AD-A241 360-

NAVAL POSTGRADUATE SCHOOL'Monterey, California

DTIC'R A AELECTv

.TAs8199110

THESIS .

OCEA\NOGRAPTIC AND ACOUSTICAL SURVEY OFTHE EAST IONIAN SEA

by

Radamanthis P. Fountoulakis

Septenber 1990

Co-Advisor Robert H. BourkeCo-Advisor Alan B. Coppens

WtFlON STAMUNT A

A b or publi r.IcbbbhtkIUIC. Unlimted

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Unclassified

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_____________________________________________ Program Element -No IProject No ITask No Work unit Accession No'11 Title (Include security class~ication) OCEANOGRAPHIC AND -ACOUSTICAL SURVEY OF THE EAST IO3NIAN SEA

-'12 Personal Author(s) Radamanthis P. -Fountoulakis13a Type of Rcport I13b Time Covered 1 4 Date of Report (year, month, day) I15 Page Count-Master's Thesis FoToISeptiTber 1990 I9816 Supplementary Notation The views expressed-in this thesis are those of the author and do not reflect the official policy or po-sition -of-the Department of Defense or the U.S. Government..17 Cosati Codes i8 Subject Terms (continue on reverse If necessary and identify by-block number)Field Group Subgroup Oceanographiic, Acoustic Survey, East Ionian Sea, PEmnodel

19-Abstract (continue on reversef necessary and Identify by-block numhber)

A-study was conducted in, an area off the I Icilenic %k est coast to examine the spatial and time variability of various oceanicparameters, with special emphasis on those effecting ASW operations. Propagation loss runs were conducted using PL andRAYMODIF modc1q The -reactions of both models to different bottom morplioluay and sound speed profiles (seasons) wereexamrined. Between the two models, the PE model was found to be closer to reality than RAY.O E Reut Ugstathe application of these models can improve the undcrstanding of sound propagation in the Hedllenic seas. The bottommodeling program, BLUG, appears to need improvement.

20 Distribution. Availability of Abstract 21 Abstract Security Classification[0 unclassified unilmited 0 same as report 0 DTIC users Unclassified22a Name or Respenjsible Individual 22b Telephone (include Area code) 22c Office SymnbolRobert -H. B~ourke (408) 646-3270 OCl~f

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Unclassified

Approved for public release; distribution is unlimited.

-Oceanographic and Acoustical Survey of the East Ionian Sea

by

Radamanthis P- FountoulakisLieutenant, Hellenic Navy

B.S., Hellenic Naval Academy

Submitted in partial fulfillment of the

requirements for the degree of

MASTER-OF SCIENCE IN ENGINEERING ACOUSTICS

from the

NAVAL POSTGRADUATE SCHOOL

September 1990

Author:

Radamanthis P. Fountoulakis

Approved by: ___________________

Robert H. Bourke, Co-Advisor

Aniony Atchley, ( irman,

Engineering Acoustics Academic Committee

ABSTRACT

A study was conducted in an area off the Hellenic west coast to examine the spatial

and -time variability of various oceanic parameters, with special emphasis on those ef-

fecting ASW operations. Propagation loss -runs were conducted using PE and

RAYMODE models. The reactions of both models to different bottom morphology and

sound speed profiles (seasons) were examined. Between the two models, the PE model

was found to be closer to reality than RAYMODE. Results suggest that the application

of these models can improve the understanding of sound- propagation -in the Hellenic

seas. The bottom modeling program, BLUG, appears to need improvement.

£1.10

IS40

Acession For

NTIS GRA&IDTIC TAB ElUnannounced 5

Justificatio

By_

Distribution/Availability Codes

Avail and/or

Dist Special

TABLEOF CONTENTS

I1. -INTRODUCTION.......................

A. GENERAL........................

B. OBJECTIVES ............................ 1

C. AREA -DESCRIPTION................................ ......... I

11. OCEANOGRAPHY ........................................... 6

A. WATER 'MASSES .......................................... 6

B. CURRETNTS................................................. 7

C. TE'MPERATURE-DISTRIBUTION .............................. 9

D. SUMMARY OF OCEANOGRAPHY ......... .................. 17

1. Winter-season.......................................... 17

2. Summ-er season .................. I......... ............... 17

III. DATA ANALYSIS ......................................... 18S

A. SOURCES OF-DATA ....................................... 18

1. Hydrographic data (Temp., -Sal., SSP vs- Depth) ................. 18

2.Bottom Characteristics ...................... .............. 18s

B. SEASONAL VARIABILITY..................................20

1. Winter........... ................................... 20

2. Summer............................................. 20

C. SPATIAL VARIABILITY OF DATA...........................27

IV. ACOUSTIC ANALYSIS......................................34

A. ACOUSTIC.MODELS ...................................... 34

I1. GENERAL............................................ 34

2. RAYMODE........................................... 38-

3. PE.Model ............................................ 39

4. Bottom-Loss Models ..................................... 41

B. APPLICATION OF DATA ................................... 43

1. Sound Speed Proflcs.....................................43

2. Bottorn Paths and Parameters..............................44

iv

3. Source, Receiver -Data..................................... 44-

C. ANALYSIS OF RESULTS..................................... 49-

I. BLUG Output ................................ 49

2. Path No 1: PE Model and-RAYMODE ...................... 49

3. Path No2: PE Model..................................... 49-

4. Path No 3.PE-ModelI ..................................... 58

5. Path No-4. PE Model and RAYMODE......................... 58

V. CONCLUSIONS .............................................. 64-

A. OCEANOGRAPHY ........................................ 64-

B. UNDERWATER SOUND ................................... 64-

APPENDIX A ................................................ 66

LIST OF REFERENCES . ................................. 86

INITIAL DISTRIBUTION 1LIST....................................89

LIST OF FIGURES

Figure 1. Local-seas and basins of the -Mediterranean Sea ........ 1

Figure 2. The location of ASW prediction areas .. .............. 4

Figure 3. Detail of coastaLarea Alpha ................................ 5

Figure 4. General circulation of the surface waters-in the Eastern ............. 8

Figure 5. Transverse cross-section of salinity ............................ 9

Figure 6. The formation areas of the Deep Waters-in the ................... 10

Figure 7. Wind-stress over-the Mediterranean Sea ....................... 11

Figure 8. Surface currents in the Hellenic Seas .......................... 12

Figure 9. Currents at the-surface and-at 200 m depth ..................... 13-

Figure 10. Seasonal Temperature, salinity and SSP in the Ionian Sea ...... 14

Figure 11. Distribution of sea surface temperature (°F), (a) .................. 15

Figure 12. Thermal surface features of the Mediterranean Sea ................ 16

Figure 13. Annual and monthly distribution of used records ................. 19

Figure 14. Bathymetry of-the Ionian Sea_(from HSHN, 19SSa) ............... 21

Figure 15. Bathymetry along the selected paths ........................... 22

Figure 16. Temperature profiles in area Alpha winter 1982 .................. 23

Figure 17. Mean profiles of temperature, salinity -and -sound speed ............ 24

Figure 18. Winter temperature profiles in area Golf ....................... 25

Figure 19. Temperature, -salinity and sound- speed profiles in areas ............ 26

Figure 20. Temperature profiles in area Alpha, summer ................... 28

Figure 21. Temperature, salinity and sound speed profiles in area ............ 29

Figure 22. Temperature, salinity and-sound speed profiles in areas ............ 30

Figure 23. Synoptic temperature profiles in area Golf ..................... 31

Figure 24. Synoptic temperature profiles in area Bravo .................... 32

Figure 25. Synoptic temperature profiles in area Golf ...................... 33

Figure 26. Partition-to wavenumber domains (RAYMODE) ............... 40

Figure 27. Simplified geo-acoustic model ............................... 42

Figure 28. Soui..d Speed Profiles for area Alpha ......................... 45

Figure 29. Sound Speed Profiles-for areas Bravo and Golf ................. 46

Figure 30. Sound Speed Profiles in areas Bravo and Golf (Detail) ............ 47

Figure 31. BLUG output for shallow and deep areas ...................... 48

vi

Figure 32. Path '1, PE Model at 1000 Hz winter.......................... 50

~Figure- 33. Path -1, PE~ M odel-at 1000 Hiz winter same depth TX -RX ........... 51

Figure 34. Path T YMODE at 1000 Hz winter. .. .. .. .. .. .. .. .. .. .. .. .52

Figure 35. Path -1, Model at 1000 Hz summer. . ................ 53

Figure 36. Path -1, PE Model at 10004Hz-summner same depth TX-RX......54

Figure 37. Path 1, RAYMODE at 1000 Hz, summer and receiver at 60 m.....55

Figure 38. Path 1, RAYMODE at 1000 Hz, summer and receiver at 10 m ....... 56

Figure 3 9. Path- 1, PE Model with- fully absorbing bottom................... 57

Figure 40. Path 4, PE Model at 250 Hz............................... 59

Figure 41. -Path-4, PE Model at I kl-z .................................. 60

Fiaure 42. Path 4. PE Model at I kl-lz................................ 61

Figure 43. Path -4, RLAYM ODE at L- kliz win ter........................... 62

Figure 44. Path 4, RAYMNODE at 1 kliz summer ........................ 63

I. INTRODUCTION

A. GENERAL

The Hellenic Navy (H.N.) recently established an underwater laboratory in order toperform tests-for underwater acoustical devices. A number-of areas-were examined in-the

region of Eastern Mediterranean and -some of them were found to comply with the

requisite specifications for low environmental noise and -a smooth, shallow sea bottom.

In the- present study one of these areas was selected for examination of-its acoustic

propagation characteristics. the-study areais located in-the Ionian Sea , specifically the

region-north of 37° O'N, a restriction to-avoid the sea lanes in the-southern IonianSea

which-cross the area in-an east-west direction. A partition- of the Ionian Sea-into smaller

sectors, each with similar characteristics, was made according to Chart No 30 published

by the Hydrographic Service ofthe Hellenic Navy (H.S.1-I.N., 19SSa). This study covers

only the eastern sectors close to'the west coast of Hellas (Greece), namely areas Alpha,

Bravo and Golf, names that will.be used in the study hereafter.

B. OBJECTIVES

The principal objective of this study is to examine the spatial and temporal vari-

ations of the oLeanic factors that affect underwater sound propagation in order that an

acoustic analysis and understanding of the sound propagation in the Ionian Sea can beperformed. In addition, an evaluation of the acoustic computer models used by the

United States Navy is performed to examine their application to this unique region of

the Ionian Sea and the -Eastern-Mediterranean in general.

C. AREA DESCRIPTION

The Ionian Sea is that part of the Mediterranean Sea lying to the west of Hellas.

The geography and partitioning of the Mediterranean Sea into regional basins is shown

in Figure I. This study covers -that part of-the southeast Ionian Sea from 370 OWN to

380 30'N and 190 30E to 210 00'E. The-total area is 20000 kin and is bounded to the

east by the I lellenic Peninsula (Fig. 2). The area can be separated into the coastal waters

lying between the mainland and-the offshore islands and the pelagic waters of the Ionian

Sea. The mainland is divided into two parts, the main Itellenic Peninsula and the smaller

Peloponnesos Peninsula. Between -the two there is a sea lane of about 10 km in width

and 165 km long which connects the Ionian Sea to the Aegean archipelagos ;ia the

Strait of Corinthos. Area Alpha is located in the shallow waters between the mainland

and offshore islands of Kefallonia and Zakynthos (Fig. 3). It-has an overall area of 1SSO

km2 and a smooth sea floor with an average depth of around 120 m. The-other two areas,

Bravo and Golf, are located west of the above islands with Bravo to tie north of Golf.

The area of Bravo is 4116 km and has bottom-depth which extends to 100 m over the

continental shelf and then falls sharply to 1500 m, eventually reaching-depths in excess

of 2000 m. Area Golf is 15000 km2 in extent and has bottom features similar to that

of Bravo but has deeper depths which extend-to 3000-3300 m.

z

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I 0-

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I -

1< Z

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I i

20

Figure 1. Local seas and basis of the M-editerranean Se-a: The eastern part iscomprised of the Ionian, Acean and Levantine Seas (froi 'vlaur,",Center for Ocea-n Science, 1974).

'33

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Fiue3. Detail of coastal area Alpha: Bathymetrv and location of the stations

used in this study are shown (from 1-SI-N, 1982).

II. OCEANOGRAPHY

A. WATER MASSESOceanographic conditions in the study area are effected by the -circulation of the

three major water -masses present in the Eastern Mediterranean Sea. These masses,

Atlantic, Levantine and Deep Water, are separated- by different depths which vary as

they move from their places -of formation towards other areas, being mixed by other

waters along- their paths. A number of authors such -as Lacombe and Tchernia (1958),

Wust (1961),-Ovchinnikov (1978) and others-have studied the water masses and the cir-

culation characteristics of the Mediterranean Sea. In general, the Mediterranean can

be divided into a western and an eastern basin, the latter being defined as the-sea east-

wards of the Straits- of Sicily. Malanotte-Rizzoli and Hecht (1988) state that the physical

mechanisms that determine the circulation patterns in the Mediterranean are still un-certain. They, as well as EI-Gindy and El-Din -(1986), have reported on a number of

studies that have been done- or are still under execution such as the POEM- (Physical

Oceanography of the Eastern Mediterranean) -cooperative program (UNESCO reports

30, 35- and 44). Part -of the uncertainty is due -to the complexity of the land barriers to

the circulation 'modeling, coupled with sometimes- conflicting reports which have ap-

peared since 1945.

The major water inflow comes from the Atlantic Ocean. This flow pattern results

from- the fact that the Mediterranean is a concentration basin wherein evaporation ex-

ceeds precipitation and runoff. Hence, Atlantic Water (AW) flows -into the

Mediterranean in order to preserve mass conservation (Bethoux, 1979 and 1980). It

flows eastward extending from the surface to 200 m and- enters the EasternMediterranean by the Straits-of Sicily. In the vicinity of Gibraltar, AW has a temper-

ature of 150C and a salinity of 36.15 psu (Lacombe and Richez, 1982). At the entrance

to the eastern Mediterranean-the salinity-has increased to 38.6 and continues to increase

eastwards until at the coast of Israel it is 38.7. In the winter the lower evaporation rate

and vigorous- mixing destroy the -upper layer quickly so the identification of AW is dif-

ficult. Frassetto (1965) reports that in wintertime some evidence of AW was observed in

the Straits of Sicily so the assumption-that this water penetrates as far east as the Ionian

Sea holds. Based upon geostrophic calculations (Nielsen, 1912), a cyclonic gyre is ob-

served to the west of Crete (Fig. 4) which carries the AW northwards into the Ionian

6

Sea. The identification of AW in summer is easier as- the high insolation and evaporation

rate, coupled with -limited wind action, create a buoyant warm- and saline layer at thesurface which preserves the low salinity influx water-found'just beneath-the surface.

Beneath the AW is Levantine Intermediate Water (LIW). This water is formed inthe Levantine basin-(Wust, 1961; Bryden and Stommel, 1982) at depths between 200-600

m and is present in -the Levantine Basin throughout the year. This water flows westward(Wust, 1961)- and upon reaching Gibraltar enters-the Atlantic Ocean where it sinks to

1,000 m. It can be traced to the east coast of the- American continent by its salinity

-maximum (Lacombe and- Tchernia, 1960). LIW also penetrates into -the Ionian and

Adriatic Seas. LIW is formed mostly in the winter and can:be identificd in the LevantineBasin by its salinity maximum (39.1) and a temperature of 15'C (Fig. 5). As it moves

-towards Crete, the salinity decreases to 38.9 and continues to reduce due to mixing as

it progresses westward. The salinity of LIW at the Straits -of Sicily is 38.7; upon exiting

the Mediterranean at Gibraltar it a has value-of 38.4.

The Deep Waters in the Eastern Mediterranean are thought to be formed in theNorth-Adriatic Sea-(Pollack, 1951; Pickard and Emery, 1982; Roether et-al., 1983), the

latter basing their hypothesis on the results of tritium tracer studies. These waLers flow

to the bottom of the Ionian Sea and then into the Levantine Basin (Fig. 6). Some of

-these bottom waters can also be formed-in the Aegean Sea but no evidence of flow into

the Ionian via the Kythera Straits has been identified (Lacombe et al., 1958). Thesewaters are characterized by a temperature-of 13.6°C and a salinity of 38.7.

B. CURRENTS

Observations of the currents in the Eastern Mediterranean Sea have lead to con-

flicting results. A number of measurements and models has been published (Malanotte-

Rizzoli and Hecht, 1988; E1-Gindy and El-Din, 1986) but no coherent picture of the

general circulation can -be drawn. This is in part because-the wind stress in the EasternMediterranean exhibits such strong seasonal variability (Fig. 7). The wind driven cur-

rents are expected to form a c.clonic g.-re in the Ionian Basin which reverses in summer

(Moslalenko, 1974), a feature which -has been confirmed by numerical experiments car-

ried out by Malanotte-Rizzoli and Bergamasso (1988). The Pilot of the Ilellenic Seas

(-ISI-IN, 1979) describes a relatively steady northward surface flow, with no seasonalvariability, atong the coastal margin of the study area (Fig. 8). More current measure-

7

-~ A

F5 -

30[

I0* li' 20 25' ,0" E

Figure 4. General circulation of the surface iiaters in the Eastern Mediterranean

(from Nielsen, 1912).

ments made by the Hellenic Navy (-ISHN, 1989) confirm the northward direction at the

surface-and at 200 m withno significant seasonal variations (Fig. 9).

The Ionian Sea and its neigibor to the north,-the Adriatic- Sea, have relatively high

surface evaporation rates both in winter, due to dry strong winds from Europe, and in

summer, due to high insolation. Assuning that the deep water, formed from the excess

evaporation flows southward, then mass conservation requires a surface replacement

inflow from the south (Atlantic and Levantine types). Thus a two-layer circulation- pat-

tern of opposing currents -is formed.

No frontal regions are located-in the study area. The Maltese Oceanic Frontal Zone

(Johannessen, Stobel and-Gehin, 1971), located east of Malta in the region between 36"

00N 170 00'E is far to the west and does -not affect the study area.

The above water masses and their circulation produce the temperature and salinity

profiles seen in Figure 10 typical of the Ionian Sea.

S

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aboue 50 meTh.nsvThe tempseratre sowlnty derassfom 15aons atter botom) of the

Sum ercodiisum are charter),izedba erh ot rseac sufae (surf)ce whereinsoation

decreaseithc follst aLepat n of ntemeit 1 CfoWed by asow de c asc e-

tweT0 n-60 LW he temperature-i arooute found in the steep water exiit istrn cesnloseri

aton Tisvribiit i ontrindtoth-srfc wtes sprfiesbeo 30 9o

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Feoe ngiguree 7.r W dpth s r teMterrnea n Sea: a-( .1). Jnaymab.Ags

The transitions between these two seasons is observed mainlsy in the depth of the

mixed layer, 10 m-in-spring and-about 25 m in the fall. Acoustic conditions during these

transition seasons arc not examined in this study, instead preferring to concentrate oil

the two extreme seazsonal conditions.

in addition to being infli1-mced by the above large-scale properties of the

Mediterranean circulation, the local tnrnpcrm w,,,_ ;jliniLtv profiles can-be affected by

meso -or small-scale activities like eddiesfN(Maianotte - Rizzoli and Hecht, 1988). For ex-

ample, El-Gindy and El-Din (1986) cite t -_ possibility of deep warm eddies-in thle cen-

tral Ionian Sea. Also a Lteilite image-of the Ionian Sea (Fig. 12) shows the presence of

warm eddies in the area under investigation.

W _W

- 4 -

rI

;Z ' - - -.J,

Figure 8. Surface -currents-in the -Hellenic Seas: top: wvinter mean, bottom: sum-

mer mean. Both seasons have a northiward direction a13flg the west-coast

of Helias with a speed of 0.2 to 0.5 Kts (from ISKN, 0979).

12

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30

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III Ito j4 314 141 380 or f 54 3* 44 34

Figure 10. Seasonal Temperature, salinity =n:-SSP in-the Ionian Sea: Both sea-

sons show the presence of the three different water masses (AW, LIW,

DW) in the Eastern .Mediterranean (from Maury Center for Ocean

Science, 1974).

14

40 76 6

I~ ~ L- _____

166

Nov

Figure 11. Distribution of sea surface temperature (OF), (a)- February, (b) .vlay, (c)

Augyust and (d) November (from Nlaury Center for Ocean Science,

1974).

15

~. NORTH

I - .

C HI~ONCL

Figure 1. Therml surfac featurs of theMeditera3enSa h rsneo

warmeddis i theeasern onin Se ar shon i.thi.plt.dei.e

fromsatelit LR.dat fro Set. 177 o frm Fb. 179 Robison

1

D. SUMMARY-OF OCEANOGRAPHY

A summary of the-preceding hydrographic analysis is presented below describing

conditions present in the study area during the -two principal seasons. Typical profiles

-for the Ionian Sea-are shown in Figure 10.

1. Winter season

The winter season, January through March, is in general characterized bystrong wind- mixing and convection which produces a well-mixed surface layer to 200

m (T = 14.00 C, S-- 38.2). Beneath this layer and extending to 600:m is LIW which flows

northward along -the west coast of Hellas. The Deep Water from the Adriatic fills thedepths from 600 m to the bottom (T L 13.5 0C, S = 38.7). The mean temperature, salinity

and sound speed--profiles of Figure 10 appear to be representative of the entire region

of the Ionian Sea. Small variations from these profiles are expected-due to local-influence

and year to year fluctuations.

2. Summer season

The summer season, July, August and first days of September, are characterizedby high insolation and limited wind mixing. This produces-a hot and saline lid at the

surface(T 25°C, S = 38.8) which overlies th clow-salinity Atlantic Waters below. The

AW is identified by its lower temperatures and salinity minimum (T --- 14°C, S = 38.2)at a depth of 20 m to 200 m. Little or no seasonal effects are expected below this depth

of 200 in.

17

III. DATA ANALYSIS

A. SOURCES OF DATA

1. Hydrographic data (Temp., Sal., SSP vs-Deptl)

Two sources of hydrographic station data were used; from the National

Oceanographic Data Center (NODC) at Washington DC -(1989) and from the

Hydrographic Service of the Hellenic Navy (HSHN) at Athens, HELLAS (1988b, 1989).

Historical- sound speed- profiles compiled by Podeszwa (1980) for the Ionian Sea were

also used for comparison, as they include more than 6000 records in this area. From the

NODC data file a total of 192 observations (Nansen and CTD) were extracted. The

majority of them (180 records) represent synoptic data collected-in areas Bravo and Golf

in "November 1980 while the rest are mostly single records taken from various sources in

the study areas between 1970 and -1983. The Hellenic Navy data included 15 CTD

stations in the coastal- area, Alpha. Additionally the Hellenic Navy provided groups of

three to four synoptic -records covering the entire area Alpha and representing all four

seasons. These records were used for spatial investigation of each group inherently and

for seasonal variation analysis, averaging records of the same group. The time interval

covered by the data from all these sources starts in 1970 and ends in 1983 and the ob-

servations are distributed over all the interested areas and seasons of the year (Fig. 13).

Sound speed profiles were calculated from these temperature and salinity data

using the Chen - Millero (1977) equation. In selecting a typical SSP, more attention was

given to the upper 600 m as this was the depth range exhibiting most of the seasonal

variability. Records having some data points lying outside the normal standard variation

were corrected by interpolation. If the number of bad data points was significant, then

the whole record was discarded. Plots and graphs were expanded to 350 m for the shal-

low area (Alpha) and to 3000 m for-deep ones, Bravo and Golf. The range 0-600 m is

also used to increase the resolution of the upper water strata.

2. Bottom Characteristics

The bottom depth in the area of interest varies such that it can be divided into

two regions. The coastal region is a part of the continental self with depths less than 200

m. The Ionian Basin is significantly deeper having depths between 200 m and 3000 m

and in some areas exceeding 3000 m. A representation of the bottom morphology can

been seenrom three transects which pass o,.er the continental shelf into deep water

is

Number of records- per year

All the months- -included

67Sg 71 7357 8ir 68 -/ 72 374' 7S 78 a 2 8

Number of records per month

-from 1970 to 1982

.5 ..... ....... ......... . .... .. *1*15....... ............. *. . .....

0-

2 .3 4 5 a 10 12

Figure 13. Annual- and monthly distribution of used records: Only non-synoptic

records are shown for each yvear and each month from all the vears

summed.

19

(Fig. 14). The continental shelf is- present-from -the mainland to the Ionian -slands; thenthe bottom precipitately descends to the Ionian Basin. The selected- study areas are dis--tributed such that are, .!pha lies above the continental shelf while areas Bravo and-Golf lie over the continental slope and- the Ionian Basin. Based -upon data from theHSHN (1988b and 1989) the sea bed is observed to be mostly mud with a small per-centage- of silt (less than 4.5%). This percentage increases- shoreward from 12.8% to29.4% depending on-the bottom slope.

Data for acoustical modeling- of the sea bed- were taken from the data basesassociated with the bottom-loss model BLUG and NAVOCEANO charts (1-1.0, 1972).

Two HSHN small scale charts, No 22-(HSHN, 1982) and No 65 (I-ISHN, 1983)_were used to extract the bathymetry along each selected transmission path for later in-sertion- into the PE transmission model (Fig. 14 and 15).

B. SEASONAL VARIABILITY

1. Winter

In the winter season (January through March) a total of -15 records were ex-amined from observations taken within the study area. These observations were takenover a time span of nine yeais. As-expected, wind and- convective mixing produce a well

mixed isothermal layer below the surface. This layer in shallow areas, like Alpha, extendsthroughout the entire water column. In area Alpha the winter is characterized by anisothermal profile of'14.5°C (Fig. 16). The mean values from these 15 records result in-

temperature, salinity and sound speed profiles as shown in Figure 17, where a low sur-face salinity (S = 38.0) is observed followed by a positivc salinity gradient. The samecharacteristics can be seen in the deep areas, Bravo and Golf, where the surface tem-perature is 15.0°C and decreases slowly with depth (Fig. 18). At 300 m the temperature

is about 14.0*C; at 1500 m-it is 13.5°C. The salinity gradient is also positive with surfacevalues of 38.44 in the deeper offshore areas. Salinity increases with depth until at 300

m it is about 38.75. This is true for both deep and shallow areas. AW is hence observed

to occupy the upper 300 m, LIW the deeper waters (Fig 19).

2. Summer

In summer a total 5 of records from the HSHN data base were examined. Theprincipal features of the temperature profile are its hot surface laer followed b a sharpdecrease in temperature. The profiles for area Alpha (Figures 20 and 21) show the pres-

20

Asp~ a, Q6 Y41"

304 P

-0 P Rw

i589571

EA vf.2S13

0 (8245) L,4im

th' i

'j 1-10

GfOAZ3551 V %

%

3360

5 2376A , km KiNnis f7776) K i"thom'

ZXkl

3062 PELOPONNESUS

-019691.

'N' Mo h.1 1 -)0 V,

97 1

35 Kalamal3408 4^407

Pa

84 i1o

Zg32112Q' 982'

3178 kra i I n iii

322 rn

ca I .,. ,

-3022 -1 r, 14

41()8 ED 24911, 190

h (de 'PloU 4r

3183 Xg421V

': -2231 100 ED

2006-Q Stool ( h,2 3 99,= 7881

2510 d121- Ln

Jjk.\ r3 0

*30,15 r 29062171

Figure 14. Bathyinetry of the Ionian Sea (from I-ISHN, 19SSa): The location

of the four transects uscd is also shown (depths in metcrs).

21

00

00

0

C)) 0+) 00 C

A0 to 0 C

C~to

Fiue15 a-yer nl i tesletdah:Teisrevlushon ee

Figurewer. Baser ilon the smoelected piate the dict aluye- shond her

areas Bravo-~and Golf.

22

oTempe rature f(0C)0 0

25-

50 E

75 af 0

100-II ~ -Mean Va Iues f r omHSHN

oeoeoRec al ( -- 2 at 2-3:00)-125 4aeaRe c a:2 (9-:2-:82 at 14: 00)

b**Rec a3 -25-2 82 at 04:0)

150-O175l

225 I

250 7

300

,325-

35or-1 of 1 1$ II IIIIIIIII-til 1 if ifII I I II 1 1111 111 111

Fioure 16. Temiperature profile in area Alphia isinter 1982: These records show

the existance of isothermal conditions with small variations from day

to day.

23

AREA RSTHiR W IIT E'R

SOUNO VELOCITY (METERS/SECONO)1500-1505 -1510 1515 IS20 1525- 1570 15-:5 '5-'g 1545 I550

.. ... . . .. .. . .. . .. .. .. . .. . .. .. ... ... . . .. .. . .. .. .. .. .. .

C

inI

.... ... ........

E-1

CM,

...... .... .....

275 77 2 .5 2.5 37

Fiue 7 ea rfie f epraue slntyadson pedpoflsinaeAlha witr The......... SS produces.... a half.................... channel... propagation...path..

ene f ixd ayrto20m.Te ixd aerisbetr evloe i te~snoti pofiles acquired~~~~~ ~~~~~~ bewe 9 an 19 Uetm~ 17 m nvlesol) thrsaadi

....... .............................. 24....

Temperature ('C)C!

0 (-d

-- N ---4 CI A- -

.10-to

50- B0

100 - 0

0"50200 .0o

250 0000031 Jan 72 at 08:00:2o5o00 9- 0009b 83z at 23:59_C, 16 Feb 76 at 03:0015 Mar 76 at 23:59-

,. 300 '.' Mean values from - records

C)50 545

400-

450-

500-

550"

600 I t I i I A I ! _1 t1 I L V I 1 1 L I I L I

Figure 1. Winter temperature profiles in area Golf: The variation between

these records taken over an interval of nine years is minimal.

25

flRCAS -BMfVO- RIN GOLF

SOUND VELOCITY (fICTERS/SECO5ID)1500 1510 1520 15130 1540- 1550 1560 - S70 -1500

.o . .

.n ........... ........... ....... ...... ............. ......

CYJ'

.0 ......... .. ............0-.............. .... . .....

In . .. . .. .. .. . .. . .. ... . .. .. .. .. .. ... . ... .. . ... . . . . ... .. . ... ..

CD.0 .. . .. .. . .. .. . .. .. .. ... .. .. .. . ... .. .. .. ... .. .. ..

CDn

0on /........ ........... ............ ....... ...... .......

0 OIn....... ............ .......

U)........ ...........................

CD,

tn. ........ ..... ...... TEMIPERATURE ................C3:

17 I110 12.5-'5 1 S 20 22.5 25 27.5- 30 32 5 3S

TEMIPERATURE (DEGREES -C)

37 37.25 37.50 37.75 38 381.25' 38 50 38 .75 39Sf9LINITY (PARTS/T1OUSHNO)

Figure 19. temperature, salinity and sound-speed profiles in areas Bravo and Golfwinter : The presence of Atlantic, Levantine and Deep (Adriatic)

Water-can be seen by the salinity minima and maxima.

26

areas Bravo and-Golf the eAistence of limited mixing is evident as the temperature at fil st

decreases- slowly -to 15 m, followed by a sharper gradient beneath. The salinity profile

in shallow waters (Alpha) shows a- nearly -isohaline condition with a sharp increase close

to the bottom, evidence of the presence of LIW flowing on to- the shelf (Fig. 21). In the

deep water areas, the salinity exhibits a minimum at 30-40 m (AW) and then increases

to 38.75 between 300 and- 500 m-(LIW). Below-the LIW it slowly decreases to values

below 38.7-(Deep- Waters) (Fig. 22).

C. SPATIAL VARIABILITY OF DATA

To investigate evidence of spatial variations in the study -areas all the available

synoptic data were-used. For area Alpha-these synoptic records-covered all the seasonsof the year; for areas Bravo and Golf the only available- data were limited between 2 and

28 November 1980.

The compilation of synoptic records -from area Alpha shows that th-e maximum de-viation of each group does not exceed 0.5"C from the mean values (Figures 16 and 20)

for both seasons, winter and summer. I lence, in area- Alpha, bathythermographic con-

ditions have negligible spatial variation for each ot- the seasons examined in this study.

For deeper areas Bravo and Golf; as noted before, -the -spatial investigation was done

using -groups of records taken bctween 2 to 28 November 1980. Again the differences

between -the -redords do not-exceed-0.5"C in temperature or 10 m in MLD.

The comparison between selected groups of records taken in area Bravo on 19 No-

vember 1989 and in area Golf on 11 and 26 November 1989 verify that area Golf is

usually 0.5 to l*C warmer than Bravo. The differences are in the mixed layer where

(Figures 23, 24 and 25) records -from Golf have a MLD of 30 m and a surface-temper-

ature 21.3 0C on 11 November. Four days later Bravo has a MLD of 30 m and a surface

temperature 20.0"C and then Golf on 26 November has a MLD of 35 m and surface

temperature 19.4"C.

The above selected records indicate a negligible spatial variation across the study

areas. -In addition the examination of single records taken in winter and summer shows

small differences between them. Records taken in these areas from the-same seasons %'ith

a five to nine year time span seem very similar and close to those compiled-by Podeszwa

(1980). Hence, the assumption that the spatial variations of temperature and salinity in

these areas are negligible is valid. Accordingly the SSPs in the study are; do not have

horizontal variations, at least for the time period for which an ASW prediction is re-

quired.

27

Temrperature (C)o ~ - D C 06 (' c CD- 0

- --- _a' (4 ('

-25

50

75 o/dI

100-Teo8 Aug 70 at 18:20

125- oeoeo19 Aurj 73 at, 14:00V- .4. Mean -values from HSHN

S175

-200[

225

250[

275

320 01.325-

350 L ' I I I , I I * I I , I I t_ I I I I Y I I I I

Figure 20. Temperature profiles in area Alpha, summer: The existence of a ixed-layer can be verified by the data collected during September 1973

(HSHN, 19SSb).

2S

AREA ASTHR SUMMER

SOUNO VELOCITY (rETERS/SECONO)3500 1505 1510 -Isis 2 1525 1510 1535 25Wo Isis 2550

....... ..... .... .... . .........

I t

....... .......

2 ...

CM ........ .... ...... ............ ......... .......... ....................................n... ..... ....... .....

......0..... ................................

............ 1:m ............. ............. ........ _ _ _ _

10 22.5 15S 17.5 20 22.5 2S 27.5 30 ~5 2

TEMIPERATURE (DEGREES C) .

27 -:7.2S . 7.0 27.75' 238 i8.25' 38.0 EO 8.75 -9

SiL I NI9 TY (PRRTc./Th-OUS=ANO)

Figure 21. Temperature, salinity and sounid speed profiles in area Alpha,

sunimcr: In this season. the formation of a mixed laver of 25-40 m

is more likely depending on wcather effects.

29

AREAS BRAVO AiND GOLF

SOUND VELOCITY (METERS/SECOND)-1500 ISM0 1520 1530- 1510- 1550 1560 1570 150

03 . ................ ..J . .......... ........ ........... .............

......... .S..... R

0 3

NLn

0

w .............. ... EPRTR .. ............ ........

8..........--to 1. 5 75 2 i. s 2.5 3 2 3

I0EPRTR DGESC

03

Temperature -('C)-6 0 0

5O~ -

0-

0

100- 00000 Rec 013 at 04:00ooaon Rec- 011 at 18:00----e Rec 176 at 20:25

150b---*Rec 1-79 at 16:00

200-

S250-

~30OF4'1 350F

40{-

550-

6 0 0r I 1 1 1 1 fly F- I IIII IIII

Figure 23. Synoptic temperature profiles in area Golf: The differences betwen

four observations taken on 11 November 1980 are less than 0.5"C. The

MLID is at 30 rn.

31

Te -perature -(0C

50 '

100 -'-Rec 131 at 07:00I 00000 Ree 133 at 12:00SRec- 134 at 14:00

150- -'+-Rec- 135 at 17:00

200

250

~4300

500-

0L- 1 1 [ i l I ,- t y I - I I I I I

Figure 24. Synoptic temperature profiles in area Bravo: The distance bCEtwq;.n

each of these observations, taken 19 November -19S0, is 30 to 50 km.

The differences are negligible.

32

TempeFrature -('C) 0

0 - N C% N N

50-

100 7o0o&0 Rec 041 at 05:00oao Rec 043 at 10:00

Rec 114 at 13:00iso --.- ~'Rec 115 at 18:00-

200

250

4 ~300i

~350_I

400 o

450-

500 I

5.50 i

600L 1)1 16: c1 ... 26 Noeme show da)ily I aII

Figyure 25. Synoptic -temperature profiles -in area- Golf: Records taken at 05:00(rec 04)and 160 (rc 15 on 26Noemer1QO ho adalvriation of 20 m in- MLD and 0.5*C in temperature.

33

IV. ACOUSTIC ANALYSIS

A. ACOUSTIC MODELS

In this study the examination of sound propagation was- performed by using two

acoustic models available at -the Naval Postgraduate School, the PE Model and the

RAYMODE -model. These two models- represent two different -techniques to calculate

the propagation loss in a given oceanographic environment. A number of runs were

compiled using both models -and relative comparison performed.

-Brief qualitative descriptions of the two models are-given below and-the implemen-

tations of bottom interactions-into each-model is discussed.

1. GENERAL

The purpose of these models -is to provide attenuation for sound propagation

into the sea. Specifically they are designed to calculate or estimate a number of propa-

gation- factors, the major ones being geometric losses (spreading and convergence),

diffraction, absorption, scattering, leakage and -boundary effects (surface and bottom).

A number- of methods, mostly empirical, for estimating -the effects of some of the above

factors in- underwater sound propagation have appeared in numerous tactical publica-

tions in the years following the close -of the second world war. Such methods were used

by surface ships and submarines to estimate, for example, detection ranges of the day

or best-depth to avoid detection.

One of the- next progressive steps was- the implementation of ray theory. An

immediate result of Snells law, raytracing is a -simple -and fast method for deteimining

soundpaths either graphically or with the assistance of a computational machine.

Sound propagation in the sea can be expressed by the linearized, lossless and

source free wave equation (Kinsler et al., 1984),

22 1 a PV-p= 2 _ (4.1)c Ct2

Ray-theory assumes that a-solution to the wave equation (Kinsler et al., 1984)

takes the form,

p(r,z) = A(r,z)eJWC - - 3-,2)

34

where A(r,z) is the spatially dependent pressure amplitude and r is a propa-

gation vector -locally -normal to-the isophase pressure surface; hence VF is the local-di-

rection of propagation,

vr = n( cosr? +-sin Oz).

where 0 is the local elevation angle- of the ray path.

This- solution, when applied-to the wave.equation (41), will result in:

V2A o02F o o 2A I "vr .vr+--- -j-- VA.o v +.Vr+vE)=o.

The above equation can- be simplified by a number of approximations- to provide

a -form known-as the-Eikonal equation,

vr vr = 2

where q1 is-the refractive index,

COI;(xjy,z) = c 'xyZ)

The assumptions and restrictions used in the derivation are that the pressure

amplitude A is significant within a finite aperture beam and that the speed of soundvaries little over distances compared to a wavelength, so

2V A (02 2 (o

and

VA .vr<1 •

The applied assumptions will make this method fail in cases whe-: the pressure

coefficient A has to deca% rapidly from the center of the ray to the-edges or the SSP has

sharp changes. Such conditions are expected in-caustics, shadow zones and the boundary

between the ocean and some type of sediments. In this theory no diffraction of the

sound at the edges is permitted as the acoustic energy trapped within a beam may not

leak out of it. In contrast -to normal mode theory, ray theory easily accepts different

35

densities in the~sediment-so reflection of-the incident rays can be calculated. In general,

ray theory is valid and useful when the frequency is high (short wavelength) and the SSP

changes slowly.

The -continued--improvements of passive- detection in frequencies -below 1 kHz

resulted in new theories -and techniques, followed by a number of complicated acoustic

models as electronic computing power became more available. Noninal mode theory

provides a solution to the wave equation 'using the summation of an orthogonal set offunctions, each -one being a solution to the wave equation; Each of these characteristic

functions, called normal- modes, has its own attenuation factor for the given -boundaries.

The final pressure field can be calculated after these modes have been combined

additively. This -solutionis formally complete but it is difficult to calculate-and interpret.

A simple approach (Kinsler et al., 1984) to normal mode theory- can be per-

formed-by inserting a source term in the wave equation

V-p 2--

- - 6(r)b(z-zo). (4.3)-C22 27rr

The solution for a sinusoidal wave propagating -into a sound channel can- be written as

the sumnation:

p(,',z,i) = ejT'Zn(r)Zn(z)

where

d 2 Z, C 2+z -2 k2,))z, = 0dz c (Z)

and

R,,(r) = -j7reC"tZn(zo) H o2 )(knr).

For r > > I the solution will be

p(r,z,t) = -7Z \/ "$n Z"(z°)Zn(z)e( + - (4.4)

36

where the -Hankel function has-been replaced-by its asymptotic form, with the use-of the

far-field approximation. The value of K and the depth dependence of Zn(z) can be eval-

uated from the appropriate boundary conditions and the given c(z).

The above values must be calculated for the whole- propagation- space -which,

due to the mathematical- complexity, makes the use -of a computer- necessary except the

most easy-cases.

An alternative method to the above solutions is an approxiffiation using a

parabolic approximation to the wave equation. The wave equation can be expressed also

as:

V2p + k2p = O,

where

c _C_k - ,k -ko, 1, = C0 '

In cylindrical coordinates and with an omnidirectional- source the wave

equation is

1 2 2Prr + ( 7 )Pr + pzz + O.

Then the acoustic pressure, p, can be expressed as

p=u(r,z)l(o2 (kor )

and use- of the far field approximation yields2 2

Urr + uzz-+ 2ikou r + ko(n -1) = 0.

Finally, the assumption that u,,<2ku, gives the form

ur = a(ko,r,z)u + b(ko,r,z)U.2

which is amenable to numerical solution.

This method was introduced-by Tappert and Hardin in 1973 who used in their

work a computational technique called the Split Step Fast Fourier Transform (SS-FFT).

The advantages of this method are that we do not need to solhe for the entire field si-

37

multaneously as in -the normal mode case but given initial=conditions at some--range r,

(close to-the source)- the solutions for larger r's can be obtained by increasing r incre-

mentally.- This is equivalent to -neglecting back scattering as the solution for any range

r has no effect on any previous range.

The disadvantages of-this technique are -errors introduced, if -the initialization

of the problem is not correctly selected, -and if the requirement of S S-FFT for continuous-

functions of depth "-' are not met. The PE is inherently restricted to narrow spectral-

angles. A number published techniques ease the initialization -problem by using

raytracing or a directional pseudosource (see, for example, a discussion from Coppens,

-1982) and~a number-of "wide angle" PE Models-have been-published.-(see, for-example,

Lee et al., -19S2).

2. RAYMODE

The passive RAYMODE model, U.S.Navy's standard acoustic model, uses an

integration method developed-in 1968 (Medeiros, 1985). A-number ofmodifications and

improvements -have been -added, such as-mode summation and low frequency paths into

the sediment. The model used-in this study was the Passive RAYMODE, specifically the

1987 baseline edition. This model utilizes ray and normal mode theories in- an- attempt

to minimize errors and- computer run time. Up to four different paths of sound propa-

gation can be formed and the total pressure field is defined by summing the contrib-

utions over all paths. Boundary interactions are calculated by invoking a number of

other rhodels, like BLUG.

The inputs to the-program are:

Sound speed profile,

Bottom depth and bottom type (BLUG or NAVOCEANO),

Wind speed,

Source depth,

Receiver depth,

Sonar D/E angle-and vertical beamwidth,

Frequency,

Range span and range step.

The program will process these data to produce a piecewise -SSP and then par-

tition it into areas of different wave nLmber (Medeiros, 1985). These wave number do-

mains represent different paths such as the surface duct or the deep sonic -layer or

sediment paths (Fig. 26).

38

Based on the entered data for each path the model will calculate two parametersthe number of cycles that exist -between source -and receiver, and- the number of modes

existing in each wave number -interval. From these parameters there are four differentways that the algorithm-will proceed. If the number of modes is less than 10, the normal

mode summation technique is used. If the propagation angle exceeds a computed limit,then a- bottom bounce integration is used. For paths employing ncither -of the abovecases the original method of RAYM ODE integration is used (Raytracing). Finally, -if thefrequency is more that 3 kHz, a fast integration subroutine is -used (High frequency

RAYMODE).For each path absorption is calculated as well as -the relative phase of each ray.

Bottom interactions are computed using models including the Naval OceanographicOffice MGS -algorithm -(NAVOCEANO charts), the BLUG (Bottom Loss Upgrade) anda low frequency bottom-loss model developed by G.Gustave (NUSC, 1987). The surface-loss is-calculated according to a- Surface Reflection Coefficient Model. For the presenttime no range dependency is assumed, neither for the SSP nor bottom depth.RAYMODE at the present time uses only a flat bottom and a single SSP.

3. PE ModelThis is a relative new model based on the parabolic equation approximation.

It is designed to operate at relatively low frequencies as computer run time for higher

frequencies, more than -1000 Hz, -increases significantly. The PE Model is not restrictedby depth dependency along the axis of propagation (range r) nor by horizontallychanging sound speed profiles. It is-best used whenever a duct-like transmission occurs,for example, in- the Arctic, shallow channels, etc., or where non-homogenities in thewater mass exist. The entered parameters in this model are:

Sound speed profiles,Bottom depth (discrete depths),SSP and attenuation profile in the sediment or bottom loss vs grazing angle

(BLUG output),

Source depth,

Receiver depth,Source vertical beamwidth,

Range step (increment).The surface is assumed to be a pressure release boundary with reflection coeffi-

cient -1, the bottom is assumed to be a continuation of the water mass. After the initial

boundary conditions haxe been speified the program will march the solution forward

39

J-1 J-2 J.

-Cl~~ -onI~e ttetr-f

CM? - . d *pe t *th. ... 'CO dtpthZ5 501CC. deptheC2Z Localvinivua.

*JZ Local *..psC4Z Local -xi-o

-Zs Itto depth

4 Im~t path stasdc

L 03- -ovftco.ln

Is~lsobots bounce@

Figure 26. Partition to wvayenuniber domains (RAYOE: Tecqiaec

between duct-like paths and rays (from Medeiros, 19S5).

40

in range using a split - step fast Fourier transform algorithm (SS-FFT) until the entire

field has been calculated (Tappert and Hardin, 1973).

4. Bottom Loss Models

The influence of the sea bottom in sound propagation is modeled using two

different approaches. The earlier encompares the geo-acoustic models where the sea

bottom is assigned a number which represents the observed acoustic behavior of the

sediment. Such a representation are NAVOCEANO curves, used by the U.S.Navy for

high frequency operation-(> 1000 Hz). Nine different loss curves are used to characterize

the-types of existing sediment. These curves represent-loss per bounce (dB;bounce) for

only reflected rays. Type 1 is the most reflective and Type 9 the most absorbing.

For low frequencies, where both reflection and refraction into the sediment have

to be calculated, the Bottom Loss Upgrade (BLUG) model is used by most of the Navy's

models. In the BLUG model a- partition of the oceans is done similar to the

NAVOCEANO model, but instead of loss curves a total of nine geo-acoustic parameters

has been assigned to each area. These parameters are used by the BLUG program to

produce a-compressional sound speed profile into the sediment-as well as an attenuation

profile (Fig. 27). This will permit the acoustical model to incorporate the bottom loss

and-refraction by using a -continuous profile into the water and sediment.

The BLUG output is given in loss per bounce vs grazing angle for each fre-

quency and it is used, in this format, in the PE Model. RAYMODE uses an internal

subroutine-for the same task. The attenuation into the sediment is calculated from the

formula (Medeiros, 1982).:

a(zJ) = a0(zj) -xfk.

This first power relationship between attenuation and frequency was-made after

the work of Hamilton in his numerous reports. Hamilton (1971 and 1974) derived this

relationship in an empirical way from the collected data over a wide but relative high

frequency spectrum, without any theoretical basis.

The density of the sediment is constant because any changes do not effect the

propagation significantly. Finally a theoretical thin lay er at the surface (Fig.27) is used

to- remove any discontinuities in the boundary between the water and the bottom.

The second and later approach is the geo-physical model where reflection, re-

fraction and attenuation into the sediment must be calculated b} physical parameters for

each area. Recently a number of authors (e.g., Kibblewhitc, 1989) reported a number

41

-!J

L.4.C-r

-IJ

3: % 0U)0_

%0

% 0 1 'iLA% - L. 1.1.- u C30

ULJ

% P-

N. W5-'~ -LI ' Uu0

% t% C%~ L

o nI nC

Fiur 2. Siplfid-eoacusicmoe: heboton SPan ateuaio po

.Vcdirs 1992).

5L1 042

of experiments where the-more fundamental analysis from Stoll (1980), using-Biot (1962)theory, seems to-bc more adequate. Stoll (1980) cites a relation of the attenuation co-efficient with the square of the-frequency for-fully saturated -sediments.

The BLUG model is undergoing reconstruction whereby the attenuation- will

be related- to frequency between the first and second power, depending on- -the bottom

quality (type and depth).

B. APPLICATION OF DATA

1. Sound Speed ProfilesTwo different sound profiles for both shallow and deep water columns were se-

lected to-represent the summer and winter seasons. Area Alpha is represented with the

profiles representing the averaging of the winter or summer records, respectively. The

main characteristic in the winter is a smooth positive sound -speed- gradient -resulting ina half channel propagation (Fig. 28). Hence only reflected surface reflected (RSR) andreflected bottom -refracted (RBR) rays will propagate. I Iigher frequencies will attenuate

more rapidly due to enhanced scattering from surface reflections (Urick, 193). In thesummer a mixed-layer exists to a depth of 20 m resulting in a slight positive sound speed

gradient, followed by a sharp decrease (Fig. 28). A weak sound channel exists with anaxis at 150 m but is bottom limited so that this will not be a viable propagation path.

Using the formula,

f= 2 x l053

DT

to determine the low-frequency cut off in the mixed layer (Kinsler et al., 1984) we find

a cut off frequency of 2.2 kIlz, a transition range of less than 200 m, and a skip distance

of about 3.6 km. For both deep areas, Bravo and Golf, the representative profiles foreach season were selected from among the existing records in order to examine the most

significant cases (Fig. 29). The selected SSPs are common for both areas as no noticeablespatial differences occur and are shown in Figures 29 and 30. In the winter isothermal

conditions will create a half channel. The presence of LIW will create a small "knee"

along the SSP between 200 and 500 m as the high salinity of LIW will increase the sound

speed -in a small amount. In contrast, summer heating changes the upper 200 m, as noted

before (Fig. 22), and increases the sound speed. lence in the surface the sound speed isabout 1540 mis and a deep sound channel is formed with a height of 2000 in and an axisat 150 n (Fig.29). In this channel the transition range is about 8 km and the skip dis-

43

tance 30 kn. For paths with more-than 2200 in depth CZ propagation is expected at a

skip distance o"29.5 km with a transition range of 8 km. The SSP used-for areas Bravo

and Golf-for winter and summer can be seen-in Figures 29 and 30 (upper 600 in).

2. Bottom-Paths and-Parameters

Using the appropriate data bases (BLUG and NAVOCEANO), all three areas

in this study were found-to be characterized- by a fast bottom with a sound speed ratio

along the boundary (water -to sediment sound speed) of 1.091 for the continental self

(Alpha) and 1.005 for the Eastern Ionian Basin. Area Alpha has a reflective bottom

(Type 2-for NAVOCEANO charts) for frequencies over I kFIz. Areas-Bravo and Golf

over the Ionian basin experience significant bottom reflection losses (Type 8,

NAVOCEANO charts). The loss per bounce for each incident-angle and frequency, as

produced by the BLUG model, canbe seen in Figure 31.

A number of paths were selected to simulate the most likely sea sediment

morphology in the region (Figs. 2-and 14). The first path represents shallow area Alpha

and is assumed to'be a flat bottom with a depth of 120 m. The second path is a com-

bined path which starts over the continental slope and- continues onto the shelf in such

way that one third is in area Bravo and the rest in Alpha. Path three is also-a combined

path which starts at a depth of 3000 m and-extends from area Bravo towards the shelf

with a sharp -rise. It is the only path were the depth at the source position exceeds 2300

m and for which- convergence zone propagation exists. Finally, path four is a sloping

bottom case which starts at a depth of 2000 m and decreases smoothl towards shore

(100 m) into area Golf.

3. Source, Receiver Data

Three depths were selected -for the source according to the M LD and bottom

depth along each path. For the shallow-paths (No I and No 2) these depths are 10 and

60 m. For the remaining paths 10 and 150 m were selected. The source frequencies are

defined to be 50, 250, 500 and 1000 1-1z. The receiver depths were also set to 10 and 60

mi. These definitions of source and receiver depth were made in-order to consider all the

available depth configurations with respect to MLD and DSC axis. A standard Figure

of Merit (FOM) of 80 dB was selected to produce relative results between seasons.

44

Sound -Speed (m/s)0' CO to # C0toC

0) ti It It V) U U) W, U)

In -to 0 ~- - In

25

50 V

75-

100-

125-

= S 10 13-e-B--Winter (Jan)'----Summer (Aug)-

175 ~Tall (Nov)

S200

225

250

275-

300-

325

.350 ~ I(lIIII511 lS1111111 ii,1111I

Figure 28. Sowid Speed Profiles for area Alpha: Summecr, fail and winter pro-files are shown for comparison.

45

Sound Speed (m/s)0~ C1 4 U t

In - to inr

250-

500-

750-

1000 Ga-e--eo Winter (Jan)Summer (Aug)

S1250

(1500

1750

2000

2250-

2500-

2750

30001

Fig-ure 29. 'Sound Speed Profiles for areas Bravo and Golf: Both summer andwincr profiles are shown for comparison.

-46

Sud Speed (ins

50

100-

150-

200-

~'-250S wo~eWinter (Jan)

- Summer (Aug)300- Fal (Nov)

~350-

400-

450

500-

550-

600 -1 1 I L i I I I I o l t I l - 1 I

Figure 30. Sound Speed Profiles in areas Bravo and Golf (Detail): The differ-

ences between the two seasons (winter-summer) seems to occur in the

upper 200 mn.

47

250-H2

-0 -[HZ

6, 4

00R

250H

so~~~s -to. 0 . 0 0 8

GGrazing Angle (Deg)

Areas ARVOanFGL

4.

C. ANALYSIS OF RESULTS

I. BLUG Output

The calculated attenuation for each selected frequency and grazing angle showsa critical angle of about 240 Tor the continental shelf(Fig. 31). This is in accordance withthe water -sediment sound speed ratio in the boundary (1.091). Rays less than thiscritical angle suffer a -loss less than 2 dB per bounce while those greater than this angleexperience a loss exceeding-6 dB. Similarly for the deep- area the critical angle- is about

60 which-is also-consistent with the 1.005 speed ratio as above.-Because no other-sourcesare available to--verify these sea sediment parameters, these data were used as input tothe two models. The BLUG uses standard values for this ratio, clc,, with no seasonalvariation. In shallow areas, where the seasonal variation- can penetrate the whole watercolum-n, this ratio changes and -some errors are expected as the -sound speed in the

sediment will -be less affected. For the most cases in this study the seasonal changes insound speeds in the water close to the bottom are -small:(Fig. 28) and errors due to theratio c./c, are negligible. This remark is only for the PE Model as the RAYMODE modelcalculates the ratio from the data given from the BLUG database directly (Medeiros,

19S5).

2. -Path No 1: PE Model and RAYMODE

This path, a flat shallow area, is used-in order to examine both-models simul-taneously -in a shallow waveguide case. (RAYMODE always uses a flat bottom). Theresults from-both RAYMODE and PE Model (Figures 32 to 34) in the winter show thelosses are in agreement within 3 dB for all the same combinations of receiver and sourcedepths. The big difference occurs in summer where the PE Model shows a sharp loss of10 dB,'kn and then a number of refracted ' reflected ra) s decreasing along the path (lessecho) (Figures 35 and 36). In contrast, RAYMODE shows a slowly increasing trans-

mission loss of about 1 dB, km (Figures 37 and 38). Also for the cases where the sourceand receiver are at the same depth, the RAYM ODE model predicts a decrease in the lossfrom 5 to 10 dB (stronger echo). The PE Model shows no difference for this case in thedirect path but the group of RBR rays is more discrete. By neglecting the bottom inter-

actions (fully absorbing), the PE Model calculated a loss similar to the previous runs forranges close to source (< 5 km), which is the direct path source-receiver (Fig. 39). Anumber of selected runs is included in Appendix A.

3. Path No 2: PE Model

In this relative shallow channel with anomalous morphology the PE Model forboth seasons shows a loss SO dB in less than 5 kin for the direct path at 50 1Iz. A number

49

* - .- -

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ot

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o W

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o N

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________ - aOoL

U= -

H:Zc.4 C, .....

WCLuaimLia-CL 1

U') (901 9901 N01SSIW9NYU81

Figure 32. Path 1, PE Model at 1000 Hz winter.

50

m

C3

C.c 0V S ...... ....... ........... .... ....... L

oC ( .i .....

UC=fl

...... ...... .... . ...... ....C:, ...... ~

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L0

C.,r U - . ..__._._._I

=)I-- -j 09 S9 OZ SL 09 so 06 so 001 SO(C)a: CC (80)- 9901 NO1I SSI9IflJI.8

Figure 33. Path 1, PE Model at 1000 Hz-winter same depth TX-RX.

51

(.0-I.

-Cto

w W

CD -LLJ WU'

-' (10r tnU

zLL.)

Iy 0 eLL >

U) U) .*

Cr) CD

>- =

I cIn

U- - Oj

I'MUELII

I 0

0U) CD C3 0 C) D

(90) 6901d0OI

Figure 34. Path 1, RAYMODE at 1000 Hz winter.

52

.1o

x InNh...to

N- ... ................ .. ........... ....... ...........

S .....

O . ~ ... ...... .... ............. ...................... C

Liz

0

C-)~

OL. LI

L............... ....... .......... ...... ....

->=~ LICD

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........ ......

LJcr

Cl Lj a_

d9.. 09 . oz S. .L S. '08 S' 06 S6 00! Solocrcr (90) ~ss0 NOISSSIW9U81

Figure 35. Path 1, PE MN-odel at 1000 Hz summer.

53

0

CU)

OLiw

o) =

8) U

C))

a- 0 E ....... .... . . ...... . .. ... ........

co - = ........ .0.. ...........

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0......

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=,-.i09 S9 0 S4 083 S9 06 56 001 Sol

(n==(90i SSOI NOISSIW.SNU81

Figure 36. Path 1, PE Mlodel at 1000 14Z summiier same depth TX-RX.

54

C) C3

010

LQ

-C

0

o :o " , .D

C 4 JU)

NN

,..o .o I

U

> ,0

190) SSOIdO8i

Figure 37. Path 1, RAYNIODE at 1000 Hz, summer and receiv'er at 60 m.

55

0

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Ln

0 (0

CD I S

U n1

CL~

0ti ~ in

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CDC*, U ...... ..... ....... .....

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(2CD) CD

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Figure 39. Path 1, PE M'vodel with fully absorbing bottom.

57

of bottom refractions occur at intervals of about 10 km. Increasing (manually) the losses-

into the-bottom, yields more attenuation of the-bottom refracted energy -and hence more-

loss in general. Also, -higher frequency gives lower propagation losses in the winter for

the direct path. In the summer the lkHz frequency wave-attenuates more than 105 dB;

at 2 km and then caustics occur from the bottom reflections which occur-at intervals-of

12 km. Selected runs for path 2 are also shown-in Appendix A.

4. Path No 3. PE Model

Results for path 3 are similar to those experienced for path 2. Increasing the

depth of both source and receiver increases the propagation loss. Bottom-bounce rays

can be-detected converging every 30 km at 50 Hz for both seasons as well as refracted

energy from the bottom. Increasing the frequency produces-lower propagation losses in

the winter. In the summer all -frequencies attenuate rapidly -and refraction- occurs every

12 ki. The higher the frequency the lower the refracted energy, due to the increased

attenuation with increasing frequency (Appendix A).

5. Path No 4. PE Model and RAYMODE

The results for the PE Model are identical to path 3 for the direct path (Ap-

pendix A) as the- SSP is the same- and the depth below the source is the same. The first

bottom return is now at 13 km (Fig. 40 to 42), about half of the path 3 range. The same

SSP and a flat bottom at 3000-r used to run the RAYMODE model. The output (Fig.

43 and 44) shows a similar high loss for the direct path as PE but after that the loss is

almost steady at 85 dB (bottom refracted) and at 38 km the convergence zone appears

with an expected gain of 6 to 9 dB depending on the frequency used.

5S

cnn

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a_Li

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Figure 42. Path 4, PE Model at I bLHz.

61

VI) (F)

0>>

0- 0 0)

u~ LJa u 6

C;D

C)C3C)

co -cco

0-

0-0

C3l

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Figure 44. Path 4, RAYMODE at 1 kHz summer.

63

V. CONCLUSIONS

A. OCEANOGRAPHY

The oceanographic parameters in the Ionian Sea affecting sound propagation in- the

sea were examined. The results show that the study area is characterized by a seasonal

thermocline of 300 m with small spatial and annual variations. In winter a -half

channelmtype SSP exists. In summer high- insolation prevents the creation- of a mixed

layer so a deep sound channel is-created:with an-axial depth of 150 m. In-areas where

bottom -depths permit, convergence zone -propagation is possible and detection- at -long

ranges can be achieved.

The sea sediment is highly -reflective in the coastal regions and-highly absorbing--in

the deeper regions. The bottom morphology and -the presence of land masses prevent

long range propagation- at all the frequencies examined.

B. UNDERWATER SOUND

The modeling of acoustic propagation results in the general observations for the

area studied. Between the two models examined, RAYMODE and PE, the- former is

adequate in presenting -the loss for areas where the bottom is smooth, flat and-deep with

respect to the acoustic wavelength. It will fail in coastal areas with anomalous

bathymetry, a situation common-to the study area-and to the Hellenic seas in general.

The PE model is better fitted-for coastal-areas because-it can treat a variable depth

morphology but -the implementation by the Split Step Fourrier transform will: create re-

strictions at sea sediment boundaries, especially in areas with sharp changes in sound

speed in, the- sediment.

Both the models- calculate propagation loss in a vertical only plane. In coastal areas

like the Ionian Sea this will not permit interactions from nearby land masses. This can

be solved -by three dimensional programs and by the implementation of better algo-

rithms.

The sea sediment models need improvement and a theoretical basis but the difficul-

-ties in collecting data from the bottom on a global basis will postpone the appearance

of such a-model-in the immediate future.

In-the study area propagation at frequr "'s between 50 and 1000 Hz is -dominated

by bottom reflection and refraction. Bo, seasons show relatively short direct path

ranges.

64

Due to the number of parameters that-can not-be defined or measured with accuracyas well as the techniques used by the models, the results of such-acoustic models must

be -treated qualitatively only.

65

APPENDIX A

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LIST OF REFERENCES

1. J.P. Bethoux, Budgets of the Mediterranean Sea. Their dependence on the local cli-mate and-on characteristics of the Atlantic waters, Oceanologica Acta, Volume 2-2,p. 157-163, 1979.

2. J.P. Bethoux, Mean water fluxes across sections in the Mediterranean Sea, evaluated)n thebasis of water and salt budgets and of observed salinities, Oceanologica Acta,Volume3-1, p.79-888, 1980.

3. M.A -Biot, Mechanics of deformation and acoustic propagation in porous media,J.Appl. Phys. 33, p.14 82 -14 98 , 1962.

4. H.L. Bryden, H.M. Stommel, Origin of the Mediterraneanoutflow, J. Mar. Res.,Supplement, p. 55-70, 1982.

5. J.Chen and I.J. Millero, Sound speed in seawater, J. Acoust.Soc.Am. 62,p. 1129- 1135, -1977.

6. A.Coppens, An introduction to the parabolic equation for acoustic propagationNaval Postgraduate Shool, 1982.

7. A. H. El-Gindy and S. H. Sharaf El-Din, Water masses and circulation patterns inthe deep layer of the Eastern Mediterranean, Oceanologica Acta, Volume-9-3, p.239-248, Jul-Sep 1986.

8. R. Frasseto, A study of the turbulent flow and character of the water masses-over theSicilian ridge in both summer and winter, p. 9-10, SACLANTCEN TM-93, 1965.

9. E.L. Hamilton, Prediction of in situ acoustics and elastic properties of marinesediments-, Geophysics 36, p.266-284, 197L

10. E.L Hamilton, Geoacoustic models of the seafloor. Physics of Sound in Marine

Sediments, p. 181-221, -Plenum, NY 1974.

11. F.D. Tappert and R.H. :Hardin, Applications of the Split Step Fourier method tothe numerical solution on nonlinear and variable coefficient wave equations, SIAMReview 15, p.423, 1973.

12. Hydrographic Service of the Hellenic Navy, Chart No 022 Ionian Sea - Northernpart, 1982.

13. Hydrographic Service of the Hellenic Navy, Chart No 030 South Ionian Sea,1988a.

14. Hydrographic Service of the-Hellenic Navy, Chart No 065 Ionian Sea, 1983.

15. Hydrographic Service of the Hellenic Navy, Pilot of the lellenic Seas, VolumeA, p. 5-9-and 20-78, Athens, 1979.

16. Hydrographic Service-of the Hellenic Navy, RESTRICTED Letter F:303/75,;88to Lt Rad. Fountoulakis, Subj. Station Data, 15 November 1988b.

17. Hydrographic Service of the Hellenic Navy. CONFIDENTIAL, Letter F:334,19,89to Lt Rad. Fountoulakis H.N., Subject: Thesis at NPS, 8 Aug 1989.

86

18. O.M. Johannessen, F. Strobel andC. Gehin Observation of an oceanic frontal- sys-tem-east of Malta-in May 1971, Technical Memorandum No 169, SACLANTCEN1971.

19. A.C. Kibblewhite, Attenuation of sound in marine sediments : A -review with em-phasis on ne;v low-frequency data, J.Acoust.Soc.Am., 86, p.7 16-738, 1989.

20. L.A. Kinsler, A.R. Frey, A.B. Coppens, J.V. Sanders Fundamentals of Acoustics,3rd edition, John Wiley & Sons, 1984.

21. H.Lacombe, P.Tchernia and G.Benoist Contribution of Atlantic water in theAegean Sea, seasonal and annually, p. 454-468, Bull Inf. COECX 10, 1958.

22. H. Lacombe, P. Tchernia, Quelques traits generaux de l'hydrologie Mediterranee,in the-Mediterranean: a natural Sedimentation Laboratory, edited by D.J. Stanley

and-others, Strodsburg, PA, 1960.

23. 1-. Lacombe, C. Richez, "The regimes of the- straits of -Gibraltar", inHydrodynamics of Semi- enclosed Seas, p. 13-74, Amsterdam, 1982.

24. D. Lee and J;S. Papadakis, A"umerical solutions of underwater acoustic wave prop-agation-problems , NUSC technical report 5929, 1979.

25. D. Lee; G. Botseas and J.S. Par~tdakis Finite- difference solution to the parabolicwave equation, J. Acoust.Soc. Am. 70, p.795-800, 1981.

26. P- Malanotte-Rizzoli,-A. Hecht, Large scale properties of the Eastern ,.Med: a re--view, Oceanologica Acta ,volume 1-1-4, p.323-335, Oct-Dec 1988.

27. P. Malanotte-Rizzoli, P. Bergamasco, Modeling- of the circulation of the EasternMed, Part I, submitted to Oceanol. Acta, 1988.

28. Maurv"Center for Ocean Science,llediterranean Environmental Acoustic Sumtmary,Long Range Acoustic Propagation Project, Report 104, -(CONFIDENTIAL),

1974.

29. R.C. Medeiros, RA Y.1ODE, Passive Propagation Loss Program, New-EnglandTechnical Services, p. 1 to -14, 15 July 1982.

30. R.C. Medeiros, A Simplified Overview of the RA-YM11ODE, New England TechnicalServices, p. 2-1S, [ November 1985.

31. L.V. Moskalenko, Steady-state wind- driven currems in the eastern half of tiheMediterranean, Oceanology, p. 494-496, 1974.

32. J. N. Nielsen, Hydrography of the Mediterranean and adjacent waters, Rep. Dan.Oceanogr. Expedition 1908-1910, 1912.

33. National Oceanographic Data Center, Letter to Rad. Fountoulakis, Subj. StationData, 18 August 89.

34. NUSC, Program Performance Specification for the 1985 baseline RA YAfODEcomputer program, NUSC, p. 1-I to 3-14 and 3-100 to 3-240, May 1987.

35. I.M. Ovchinnikov, The second (Mediterranean) trip of R; V Prof. BogorovOceanology Volume 18, p.165-168, 1978.

36. G.L. Pickard, W.J. Emery. Descriptive Physical Oceanography, 4th editionp.164-16S. Pergamon, 1982.

87

37..-E.M. Podcszwa, Sound- Speed Profiles for the Mediterranean, N1'USC,-p.l-14 and-29-42, 1980.

38. M.J. Pollack, The sources of deep water of tlze Easemn Jiediierraneaui, J. Mar.fees., Volume 10-1, p.128-152, 1951.

39. I.S. Robinson, Satellite Oceanography , p.240- 242, Halsted press, 1985.

40. XvV. Roether and-others, Transient tracer studies of the-thermn~cline circulation- ̂ theMfediterranean, MS for Mediterranean circulation,-NATO-Workshop, p.44, 1983.

41. R.lD Stoll, Theoretical aspects of sound transmission -in sediments, J.AcoustoSc.Am., 68, p.1341-l 1980.

42. -UNESCO Reports in Marine Science, POEM :an overview and research plan,Report No 30, p.16, 19S4.

43. UNESCO Reports -in -Marine Science, POEM,. a research programmne, Repor N-35, p.67, 1985.

44. UNESCO Reports in Marine Science, POEM, Initial -Resul.ts , Report No 44,_p.92, 1986.

4.R.J. Urick, Principles of Underwater Sound 3rd edition, McGraw-Hill, NY 1983.

46. U.S. Flydrographic Office, A4S IV prediction areas , Naval Warfare Planning ChartBase (NWPCB), NAS p.2401, 1972.

47. G. Wust, On the vertical circulation of the Mediterranean Sea, J.Geophys. Res,-p.3261-3271, 1961.

4S. ', Telephone conversations between D.W Collins, Occanogr. Services Branch,NODC and the author, of 8'1_3',2 and 3/1'1189.

883

INITIAL DISTRIBUTION LIST

'No. Copies

1. Defense Technical Information Center 2Cameron StationAlexandria, VA 22304-6145

2. Library, Code 52 2Naval Postgraduate SchoolMonterey, CA 93943-5002

3. Professor Robert H. Bourke INaval Postgraduate School, Code OCBfMonterey, CA 93943

4. Professor Alan B. Coppens 2Naval Postgraduate School, Code PICzMonterey, CA 93943

5. Embassy of Greece 2Naval Attache222S Massachusetts Av., NAV.Washington D.C. 20008

6. Lieutenant R. Fountoulakis 2Grigoriou Lampraki 1518120 Kordalos, AthensGreece

89