a hydrographic and acoustic survey of the persian gulf

A HYDROGRAPHIC AND ACOUSTIC SURVEYOF THE PERSIAN GULF - PART I

Jay Lee Wri ght

DUDLEY KNOX LIBRMW

NAVAL POSTGRADUATE SCHOOL

MONTSRSY, CALIFORNIA BB949

NAVAL POSTGRAD Oil

Monterey, California

THA HYDROGRAPHIC AND ACOUS TIC SURVEY

OF THE PERSIAN GULF - PART I

by.

Jay Lee Wright

!

September 1974

Th ssis Advisor: R. H. Bourke |

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A Hydrographic and Acoustic Survey of the

Persian Gulf - Part I

7. AUTHORfa;

Jay Lee Wright

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20. ABSTRACT (Continue on reveree eide It neceeemry and Identity by block number)

A survey of literature and historical data is utilized to investigate the

seasonal variations in the hydrographic and acoustic properties of the

Persian Gulf.

The Gulf has a year round salinity of about 40 o/oo. The surface temp-o _ o .

erature varies from 30 C in summer to 20 C in winter. An area of

significant importance in the Gulf is near the Strait of Hormuz where the

Persian Gulf water encounters the warmer less saline water of the

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20. (continued)

Arabian Sea.

Utilizing the FACT acoustic transmission loss model detection rangesfor diesel and nuclear submarines are investigated. Generally, rangesappear to be greater in winter due to increased vertical mixing, creat-ing strong positive sound speed gradients.

DD Form 1473. 1 Jan 73

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SECURITY CLASSIFICATION OF THIS PAOECWhmr. Dml* Enfrtd)

A Hydrographic and Acoustic Survey.

of the Persian Gulf - Part I

by

Jay Lee WrightLieutenant, United States Navy

B.S., United States Naval Academy, 1968

Submitted in partial fulfillment of the

requirements for the degree of

MASTER OF SCIENCE IN OCEANOGRAPHY

from the

NAVAL POSTGRADUATE SCHOOLSeptember 1974

S394Q

ABSTRACT

A survey of literature and historical data is utilized to investigate

the seasonal variations in the hydrographic and acoustic properties of

the Persian Gulf.

The Gulf has a year round salinity of about 40 o/oo. The surface

temperature varies from 30 C in summer to 20 C in winter. An area

of significant importance in the Gulf is near the Strait of Hormuz where

the Persian Gulf water encounters the warmer less saline water of the

Arabian Sea.

Utilizing the FACT acoustic transmission loss model, detection

ranges for dies el and nuclear submarines are investigated. Generally,

ranges appear to be greater in winter due to increased vertical mixing,

creating strong positive sound speed gradients.

TABLE OF CONTENTS

NOTE: Portions of this thesis are contained in Part II.

I. INTRODUCTION 11

II. LITERATURE SURVEY 12

A. AREA DESCRIPTION 12

1. Political 12

2. Geography 13

3. Climatology Part II

4. Hydrology 15

B. RELEVANT OCEANOGRAPHIC CHARACTERISTICS--- 15

1. Physiography Part II

2. Bottom Sediments Part II

3. Currents Part II

4. Sea and Swell Part II

5. Temperature, Salinity, Density and Circulation 15

6. Sound Speed Part II

7. Biologies Part II

III. HYDROGRAPHIC INVESTIGATION 18

A. SOURCE OF DATA 18

B. PREPARATION OF DATA 18

C. APPLICATION OF DATA 19

D. ANALYSIS OF "WINTER CONDITIONS - 24

E. ANALYSIS OF SUMMER CONDITIONS 30

F. SUMMARY OF HYDROGRAPHIC INVESTIGATION 31

IV. SOUND PROPAGATION INVESTIGATION 36

A. PROPAGATION LOSS ANALYSIS 36

B. SUMMARY OF SOUND PROPAGATION 41

v 1. Passive Case 45

2. Active Case 46

V. CONCLUSIONS 47

APPENDIX A NODC Tape Data Transfer Program 49

APPENDIX B Propagation Loss Profiles 50

APPENDIX C Figure of Merit Computations 70

APPENDIX D Sound Speed Profiles 72

LIST OF REFERENCES 86

INITIAL DISTRIBUTION LIST FOR PART I 87

LIST OF TABLES

Table Page No.

1 FACT model input parameters 37

2 Figures of Merit for summer and winterfor the passive and active cases 42

3 Passive detection ranges (nm) for

summer and winter 43

4 Active detection ranges (nm) for

summer and winter 44

LIST OF FIGURES

Figure Page No.

1 Persian Gulf with bordering countries andimportant cities 14

2 Diagramatic representation of watercirculation in the Persian Gulf 17

3 Distribution of all known hydrographic data by-

month for each 1 square 20

4 Confidence level based on the number of

observations for the months of February andJuly 21

5 February transect and data points for

1 squares 22

6 July transect and data points for1° squares 23

7 Vertical cross section of winter temperature(°C) along the transect 25

8 Vertical cross section of winter salinity (o/oo)

along the transect 27

9 Nested winter TrS profiles along the

transect 28

10 Vertical cross section of summer temperature(°C) along the transect 29

H Vertical cross section of summer salinity (o/oo)

along the transect 32

12 Nested summer T-S profile along the

transect 33

13 Modification of Sugden's 1963 diagramaticrepresentation of water circulation in the

Persian Gulf 35

14 Winter areas of acoustic similarity 39

15 Summer areas of acoustic similarity 40

ACKNOWLEDGMENTS

Many people have been helpful in the research and composition of

this paper and I thank them all. In particular I wish to thank Assistant

Professor R. H. Bourke and Lieutenant Commander D. C. Honhart, USN,

for without their guidance this project could not have been completed.

Finally, I wish to thank the person who has provided constant encourage-

ment throughout this undertaking, my wife, Penny.

10

I. INTRODUCTION

It has been proposed that in 1975 the Naval Underwater System

Center with the assistance of the Naval Postgraduate School, conduct

a hydrographic and acoustic survey of the Persian Gulf. The purpose

of this cruise will be to prepare a report for the Shah of Iran on sound

propagation conditions within the Gulf applicable to specific sound sur-

veillance systems.

This thesis is submitted as a pre-cruise report to aid NUSC in

planning the expedition. The objectives of the thesis are: 1) to provide

a survey of the literature, consolidating relevant material into a single

reference; 2.) to report on the hydrographic structure of the Gulf, using

historical data; and 3) to report on sound propagation conditions in the

Gulf, based on the hydrographic structure. Hopefully, realization of

these objectives will enable NUSC to determine areas in the Gulf where

study should be concentrated.

11

II. LITERATURE SURVEY

A. AREA DESCRIPTION

1. Political

Only in recent years has the importance of the Persian Gulf

region come to the attention of the general public, this resulting

mainly from the highly journalized oil export practices of the Gulf

countries. The United States Government in recognizing the strategic

importance of the Gulf region as a source of petroleum has been vitally

interested in maintaining a friendly relationship with the one politically

neutral country bordering the Gulf, Iran.

Iran has become immensely wealthy through its oil industry,

and under the strong leadership of Shah Mohammed Reza Pehlavi, it

is emerging rapidly as the key to stability in this area of the world.

Because Iran maintains friendly relations with both the United States

and the Soviet Union, it enjoys the technical assistance and material

support of both nations. In particular the United States is providing

current weapons platforms, such as the F-14 and P-3 aircraft, and

the Spruance class destroyer. Furthermore, the proposed NUSC cruise

will be undertaken at the request of the Shah, who has recognized that

in order for Iran to assert itself as the "peace keeper" of the Persian

Gulf, it must have a thorough understanding of the ocean environment

to make optimum use of new weapons platforms.

12

2. Geographical

The Persian Gulf is a shallow basin 500 miles (311 km) long

by 200 miles (124 km) wide which separates the Arabian Plateau from

Iran. Its deepest channel, seldom deeper than 50 fathoms (91 m) lies

close to the Iranian shore. Along the coastal areas of the Gulf there is

scant vegetation, a result of a meager annual rainfall of only 3 to 1

1

inches (7. 6 to 28. cm) most of which falls during the winter months.

o o o oTemperatures averaging 90 F (33 C) and exceeding 120 F (48 C) in

some locations are common during the summer months. During the

o owinter season temperatures are cooler averaging 70 F (21 C), with

o onighttime lows of 40 F (5 C) in the western part of the Gulf.

Viewed from Iran, the Persian Gulf appears as a remote region,

kept inaccessible by the vast arc of the Zargos Mountains (Figure 1).

Only in the northern part of the Gulf, where Iran's oil-rich Khuzestan

plain merges with the Shatt-al-Arab River to form a delta is the Gulf

easily accessible. The 120 mile-long (75 km) Shatt-al-Arab River is

formed by the confluence of the Tigris, Euphrates and Karun Rivers.

It provides a waterway to the main port of Iraq at Basra. The Karun

River, which joins the Shatt-al-Arab downstream from Basra provides

access to the major Iranian ports of Khoiramshahr and Abadan. On

the coast, northeast of the mouth of the Shatt-al-Arab, lie the Iranian

sea ports, Mashur and Shapur. To the south lies the brief, open coast

of Iraq.

13

SAUDI

ARABIA

Figure 1. Persian Gulf with bordering countries

and important cities (modified from Emery, 1956)

14

The western side of the Gulf is 1300 miles in length from

the Shatt-al-Arab to Oman on the Musandam Peninsula. The coast line

is ill defined in this region and navigation is hazardous due to the

presence of numerous shoals, reefs, and islands.

4. Hydrology

Although the Persian Gulf acts as the drainage center for most

of Arabia, all of Iraq, parts of Syria, Turkey and Iran, little fresh

water flows into the Gulf except at the northern end via the Shatt-al-

Arab River [Sugden, 1963]. This inflow amounts to about 45 cubic kilo-

meters per year, most of which occurs during the flooding season

(January-March). Flow rates during the flood season are affected by

the yearly variation of rainfall. For example, in 1929 the maximum

3flood discharge rate of the Euphrates River was 4, 700 m /sec, while

3in 1930 the rate was only 650 m /sec.

Occasionally there is an additional fresh water input along the

coast of Iran as a result of flood discharge during the winter [Sugden,

1963].

B. RELEVANT OCEANOGRAPHIC CHARACTERISTICS

5. Temperature, Salinity, Density and Circulation

During the summer Emery [1956] found a general increase in

o osea surface temperatures from 75 F (24 C) in the Arabian Sea to more

o othan 92 F (33 C) in the Persian Gulf. Temperatures of the winter are

o ofar different from those of summer, with values of only 60 F (16 C) at

15

o °the head of the Gulf, increasing to about 75 F (24 C) in the Arabian

Sea. Thus the water at the head of the Gulf undergoes an annual change

of at least 30°F (17°C) [Emery, 1956].

The summer surface salinity increases from about 36. 5 o/oo in

the Gulf of Oman to over 42 o/oo in the Persian Gulf. Winter salinities in

the Gulf of Oman differ little from their summer time value. The Persian

Gulf, on the other hand, is diluted in winter by the increased flow of the

Shatt-al-Arab River and therefore has a salinity of 40 o/oo or less

[Emery, 1956].

The Gulf is similar to a land-locked sea in which evaporation

exceeds precipitation [Sverdrup, Johnson and Fleming, 1942], The

water loss due to evaporation is made up by the inflow of water from

the open ocean through the Strait of Hormuz. This water moves on the

surface toward the Gulf's coastal margins gradually increasing in

density. Eventually the water sinks to lower levels where it flows out

of the Gulf below the incoming water. Figure 2 is a schematic of this

density driven circulation [Sugden, 1963]. Summer in the Gulf is sub-

stantially hotter than the winter, but the differences between summer

and winter surface salinities are not great; hence, evaporation evidently

continues at a high rate throughout the year [Sugden, 1963]. The

control of density by salinity is, of course, modified by temperature

changes which vary in effect according to the seasons. However,

salinity, as opposed to temperature, is a much more important

determinant of density than in the open ocean [Sugden, 1963],

16

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Figure 2. Diagramatic representation of water

circulation in the Persian Gulf (Sugden, 1963)

17

III. HYDROGRAPHIC INVESTIGATION

A. SOURCE OF DATA

The National Oceanographic Data Center, NODC, provided all

known temperature and salinity observations for the Persian Gulf,

which consisted of bathythermograph and hydrographic observations.

Only hydrographic data were used in this thesis. Specifically, only

values of temperature, salinity and sound speed, as calculated by

Wilson's equations, were utilized. The data were converted to printed

output and punched cards through the use of an existing FORTRAN

program utilizing the Naval Postgraduate School IBM 360 Computer

(Appendix A).

B. PREPARATION OF DATA

In order to make optimum use of the data the Gulf was divided

o ointo 1 (one degree) squares, with 60 E as the eastern boundary, thus

including the Strait of Hormuz and the Gulf of Oman in this study. Fur-

ther, since a seasonal description of the oceanographic character of the

Gulf was desired, two months from each of the primary seasons, winter

and summer, were chosen for investigation. The months of January

and February were chosen as characteristic of winter conditions while

July and August were chosen for summer. After sorting the data, as

described above, it became obvious that there is a paucity of data from

18

the Persian Gulf area. Figure 3 shows a plot of the monthly distribution

for each one degree square. The number of observations in the squares

vary from 100 to 0. The data were concentrated to some degree within

the months of February and July; therefore, the other months were

eliminated from further consideration. Figure 4 entitled, "Confidence

Level" assigns an arbitrary value of good, fair or poor according to

how many observations are in each square for February or July.

The next step in the data preparation was to perform a statistical

analysis of the data for February and July. The monthly mean temp-

erature, salinity, and sound speed were determined at standard depths

for each 1 square. As the data were sparse for most 1 squares, the

mean was the only meaningful statistic obtained. The resulting values

of mean temperature, salinity and speed sound were used to investigate

the hydrography of the Gulf.

C. APPLICATION OF DATAo

Rather than describe the hydrographic character of each 1 square

within the Persian Gulf, the analysis has been concentrated on a transect

located in the center and deeper portions of the Gulf which are considered

more representative of areas in which ASW operations might be conducted.

Figures 5 and 6 show the transect and the location of the associated data

points for February and July. It is important to note that the positions

of the data points in these figures are arbitrary because the values they

represent are based on data taken throughout each 1 square. Vertical

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oplots of mean temperature, salinity and sound speed for each of the 1

squares along the transect were then constructed. In order to establish

the continuity of this technique verticle cross sections of temperature

and salinity and a nested temperature-salinity plots were constructed.

D. ANALYSIS OF WINTER CONDITIONS

During winter the Gulf is affected by the influx of water from the Shatt-

al-Arab River at the northwestern end of the Gulf (Figure 7). The cooling

effect of the river is seen as far south as point 8, where the 20 C water

ounderlies the 21 C water. This finding agrees with Sugden [1963], as

he states that the river discharge can influence the temperature of the

Gulf water at least as far south as 28 N, 50 E or near point 4.

The temperature generally decreases with depth until point 9-

oBetween points 9 and 12 the temperature increases from about 22 C at

the surface to 23 C at depth. The reason for this increase with depth

is unclear. Emery [1956] found that in this area, between points 9 and

12, there is mixing between the warmer Arabian Sea water entering the

Gulf, and the cooler Persian Gulf water, exiting the Gulf. However, the

strong halocline which exists during winter in this area would prevent

mixing below about 30 meters (Figure 13). As points 10 and 11 are

based on only two observations taken by the same vessel during February

oof 1961, the 23 C temperature could be an anomalous condition or erro-

oneous data. Since there are no other sources of water as warm as 23 C

in the Gulf, the data are probably in error.

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The temperature between points 13 and 18 decreases from 22 C at

the surface to 13 C at 500 meters. A warm nose of 22 C water can be

seen at point 13, where the Persian Gulf water spills over the sill into

the Gulf of Oman.

Figure 8 shows that the surface salinity decreases from 41 o/oo at

the head of the Persian Gulf to 37 o/oo in the Gulf of Oman. This is

contrary to Emery [1956]; however, it is in agreement with Schott's

data of 1918. Both of these reports were based on single cruises

indicating possible year-to-year variability. Salinity generally increases

with depth at all locations until point 8. At points 8 and 9 the water

column is nearly isohaline at 39 0/00. This is indicative of a well mixed

area, as is expected, since this is the region in which Arabian Sea water

and Persian Gulf water encounter each other and mix.

The salinity increases with depth between points 10 and 13 from

37 0/00 at the surface to 39 0/00 at depth. This confirms the inflow of

the lower salinity, Arabian Sea water at the surface and the outflow of

the more saline, Persian Gulf water at the bottom. Further, at point

13 the Persian Gulf water can be followed over the sill in the Straits of

Hormuz and down the slope into the Gulf of Oman. There it reaches an

equilibrium depth between 200 and 300 meters, thus confirming what

Emery [1956] found.

The winter picture is further clarified by examining the nested T-S

diagram shown in Figure 9. At data points 1 through 7 the water column

is approximately homogeneous with temperatures between 15 C and 20 C,

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and salinities near 40o/oo to 41o/oo. The water column is also stable

at these points as density increases with depth. The characteristic at

points 8 and 9 is the instability of the water column at the surface

changing to neutral stability at 50 meters. This further substantiates

the idea that Arabian Sea water and Persian Gulf water mix in this region,

however, only in the first 30 meters. Points 12 through 18 show the nose

of the warmer, less saline Arabian Sea as it pushes into the Persian Gulf

between the surface and about 200 m.

E. ANALYSIS OF SUMMER CONDITIONS

During summer the flow of the Shatt-al-Arab River is less than in

the winter and the Gulf is relatively unaffected by the influx of water.

The temperature is stratified throughout the Gulf, always decreasing

with depth. There are several abnormally hot areas present, one of

32 C located at point 4 and a 30-32 C area located between points 13

and 16 (See Figure 10). These areas are similar to those found by

Emery [1956]. The mean temperature for point 4 is based on 15 obser-

vations. However, the reason this point is relatively hot is unclear.

The possibility that a majority of the observations were made in the

onearshore portion of the 1 square was investigated and found to be

invalid. Also a search of any reports of local heating phenomenon was

conducted with no success.

The hot area in the vicinity of points 13 through 16 is in the region

where Persian Gulf water and Arabian Sea water meet. During the

summer months there appears to be little vertical mixing, and hence

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strong stratification occurs. A thin quiescent surface layer is formed

which is heated considerably by insolation.

The salinity in summer is well stratified throughout the Gulf. A

low salinity region of less than 39o/oo is associated with the intrusion of

the Shatt-al-Arab River at the extreme northwest end of the Gulf as

depicted in Figure 11. As in winter, in the vicinity of the Strait of

Hormuz, points 12 through 15, the 37-38 o/oo salinity nose of Persian

Gulf water is observed to descend to about 300mdepth as it passes over

the entrance sill of the Gulf.

The nested T-S diagram in Figure 12 reveals the cooling effect at

the surface of the Shatt-al-Arab River at point 1. Looking at points

2 through 11 the Gulf is seen to be well stratified. Points 12 through

14 show the intrusion of less saline Arabian Sea water between the

surface and 30m in the Gulf. Points 15 and 16 show that the Arabian

Sea water in the northern part of the Gulf of Oman is contained in a

layer from the surface to approximately 200m.

F. SUMMARY OF HYDROGRAPHIC INVESTIGATION

In summary, the water temperature throughout the Gulf is almost

o10 C warmer during summer than in winter. The salinity behaves

differently, as it remains fairly constant throughout the year. The

only exception occurs during the winter months when the discharge of

fresh water from the Shatt-al-Arab River reduces the salinity in the

northern portion of the Gulf.

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During both seasons there exists a region in the vicinity of the

Straits of Hormuz, where the Persian Gulf water and Arabian Sea water

meet causing an area of horizontal stratification. It is in this region

where the most irregular temperature and salinity structures occur.

The basic circulation of the Persian Gulf appears to remain the same

year round; that is, there is an outflow of the Persian Gulf water at

the bottom, through the Straits of Hormuz and an inflow of Arabian Sea

water at the surface. In Figure 7 the basic circulation was shown as

described by Sugden [1963]. The analysis of the mean values of temp-

erature and salinity would suggest a slightly different circulation pattern.

Figure 13 shows this alternative. As stated by Emery [1956], in the

Strait of Hormuz there is an area of convergence where Persian Gulf

water and Arabian Sea water come together. It appears from this

analysis, based on mean temperature and salinity profiles, that Arabian

Sea water does not penetrate into the Gulf very far beyond this point.

Without question the winds play a very important role in the circula-

tion of the Gulf. Their actual influence has not been investigated; how-

ever, as surface winds are generally northwesterly throughout the year,

the concept of a southerly surface flow in the Gulf is most logical.

34

PersianGulf

Strait of Gulf of

Hormuz Oman

Figure 13. Modification of Sugden's 1963

diagramatic representation of water circulation

in the Persian Gulf

35

IV. SOUND PROPAGATION INVESTIGATION

A. PROPAGATION LOSS ANALYSIS

The Fast Asymptotic Coherent Transmission model (FACT), a low

frequency ray-acoustic model, was utilized to analyze acoustic propaga-

tion conditions in the Persian Gulf. This particular FORTRAN library

program has been adapted to provide both graphical and numerical out-

put of propagation loss versus range, as a function of a variety of input

parameters shown in Table 1. Note that the bottom loss index, scale

1-5, is based on bottom composition, roughness, and slope. Basically,

the Persian Gulf has a flat mud-sand bottom for which an intermediate

loss index of 3 is assigned. In the Gulf of Oman a slope is present:

however, the bottom is almost entirely sand, so the value of 3 is

assumed to be valid.

Although salinity is high in the Persian Gulf (40o/oo), it does not

appear explicitly in the expression for absorption, but it is recognized

that high values of salinity would increase the effect of the MgSO

relaxation process. Preliminary computations indicated that absorption

coefficients would increase approximately 0. 05% from the standard ex-

pression when considering a body of water with an average salinity of

40 o/oo. Therefore, this additional absorption factor was ignored.

After running the FACT program for the six frequencies at each

point along the transect, it became obvious that an unwieldy number of

36

TABLE 1

FACT model input parameters

PARAMETER VALUE HOW DETERMINED

1.

3.

4.

Layer Depth(ft)

Bottom Lossindex

Sea State

Varied

Frequencies(Hz)

Source/Receiver DepthCombinations(ft)

February 2

July 3

300, 500, 1000, 35005000, 8000

60/60300/30060/300

Determined fromtemperature profile

Based on scale of

1 to 5 (1 low, 5 high)

Based on scale of

2, 3, 4. Winds light in

winter; shamal windspresent in summer

Arbitrary.

These frequencies

cover most passive

and active detection

systems

ArbitraryThese combinationsprovide for source/re-

ceiver in layer, belowlayer, and mixed

37

graphical representations would be required to describe the results.

Thus, methods of reducing the amount of data without loss of continuity

were investigated. Utilizing the nested T-S plots and FACT generated

propagation loss curves, it was found that large areas of the Gulf along

the transect were acoustically similar. Hence the transects were

portioned into areas according to their hydrographic and acoustic

similarities. These acoustic areas are shown in Figures 14 and 15 with

the 10 fathom (18m) bottom contour defining the approximate limits for

a submerged submarine. A further reduction of data was made in that

the 500Hz and 5000Hz propagation loss curves could be eliminated from

the investigation, because they closely matched those of 300Hz and

3500Hz, respectively.

The resulting propagation loss curves for summer and winter

conditions are shown in Appendix B.

In order to use the propagation loss profiles to determine the

seasonal and geographic variability of detection ranges, it was neces-

sary to generate Figures of Merit (FOM) for passive and active detection

systems. It was assumed that passive detection would be made by an

air dropped sonobuoy and active detection would be made by a hull

mounted sonar. For the passive analysis FOM's were computed for

300Hz and 1000Hz signals, for both a snorkeling diesel and a nuclear

submarine. The FOM's for the hull mounted sonars were based on

38

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39

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aaninvi

40

transmission frequencies of 3500Hz and 8000Hz. The resulting FOM's

are shown in Table 2. The sonar equations and associated calculations

are located in Appendix C.

Tables 3 and 4 show the passive and active detection ranges for the

winter and summer seasons, respectively.

B. SUMMARY OF SOUND PROPAGATION

Several important points can be gleaned from the propagation loss

profiles in general. The most significant point is to question what phys-

ical processes or mechanisms permit detection ranges of 30 nm, seen in

Tables 3 and 4, to exist in a "hot bath tub" like the Persian Gulf. The

propagation loss profiles in Appendix B show that there is little loss of

acoustic energy in Areas 11. IV, V, VI, VII in winter. The loss that

does occur falls in the range between cylindrical and spherical spreading

favoring the former. In these areas the positive sound speed gradients

present cause the Gulf to act as a waveguide with minimal surface

scattering and bottom loss. The propagation loss in areas I, III, VIII

in winter and in areas I, II, III, IV, V in summer is greater because

there is no capability for channeling of sound energy. Losses greatly

2 3exceed spherical spreading (1/r ) often approaching a 1/r loss rate.

Again, this is a function of the sound speed profile, which in these

areas has a negative gradient (Appendix D). Comparing the two situations

it is apparent that the shape of the sound speed profile, and not the high

41

TABLE 2

Figures of Meritfor summer and winter for the passive and active cases

Passive FOM's (db re 1 u bar) for summer and winter

Nuclear Submarine

FREQ FOM300 Hz 82 db

1000 Hz 92 db

Snorkeling Diesel Submarine

FREQ FOM300Hz 85 db1000Hz 95 db

Active FOM's (db re 1 u bar) for summer and winter

FREQ FOM3500Hz 195 db8000Hz 180db

42

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water temperature, appears to be the controlling factor of sound propaga-

tion in the Persian Gulf.

Another feature of the propagation loss profiles is its "spikey"

appearance. This is a result of surface-image interference, and the

spikes represent areas of higher energy in which direct path or refracted

rays are reinforced by surface reflected rays. The position of the spikes

on the propagation loss profile does not guarantee their presence in a

particular location. This is because the FACT model calculations are

based on a flat bottom, thus, any bottom irregularities or bottom slope

would cause the spacing between spikes and their relative intensities

to vary from that predicted.

As the sound propagation investigation is based on a mean analysis,

the effects of perturbations caused by a shamal wind are not considered.

However, it can be assumed that a shamal wind would reduce detection

ranges by increasing surface scattering and surface reverberation.

1. Passive Case

Generally, the longest detection ranges can be expected to

occur during the winter season. This was anticipated because of the

greater evaporation rate in the winter, causing convective mixing which

breaks down the stratification of summer and creates positive or iso-

thermal sound speed gradients. On the other hand, shorter ranges are

observed during the summer season due to the negative sound speed

gradients resulting from stratification of the Gulf waters. Appendix D

contains the sound speed profiles for both seasons.

45

Geographically, there is little pattern to the passive detection

ranges. An exception occurs during the winter months when throughout

the entire Gulf, excluding the northern end which is affected by the

Shatt-al-Arab River, approximately the same detection ranges exist

for both frequencies investigated.

2. Active Case

With regard to active detection ranges, the seasonal trend is

similar to that of the passive case, note Table 4. Also, as expected,

the detection ranges are less for the 8000Hz signal due to the increased

attenuation associated with higher frequencies. There are no trends

in the geographical locations of the sampled areas.

The extremely long active detection ranges found in Table 4

are questionable. As mentioned previously, the volume reverberation

in the Persian Gulf is essentially an unknown quantity. Without know-

ledge of this factor the Figures of Merit may be excessive and thus

detection ranges overly optimistic.

46

V. CONCLUSIONS

The Persian Gulf has received little attention in oceanographic

literature, and what scant information is available is derived from geo-

physical and oil exploration cruises where ocean acoustics has played

a minor role. Although, these surveys are extremely valuable tools to

the descriptive oceanographer, the approach to these surveys has been

neither coordinated nor systematic. Thus, a description of the seasonal

and geographic variation of oceanographic parameters is difficult in

certain areas of the Gulf.

The Gulf can be characterized as being well mixed during the winter

due to convective mixing caused by a high evaporation rate. In the

summer the Gulf is stratified and what mixing occurs is caused by wind.

The most interesting region in the Gulf area is in the vicinity of the

Strait of Hormuz where Persian Gulf water encounters Arabian Sea

water.

Acoustically the Gulf has long detection ranges during the winter

resulting from isovelocity and positive sound speed profiles. During

the summer the sound speed profiles have negative gradients, thus

detection ranges are reduced. Although in comparison with the major

world oceans, the Persian Gulf is unusually warm and saline, the

controlling factor in sound propagation appears to be the shape of the

sound speed profile.

47

The Persian Gulf has become strategically important to major

powers of the world. Therefore, it follows that the oceanographic

characteristics of the Gulf, and their influence on military operations,

will be studied in depth. A Naval Underwater Sys terns Center cruise

in 1975 would be just a beginning.

48

APPENDIX A

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49

APPENDIX B

Propagation loss profiles for winter and summer. Each profile is for

a particular area of acoustic similarity. (Refer to Figs. 19 and 20).

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APPENDIX C

FIGURE OF MERIT

Passive February and July

FOM = SL - NL - RD + DI = TL

Source Level

SL - 52 db. for a typical nuclear submarine

SL = 55 db for a typical snorkeling diesel submarine

Noise Level

Considering air dropped sonobuoys, the noise level is primarily a

function of ambient noise. The Wenz Curves provide an approximate

ambient noise level. As the Gulf is an area of heavy shipping and has

an average sea state of 2, the following values were obtained

NL = - 30 db for 300Hz

NL = - 40 db for 1000Hz

Recognition Differential

RD = for 50% probability of detection

Directivity Index

DI = for omnidirectional passive sonobuoy

FOM = 82 db for 300Hz, nuclear sub.

FOM = 85 db for 300Hz, snorkeling diesel sub

FOM = 92 db for 1000Hz, nuclear sub

FOM = 95 db for 1000Hz, snorkeling diesel sub

70

Active February and July

.FOM = SL _ NL + TS + DI - RD = 2 TL

Source Level

SL. = 145 for 3500Hz of typical sonar

1 = 130 for 8000Hz of typical sonar

Noise Level

The NL for a destroyer operating at 16 kts is approximately -35 db

(re 1 ju bar) at 1 yard.

NOTE: NL is difficult term to determine in the active equation as it

is composed of three terms; self noise, ambient noise, and reverberation

level. The main difficulty lies in the reverberation term as knowledge

is lacking on reverberation levels expected in the Persian Gulf. As a

result, the FOM may be too large and the resulting detection ranges

overly optimistic.

Target Strength

TS = 10 db for a bow or stern aspect

Directivity Index

DI - 25 db for a typical hull mounted sonar

Recognition Differential

RD = db for 50% probability of detection

FOM = 195 db for 3500Hz

FOM = 180 db for 8000Hz

71

APPENDIX D

Sound speed profiles for winter and summer . Each profile U fo

particular area of acoustic similari ly . (Refer to Figs . „ and ,

r a

72

V10

*

20

30

DEPTH

(ft)

o

«

*

75 --

%

100 — i i.

5260 5280 5300 5320

SOUND SPEED (ft/sec)

5340

Figure D- 1 . Sound speed profile, winter, area

73

5280 5300 5320 5340SOUND SPEED (ft/sec)

5360 53S0

Figure D- 2 . Sound speed profile, winter, area li

74

10 /

?0/

30J

«50 J-~>H

75

1005260 5300 5320 5340

SOUND SPEED (ft/sec)5360 5380

Figure D- 3 . Sound speed profile, winter, arealli

75

.

10'1

?0

30

/~\ \*J \«H \V-*

\W 50 \Ha.w«

75

100 . —5280 5300 5320 5340

SOUND SPEED (ft/sec)5360 53S0

Figure D-4 . Sound speed profile, winter, area iv

76


5360 5380

Figure D- 5 . Sound speed profile, winter, area

v

77

10

20

30

W50 1

H

75

1005:220 5240 52 60 5 280 5 300 53:


Figure D- 6 . Sound speed profile, winter, area VI

78

o

10 \

•.

70 \

30

4J

W50 /•

H

//0,

a,w 200 sQ

400

600

/

75

100

5 50C 5 00 5200 5300

5 280 5300 5:120 5; 40 53 60 53


Figure D- 7 . Sound speed profile, winter, area vn

79

o

10

20 /

10 /

/

K50

|

H

\

o 1

feu 200

400

600

>Q

/

75

100

4900 2000 3100 5 200

53s:>60 5:280 5 300 £320 53 40SOUND SPEED (ft/sec)

Figure D- 8 . Sound speed profile, winter, areavu

80

5400 5450SOUND SPEED (ft/sec)

55005300 5350

Figure D- 9 . Sound speed profile, summer, area l

5550

81

10

70

III | II 1 HI

•

30

+J

<4-l

K50HCMWO

75

1005300 5350 5400 5450 5500 5550


Figure D-10 . Sound speed profile, summer, areall

82


5500 5550

Figure D— 11. Sound speed profile, summer, area

83

5300 5350 5400 5450 5500 5550


Figure D-12. Sound speed profile, summer, area iv

84

100f_5300 5350 5400 5450 5500

SOUND SPEED (ft/sec)5550

Figure D-13. Sound speed profile, summer, area

v

85

LIST OF REFERENCES

1. Crozait, V. J. , 1973. Stability in the Persian Gulf. U. S. NavalInstitute Proceedings, vol. 99 (7/845):48-59.

2. Dubach, Harold W. , 1964. A summary of temperature - salinity

characteristics of the Persian Gulf. NODC Pub. G-4 Washington,Naval Oceanographic Office. 233 p.

3. Emery, K. O. , 1956. Sediments and water of the Persian Gulf.

Bulletin of the American Association of Petroleum Geologists,

vol: 40 (10):2354-2383.

4. Honhart, D. C., 1974. Acoustic forecasting notes. U. S. NavalPostgraduate School, Monterey. 163 p. Unpublished.

5. La Violette, Paul E. and Theodore R. Frontenac, 1967.

Temperature, salinity, and density of the world's seas:

Arabian Sea, Persian Gulf, and Red Sea. U. S. NavalOceanographic Office IR No. 67-49, Washington. 105 p.

6. National Oceanographic Data Center, 1973. Bathythermographdata tape for marsden squares 102 and 103. Washington.

7. National Oceanographic Data Center, 1973. Hydrographic data

tape for marsden squares 102 and 103. Washington.

8. Sugden, W. , 1963. The hydrology of the Persian Gulf and its

significance in respect to evaporite deposition. AmericanJournal of Science, vol. 26 1(8):741-755.

9. Urick, Robert J. , 1967. Principles of underwater sound for

engineers. McGraw-Hill, New York. 342 p.

10. U. S. Defense Mapping Agency Hydrographic Center, 1973.

Persian Gulf, 3rd ed. Chart N. O. 62032. Washington.

11. U. S. Naval Oceanographic Office, I960. Sailing directions for

the Persian Gulf, 5th ed. H. O. Pub. No. 62. Washington. 404 p.

12. U. S. Navy Hydrographic Office, I960. Summary of oceanographic

conditions in the Indian Ocean. SP-53. Washington. 142 p.

86

INITIAL DISTRIBUTION LIST FOR PART I

No. Copies

1. Defense Documentation Center 2

Cameron Station

Alexandria, Virginia 22314

2. Library, Code 0212 2

Naval Postgraduate SchoolMonterey, California 93940

3. Professor R. H. Bourke, Code 58Bf 3

Department of OceanographyNaval Postgraduate SchoolMonterey, California 93 940

4. LCDR D. C. Honhart, USN, Code 351 1

Naval Postgraduate School

Monterey, California 93940

5. LCDR Glen Eubanks, USN 1

Naval Underwater Systems CenterNew London LaboratoryNew London, Connecticut 06320

6. Office of Naval Research 1

Code 480DArlington, Virginia 22217

7. Oceanographer of the Navy 1

Hoffman Building No. 2

200 Stovall Street


8. Naval Postgraduate School 1

Code 58


9. LT J. L . Wright, USN 3

U. S. Naval Facility Box 100

FPO San Francisco, California 96614

10. LCDR L. E. McGovern, USN 1

Naval Postgraduate School, SMC #2347Monterey, California 93 940

87

11. LT J. G. Bodie, USNDefense Mapping AgencyHydrographic CenterHonolulu Office

Box 116

FPO San Francisco, 96610

12. Dr. Robert E. StevensonScientific Liaison Office, ONRScripps Institution of OceanographyLa Jolla, California 92037

13. Naval Oceanographic Office

Library, Code 3330Washington, D.C. 20373

14. S.I.O. LibraryUniversity of California, San DiegoP. O. Box 2367La Jolla, California 92037

15. Department of Oceanography LibraryUniversity of WashingtonSeattle, Washington 98105

16. Department of Oceanography LibraryOregon State UniversityCorvallis, Oregon 97331

17. Commanding Officer

Fleet Numerical Weather Central


18. Department of the NavyCommander Oceanographic Systems Pacific

Box 1390FPO San Francisco 96610

19. LCDR John Ciboci, USNOcean Operations Division

Fleet Numerical Weather CentralMonterey, California 93 940

20. Commanding Officer

Environmental Prediction Research Facility


88

21. Oceanographer of the NavyHoffman Building No. 2

200 Stovall Street


ATTN: CDR Applegarth

22. LCDR S. E. Wheeler, USNPatrol Squadron ONEFPO San Francisco 96601

89

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a hydrographic and acoustic survey of the persian gulf

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