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Backscatter Measurements by Seafloor-Mapping Sonars Guidelines and Recommendations

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Backscatter Measurements by Seafloor-Mapping Sonars Guidelines and Recommendations Backscatter measurements by seafloor-mapping sonars Guidelines and Recommendations A collective report by members of the GeoHab Backscatter Working Group Editors Xavier Lurton and Geoffroy Lamarche Authors: Xavier Lurton1 Geoffroy Lamarche2 Craig Brown3 Vanessa Lucieer4 Glen Rice5 Alexandre Schimel6 Tom Weber7 Affiliations 1 – IFREMER, Centre Bretagne, Unité Navires et Systèmes Embarqués, ZI Pointe du Diable, CS 10070, 29280 PLOUZANE, France 2 - National Institute of Water & Atmospheric Research (NIWA), Private Bag 14901, Wellington 6241, New Zealand 3 – Applied Oceans Research, Nova Scotia Community College, Waterfront Campus, 80 Mawiomi Place, Dartmouth, Nova Scotia B2Y 0A5, Canada 4 – Institute for Marine and Antarctic Studies, University of Tasmania, Private Bag 49, Hobart, TAS, 7001, Australia 5 5 – National Oceanographic and Atmospheric Administration, Center for Coastal & Ocean Mapping/Joint Hydrographic Center, 24 Colovos Road, Durham, NH, 03824 6 – Deakin University, Warrnambool Campus, PO Box 423, Warrnambool, VIC 3280, Australia 7 – Center for Coastal & Ocean Mapping/Joint Hydrographic Center; 24 Colovos Road, Durham, NH, 03824 Corresponding authors Xavier Lurton <[email protected]> Geoffroy Lamarche <[email protected]> Assistant Erin Heffron QPS US, 104 Congress Street Suite 304 Portsmouth NH, 03801 [email protected] Page | ii Acknowledgements We are hugely grateful to Erin Heffron, QPS, for the support and help she provided throughout the process of the BSWG, including organizing video-conferences, managing the Dropbox, keeping minutes, and proofreading all the chapters. A massive task that she undertook with class and good humor at all time. Our recognition goes to the authors of the six chapters, and in particular to the first authors: Tom Weber, Vanessa Lucieer, Craig Brown, Glen Rice and Alexandre Schimel. The support of the GeoHab group has been also determinant in this endeavor, and we are grateful to Andrea Fiorentino and Daniel Ierodiaconou for programming the special session about backscatter during the annual GeoHab meeting they chaired in Rome (2013) and Lorne (2014, Australia), respectively. Thank you also to Alex Bastos for enabling the launch of the document at the GeoHab 2015 in Salvador, Brazil. Cover image: Disclaimer Suggested reference Lurton, X.; Lamarche, G. (2015) Backscatter measurements by seafloor-mapping sonars. Guidelines and Recommendations. Geohab Report. 150p. Page | iii TABLE OF CONTENTS Chapter 1 - Introduction to backscatter measurements by seafloor-mapping sonars ........................ 1 1.1 Background and objective of the report ................................................................................. 1 1.2 Limitations ............................................................................................................................... 3 1.3 Basic concepts ......................................................................................................................... 4 1.4 From backscatter to seafloor characterization ....................................................................... 8 1.5 A short historical perspective .................................................................................................. 9 1.6 Document layout ................................................................................................................... 11 1.7 References ............................................................................................................................. 13 Chapter 2 - Background and fundamentals ....................................................................................... 16 2.1 Objectives .............................................................................................................................. 16 2.2 Echosounding of the Seafloor ............................................................................................... 16 2.3 The sonar equation and Target Strength (generic) ............................................................... 18 2.3.1 Active sonar equation ................................................................................................... 18 2.3.2 Target strength of discrete targets ................................................................................ 22 2.3.3 Target strength of extended volume targets ................................................................. 23 2.3.4 Target strength of the seafloor: an extended surface target ........................................ 24 2.4 Physical phenomena in seafloor backscatter ........................................................................ 27 2.4.1 Reflection from a smooth surface ................................................................................. 27 2.4.2 Seafloor roughness and its impact ................................................................................ 29 2.4.3 Coherent reflection coefficient ..................................................................................... 32 2.4.4 Basic treatments of rough interface scattering ............................................................. 33 2.5 Putting it all together: what we observe from the seafloor ................................................. 35 2.5.1 Combined model ........................................................................................................... 35 2.5.2 Angle dependence ........................................................................................................ 36 2.5.3 Frequency dependence ................................................................................................. 37 2.5.4 Seafloor-type classification ........................................................................................... 38 2.5.5 Individual echo structure .............................................................................................. 39 2.6 Summary ............................................................................................................................... 40 2.7 References ............................................................................................................................. 43 Chapter 3 - Seafloor backscatter user needs and expectations ......................................................... 44 3.1 Introduction .......................................................................................................................... 44 Page | 4 3.2 Users of backscatter- the past, the present and the future .................................................. 45 3.3 Objectives .............................................................................................................................. 49 3.3.1 Mapping for discovery (relative level- single survey). ................................................... 50 3.3.2 Mapping for monitoring (calibrated –absolute level) ................................................... 50 3.3.3 Spatial resolution .......................................................................................................... 54 3.4 Processing procedures .......................................................................................................... 54 3.4.1 Signal processing ........................................................................................................... 56 3.4.2 Image processing........................................................................................................... 59 3.5 Applications ........................................................................................................................... 60 3.5.1 What do we want to achieve from the different backscatter processing procedures? 60 3.5.2 Discipline requirements ................................................................................................ 61 3.6 Ancillary data ........................................................................................................................ 64 3.6.1 Data types ..................................................................................................................... 65 3.6.2 Ground truthing data .................................................................................................... 66 3.6.3 Water column data ........................................................................................................ 66 3.7 Data formats ......................................................................................................................... 66 3.7.1 Common data format for archiving ............................................................................... 66 3.7.2 Metadata requirements ................................................................................................ 67 3.8 Current needs........................................................................................................................ 68 3.9 References ............................................................................................................................. 70 Chapter 4 -Backscatter measurement by bathymetric echo sounders ............................................. 73 Acknowledgement ............................................................................................................................. 73 4.1 Objectives .............................................................................................................................. 73 4.2 Backscatter measurement: today’s MBES state of the art ...................................................
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