOSS
Sound velocity profiles used for ray tracing and propagation loss curves
for sections A, B, and C are shown in figures 2, 3 and 4 respectively.
Profiles used for sections A and B are in Sargasso Water. Profiles used for
Section C are in Slope, Gulf Stream, and Sargasso waters.
Ray traces and transmission loss curves for sections A and B are shown
in figures 5 and 6. In both cases a 290 Hz source at 137 mn depth was used
wit an 88 in deep receiver. Comparison of ray traces shows that in section A
most of the transmission Is through bottom bounce and deep sound channel,
whereas in section B, considerable amount of transmission is through the
near-surface duct. Inspection of the sound velocity profiles used in these
sections (figures 2 and 3) reveals that the only difference is in the near-
surface sound velocity and in the sonic layer depth (SLD). In section A,
the SLD was 240 mn at the source and 105 in at the receiver, whereas in section
B, the SLD was 200 m both at the source and the receiver, resulting in well
* defined surface ducting. This surface duct transmission resulted in 4 dB
higher sound level at 50 nmi(92.6 kin) range.
Ray traces for section C, crossing the Stream from Slope to Sargasso
Water, with a source at 137 m depth are shown in figure 7. Sound velocity
profiles used for this section (figure 4) show that in addition to near-
3
surface sound velocity diffences, the depth of the deep sound channel axis
(DSC) deepens from 700 m in Slope Water to about 1200 m in the Gulf Stream
and 1400 mn in the Sargasso Sea. En addition, sound velocity in the upper
layer of the sound channel is higher in the Stream and Sargasso than in the
Slope Water. Deepening of the sound channel and the increasing sound velocity
across the Stream are reflected in the ray traces, where it can be seen that
after the rays cross the North Wall of the stream, they are channeled downward
into the deep sound channel. Similar results were shown by Germnill (1974).
Propagation loss curves in figure 7 show the intensities for a 290 Hz
source at 137 mn and a. receiver depth of 88 m. The solid line shows the
intensities from the eight profiles used in the ray trace, and the dashed line
shows the intensities using only the first Slope Water profile. The difference
between the two curves, therefore, represents the difference caused by the
fronts. When the North Wall is crossed, the sound rays become out of phase
and the convergence zones which are typical for a single water mass are
destroyed. The resultant effect on intensities is seen by the significant
difference between the two curves when the North Wall is crossed. The
maximum difference of 11 dB occurs at 90 mud (166.7 kin) range.
Figure 8 shows the ray traces and propagation loss curves for Sargasso
to Slope transmission across the Stream using sound velocity profiles shown
in figure 4. Ray traces were calculated for a 137 mn source depth. When
the South Wall of the Stream is crossed the surface bounce propagation is
terminated. This is because the SLD is considerably shallower in the Stream
than it is in Sargasso Sea as shown in the sound velocity profiles. When
* sound travels across the North Wall, convergence zones become reinforced with
significant effect on propagation loss values. Propagation loss curves in
figure 8 were calculated for a 290 Hz source at 137 mn and a receiver depth
4
of 88 m. The solid line represents the propagation loss calculated from
1 6. the eight profiles used in the ray traces and the dashed line shows the loss
for identical conditions using the first Sargasso profile. The difference
between the curves, therefore, is caused by the fronts. It can be seen
that the South Wall of the Stream has only a minimal effect (4 dB) on
propagation; the North Wall of the Stream, however, has a very significant
effect which is caused by reinforcement of convergence zones. For example,
at 130 nmi (241 kin) range, transmission is decreased by 14 dB, but at 135 ni
range, transmission is improved by 8 dB because of the front.
IV. 3-D PROPAGATION LOSS REPRESENTATION
Three-dimensional propagation loss diagrams serve primarily as visual
aids to show the variation of intensity at several receiver depths. Exact
values of propagation loss, for any receiver depth and range, however, can
* also be determined from the diagrams.
The following example, using figure 9, will illustrate how to read the
propagation loss value at any point. To determine the propagation loss
value at 40 nmi (74 kcm) range and 200 meter receiver depth, the following
procedure can be used:
1) Find the propagation loss vs depth line at 40 nini range (see arrow in
figure 9).
2) Find the propagation loss vs range line at receiver depth of 200 mn.
3) Find the intersection of the two lines.
4) Draw a line from this intersection parallel to the range axis and
equal in length to 40 nmi on the range axis scale. The resulting propagation
loss value of -88 dB can be read on the propagation loss plane. In cases of
5
complex sound velocity structure, parts of the propagation lass surface
may be hidden and it may not be possible to read some points from the
diagrams. In these cases, a new viewpoint can be selected to display
hidden regions of interest.
The 3-D propagation loss diagrams for sections A and B are shown in
figures 9 and 10 respectively. The two diagrams are similar except for
the near-surface values, caused by the near-surface sound duct as described
in Section III. In both cross sections, which are in the Sargasso Sea, there
is a significant decrease in propagation loss between the su-'ace receiver and
a receiver at 25 m; below 25 m propagation loss vs depth is arly constant.
(The abrupt change at 25 m has no physical significance; it caused by
computational procedures where receiver depth was increment- . 25 m intervals.
The 3-D plots of propagation loss across the Stream for section C, from
Slope to Sargasso and from Sargasso to Slope, are shown in figures 11 and 12
respectively. When sound travels from cold Slope Water to warmer Gulf Stream
and Sargasso Water the convergence zones become weaker as sound is deflected
into the deep sound channel (see the ray traces in figure 7). The effects of
weaker convergence zones are reflected in the 3-D plot of propagation loss
in figure 11, and it can be seen that the effects of the front persist at all
receiver depths from sea surface to 1000 m. When sound travels from
Sargasso to Slope Water, the convergence zones are reinforced as can be
seen in the ray trace in figure 8 and the resultant 3-D propagation loss
plot in figure 12. Effects of the front are again evident at all receiver
d depths.
Note that reciprocity of sound pressure level exists because the ocean
system is linear with respect to the pressure levels encountered in acoustic
surveillance. Therefore, identical propagation loss curves would be obtained
if the source and receiver locations were interchanged. In the case of 3-D
6
diagrams, this means that identical propagation loss surfaces would be
obtained for variable source levels and a single receiver depth as for single
source depth and variable receiver depth. The 3-D diagrams in figures 9 to
12, therefoi.e, can also be used for receiver depth of 137 m and variable
* source levels from 1-1000 m. In order for reciprocity to strictly hold, all
possible rays must be used in intensity calculations. In the above figures
an envelope of rays from +200 to -2Oo from the horizontal at 10 increments
was used, and rays were terminated after 5 bottom bounces and 10 surface
I bounces. This has little effect on propagation loss values and reciprority,
but provides considerably clearer ray diagrams
V. CONCLUSIONS
1) For shallow source and receiver depths,* there was a 4 dB difference
in intensity at 50 nmi (92.6 kmn) range between the two cross sections in
the Sargasso Sea. The difference was attributed to a deeper SLD in one of
the sound velocity profiles which resulted in r.ows being trapped in the
surface duct.
2) When sound is propagated from Slope Water across the Gulf Stream into
Sargasso Sea, recurrent convergence zones which are'typical for a single
water mass are destroyed. When sound is propagated from the Sargasso Sea,
across the Stream, into the Slope Water, the convergence zones are reinforced.
This has a significant effect on propagation loss values.
3) When propagation loss for shallow stource and receiver depths calculated
for a single Slope Water profile is compared to propagation loss values
calculated from multiple profiles across the Gulf Stream under similar
conditions, a maximum of 11 dB difference occurs. When propagation loss is
calculated from a single Sargasso Sea profile and compared with propagation
loss values using multiple profiles across the Gulf Stream, a maximum difference
7
of 18 dB results. In both cases the difference is caused by the Gulf Stream
North Wall.
4) The effects of the North Wall on propagation loss are evident at all
receiver depths from sea surface to 1000 meters.
* ..
!8
I
ACKN OWLEDGEMENTS
The authors acknowledge the efforts of Mr. Mitchell K. Shank, Jr., for
his administrative and technical guidance; Mr. Rudolph Michlik for his
assistance in developing the 3-D plots; Mr. Glen Voorheis, Mr. Randy Doblar,
and Ms. Kay Collins for their help in preparation of the manuscript.
- .9
i9
REFERENCES
Cornyn, John J.,(1973) GRASS: A Digital Computer Ray-Tracing andTransmission Loss Predicted Model. Vol. I, Overall Description,NRL Report 7642.
Cornyn, John J., (1973) GRASS: A Digital Computer Ray Tracing andTransmission Loss Prediction Model. Vol. II, Users Manual, NRLReport 7642.
Fisher A., (1976) Selection of Water Mass History from BathythermogramCharacteristics. U.S. Naval Oceanographic Office TN 3700-51-76.
Ge-,nill, W.H. and E. Khedouri (1974) A Note on Sound Ray Tracing Througha Gulf Stream Eddy in the Sargasso Sea. U.S. Naval OceanographicOffice TN 6150-21-74.
Gemmill, W.H., (1974) A Note on Sound Ray Tracing Across the Gulf Stream.U.S. Naval Oceanographic Office TN 6150-27-74.
Hanssen, G.L. and W.B. Tucker, III. (1975) Interim Integrated CarrierAnti-Submarine Warfare Prediction System (ICAPS) manual, (CP 642-BComputer System) (U), U.S. Naval Oceanographic Office, ReferencePublication 10, Washington, 100 pp, (CONFIDENTIAL).
Levenson, C. and R.A. Doblar (1976) Long-range Acoustic PropagationThrough the Gulf Stream, Acoustical Society of America, 59 (5), 1134-1141.
1
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38N
A LOCATION OF AXBT PROFILES
19
S 20
211
36 1
20 Setion0
211
~~. 21.22d
VELOCITY (m/sec) VELOCITY (m/sec)
1400 1500 1600 1400 1500 1600b•.:oI I
0- 0-
1000 1 1000 2
Sargasso Sargasso
SLD = 240 m SLO =105, m
2000 DSC =1400 m 2000 =1400m
3000 o 3000
4000 4000
5000 - 5000 -
Figure 2 - Sound Velocity Profiles for Section A
VELOCITY (m/sec) VELOCITY (m/sec)
1400 1500 1600 1400 1500 1600
0 0
1000 3 1000 4
Sargasso Sargasso
SLD = 200 m SLD = 200 m2000 DSC =140 m 2000 DSC = 1400 m
• E
' 3000 -' 3000
4000 4000
5000 - 5000
Figure 3 - Sound Velocity Profiles for Section B
12
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0 .
o
Lu>
0 0 0 0 0 0 0 00 0 0 0 0 0 0 0 0o 0 0 0 0 0 0 0 0
(wu) HIM3 H1430)
0 0
000 C-4-0
Lu4
0 0.
o 00
o 0 0 0 0 0 0 0 0
- C, C.) v &M - Of (l) fln
(w14Id3U (W) Hid3a
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CL 0
0- 00
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00 0 0 0 0 0 0 0 0 0 0 >0 0 0 0 0 0 0 0 0 0 41o 0 0 0 0 0 0 0 0 0
C1 C- 4 ~ n C4d m~ le "
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00
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- C4 M -W n - 04d ( ~ S
()H~d3O (W) Hld~a
13
RAY TRACE (SARGASSO)
1000
92000
3500
4000
0 5 10 Is 20 25 30 35 40 4S soRANGE (nmi)
PROPAGATION LOSS (SARGASSO)
.55. I
.60 SOURCE DEPTH =137 mn-65 -RECEIVER DEPTH = as m
.~- .0 -FREQUENCY 290 Hz
-75
0
-90
-95
.100
0 5 10 Is 20 25 30 35 40 45 soRANGE (nmti3
Figure 5 -Ray Trace and Propagation Loss for Section A
NAVOCEANO TN 3100-55-76
RAY TRACE (SARGASSO)
50a-I
SOURCE
no 2500A
2500
4000 w
0 s 10 is 20 2S 30 35 40 45 soRANGE (nm4;
PROPAGATION LOSS (SARGASSO)
-60 -SOURCE DEPTH 137 mn.65 -RECEIVER DEPTH =88 m
.70
-75
0-6.0
0
-90
-95
0 s 10 15 20 25 30 3S 40 4S soRANGE (nmi)
Figure 6 -Ray Trace and Propagation Loss for Section B
Nlk/U(CLtliU IF) Ji1UU-D:-/ti
RAY TRACE FROM SLOPE TO SARGASSO0 WATER
0I
2500
1'y
3500
N ORT H RANGE (nm.3) SOUTHSLOPE WALL STREAM WALL SARGASSO
PROPAGATION LOSS FROM SLOPE TO SARGASSO WATER
-60 I I
-65
-70 SOURCE DEPTH 137 m
JS5 RECEIVER DEPTH = 88 mFREQUENCY = 290 Hz
-80.- ---0 SINGLE PROFILE1 .65 *-- MUTPEPOIE
o-90
-100
-105
.110
-115 -
r -- 120 -
0 10 20 30 401 SO 60 70 40 90 1001 110 120 130 140 150
*NORTH RANGE (nhl) SOUTHSLOPE WALT, STREAM "ALL SARGASSO
Figure 7 -Ray Trace and Propagation Loss for Section C.Slope to Sargasso Water
* 16
......
RAY TRACE FROM SARGASSO TO SLOPE WATER
300
3000
-S
-0
SORE ETH=13
.0
-60-6S9
-90111TIL
.125
2.130
.130
-140 -
0 10 20 30 40 s0 60 70 s0 90 100 110 120 130 140 I50
RANGE (,smi)
SARGASSO SOUTH STREAM NORTH SLOPE
WALL WALL
Figure 8 -Ray Trace and Propagation Loss for Section C.Sargasso to Slope Wqater
17
SOURCE DEPTH =137 m
- -FREQUENCY =290 Hz
-. 60
-10
0~o'
-10Figue 9 0 %- rpgto os o eto
S. . ...
S - - -- .-. - - - .t.2 l NO
NAVUCEAI'U ITN J/UU-'-rb-
SOURCE DEPTH =137.1 :.FREQUENCY =290 Hz
0 60<
0* -7,I-90
-flu
*11
0t
Fiur 0 - PopgtinLos o Sci.
i~V'JLt~Li Ld JiU ) .j tb
SOURCE DEPTH =137m
- FREQUENCY =290 Hz
.4o
010
Figure 11-3-D Propagation Loss for Section C.Slope to Sargasso'a t er
20
SOURCE DEPTH =137m
-FREQUENCY 290 Hz
4r
"14
21
DISTRIBUTION LIST
!- .DATE: GERIAL"DAE: September 1977 R NAVOCEANO TN 3700-55-76
. SUBJECT: Acoustic Ray Tracing and Three-Dimensional Propagation Loss
Representation in the Gulf Stream Region
-TECHNICAL NOTE -EVALUATiON REPORT3700-55-76
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