Contractor Report Series 80-4 [Pt. 1 of 4]

EVALUATION OF SONAR EQUIPMENT AND TECHNIQUES FOR APPLICATION IN THE BEAUFORT SEA FINAL REPORT • VOLUME 1

by R. Hutchins Huntec ('70) Ltd.

INSTITUTE OF OCEAN SCIENCES Sidney, B.C. Contractor Report Series 80-4 (Pt.l of 4)

011909

EVALUATION OF SONAR EQUIPMENT AND TECHNIQUES FOR APPLICATION IN THE BEAUFORT SEA FINAL REPORT - VOLUME 1

by

R. Hutchins Huntec (' 70) Ltd.

Institute of Ocean Sciences Sidney, B.C. 1980

. .. This report was prepared by R. Hutchins of HUntec ('70) Ltd., Scarborough, Ontario, under contract DSS No. 07SB. FP833-9-l43l. The contents of this report are the responsibility of the Contractor. 0-0

EVALUATION OF SONAR EQUIPMENT AND TECHNIQUES FOR APPLICATION IN THE BEAUFORT SEA FINAL REPORT

July 1980 DSS No. 07SB. FP833-9-1431

Prepared by R.·Hutchins. Huntec ('70) Limited 25 Howden Road, Unit 8 Scarborough, Ontario, Canada, M1R 5A6 Telephone: (416) 751-8055 Telex: 06-963640 Cable: HUNTOR, TORONTO

0-1 TABLE OF COBtERtS PAGE TITLE PAGE 0-0 TABLE OF CONTIRTS 0-1 SUMMAIlY 0-5 BlBLIOGKAPBT 0-6 SECTION

1 GENEKAL 1.1. Introduction ·"1·' 1.2. Objectives of the Stuely 1 1.3. Area of Concetn 1 .1.4. Information Source. 2 1.5. Study Method '2- 1.6 Sea Ice anel Weather 4

2 THI ENVIIlONMlNt 2.1. Introduction 6 2.2. The Sea.F1oor 6 2.3. Target Features 6 2.4. Water Co1:umn 8

3 NOISE 3.1 Introduction 11 3.2 Sea Surface Sc.tter,•• 11 3.3. Volume Reverberation , 12 3.4. Ambient Sea Roise 12 3.5. Flow Noise .. . ·13 3.6. Ship Noille 13 3.7. Total Noiae 15 3.7. Optimum Frequency 16 0-2

TABLE OF CONTENTS

SECTION

4 SYSTEM CONSIDERATIONS 4.1. Introduction 17 4.2. Requirements 17 4.3. Side Scan Sonars (Cl~ss C Systems) 18 4.4. Echo" S~unders: (ciass A Systems) 23 4.5 • Multi Beam (Class B Systems) 24 4.6. Refraction Effects in the Mixing 28 Layer 4.7. Discussion of Results 28 (1) Side Scan Sonars 28 (2) Multi Beam System 30 4.8. "System Paramkters fo~ Side Scans 31 4.9. Selection Criteria 32 4.10. Motion Compensatio"n 32

5 • ALTERNATE SYSTEM CONCEPTS 5'1. Introduction 36 5 .2. Underwater Towed Sonars 36 5 . 3. Topographic Mapping Sonars 38 5.4. Parall"eX' Measurement 47

6 • CONCLUSIONS AND RECOMMENDATIONS 5; 1. Conclusions 48 5 .2. Recommendations 49

FIGURE LIST OF FIGURES 1 Map of Median Number of Weeks in 5 Operating Season - under 2/10 of ice. (After Markham (14».

2" Seismic Reflection trace over one 7 of the Pingo-like Features. (After Shearer et al (27». 0-3

TABLE OF CONTENTS

FIGURE LIST OF FIGURES PAGE

3 Target Model with Moat 8

4 Slope Detection 19

5 Change in Signal Strength db at 20 a Slope Break Point Vs Dimension- less range.

6 Operation of a Hull Mounted Echo 23 Sounder

7 Operation o~ Hull-mounted Mills-Cross 24 Array Sonar Systems

0 8 21 Beam Mills Cross Se = 105 PLF 26 Diameter D - Side Slope as in Water Depth 30m, w = 78 m.

9 Geometry for Combined Systems Hydro 31 Chart as = 105 0 and UDI 48 kHz Side Scan at Elevation 10 meters.

10 Geometry of Lloyd Mirror System 38

11 Two Transducer Systems 42

12 Directivity Pattern of the Two 44 Transducer Array drawn on a Linear Amplitude Scale

13 The Lloyd Mirror Effect Applied to a 46 Reflector or Transducer or to Two Transducers.

TABLE LIST OF TABLES PAGE

1 Spread, Average and Standard 7 Deviation of PLF Physical Dimensions

2 Representati ve Sound Speed Versus 8 Depth Profiles

3 Water Models 10

4 Survey of Side Scan Sonar Systems 21 & 22 APPENDICES

Appendix l Sonar System Equations for Oblique AngLe Bottom Scattering Sonars

Appendix 2 Environmental Considerations

Appendix 3 Statement of Work and Related Contractural Matters

Appendix 4 Interim Report and Activity Reports

Appendix 5 Glint, and its Effect on the Accuracy of a Sea Bed Profiling Sonar

Appendix 6 Ray Tracing and Signal Modelling Results 0-5

SUMMARY

This is a study of sonar systems which may be ~uitable to~ the detection and 'mapping 0.£ pingo-like features in the south eastern 'Beaufort Sea no,rth of the 20 meter isobath to the edge of the shelf. The major portions of the area of concern lies in water depths in excess of 30 met~rs.

Pingo like features (PLF's) are conical mounds rising from the surrounding sea floor t6 a maximum height rang{ng from 5 to 40 meters. They have base diametersr~nging from 100 to over 2000 meters .and are sometimes surrounded by a shallow moat. Their density is not uniform throughout the area of concern, and they may occur singly, or in clusters of 3 to 7 within a radius of 2 to 3 kilometers.

The adjacent sea floor is generally devoid of topographic features On a comparable scale and is acoustically uniform ov~r distances of several tens of kilometers; Other acoustic features are ice scour furrows which form striated patterns generally in a NW-SE direction readily distinguishable on side scan sonar recordings.

The significant acoustic features whereby aPLF may be detected against the sea floor backgroundi~ the regional change in sea floor,slope, and the moat when present. These moats, unlike the ice scour ma~ks, do not have side ridges.

Refraction. effects due to la~ge sound speed/depth gradients within the first few meters of the sea surface are highly variable both in time and space. During the summer months when the polar ice pack has retreated from the area, the conditions are determined by the wind., which influences the distribution of the discharge ~aters of the Mackenzie River. Below the halo cline generally at 5 to 15 meters conditions are satisfactory for side scan sonar deployed from underwater platforms towed behind surface vessels.

Specially modified side scan sonar towed at an elevation of

5 to 15 meters. .. above the sea. floor should be. capable . of.0 detectingo . changes in across. track bottom slope of from 3 to 5 .at ranges . out . to 300 meters each side of the vessel.. 0 A combined system utilizing a UDI 48 kHz wide beam (3.2 ) side scan .tow fish and a General Instrument Hydro Chart multi beam system will provide an area' coverage rate of approximately 3000 meters 2 per second at 8 knots.

Two alternative concepts for wide swathe topographic mapping sonars to be deployed in tow fish are discussed - whilst such systems appear feasible, lead time for development is of the order of five years. 0-6

BIBLIOGRAPHY

1. O'Rourke, J.C., "Inventory of Physical Oceanography in the Eastern Beaufort Sea", 1974, Proceedings of a Symposium on Beaufort Sea Coast and Shelf Research Reed, J.C. and Slater, J.E.(ed) The Arctic Institute of North America.

2. Milne, Allen, "Oil, Ice and Climate Changes The Beaufort Sea and the Search for oil" ed. R.J. Childerhose Beaufort Sea Project.

3. Herlinveaux, R.H., de Lange Boom, B.R., "The Physical Oceanography of the South-Eastern Beatifort Sea" 1975, Beaufort Sea Project Report No. 18

4. Lewis, C.P.,and Forbes, D.L. "Coastal ~edimentary Pro­ cesses and Sediments,Southern Canadian Beaufort Sea" 1975, Beaufort Sea Project Report No. 24.

5. Walker, E .R., "Oil, Ice and Climate in tOhe BeaufortO Sea" 1975°;'; ° Beaufort Sea Project Report No. 35.

6. Wadhams, Peter, "Sea Ice Morphology in the Beaufort Sea" 1975. Beaufort Sea Project Report No. 36.

7. Doires, K.F., "Mackenzie River Input To the Beaufort Sea" 1975, Beaufort SeaoProject Report No. 15.

8. Huggett, W.S., Woodward, M.J., Stepehenson, F., Hermiston, F.V., Douglas, A., "Near Bottom Currents and Offshore Tides" 1975, Beaufort Sea Project Report No. 16.

9. MacNeill, M.R., Garrett, J.F., "Open Water Surface Currents in the Southern Beaufort Sea" 1975, Beaufort Sea Project Repor t No. 17.

10. Henry, R.F., "Storm Surges" 1975, Beaufort Sea Project Report No. 19.

11. Berry, M.V., Dutchak, P.M., Lalonde, M.E., McCulloch, J.A.W., Savdie, I., "A Study of Weather, Waves and Icing in the Beaufort Sea" 1975, Beauofort Sea Project Report No. 21.

12. Pelletier, B.R., "Sediment Dispersal in the Southern Beaufort Se~" 1975, Beaufort Sea Proj.ct Report No. 25a.

13. Bornhold, B.D., "Suspended Matter in the Southern Beaufort Sea" 1975, Beaufort Sea Project Report No. 25b. 0-7

14. Markham, W.E., "Ice Climatology of the Beaufort Sea" 1975, Beaufort Sea Project Report Bo.26.

15. Ramsecer, Rene 0., Vant, Malcolm R., Aisenault, Lyn W., Gray, Lawrence, Gray, Robert B., Churdobrak, Walter J., "Distribution of the Ice thickness In the Beaufort Sea" 1975, Beaufort Sea Technical Report No. 30.

16. Marko, J.R., "Satellite Observations of the Beaufort Sea Ice Cover" 1975, Beaufort Sea Project Report No. 34.

17. Cooper, P.F., "Movement and Deformation of the Landfast Ice of the Southern Beaufort Sea" 1975, Beaufort Sea Project Report No. 37.

18. Young, F.G., Myhr, D.W., Yorath, C.J., ~Geology of the Beaufort-Mackeqzie Basi~ 1976, Geological Survey of Canada Paper 76-11. Department of Energy Mines and Resources, Canada.

19. Grainger, E.H., Lowrety, J.E., "Physical and Chemical Oceanographic .Data from the Beaufort Sea 1960 to 1975'~, Fisheries and Marine Service Report No. 590 Department of the Environment, Canada.

20. Kovacs, Austin and Mellor, Malcom, "Sea Ice Morphology and Ice as a Geologic Agent in the Southern Beaufort Sea" 1974, Proceedings of a Symposium on Beaufort Sea Coast and Shelf Research, Reed, J.C., and Slater, J.E .. , (ed) The Arctic Institute of North America.

21. Reimnitz, Erk, and Barnes, Peter W., "Sea Ice as a Geologic Agent on the B~aufort Sea Shelf of Alaska" 1974, Proceedings of a Symposium on Beaufort Sea .Coast and Shelf Research, Reed, J.C., and Slater J.E., (ed) The Institute of North America.

22. Lewellen, Robert I., "Offshore Permafrost of Beaufort Sea, Alaska" 1974, Proceedings of a Symposium on Beaufort Sea Coast and Shelf Research, Reed, J.C., and Slater, J.E., (ed) The Arctic Institute of North America.

23. Judg., Alan "Occurrence of Offshore Permafrost in Northern Canada" 1974, Proceedings of a Symposium on Beaufort Sea Coast and Shelf Research, Reed, J.C., and Slater, J.E., (ed) The Arctic Institute of North America. 0-8

24. Pelletier, B.R., "Discussion of Papers on Geological Action of Sea Ice, Sedimentation, and Sea Floor Morpho­ logy" 1974, Proceedings of a Symposium on Beaufort Sea Coast and Shelf Research, Reed, J.C., and Slater, J.E., (ed) The Arctic Institute of North America.

25. Panel Discussion Proceeding~ of a Symposium on Beaufort Sea Coast and Shelf· Research, Reed, J.C., and Slater, J.E., (ed) The Arctic Institute of North America.

26. MacKay, J. Ross, "Pingos of the Tuktoyaktuk Peninsula Area, .Northwest Territories" 1979, Geographie Physique et Quaternaire Vol XXXIII No.1, p. 3-61.

27. Shearer, J., "Submarine Pingos in the Beaufort Sea", Science Vol. 174.

28. Milne, A.R., "A Seismic Refraction Measurement in the Beaufort Sea" 1966, Bulletin of the Seismological Society of America, Vol. 56, No.3, pp. 775-779.

29. Herlinveaux, R.H. de Lange Boom, B.R., Wilton, G.R., "Salini icy, Tempera tu re, Turbidity and Me teorological Observations in the Beaufort Sea: Summer, 1974, Spring and Summer 1975", Institute of Ocean Sciences, Patricia Bay, Victoria, B.C., Pacific Marine Science Report 76-26.

30. "Beaufort Sea - Mainland Coast of Canada from Demarcation Point and Herschel Island to Cape Bathurst" - Pilot of Arctic Canada Chap. 1 Vol. III. Second Edition 1968. Canadian Hydrographic Services.

31. "Study of Pingo-Like-Features Detected in the Beaufort Sea" (Contract Serial No. 03B79-00234 Final Report) Coast Pl~t Ltd., Box 2545, Sidney, B.C., V8L 4B9. Department of Fisheries and Oceans, P.O. Box 6000, Sidney, B.C., V8L 4B2.

32. (a) (b) "Ice Observations 1965(a), 1970(b), Canadian Arctic" Environment Canada Atmospheric Environment.

33. Ship's Meteorological Log MV Theta from July 1st to September 6, 1974, Transport Canada - Meteorological Branch.

34. "Ships Meteorological Log MV Pandora II from July 3 to Sept. 5, 1974", Transport Canada Meteorological Branch.

35. "Tuktovaktuk - Hourly Wind Summary Summer 1974, Spring and Summer 1975", Transport Canada, Meteorological Branch. 0-9

36. Launch Petrel - Sound Range Report January 24, 1978, Naval Engineering Unit (Pacific) Esquimalt, B.C., Report Serial No. El19.

37. Ship MV Pandora II - Sound Range Report August 13, 1976, Naval Engineering Unit (Pacific) Esquima1t, B.C., Report Serial No. E103.

38. O'Connor, A.D., (letter) - Supplementary Pingo statistics for inclusion in the photographic record of PLF echo graphics (reference 3l),Institute of Ocean Sciences.

39. Canadian Ice Charts for August 1, 1975 to Sept. 5, 1975 and Aug. 6, 1974 to Sept. 10, 1974, Ice Forecasting Central - Ottawa.

40. Grainger, E.H., Lovitz, J.E., Physical and Chemical Oceanographic Data from the Beaufort Sea 1960 to 1975, Fisheries and Marine Source Technical Report No. 590, Canada.

41. Wilson, Wayne D., Equation for the Speed of Sound in Sea Water, Journal of the Acoustical Society of America, Vol. 32, No. 10, 1357, Oct. 1960.

42. Urick, R.J., Principles of Underwater Sound, McGraw-Hill, 1975.

43. Bryant, R.S., Sidescan Sonar for Hydrography - An Evaluation by the Canadian Hydrographic Service, International Hydrographic Review, volume 52, No.1, 1975, pp. 43-56.

44. Schulkin, M., and Shaffer, R., Backscattering of Sound from the Sea Surface, J. Acoust. Soc. Am., Vol. 36, No.9, 1964, pp. 1699-1703.

45. Haddle, G.P., and Skudrzyk, E.J., The Physics of Flow NoiSe, J. Acoust. Soc. Am., 46: 130, 1969.

46. Nishi, R.Y., Stockhausen, J.H., and Evensen, E., Measurement of Noise on a Underwater Towed Body, J. Acoust. Soc. A~. 48: 753, 1969.

47. Launch Petrel, Sound Range Report January 24, 1978, Naval Engineering Unit (Pacific) Esquimalt, B.C;, Report Serial No. El19.

48. Ship MV Pandora II - Sound Range Report August 13, 1976 Naval Engineering Unit (Pacific) Esquima1t, B.C., Report Serial No. El03. 0-10

49. Clay, C.S., and Meldwin, R., Acoustical Oceanography, John Wiley & Sons, 1977.

50. 'Officer, C.B., Introduction to the Theory of Sound Transmission, McGraw-Hill, 1958.

51. Anderson, G., Gocht, R., and Sirota, D., Spreading Loss of Sound in an Inhomogeneous Medium, J. Acoust. Soc. Am., 36: 140, 1964.

52. Ross, Donald, Mechanics of Underwater Noise, Pergammon Press, 1976.

53. Fronz, G.J., Splashes as Source of Sound in Liquids, J. Acoust. Soc. Am., 1080-1096,1959.

54. Heindman, T.E., Smith, R.R., and Arne.on, A.D., Effects of Rain upon Underwater Noise Levels, J. Acoust. Soc. Am., 27, 378-379, 1955.

55. Bonn, N., Effect of Rain on Underwater Noise Level, J. Acoust. Soc. Am., 45, 150-156, 1969.

56. Pelletier, B.R., and Shearer, J.M., 1972, Sea Bottom Scouring in the Beaufort Sea of the Artic Ocean, in Proceedings of the 24th International Geological Congress, Montreal 1972, Section 8, p. 251-261.

57. Lewis, C.F.M., (1976), Bottom Scour by Sea Ice in Southern Beaufort Sea. Beaufort Sea Project Technical Report #23, Department of Environment, Victoria, B.C.

58. Wilson, Wayne D., (1960), Equation for the Speed of Sound in Water, JASA Vol. 32, No. 10, 1357, October 1960.

59. Parrott, R., Dodds, J., King, L.R., Simpkin, P.G., (1979), Measurement and Evaluation of the Acoustic Reflectivity of the Seafloor, Canadian Journal of Earth Sciences in Press.

60. General Instrument Corporation, Harris, A.S.W. Division Hydro Chart and Sea Beam Systems brochures and price quotation.

61. Cutrona, L.J., Comparison of Sonar System performance achievable using synthetic-aperture techniques with the performance achievable by more conventional means, JASA, Vol. 58, No.2, August 1975, p. 336-348.

62. Chesterman, W.D., et aI, Acoustic Surveys of the sea floor near Hong Kong, International Hydrographic Review, Vol. 44, No.1, 1967, pp. 35-54. 0-11

63. Stubbs, A.R., et aI, Telesounding - A Method of Wide Swathe Depth Measurement, International Hydrographic Review, Vol. 51, No.1, January 1974, p. 23-59.

64. Denbigh, P.N., Glint and Its Affect on the Accuracy of a Seabed Profiling Sonar presented at a symposium at Loughborough University of Technology, 1980. j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j - 1 -

SECTION 1

GENERAL

1.1. Introduction

This is the Final Report of a study of the applicability of sonar systems to the identification ~nd mapping of Pingo Like Features (PLF's) in the south eastern Be~ufort Sea. For the sake of completeness 'and context, some of the results documented in the Interim Report dated Ma~ch 31st, 1980* are repeate~ in this present repoyt. In addition the sonar system equations given in theInte~im Report ha've' been developed along slightly different lines in Appendix I, which re'place's corresponding portions, of the Interim Report in particular the equa'tions for volume reverberation noise. '

1.2. Objectives Of The Study

(1) To examine the applicability of, and to assess the relative effectiveness of, sonar systems for identify­ ing and mapping Pingo Like Features (PLF's)' in the Area of Concern.

(2) To compare the performance of existing sonar systems d~p16yed from surface v.ssels or from underwater i?strument packages towed by surface vessels.

(3) To identify alternative system concepts which will be more cost effective than existing systems.

The Statement of Work givert in the contract document is included in Appendix 3.

1.3. Area Of Concern

The area of concern is in the south eastern Beaufort Sea, just north of the TuktoyaktukPenirtsula extending from Cape Bathurst westwards to Macken'zie Bay and from seawards of the 20 meter isobath a distance of 110 to 150 kilometers north to the edge of the continental shelf. A description of the area and the environmental factors which affect sonar perfo~mance is given in Appendix 2, "Environmental Considerations".

* Included ih Appendix 4 - 2 - 1.4. Information Sources

Information regarding the distribution of PLF's, their size and existing bathymetric coverage of the area wa. provided by the scientific authority (31)(38).

A computer assisted literature search On the oceano­ graphy and geology of the area was undertaken and is included in the bibliography. The bulk of the data used in this study was obtained fro'm the published reports in the Beaufort 'Sea Project series; by discussions with Dr. J. Shearer, Dr. Steve Blasco, Dr. C.F.M'. Lewis and Dr. B. Pelletier as well as the scientific authority and members of the Canadian Hydrographic Service - Unpublished data including extracts from ships' logs, meterological and ice observations were obtained from Ice Forecasting Central in Ottawa and the Atmospheric Environment Service at Downsview.

Bracketed numbers used throughout this report and appen­ dices refer to the corresponding items in the bibliography.

1.5. Study Method

This study considers two distinct problems:

(I) Comparisons between different sonar syst.ms within the same class for three different classes as follows:

Class A - Normal Incidence Echo Sounders~

Class B - Multi Beam ship mounted oblique angle (wide swathe) sonars such as the General Instrument Corporation Sea Beam and Hydro Chart systems, and

Class C - Tow Fish Deployed Side Scan Sonars.

(2) Comparison betw,een the three classes in terins of mission performance considering cost and other constraints.

,The £irst problem is straight forward. Comparisons within the sonar class can be made in terms of signal to noise ratio ~f each set as a function of its area coverage rate. The system parameters of beam width, range and resolution being constrained by the target feature statistics. It is worth noting that Class A and Class B are aperture limited sonars, whilst Class C sonars operate in the pulse length limited mode. - 3 -

The second problem is not so straight forward in this particular case. The target features, those charac­ teristic of the PLF's which cause them to produce an acoustic response different from the background response of the sea floor, do not produce comparable responses in all three systems. Class B systems give a positive response to across track bottom slope, whereas Class C s'ystems as they pre­ sently exist do not. On the other hand, area coverage rate for Class C systems is very much higher than it is for Class A, and moderately higher than for Class B. Their cost on the other hand is very much lower than Class B and moderately higher than Class A.

A model of mission cost effectiveness is of limited value under the circumstances when the missioni are not equivalent for each of the three classes of systems. We have redirected our effort therefore to an examination of ideas for improving the across track slope detection capability of Side Scan Sonars, since we believe that this technology has considerable potential for development into an effective method for wide swathe bathymetry particularly suited to the Beaufort Sea environment.

In order to compare the relative performance of existing Class C systems - that is oblique angle tow fish deployed pulse length limited side scan sonars we have developed equations (Appendix I) which predict the performance of such sonars for iso velocity water conditions. In addition to prediction these expressions may also be used for the system design problem, where, given the mission requirements, a set of sonar system parameters may be selected which are best matched to the requirements.

Class C systems are most sensitive to refraction of the oblique angle rays in the water caused by sound speed depth gradients whereas Class A systems being normal incidence are not significantly affected, and Class B systems are affected to a small extent •

. In order to include refraction effects in these sonar models two computer programs to run con­ currently have been written. The first of these is a ray path program which gives the range, depth, intensity, path length forward spreading loss and backward spreading loss for a ray originating at a specified point in the water at a specified initial angle given the temperature and salinity and depth or simply the sound speed versus depth. - 4 - The output of this program is printed in tables and plotted graphically. In addition the results are stored on disc for use in the second program, the scattering program. This Scattering program allows for absorption loss and gives the intensity of the signal back scattered to the source from the sea floor using a simple Lambert scattering model. The results are output to tables and are also plotted as normalized signal versus time and horizontal range for the two cases, PLF present and PLF not present.

A model for estimating noise that is not coherent with the transmissions, that is the noise level is constant during arrival of the back scattered sea floor signals, is developed for use with the scattering model results to determine maximum range for a specified detection threshold.

The program outputs for the various models are presented in Appendix 6 together wi·th the program documentation. This po.rtion of the work was carried out under the MOSAICS project supported by the PILP/COPI Funding from Energy, Mines and Resources and Fisheries and Oceans.

The environmental and geological data were used to derive parameters for several representative cases for both the ray path model and the scattering model and for identifying features of the targets which would provide an acoustic response.

1. 6. Sea Ice and Weather

The influence of sea ice, weather and other periodic (seasonal) and non periodic variables on temperature and salinity is discussed in Appendix 2, "Environmental Considerations".

In addition to their effect on the water model these variables determine how many survey days a season are probable - and thus impose an operational constraint which must be considered in planning a multi year PLF mapping program.

Figure 1 shows the area of concern and contours of the median duration of the operating season from a report by Markham (14). Storms, waves and surface temperature and other operational data are given in a report by Berry et al (11). Storm surges are described by Henry (10).

The scope of this study does not include consideration of the operational constraints noted above except where they have direct impact on the enviromental model. .. • ---'--1', . ' 0::' l..J'

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SECTION 2

THE ENVIRONMENT

2.1 Introduction

This section considers those aspects of the sea floor, the PLF's and the water column which influence sonar performance. Appendix 2 provides a comprehensive discussion of the various environmental factors and their inter-relationship and should be consulted for more detailed information.

2.2 The Sea Floor

The Sea Floor is described by Pelletier (24)(12). Sea ice as a geologic agent and ice gouges on the sea floor are described by Pelletier (56) Lewis (57) and Kovacs (20).

The conclusion from Appendix 2 is that a normal incidence back scattering strength of -30 db can be taken as representative throughout the area of concern.· The major sea floor features from an acoustic point of view being ice scours and PLF's.

2.3 Target Features

The primary target features are side slope, and base diameter. Secondary features are moats and possibly ice keel scours. The surface roughness and back scattering strength at normal incidence of the targets are not expected to differ substan­ tially from the adjacent sea floor.

Figure 2 shows a seismic reflection trace of a PLF taken during the mapping cruise of 1970 by the Canadian Hydro­ graphic Service at location 70 0 51'N and l310 72'W. The PLF shown has a base diameter of about 350 meters is about 25 meters high in a water depth of 50 meters. Based on the extensive surveys of the PLF's in the period 1970-1972, photographic and statistical records were co~piled on a total of 205 PLF's (31)(38). These records indicate that PLF's come in all sizes with base diameters varying from 100 meters to 2150 meters and heights varying from 3.~ meters to 50 meters. The slopes vary from 1:128 (0.5 ). The spread, average and standard deviation of these PLF's in terms of base diameter, height and slope are given in Table 1. - 7 - Table 1. Spread, Average and Standard Deviation of PLF Physical Dimensions.

~tatistical Spread Average Standard Deviation Physical Parameterp!~

Base Diameter 100m - 2150m 601.Bm 433.5m

Height 3.4m - 50m 16.1m B.6m

0 0 0 Slope 0.5 - IB.4° 4.7 3.6

Fro.ml Table: 1, it is seen that the average is skewed towards the lower end of the distribution - it thus tends more towards a Rayleigh rather than a Gaussian Distribution.

According to Shearer (private communication) the depression (moat) surrounding the PLF in Figure 2 is usually present. When present the moat should be an important target feature.

From the data in Table 1, we settle On a target model as shown in Figure 3. The diameter is 500 meters, height 20 meters, side slopes 50 moat side angles 50 depth below surrounding sea floor of moat 10 meters.

300 600 In 9001 o 1200 I~OO o· ·_-.COO=---'2Cr.-J i .., J~7.I -­. o ... - 0 • • <000• 50!" I

100 .---" i. -=-~-=:'''':=-:..--= '::"''''::''''':::::;: ."'!.-:;: ...... _ • J , Pro&:osed otls,"al Jake ba5i~ r _ t ~_>;'"" ,. " Bo:tom s. •...... " / ;:=._-....;:.-.-..--... -""Ii'.':;::Oo _.,-,,_1 __ -- _ •. 0 ..... _ .. " • . " ... ".. ::. " ---;.; .. -:.::..--' .... # • - - Sub·bottom rellectors

; --~--~ ___L-.

Figure 2 Seismic reflection trace over one of the pin go-like features.

(After Shearer et a1 (27)) - 8 -

I \ . \".. -----...... ,- ~t;;;.;. . /...:;;...-::::..---~~.,...--..;.,.-...:::..... c ." - s· ~ ...-·I ..~=--- 1 Figure 3. Target Model With Moat

This target model and variations of it (moat removed) are used in the simulation modelling to show. the changes in response of a side scan SOnar system at various ranges and towing heights.

2.4. Water Column

Figure A-2-26 of Appendix 2 shows the sound speed depth profiles for data collected during .summer 1974 and spring and summer of 1975. These profiles can be assigned to one of the three general types shown in Table 2.

Tab Ie 2 REPRESENTATIVE SOUND SPEED VS DEPTH PROFILES

Number of Examples Summer Spring Summer Total 1974 1975 1975 ,/ R I 1 2 11 8 21 1'>, '"\C,

\ 1\,- \ 2 ~~~ - - 10 10 c ...

, , 1\ ~ \ 3 \&} 11 9 1 21 - 9 -

Conditions in the surface mixing layer indicated by the dashed lines in the sound speed depth profiles "a~e highly variable in both space and time - There may be several maxima and minima within the first 10 meters - Only that portion of the profile below the surface mixing layer are relatively stable. The problem thus reduces to identifying the transition points. The term high or low as related to sound speed, means with respect to a nominal valu~ of 1437 meters/sec.

For all three types, below the surface mixing la~er (Point B) at a depth of 5-15 meters the sound speeds tend toward a value between 1433 to 1440 meters per second, representing the sound speed in the surface layer of the Arctic Water Mass. In the area of concern the Arctic Water Surface Layer extends in depth from below the surface mixing layer at 5" to 15 meters to a depth of 25 to" 50 meters. The Arctic Surface Water is nearly isohaline with salini~ies between 0 28.5 to 35.5 /00 and temperatures less than OOC generally near the freezing point*.

Below the Arctic Surface Water at a depth of 25-50 meters is the Arctic Sub-Surface layer extending in depth to 100-150 m. In the Canadi~n Basin this water is charac- terized" by" a . slight temperature maximum (0.5 to 1.0 0 C) at 75 to 100 meter depth decreasing again to. -1.4 to :'1.50 C at 150 m. This maximum is attributed to summer Bering Sea water that is advected around the Beaufort Gyre (3).

Within the area of interest the sound speed gradients below 30 meters are slightly positive and for our purposes can b"e ignored. What concerns us are the highly variable conditions within the surface mixing layer to depths of 5 to 15 meters. "Are these sufficiently serious to affect surface deployed systems? Side scan SOnars " operated within these layers wili give highly var:j.able results.

Type 1 profiles are characterized by a velocity low (point Bi of Table 2) of 1410 m/sec to 1415 m/secwithin the first 5 to 10 meters followed by a steep positive gradient due to the sharp halo cline to the deeper water speed in excess of 1430 meters per second within the next 5 meters of depth (point Cl of Table 2). Type 1 profiles occur most frequently in the spring and early summer as indicated in Table 1.

The freezing point of salt water is nearlg a linear function * 0 of salinity, with a value of -1.5 C at 28 /00 decreasing to o 0 -1.9" C at 35 /00. - 10 - Type 2 profiles are characterized by a velocity high (point B2 of Table 2) of 1450-1465 m/sec at a depth varying from 5 to 15 meters and a steep negative gradient within the next 3 to 5 meters to a value between 1430-1440 m/sec. Type 2 profiles are found in late summer. Table A-2-l and Figure A-Z-26 (Appendix 2) illustrate the extreme variability of the surface mixing layer.

Type 3 is representative of spring, and early summer. There is a generally positive gradient of zero to .5 meter/sec/meter ·to the deep water value. at 20 meters.

For purposes of this study we shall consider Three Water Models shown in Table 3. For water depths in excess of 30 meters environmental factors have decreasing influence on system .performance.

TABLE 3 Water Models

Model Type Depth Sound Speed m m/sec

1 0 1450 7 1434 20 1438 30 1439

2 0 1450 15 1465 20 1438 30 1439

3 0 1410 10 1430 20 1438 30 1439

- 11 -

SECTION 3

NOISE

3.1. Introduction

We shall consider two classes of noise, time coherent noise such as volume reverberation or sea surface back scattering or any type of noise which is synchronous with the outgoing signal, and noise which is not time coherent with the signal. The following discussion of noise together with that given in Appendix I assumes that the noise field has low spatial coherency, is isotropic and of uniform intensity through­ out the water column.

These approximations are not always valid and estimates based upon them should only be used for- a preliminary assess­ ment of system performance and even then, only if the physics of the particular situation indicates that they are reasonable.

Our objective in this section is to define analytic express­ ions for the different noise contributions which will be useful for preliminary system design.

3.2 Sea Surface Scattering

Back scattering from the sea surface has not been included in the noise model. It is assumed that son~rs deployed on the surface will have rays steeper than -10 from the horizontal. Side scan sonars will be below the surface mixing layer and have their transducers adjusted so the main beam rays do not intercept the sea surface.

Scattering from minor lobes of the main beam may be a problem, particularly with side scan sonars operating close to the surface. Their effects have not been accounted for explicitly in the sonar models developed in Appendix I in order to keep the model simple. Generally detailed data on individual sets is not readily available from some manufacturers.

Side lobe suppression through shading or some other means is an important consideration in the selection of side scan sonars for all applications, but in particular for this application when the transducers will be deployed close to the bottom, or close to sound channelling layers. - 12 -

3.3. Volume Reverberation

The expression for volume reverberation (equation 37 Appendix I)has its highest value at Rmin (See Appendix 1 Figure A-1-9). The volume scattering parameter Sv due to suspended matter in the Mackenzie River water is given by equation 36 Appendix I S 1T2nVs2' 5 2 ,1 v = A4 (2) At 100 kHz, A= .015 m, a clay particle is typically <4 x 10-6 meters in diameter and n can be taken at 106 particles per cubic meter (13). Thus 10 log Sv = -104 db//l ppa This estimate is probably high since the clay particles do not act as the rigid spheres assumed for rayleigh scatter­ ing.

For the purposei of this study, we consider that volume reverberation noise is not a significant factor in comparison to other noise and its exclusion from this model is a welcome simplification.

3.4. Ambient Sea Noise

Unlike sea surface scattering and volume reverberation, ambient sea noise and system self noise are time incoherent with the transmissions - that is to say, they have a steady threshold and are not range (time) dependent.

Ambient sea noise is a function of surface roughness (sea state). Rain and other forms of precipitation has a large ,effect. A discussion of ambient noise is given in Appendices 1 and 4. In this study we have taken sea state 3 as representative. The spectrum level at 10 kHz is taken at 48 db/1Ppa/IHz plus 6 db for shallow water for a spectrum level of 54db//lppa/ 1Hz at 10· kHz; A roll-off of -5 db/octave is assumed as describe~ in Appendix 4.

Under the above conditions, an analytic expression for ambient sea noise * is na = 1.17 x 1012f-l.667 2 where 10 log nais the pressure spectrum level _

i.e. 10 log na is db//]Jlpa!lHz - 13 -

the ambient sea noise intensity is

I = I n' o a 0 a 3 1 B = .667 x 10-' na watts/m2 /IHz 'ba~dwid-th

3.5~ Flow Noise

From Appendix 4, at 10 knots the flow noise spectrum level at 10 kHz is 60 db/l ppa/IHz and at 5 knots it is 42 db Since the falloff is 9 db/octave the system will be ambient sea noise limited. At 50 kHz the flow noise spectrum level is 30 db at 10 knots, and the ambient sea noise is 38 db. Flow noise can be neglected for side scan SOnars at speeds to 10 knots. An ana.~ytic expression for flow noise is 3 . n = 5.8186 x 1013 6f- f V 4 for O1000 Hz ~ - is the flow noise' pressure spectrum level in db/ /lppa/IHz. 10 log n f )J = Free Stream veLocity m/sec

£ = frequency in Hi

Equation 4 fits the data in Figure A-1-7 of Appendix 1.

3.6. Ship Noise

Equation 20 of Appendix I gives the spectrum level noise for ships 7fnD B Ls = 195 + 60 log (-zs) + 10 log 4 - 25 log f

In non- logarithmic form 2 • 5 n = 3.162 (referenced to 1m) 5 s 6 (7fDR) 6 where K = 25 7 R = n/V Revolutions/sec/m/sec

= blade cavitation pressure spectrum level referenced to 1 meter

D = Propeller diameter in meters

B = Number of blades

.n = Revolutions per second of propeller

v = ship speed in m/sec - 14 -

Launch Petrel

n = 2~~0 = 41.67 revolution per second

D = 19" = .483 m.propeller diameter

B = 4 = number of propeller blades 3 f = 16 x 10 Hz

L 195 + 24 - 0 - 105 114 db/l~pa/IHz s = = at 1 m at f = 16000 Hz

Measured value for Petrel (37) at 2500 rpm is 122 db/. Since the error in equation A-1-20 can be as high as 10 db this is. in line wi th the measure.d values which we shall use.

At sonar frequencies of 50 kHz Petrel's noise should be down an additional 12 db giving a calculated spectrum level of 102 db versus a measured value of 110 db. - 15 -

Pandora II

Measured noise data for Pandora 2 at 5 knots is 133 db at 15 kHz. This ship is substantially noisier than Petral.

We shall use a spectrum level of 100 db/Illpa at 1 meter at 50 kHz for ship noise at speeds to 10 knots. Fortunately the seabed is quite lossy (-30 db) and multiple reverbera­ tions between the sea-floor and sea surface should attenuate quite rapidly. If we provide a separation of 15 meters to the sonar vehicle, the effective ship noise at the sonar will be down 20 log 15 = 23.5 db leading to a final figure of 77 db for ship noise at 50 kHz some 40 db in excess of ambient sea noise.

3.7. Total Noise

The above considerations lead to the following expressions for noise -

8

10 log is the total noise pressure spectrum level. n t 10 log na is the ambien t sea noise pressure spectrum level. y is the distance from the propeller to the sonar receiver* . y» than the propeller diameter.

10 log ns is the ship noise pressure spectrum referred to 1 m from the prop.llers.

The individual expressions are 1012 -1.667 na = 1.17 x f from equation 2 _3 5.819 x 1013v6 f from equation 4 n f = 1019 6 t2: 5 ns = 3.162 x KV from equation 5 6 B K = ('TDR) from equation 6 25 4 R = n from equation 7 v substituting the above expressions in equation 8

9

* This expression must be used with caution in shallow water· and rock bottoms. The expression for ship noise is only valid when blade cavitation dominates which is usually the case. - 16 -

Example

If Petrel has a speed of 15K (7.5 .m/s) at 2500 RPM (41.67 RPS) then n 41.67 R = = 5.556 v = 7.5 1T x .483 x K = ( 5.556) 6 (4) 25 4 = 1.471 x 10-3

At f ~ 50,000 Hz, v = 7.5 mls ( = 15 Kn)

and the side scan sonar tow fish is 15 meters astern of the ship then

5 6 x = 65816526 n t = (2951 x 10 + 6864 x 10 + 4332 1012)~ 10 log ,n t= 78.18 db

10 log n = 42.3 db 8

10 log n f= 49.2 db

(n y-2) =' 78.18 db' 10 log s 3.8 The Optimum Freguencl

Appendix I demonstrates 8 formal procedure for selecting the opiimum fr~quency given an analytic function for the noise such 8S equation 9. In the appendix the system was consi­ dered to be se'a noise limited. In the example just given, the system is ship noise l'imited, thus the expression for optimum frequency for a given range will be different from the sea noise limited case given in Appendix I.

The influence of ship boise can be reduced by reducing the speed and increasing the distance of the side scan sonar from the ship. For example at a speed of 2.5 mls (5 knots) and a distance astern of 30 meters

10 log n a = 42 db unchanged

10 log nf = 21 db down 28 db 10 log y-2) = 44 db down 34 db Cn s Under these conditions, the optimum frequency expression given in Appendix I is valid. - 17 -

SECTION 4

SYSTEM CONSIDERATIONS

4.1. Introduction

In this section we consider the mission requirements and the suitability of the three different classes of systems. Also included is a tabulation of the main characteristics of a number of commercially available side scan sonars.

4.2. Requirements

The sonar system must be able

(1) to detect, identify and locate all PLF's within the area of concern*.

(2) Do this rapidly, that is, have a good area.coverage rate.

In addition it must meet all the other requirements and constraints imposed by the environment and nature of the operation,

As discussed in· Appendix 2 the target distribution is not uniform throughout the area of concern and PLF's appear to occur in clusters. Under such conditions, a reasonable mapping strategy is to map pingos in two main phases: -

(1) Detection Phase - Objective to identify sea floor anomalies that are associated with PLF's. These are the target features which consist of (a) moats associated with a change in sea floor slope on a scale comparable to a pingo base radius. (b) A change in sea floor slope without a moat. (cl A change in the pattern of ice keel furrows indicative of a slope change or a furrowed peak.

(2) Mapping Phase - Objective to map the sea floor topo­ graphy within areas identified in the detection phase at a line spacing sufficient to delineate the PLF's and to define their shape and position of their peaks.

* See Statement of Work Appendix 3 - 18 -

PLF 064 (31) has a base diameter of 100 m a side slope of 1.3.and a maximum height above the surrounding sea floor of 79.5 m. The minimum water depth being 18.6 m*. Thirty one of the 205 PLF's given in (31) have base diameters of 200 meters or less. Reliable mapping of these "small diameter" PLF's requires a line spacing of the order of 50 meters.

A mapping strategy based on 50 m line spacing throughout the area will meet the requirements. For the purposes of this study however we shall assume the two phase strategy and confine our attention to the Detection Phase, a situation where we must find all of the targets whilst minimizing the false alarms.

4.3. Side Scan Sonars (Class C Systems)

The output display of a properly adjusted side scan sonar system operating below the mixing layer will indicate a response to the target features as follows:

(1) A change in sea floor slope (86 ) on a scale larger than a resolution cell results in an increase in the detector output at the point of increased slope. Similarly a negative change in sea floor slope results in a corresponding decrease in the detector output.

(2) Physiographic features on a scale equal to or larger than a resolution cell will modulate the detector output. Examples of su.ch features are moats and ice keel scours. A change in the pattern or the intensity (dark-light sequence) may be associated with a PLF (an ice scoured peak but no scours on the adjacent sea floor).

(3) Changes in texture or rock type, (lithologic changes) on a scale larger than a resolution cell. From the information given in Appendix 2 we should not expect to find any· such changes associated with PLF's.

* The tabulated dimensions for this PLF (31) do not agree with the echo-graph. It appears that height and minimum depth columns are transposed. - 19

Response to Slope Changes

Equation A-l-ll (Appendix I) is the expression for the target strength. The slope dependent term is given by

f (a) .= 10 differentiating 10 with respect to a we obtain

2 ili(a} = {2 + tan a}Sin a 11 da Consider Figure 4

.Figure 4 Slope Detection

There is a change in sea-floor slope 6a at p.Rp(-) is the sl~nt range just short of p (half across track resolution cell) Rp(+) is the slant range just beyond P.

The relative change in signal strength is given by Sl = f(a) + lIf(a) f(a) 12 - 20 -

Substituting 10 and 11 in 12 we obtain for the proportional change in signal

S1 a {I + (2 C~t 6 + tan 6)~6} 13a

= t1 + (2+02)~e} for e « 1 13b 6

1 Figure 5 shows 10 log S versus dimensionless range xh- 1 for various breakpoints (~e) in sea floor slope.

;z.o

IS' 10' • ..

o 4-0

Figure 5 Change in Signal Strength db at a Slope Break Point Vs Dimensionless Range.

From Figure 5,·for h = 10m a 3 db signal change is detectable o 0 at 55 m for a 5 slope change and at 90 m for a 3 (1:19) change. Of the 205 PLF's in (31) 115 of them have slopes > 1:19.

The other target features moats and ice furrow patterns are not always present. It has been shown earlier that acoustically significant lithologic changes do not occur in the area. r'- ______1...... _____ ..., _.l-!""--'-"______1__ ...... ,··· '1. •.. -...... - I'· . ... · ....-..------.-- · l..-.!..- ••• - ...... ------• .. _ .. I·.... •· ...... ·I_ .. •·•·.• .... _- _I·'---r-"--.. ,.... ,·L-!.... - ... ow .....~~:.. ___ 2-121u~ 600 ~'l'0\1 I~bh Coll 128dU L 2° 120° ,50" 10~20'~ EOW Nat·k 1 B 105 50-500 0.1 - sIan protecti 0 ,., 128dB . 1. 2 120° ,50010~20 2-151ui 600 ~*SltInt range .IEO&G SNS9GO 105 100-500 0.1 .- Speed Correc­ tion (Addi tional Inputs) ·*Positive or Negotive Dis plllv 2290 ~Modules. avat 12SdB 1.5° 40° '10°20 0-161tr .CKlein 422S-0010 50 800-1200 0.2 6 - 2290 able for sl~ 128dB 0.75° (10° 10 0-16kr Klein 422S-001 100 400-1000 0.1 0 2290 range & spe~ 0.1 128dU 1 40° 10° 0-16k~ K1e11. 422S-001A'~ 100 1,00-1000 0-16kl 2290 correction 1,00-1000 0.1 128dn .1° 20°,"'0° 10~20 Klein (,22XS-OOIA 100 1.5° 40°· 10 0-161tT 2290 . (Additionol 100 1,00-1000 0.1 128dn - Klein 422S-0011l 0 2290 inputs) Klein 422S-001E 500 .50-200 0.02 116dU 0.2 1,0° l.Oo - 0-161tn ,._Optionol CRT Display .. Subbottolll pro filer Dttachment _Additional signal procef sing modules J- TOI~ Fish Colli sion protection ° 15knl 6101-Subbottom pZ'l 117dU 2° 50° .. Ello Hes te rn 60M 100 50-400 0.1 - fiter .attach< ment· I-Positive or Negative Disploy o o 1° JO 5S 110 6500 I Add-on optio~ 0.• 1 129dB ~ . - :Ocelln Research 100 - - . for 5111n t ' ~quipmc!nt Mod.e1 . range and i 1500 , spced corrcc-(, tion i (Additional 'Innll~!I

~"" '':' "_", '•• ~".-" .....~TI\ .....,' .,.,..,"J'-".-s'; ,-.,r,.,""-'_:_'""'" . u",-,,. "' ....,.,,~ "~".' .••• , _ •• _ .•• _ •• _l_ .------e------)~ S~ Soe:" se~_O/Q~co~ue~. F~ C- .__

Equipm!!nt Frequency Uance' Pulse Source Level ncmnwidth (Dc \) 'l'~w 1·'1 s 11 . Special _c \til 1.) (mccr~ Lcnr.th(ms) \lll:n'CH iii IIll1'/lI / V~'l.'t.tc:ll 'fll t }'(, a t u t:tt-J!. -- ~--~ :! ~E.llitiY lil!.!!£.'L ~!!h

Ocean Research -Subbotto~ - (con tinued) profllcr I· . AttachmcJ -Tow Fish . , .. Collisio!

. protcctio

; -unI kS.350A 48 kHz .15 123 - 1.7 60(1) 762m dual Side Scan - - • Sonar Twin 3.2 60(2) , fish , .;,;

. (1) Narrow 1;leam-transducer length 508 mm - multi element array" . , , (2) Wide bea~ transducer length 254 rom - multi element array "

,

.

-. "

.1". • , •

-" \: - 23 -

We conclude that Side Scan Sonar technology of the type considered in Appendix I and listed in Table 4 is not capable of reliably detecting changes in acrosso track sea bed slope associated with PLF's of le·ss than 5. Improved signal processing using digital techniques would improve the ability of the system to resolve small slope changes.

4.4. Echo Sounders (Class B Syst~ms)

Figure 6 shows the operation of a hull-mounted echo sounder of beamwidth 8e and height h above the sea floor. The insonified swathe length is W.

A

.•

h

1-J// ~-- :'~I_- \ I .

Figure 6. Operation of a Hull-mounted Echo-Sounder

Let the surface of the transducer be at depth d beneath the sea surface. Let the water depth be H. Since h» d, H - hand W=2h tan8e/Z-2Htan8e/a.If the vessel speed is V, the area coverage rate Xe is given by:

14 = 2VH tan8e/Z Echo Sounders for shallow water applications are typically o 0 0 15 - 33 , e.g., Kelvin Hughes Sounder is 15 , Ross Sounder 0 0 0 22 and Alpine Sounder 33 • We shall let 8e = 33 for Echo Sounder and thus o Ae = .6VH 15

- 25 -

The projector and hydrophone in Mills Cross Array SonAr Sys tems: 'are long l:tne - transducers,: placed orthogonal to each other. The projector sends out a narrow ' beam in the along axis direction but wide in the across axis direction to cover the angular scanning sector of interest. The hydrophone, being orthogonal to the pro­ jector, has beamwidth narrow in its along axis direction but wide in its across axis direction and thus intersects the projected beam along narrow angular segments. The outputs from each receiving hydrophone element are then phase-shifted and summed in a beamformer receive'r to form N directional output beams. The number of directional output beams N depends on the cross-tr,ack beamwidth ,of the projector Se and the resolution beamwidth of the hydro- phone SH such that: ' S e N = 16

If the survey vessel is moving a constant speed V and the depth of the water is H the area coverage rate Amx for the Mills Cross Array Sonar System is given by: o A ,=,2VH tan (S /2) 17 mx e o Thus A mx increases with Se • There is a limit on how large 0e can be. To form the direction beam-S off the broadside axis of the hydrophone, the increase in beamwidth due to the beamforming process is given by: Sb Sbo = 18 CosS where Sbo is the beamwidth in the broadside direction which is a function of the physical dimension of the projector array and the frequency of operation and Bb is the beamwidth as a result of phase shifting the hydrophone array outputs to form the beam at an 1e B from the normal of the hydrophone array. For B = 45 B(i.e. a scanning angular sector of 90° or in our notation Be= 90°), the increase in beamwidth is 41% at the outermost beams.

The General Instrument Hydro Chart Bathymetric Swathe Survey System (60) is a Mills-Cross Array System with Se = 1050 and BH= 50 giving a 21 beam output at 36 kHz. The Hydro Chart system is claimed to be capable of operating on a ship whose motion is ± 10 degrees in pitch and 20 degrees in roll at speeds in excess of 10 knots. The pulse length can be varied from 1-24 msec to accommodate a variety of water depths. The Hydro Chart System is recently quoted at US$435,000.

Using the above values in equation 17, we have o A = 2.61VH mx 19 - 26 -

Resolution

Figure 8 shows a PLF of base diameter D and side slope at the seabed a6e • The PLF is insonified by some or all of the beams of a Mills-Cross with a total of N Beams.

For 100% survey coverage, the adjacent line spacing is equal to the swathe width W. For modelling purposesPLF's are symme trical cones (possibly t runca ted) wi th a side. slope of ~6 with respect to the adjacent sea floor, a6 is thus the slope change at the "break point".

For D ~ W at least half of the PLF is illuminated at least once during two adjacent line sweeps as illustrated by the dotted PLF in Figure 8.

Thus, the number of beams intercepted by a PLF during 100 percent coverage is

'.---

. -

I~

. J . ., . -:--'-:---~--=-- r- •. __ : .• r...... _ .. -.-:.-.~---.--:---.--- .--- . I - , .r ---:------.- ..... - -.' "' .-".~-. ------=- .. ---~ ,. ~:~~.--. :. ... -- ,. : - -..

. "__ ..• ,_ .,-" __.. _0. ___"_--- ___ ._}--...- ..

Figure 8. 21 Beam Mills Cross 6e = 105~ PLF Diameter D. Side Slope liS In water depth 30 m, W = 78m. - 27 -

N D 19 N = - P W x 2' Where N is the numb'er of beams falling on the PLF and N are tEe number of beams comprising the swathe Wand D is the base diameter of the PLF. The above expression does not allow for the decrease in beam density at the edges of the swathe which would require an additional variable to include the position of the target within the swathe.

The change in elevation ~z across the target is D ~Z ~ 2" tan (89) 20 and the change in elevation per beam is

~! c Wtan(~9) 21 Np N We note that equation 21 is independent of the pingo diameter D.

Now the outermost beam has an angle with respect to the 0 0 horizontal of 37.5 (9 = 105 ) and thus e 0 z ~ R Sin 37.5 22 where Z' is the depth and R is the measured slant range along the outermost ray (worst case).

The slant range resolution for bottom detection of the hydro-chart system depends upon the rise time of the 1 millisecond pulse. The operating frequency is 36 kHz and if the transducer q is 10 then the effective bandwidth is approximately 4,kHz giving a resolution in arrival time of 2.5 x 10-~ seconds corresponding to about 20 cms.

For the outermost beam this corresponds to a depth resolution per beam of ~: ~ .2 Sin 37.5 ~ .12 meters 23 Substitute this value in equation 21 and solve for the PLF side slope

24

Pingoes with slopes less than this will probably not be readily detected. 28 -

4.6. Refraction Effects In The Mixing Layer

The amount of ray bending and refraction depends on the exit angle as the ray departs from the source and the sound speed depth profile. The amount of ray bending and refraction is most severe at low exit angles. Inspection of the water models indicates tht if the exit angle from the source is not smaller than 10 degrees, the amount of ray bending and refraction is acceptable.

Thus neither Class A or Class B systems will be limited by refraction.

4.7. Discussion of Results

(1) Side Scan Sonars - Whilst unmodifie~ conventional side scan sonars operating alone will not. reliably detect all PLF's on the basis of side slope changes only. they will indicate the other target features when present (moats and ice keel furrows). Side 'slope changes of greater than 50 at .a range of 150 meters .either side in water depths at 30 meters and a tow fish elevation of 10 meters will be detected providing careful tuning and set up of time variable gain is closely monitored and controlled and speeds are held to 5 knots or less.

Thus, side scan sonar technology provides a valuable adjunct to survey systems employing echo sounders ,or the Hydro-Chart Mi11s~Cross system. The latter. unit is limited to 10 knot speeds •

. The dynamic range of the gray scale of the graphic recording medium is about 12 db whilst that of the bottom back scattered signal is generally greater than 130 db. The ability of the human eye brain 'combination to discriminate small changes in gray scale is dependent upon the area or spot size as well as the gray scale change itself. Whilst all commercial side scan have some form of time variable gain, the TVG function must be fine tuned by the operator.

The quality of the resulting graphic record is thus very dependent upon these adjustments, and the judgement of the operator of what constitutes a satisfactory output.

Digital processing based on a signal model to adaptivity correct for the change in grazing angle and area beinJ inson!fied offers a large - 29 potential for improved discrimination for detecting changea in bottom slope especially when the back­ ground noise from lithographic changes is low. Such a processor should probably utilize a combina­ tion of gray scale and spot size modulation in the graphic output to maximize the eye brain response.

Signal to noi~e ratio is a primary criterion in selecting a side scan sonar for this application. The sonar system should have a resolution adequate for revealing ice keel furrows and .moats. Higher resolutons than required for this purpose will compromise performance in other areas.

Side lobe suppression is of particular importance. Transducers which consist of arrays of small elements or motors as they are sometimes called, whilst more expensive, can utilize shading in order to suppress side lobes. The design should show evidence that care has been taken to reduce as much as possi~le excitation of modes other than the" primary (piston) mode.

Many sets employ a capacitor discharge to excite the transducer, allowing it to ring. Whilst such a drive arrangement is very simple, reliable and cheap, it is unsatisfactory since it excites the other modes resulting in loss of efficiency, spurious response, increased reverberation noise and additional side lobe generation.

Selection Criteria Summary

Side scan sonars selected for this mission should have the following features:

* Operating frequency 50 kHz or lower for best signal to noise.

* A resolution capable of mapping ice keel furrows and mo.ts, but no finer.

* A remote tilt adjustment for altering the depression angle of the main beam.

* Side lobe suppression by shading or other means. * A sound velocity meter, or a temperature and salinity measuring system to compute sound speed so that the best towing height can be chosen. - 30 -

* Motion compensation for heave and sensing for roll and pitch.•

* Adaptive on-line Digital signal processing to remove the effects of water column loss and grazing angle changes for a flat bottom model. No operator adjustments should be required. Full Tange tape recording should be provided for post processing.

* The transducers should be driven at their piston' mode resonance with a phase coherent gated tone burst.

* Transducers consisting of arrays of smaller elements and which employ shading to suppress side lobes are to be preferred.

Candidate Systems

Commercial sidescan sonars for the most part have been developed for a general purpose price sensitive market. The appliCati0n which concerns us here, across track slope detection, is a difficult requirement to meet with the technology, and none of the systems listed in Table 4 is completely satisfactory and would require extensive modifica­ tion.

The most critical part of the side scan is the tow fish transducers and driver arrangements, since they do not provide much scope for modification. From this point of view, probably the best off the shelf candidate side scan sonar is the UDI 48 kHz - broad beam version. It employs a 48 kHz trans­ ducer multi-element array driven with a gated tone burst.

(2) Multi Beam Systems

The Hydro Chart system supplied in a dome enclosure is a good candidate giving a swathe of 78 meters total in water depths of 30 meters. The operating frequency at 36 kHz is well removed from the side scan at 50 kHz. Price of the Hydro-Chart system is quoted at US$435,OOO, delivery 9 months (60). - 31 -

From equation 15 and 17 25 ~ mx'='2~6lVlH = 4.35VV21 ~ .6V2H Where VI the speed for the Hydro-Chart is limited to 10 knots. If we accept 15 knots as allowable for the echo sounder then the' area coverage rate improvement for the Hydro Chart over the echo sounder is

41.5 ~ 35 = 2 ..9'1 26 4.8. System Parameters for Side Scan

Consider the geometry of Figure 9 which illustrates simul­ taneous coverage with a ship mounted echo sounder or multi­ beam system and a side scan SOnar

, -"r- j ---_._.- . --I-'; -.-:~-~------!I ~ I" ' , -_.. ---. -- -,--:.-.-~. --. --+-+-~-, ,~. - "7'----' - --,-_·t---;---j- , • I . -i--L_.I---L__ .-c-~-o--;----,'--:~~ -_.-" ---,_.-:.. -,-~.--~.- ...-~-;--'':-..,.-+- I iii ! , ! f- ., . f ~ i : ~ . I' -i,f-. ~,.-+--'--,--I-----~- C '~-'--~-~'-~~'!~I'~"I~'..,-'. 'f-,-:,~';) I , ! !- I 1 • . .I .. ,,:. 1 . ! '­ i __ ~-~~-~.-~_7--~L--Li-+_-----~_+I--.,~'--.-:-~:~~~:~~; I • i '. ;-- ·~~.. ~r _ •. _.-~-.,_. "'.--" _ •. C__ L - .. -._.--1-....,..--.;-,....;-;-'4 ~ ;' : . -,!_-,Jf---I-'-_'-,:,,:'+..;'.'_~;:'-'4 .' , ... --~~~--~~~I---' l--.....;~, ~-,;,...!.!.,-;.;., -j.-...l....<:::;;c(",~:..t.~~.:.....t.~:....~?t.Z~..:...-.:-:----. _~._ ~:~~+: .,,, __{_-'-_ ._I_L .. ,. __ '__ · _-'--,' ;...'---''--..L _.... ._ .. _, .. _. : : I

Figure 9'. Geometry For Combined Systems. 0 Hydro Chart Be = 105 an~ rrDl 48 kHz Side Scan at elevation 10 meters.

In this illustration the water depth is 35 meters and the side scan is flown at a maximum elevation above the sea floor of 10 m, or below the lower knee in the sound speed depth profile. That is to say, in the minimum water depths of 20 m, the side scan may only be at 5 meters elevation. In the deepest parts (150 m) the side scan is maintained at 10 meters elevation. - 32 -

The depression angle ~ of the side scan transducers is adjusted so the upper limit of the beam is horizontal, that is to say a grazing angle very nearly zero, thus providing maximum sen~itivity to bottom slope changes.

The above configuration provides positive slope detection within the swathe width of the ship mounted systems, wher~ the side scan grazing angle is steepest and there­ fore it is least sensitive to slope change. At longer horizontal ranges, the side scan sonars sensitivity to slope changes improves. The side scan will provide positive detection of any large scale feature whose elevations exceed 10 meters to its maximum range limit.

4.9. Selection Criteria

1. For maximum range in the ambient sea noise limited case. we selec~ the lowest frequency consistent with adequate resolution ~or detection of ice scotir furrows, the smallest significant target feature.

2. For high tow speeds (maximum area coverage rate) we select a wide horizontal beam width (21jJR') consistent with resolution of small scale features (furrows). (If Hydro-chart multi beam system to be used - speed is limited to 10 knots).

3. For minimum noise aperture we restrict vertical beam width ·(2WD)tO .minimum overlap with echo sounder or Hydro-chart.

4.10. Motion Compensation

Relative movement between ship and tow fish will degrade the combined systems' cap.abi1ity to detect and locate targets •. The Hydro-char.t.system is corrected for roll and pitch. Vertical motion (heave) should be added.

In the tow fish pitch roll and heave should be sensed. All data should be corrected for their motion, pre­ ferably on line.

Whilst a detailed examination of the motion sensing and correction problem is outside of the scope of this study, experience with strap down sensing systems indicates that a satisfactory solution within a reasonable time frame is possible * ..

* A heave sensing and correction system for heave and depth change of the tow fish and for heave of the ship presently exists. - 33 -

4.11. Side Scan Transducers

The performance of a side scan sonar system is dictated by the geometry of its acoustic beam, which depends upon the transducer design. This is the most expensive component, it is not easily modified and realization of new designs have a long lead time.

Of the sonar systems identified in Table 4, the UDI 0 350 wide beam ( 2 •. =3.2 ) a~pe~rs to be a good candidate. H·

(a) It has the lowest frequency 48 kHz.

(b) The transducers are available as single units - i.e. one for each side. o (c) The 3.2 horizontal beam width has adequate resolution.

(d) The transducer is driven with a gated tone burst - the power amplifier being located in the tow fish.

On the negative side, its vertical beam width of 0 0 60 is greater than the desired 40 •

4.12. Performance Estimates

(1) Maximum Range (Rmax)

For the Sea noise limited case, the range far maximum signal to noise ratio is given by equation 78 of Appendix I (e.g. A-1-78) 2.647 x 10 10 R = --~f~2r..{~~~1~~------~1~6~.-4~x--.l·~0'5------~---- + +·2.75 x 10-3} {(1+10-~f2)2: (4100+l0-6 f2)2 } 3 set f = 48 x 10 and

R '" 270 meters

The signal to noise ratio of the frequency dependent cluster of terms in eq •. A-1-73 will be 3 db down from the maximum at a range of 360 meters. Thus, set Rmax = 360 meters. - 34 -

(2) Resolution

The worse case along track resolution is given by eq. A-I-56 with R = Rmax

l1y = 2 RmaxWH

= 2 x 360 x 1.6 x 1T 180

= 20.1 meters

(W is half of the horizontal beam width of 3.20) H (3) Maximum Pulse Repetition Rate (PRF)

For range ambiguity avoidance, the maximum and minimum pulse repetition rate are constrained by A-I-51 and A-I-52. There is no integral solution >1 for these equations and therefore M = 1 and thus from A-I-51

T = 2 Rmax C

'" .5 seconds 1 and the PRF, n = = 2/sec. T (4) Maximum Towing Speed and Minimum Range

At minimum range the along track resolution cell should be illuminated at least once.

If the multi beam system is to be used in a water depth of 25 meters and no overlap is provided by side scan the multi beam half swathe width is

X = 25 tan 55 mb = 35.7 m

and the minimum slant range for the side scan at 10 m elevation is

Rmax =.; 10 2 + 35.7 2

= 37 m - 35 - and along track resolution cell is

= 2 x 37 x 1.671 180

= 2.1 m and the maximum towing speed for 100% insonification of the target at minimum range is

v = 2 x 2.1 = 4.2 meter/sec = 8 knots (5) Area Coverate Rate

The effective area coverage rate for the combined system at 8 knots is g~ven by 0 A = 2 x Rmax x v = 2 x 360 x 4.2 2 = 3024 m /second

in 25 meter water depth.

(6) The Hydro-chart system will pzovide a half width swathe of 360 meters in water depth.s given by

z = 360 tan 55

= 252 m

_._.-._ ...... _----- .-.--...._- ._-_ .. _..... -.------_._--

- 36 -

SECTION 5

ALTERNATIVE SYSTEMS CONCEPTS

5.1 Intraduction

Sonar systems deployed from underwater towed platforms appear to offer a number of advantages over surface vessel mounted sonars for wide swathe bathymetry}. Kowever there are· no readily available sonars mounted in tow fish that provide a wide swathe topographic mapping capability.

One approach is to adapt conventional beam forming tech­ niques to a tow fish. In addition, there are at least two other approaches which have been proposed and we understand are under active investigation in the United Kingdom.

In this section, we outline two concepts for what are generally referred to as topographic mapping sonars (TMS). Either of the concepts would require a commitmen.t to.a five year development program for realization of an operational system.

An effective contribution to the PLF mapping mission requirement is not likely to result from such a program. because of time constraints. However, such systems would appear to offer substantial advantages for hydrographic charting and geological mapping under Canadian conditions as they are presently understood.

A fifth concept known as synthetic aperature side scan sonar (61) offers the only known means for obtaining high area coverage rates with sonar especially in deep water. Such sonars in addition have the very substantial advantage that resolution is not speed or range dependent, but is uniform throughout the area swept.

A synthetic aperature side scan sonar for water depths in excess of 200 meters would be a logical successor to the topographic mapping sonar systems. For the purposes of the PLF mission requirement, there are no concepts for sonar systems that could be developed and usefully deployed in the .artie earlier than April 1982.

5.2 Underwater Towed Sonars

Advantages and disadvantages of underwater towed platforms over surface deployed (including ship mounted) sonar systems for sea bed surveys are: - 37 -

Advantages

(1) Can be installed and removed quickly from a wide variety of non specialized vessels.

(2) In deeper water, systems are in a reduced noise envi ronmen t.

(3) The sonars are removed from the surface mixing layer, generally 20 to 30 meters in depth where water conditions are more stable.

(4) Cavitation thresholds increase with pressure and decreased gas content in the water allowing for higher intensities (dr1ve powers) and thus stronger signals.

(5) Systems are closer to the bottom and provide correspondingly improved resolution.

(6) Makes possible new system concepts not feasible from surface vessels.

(7) . Systems may be placed at most advantageous height in the wat~r column to take advantage of sound speed/dep~h profi1~s •

(8) Body motion compensation makes possible operation in heavy ~eather where surface deployed systems would be affected by surface wave action and aeration in the mixing layer. Essential where trace to trace coh~rence required for imaging such as sub-bottom profiling and side scan sonar.

(9) Surface ghosting and sea surface back scattering effects generally less troublesome.

Disadvantages

. (1) Special handling gear necessary.

(2) Tow speeds (generally 12 knots or less) in deeper water >500m if close bottom clearances are necessary.

(3) System must include. means for knowing position qf the tow fish for h.igh precision work.

(4) Generally systems are more costly and require more maintenance. - 38

Sonar systems generally have a large potential for further improvement due to the dramatic increase in the accuracy bandwidth product per unit of cost arising from use of modern LSI technolgy. This makes it possible to realize real time operating systems employing modern methods of digital signal processing.

5.3 Topographic Mapping Sonars

Lloyd Mirror Approach

The following description is excerpted from a previous company study entitled"Side Scan Sonar Systems Study" - November 30, 1979, carried out under the Mosaics Project (PILP-COPI) .

"In order to deduce information on" the bottom slope, it is necessary to use a multi-beam system. An early application of such a technique to side scan sonar' is to use the sea surface as a virtual source so that when the towed transducer is run close to the sea surface, the direct arrival and the surface reflected arrival from the sea bottom would create an interference pattern on the recorder paper (Lloyd mirror) whereby the slope of the sea floor can be deduced."

Consider the geometry shown in Figure 10.

j). L

Figure 10. Geometry of Lloyd Mirror System - 39 - o is the transducer at depth t below the sea surface NN' , assumed to be flat. Pi is a reflector at slant range Ri from the transducer 0, at horizontal range hi from 0 and at depth Di below the sea surface.

Since the sea surface is a good reflector, the pressure field at 0 will be the sum of the signal over paths PiO and Pi or PiNO' where 0' is the virtual source. The path difference is given by:

11 = O'p - ~ 2 From O'P = (D~ + tj2 + Rl - (Di t)2

2 2 2 2 2 = Di + 2Dit + t + Ri - Dr - t + 2Di;t

2 = 4tDi + Ri 27 or O'p = IR/ + 4Di t

4Dit ~ 3 Ri (1 + ~)

~ R:~ ~ 28

, ignoring higher order terms in (t/R).

= 2Di t Thus, 11 29 Ri Since the sea surface is a pressure release surface, the signal will undergo .a 1800 phase shift upon reflection. The received signal will be a maximum when the path difference is a multiple of an odd number of wavelengths i.e.

= 2Dit (n - ~)A R 30 i The received signal will be a minimum when the path difference is a multiple of an even number of wavelengths 2D.t nA = ~ 31 R i As Pi moves along the bottom contour, conditions (30) and (31) will be alternately satisfied. The received record will thus display an interference pattern.

Let the grazing angle be ~i' then Ri = (Di - t)/Sin ~i. - 40 -

For a flat bottom D~ = D is a constant. Thus the received signal maximum occurs when:

(n - ~)A =

= 2Dt Sin u i 32 (D - t)

Thus n ~ Sin a i for a limited range of a i (between 0 and n/2) , Sin a. can be approximated by a straight line. The spacing betwe~n signal maximum is therefore regular for a flat bottom.

For an upward slope, Di decreases as ai',g~ts smaller and from the condition of signal maximum: 2p. t = _,,_1_ (n - ~)A Ri ,2D. t , 1 = 33 (D' - t) i We observe that the spacing between signal maxima decreases at smaller ai" The opposite is true for ~ downward slope, where the spacing increases at smaller ai.

This can also be observed from the following: 2Dit From (n - ~) A =

dn 2t dR :,-1 = -r-{Di - 34 1 dDi. dn 2tD .­ For a flat bottom = 0 and Di" = -A- dRi = D, thus dR.:-l 1 dD For an upward slope dR i .is negative and for a downward dD. slope ~d1 is positive • Ri To apply the result to determine the depth D .• we note that for a fixed A, t and R. (rea~ off fro~ th. r~torder record), Di can be calculated o~ce we know n, the fringe order. Only one value of n is required, since adjac.nt fringes differ by + 1. n can be obtained by calibrating the depth at one known point on the side scan record (using e.g. a depth sounder). - 41

Alternatively, the depth can be estimated from known values' of a i • From the condition for signal maximum: 2tDi (n ~p. - = R 35 i d n i 'Ii t Since D. ::» t , we let 1. Ri '" Ri Sin a = i (n 2t Sin Then: - ~)A = a i 36 Let N be the value of n corresponding to

a = If/2 i Then: (N - ~)A = 2t 37

= 2 tOi Thus: (n - ~)A R{

= 2t sin a. 1.

= (N - ~)A

(n - ~) or Sin a = i (N ~) 38

~)2 ~ and Cos = {I {n 39 a:i. - (N - ~)2} Then: Z = R Sin a 40 i i i Cos a 41 hi = Ri i This also requires a single point determination of a i at a given Ri· Instead of using the sea surface as a virtual source, a second transducer can be used in its place, thus eliminating the reliance on surface reflection. This is reported in the Telesounding work (63). We shall first derive expressions for the far-field pressure functions for an i.n-phase and anti-phase two transducer system. The geometry and notations are shown in Figure 11.

. -.-~-- "--' -'------,.. -. " ...... _.. ' .. --""-.- '"-:"" ," I

- 42 -

Figure 11. Two Transducer Systems

The two transducers dimensions a ar~distance d apart. We shall take the origin mid-way between the two transducers. The X-axis is taken along the dimension of the transducer the Y-axis is taken normal to the transducer as shown.

The pressure field at a distance ro from the origin and incidence angle a is given by (assuming an infinite baffle and uniform velocity distribution across the transducer face):

Pero ,a) = 42 where A is a ·proportionality constant, k = 2n/l and w is the frequency in rad/sec and r is the distance of P from the transducer. Since r»d, we can approximate:

eiwt ikr P(ro ,a) '" !!r : Ix e- ... dx ikr = A' f e- dx 43 x - 43 -

For an inphase system, the output from the two transducers are summed coherently. Thus: (_¥ (dlo {.z ze~i-l~(ro+sinalx + z 2 - ik(ro-xsina)dX} Pdro,a) e J :..L~ , .L.!!. z z z z

l -ikro ikasina/z -ikasina/z ikdsina/z+ -ikdsina/2 = A e {e ,..e } . {e e } ik sin a

-ikro = 2A 1 e sin(kasin

= B sin(ka sina/2) (kd i 12) . (ka sinaal Z) cos s na

For an antiph.se system, the output from one transducer is at 180 o phase reversal from the other i.e. _ ¥ (dla ( z z z z P (ro,a) = Al { . e-ik(ro+xSina)dx _ e-ik(ro-xsina)dX} 2 ) - ~-.!!. ) ~.!!. z z z z

Al -ikro ikasina/z -ikasina/z ikdsina/z -ikdsina/z = e {e iks~:a } {e ~e } -ikro =2iAlesin(kasina/z) sin (kdsina/z) (kasinal z)

= iB sin(kasina/z) sin (kdsina/z) (kasina/z)

= B sin(kasin /z) sin (kdsina/z) 45 (kasin Iz)

From k = 2~/A we can rewrite (42) and (43) as

~ -- B sin(,asina) Cos ( ~d ina) Pl(ro,a) 1\ I s ~ (Iasina) 46

~ (Ia sinal i P 2 (ro,a)= Bl sin s n 47 (~a sinal A

. -.. -.- --,-~ - 44 -

For d =20 cm, A = 0.6 cm (250 kHz) and a = 1.5 cm (corresponds to 35 0 beam), a computer plot of the anti-phase beam pattern (magnitude of equation 36) is given by (63) and shown in Figure 12. It should be noted that the envelope of the function is dictated by sin x/x term due to transducer aperture whereas the finenstructure under the envelope is governed by the sin (,d sinal term due to the spacing. In the figure, the bea~ is tilted 0 downward at 20 • Note that the in-phase beam pattern is identi cal to tha t of the an ti-phase excep t fo r a phase shift. '

By summing the transducer outputs at 'anti':"phase orinphase, the received record will show alternate bands of signals separated by shadow corresponding to nulls on the radiation pattern. The result is thus identical to that of using a reflector. Another point worth noting is that the 'transducer arrangement' here is quite different from the transducer array arrangement 'used in other sonar systems" In these other applications, the array spacing is restricted to so that n n Al2 the term cos(Idsina) orsin(Idsina) is a slowly varying funct16n of a and is a single-valued function o£ a ,for a 0 between 0 'and 90~ SO that the fine structure under, the Sin x/x envelope does not exist. In the side scan application,

80 60 40

o

10

.'

40 Figure 12. Directivity Pattern of the Two Transducer Array drawn on a linear amplitude Scale. - 45 - the purpose is to generate a multitude of beams with two transducers. In this case, the arr~y spacing is mUih larger than A so that the term cos(rdsina) or sin (rdsina) is a rapdily varying function of" a and is a multiple- " 0 0 varied function ~f a for a between 0 and 90 thus accounting for the periodic fine structure under the sinx/x envelope.

In a development being pursued at Bath "University under government (British) funding, a third transducer is located in the same plane so as to generate a coarser beam pattern the spacing of the two receiving transducers in relation to the transm-itter and receiver being chosen to provide a vernier effect thus giving a very high resolution to the beam angle.

Application of the two transducer system to side scan ~s shown in Figure 12. The tra.nsducers T and T 1 are separated by a distance d. C is shown to illustrate. the resemblance to the Lloyd Mirror method. The reflector P on the sea floor is illuminated by the nth beam and is at range Zn (depth) "and Yn (horizontal range) from the transducer c!'ntre. The grazing angle is indicated by en. The condition for the point P to be an enhanced beam is (for the antiphase system):

(n - I:;}A = d. sinen = d ZII/Rn 48. 0 Let N be the number corresponds to en = 90 , or (N - 1:;) = d/A 49 Thus, Sin en = (n - 1:;) I ( n-I:;) 50

Cos en = {I - (n-I:;) 2.1 (N-I:;) 2"} l:; 51

Zn = Rn (n-l:;) I (N-l:;) 52 {I - (n-l:;)2 (n_l:;)2}l:; Yn = Rn I 53 - 46 -

Similarly, the condition for the point P to be an enhanced beam for the in-phase systems is:

nA = d sina n = dZ ./R n n 54 Sin a = n/N n 55 Cos a = {l n2/N2}~ n - 56 Zn = Rn N/ N 57 2 2 1 Yn ={Rn l_n /N }'1 58

I T "- \ " • T \ d " C 1 T ~..

Figure 13. The Lloyd Mirror Effect Applied to a Reflector or Transducer or to two Transducers. - 47 -

Thus Zn.and Yn can be computed once n is known. Only one value of n need to be ascertained since neighbouring values differ + 1.

5.4. Parallex Measurement

Denbigh in a recent paper (64) describes a TMS sonar employing two receiving transducers and a transmitter. The concept reduces essentially to determining the arrival time difference of pulses originating from the same set of seabed scatterers at two receiving transducers by measuring the phase between the two signals.

The resulting phase difference yields the late rial distance off track and the elevation of the scattering region as a continuous function of time following transmission.

Denbigh's results show that the transmitted pulse length in water should be about one fifth of the desired depth accuracy. On this basis, a 100 microsecond pulse should provide depth accuracies of the order of better than 1 meter. A copy of Denbigh's paper is included as Appendix 5. - 48 - SECTION 6

CONCLUSIONS AND RECOMMENDATIONS

6.1. Conclusions

(1) The sound propagation conditions in the first 10 to 15 meters of depth in the south eastern Beaufort sea are not favourable for operation of oblique angle sonar systems. Stable conditions are found below the halocline generally at 5 to 20 meters.

(2) Oblique angle sonar systems can be successfully operated in the Beaufort Sea from surface vessels providin§ the ray exit angles .are steeper than 10 to 15 below the horizontal.

(3) Side scan sonars cannot .be operated reliablyunles's they are placed below the surface mixing layer. This can usually be identified by the haloclime.

(4) Side scan sonar technology should be potentially capable of detecting changes in bottom slope of as little as 3q at ranges to 300 meters each side.

(5) Candidate side scan ,sonars should have the charac­ teristics desctibed in sections 4.7(1) and 4.8 through 4.12 inclusive. These are highlighted below:'

(a) Driven transducers

(b) Operating frequency below 50 kHz

0 (c) A horizontal beam width of about 4 • o ( d) A vertical bandwidth of about 40 •

(e) Be fitted with a continuous reading STD.

(f) Have means for tilting the transducers (± lOowould be satisfactory) (g). Employ full dynamic range digital signal processing to recover absolute magnitude of signals.

(h) Generate synthetic graphics for display from digital signals to obtain best recognition of small signal changes. For example, simultaneous modulation of both spot size and gray scale.

LlGRAR'I' F OCEAN SCIENCES INSTIT UTE O 49 -

(i) Be fitted with body motion sensors and motion compensation on display.

(j) Employ wide dynamic range recording on magnetic tape for off line 'processing.

(k) Hard copy on line graphic recorders should be as nearly· automatic as possible to reduce the necessity of intervention by the operator.

(6) The maximum range of a 48 kHz system will be about 300 meters each side with positive detection of any PLF with a height exceeding the two fish elevation - (5 meters).

Operations in water depths as shallow as 20 meters shotild be possible under almost all conditions extept possibly. following storm surges; when bottom of surface mixing layer may extend deeper than 15 meters.

6.2. Recommendations

(1) A detailed assessment of the UDr 48 kHz wide angle sys~em and of other comparable systems should be undertaken in order to make a final selection of the essential components.

(2) Uridertake' Development of a digital signa1 processor for both recording on magnetic tape and signal conditioning for hard copy graphic recorders in parallel with above 6.2(a).

(3) Two sets of purchased components be acquired and integrated into two systems meeting the requirements identified in 6.1.(5)

The two £ystemapproach makes it possible to dedicate at least one system full time to development. Full integration of the system components may not be possible by March 31, 1981 - however, an operational system with the essential capabilities of transducer tilt adjustment and controls could be provided and possibly a digital signal processor could be provided subject to resources being allocated without delay.