This is an introductory text that presents the major optical ocean sensing techniques. It is suitable for professionals and managers in related disciplines, as well as students who are interested in exploring career paths in remote sensing of the ocean or ocean engineering.
The book starts with a brief overview of the main branches of ocean research, including physical, chemical, biological, and geological oceanography, as well as biogeochemistry. The basic optical properties of the ocean are presented next, followed by underwater and remote sensing topics, such as diver visibility; active underwater imaging techniques and comparison to sonars; ocean color remote sensing theory and algorithms; lidar techniques in bathymetry, chlorophyll, temperature, and subsurface layer explorations; microwave sensing of surface features including sea surface height, roughness, temperature, sea ice, salinity and wind; as well as infrared sensing of the sea surface temperature.
Platforms and instrumentation are also among the topics of discussion, from research vessels to unmanned underwater and aerial vehicles, moorings and floats, and observatories. Integrated solutions and future sensing needs are touched on to wrap up the text. A significant portion of the book relies on sketches and illustrations to convey ideas, although rigorous derivations are occasionally used when necessary.
- List of Acronyms and Notation
- 1 Oceanography Overview
- 1.1 Introduction
- 1.2 Planet Earth and the Oceans
- 1.3 History of Oceanography
- 1.4 Branches of Oceanography Research
- 1.4.1 Chemistry
- 1.4.2 Physics
- 1.4.3 Biology
- 1.4.4 Geology
- 1.4.5 Biogeochemistry: global carbon budget
- 1.5 Summary
- 2 Basic Optical Properties of the Ocean
- 2.1 Introduction
- 2.2 Light Attenuation: Absorption and Scattering
- 2.2.1 Absorption
- 2.2.2 Scattering: volume scattering function, backscattering coefficient
- 2.2.3 Beam attenuation
- 2.3 Light Propagation
- 2.3.1 Basic definitions
- 2.3.2 Snell's law
- 2.3.3 Radiative transfer equation
- 2.4 Light Generation: Solar Radiation, Fluorescence, Bioluminescence, Raman Scattering, Brillouin Scattering
- 2.4.1 Solar radiation
- 2.4.2 Fluorescence
- 2.4.3 Bioluminescence
- 2.4.4 Other inelastic scattering: Raman and Brillouin
- 2.5 Summary
- 3 Underwater Sensing: Diver Visibility
- 3.1 Introduction
- 3.2 Point Spread Functions and Modulation Transfer Functions
- 3.3 Point Spread Functions in the Ocean
- 3.3.1 Historical forms
- 3.3.2 A simplified form
- 3.4 Modulation Transfer Function of the Ocean Waters
- 3.5 Impacts from Underwater Turbulence
- 3.5.1 Theoretical treatment: the simple underwater imaging model (SUIM)
- 3.5.2 SUIM validation
- 3.6 Secchi Disk Theory Revisited
- 3.7 Through the Sensor Technique
- 3.8 Summary
- 4 Active Underwater Imaging
- 4.1 Introduction
- 4.2 Active EO Systems
- 4.2.1 Separation of source and receiver
- 4.2.2 Time of flight and range gating
- 4.2.3 Synchronous scan: spatial filtering
- 4.2.4 Temporal modulation
- 4.2.5 Imaging underwater particles
- 4.3 Comparison to Active Acoustical Systems
- 4.3.1 Active acoustical systems
- 4.3.2 Depth sounder
- 4.3.3 Sidescan sonar
- 4.3.4 Imaging sonar
- 4.4 Summary
- 5 Ocean Color Remote Sensing
- 5.1 Introduction
- 5.2 Basic Principles of Ocean Remote Sensing
- 5.3 Atmospheric Correction
- 5.4 Ocean Color Sensors
- 5.4.1 CZCS
- 5.4.2 SeaWiFS
- 5.4.3 MODIS
- 5.4.4 MERIS
- 5.4.5 HICO
- 5.4.6 GOCI
- 5.4.7 VIIRS
- 5.5 Retrieval Algorithms
- 5.5.1 Empirical methods
- 5.5.2 Semi-analytical methods
- 5.6 Calibration and Validation
- 5.6.1 Basic calibration: radiometric and spectral
- 5.6.2 In-orbit calibration: vicarious method
- 5.7 Summary
- 6 Ocean Lidar Remote Sensing
- 6.1 Introduction
- 6.2 Basic Components and Principles of Lidar Remote Sensing
- 6.3 Lidar in Depth Sounding: Altimetry and Bathymetry
- 6.4 Lidar in Temperature Measurements
- 6.5 Lidar Fluorescence of Subsurface Chlorophyll and CDOM
- 6.6 Other Lidar Applications
- 6.7 Summary
- 7 Microwave Remote Sensing of the Ocean
- 7.1 Overview
- 7.2 Passive Sensing of the Sea Surface Temperature (SST), Salinity (SSS), and Sea Ice
- 7.3 Active Microwave Sensing of the Ocean
- 7.3.1 Altimeter
- 7.3.2 Scatterometer
- 7.3.3 Imaging radar
- 7.3.4 Synthetic aperture radar (SAR)
- 7.4 Summary
- 8 Infrared Remote Sensing of the Ocean
- 8.1 Overview
- 8.2 Sea Surface Temperature (SST): Definition
- 8.3 Basic Principles
- 8.4 SST Sensors and Algorithms
- 8.4.1 AVHRR
- 8.4.2 MODIS
- 8.4.3 Transition to VIIRS
- 8.5 Cloud Detection
- 8.6 Summary
- 9 Platforms and Instruments
- 9.1 Overview
- 9.2 Platforms: from the Bottom to the Surface, to Air, to Space
- 9.2.1 Surface platforms: ships
- 9.2.2 Remote sensing platforms
- 9.2.3 Subsurface vessels: unmanned underwater vehicles
- 9.2.4 Floats, buoys, moorings, observatories, and shore systems
- 9.3 Common Instruments for Ocean Observation
- 9.4 Summary
- 10 Integrated Solutions and Future Needs in Ocean Sensing and Monitoring
- 10.1 Overview
- 10.2 Tactical Ocean Data System
- 10.2.1 Glider optics
- 10.2.2 OPCAST
- 10.2.3 Optimization and 3D optical volume
- 10.2.4 TODS summary
- 10.3 Future Ocean Sensing Needs
- 10.3.1 Short- to mid-term outlook
- 10.3.2 Long-term outlook
- 10.4 Concluding Remarks
This book covers general topics related to optical techniques in ocean sensing and monitoring applications. It is written as a tutorial text, primarily for those with undergraduate training in science and engineering who wish to gain a broader understanding of these topics. The book is designed to serve as a stepping stone for more advanced topics in specialized disciplines. It can also serve as a refresher for those who are already familiar with some of the topics but wish to have a peek at recent advances, trends, and new challenges. It provides readers with the necessary expertise and technical know-how to understand the current issues at hand and is a great tool for managers as well as graduate students and young professionals to gain a sense of their whereabouts in the newer aspects of this field.
There are many specialized areas in oceanography, and it would be impossible for this tutorial to include them all as well as their fundamentals and field data. Instead, this book takes a more narrative approach and focuses on the science and reasons behind methods and approaches, rather than on the precise details of issues (unless they are key to understanding the problems). Bibliographical entries are given for readers who have strong interests in certain topics.
Although this introductory text mainly focuses on the area of optics in oceanography, a bigger picture of oceanography in general must be provided. Chapter 1 starts with the history of the oceans and ocean research, followed by outlines of the major ocean research disciplines and topics they cover, including chemistry (often referred to as chemical oceanography or marine chemistry), physics, biology, and geology. One specific interdisciplinary area, biogeochemistry, is included due to elevated interest and associated pressing issues related to global climate change.
Optical properties of the ocean are the focus of Chapter 2, which lays the foundation for discussions in later chapters. This mainly covers the small transmission window in the visible wavelengths of electromagnetic bands, especially when in-water transmissions are considered, due to the strong absorption by the water in longer (infrared, microwave, radio) and shorter wavelengths. The properties associated with other bands are discussed in corresponding chapters when necessary.
Chapter 3 covers diver visibility, or passive optical imaging issues. This is one of the oldest topics in oceanography research. Historical approaches as well as state-of-the-art methods are covered in detail, which is the reason for including precise mathematical formulas. The influences of both turbidity and turbulence are discussed in this chapter.
Chapter 4 extends the discussion of underwater imaging to active methods, covering both optical (light detection and ranging, known as lidar) as well as acoustical approaches (sound navigation and ranging, known as sonar), although the main focus remains on optical systems. Major approaches to enhancing optical imaging range and resolution are covered, including source-receiver separation, range gating, synchronous scan, and modulation. Several techniques discussed are current, active research topics that show very promising potential for future underwater sensing capabilities.
Remote sensing of the ocean is one of the key advances in modern oceanography. The next four chapters discuss the major areas of research and applications in this field. Chapter 5 focuses on ocean color remote sensing, which is the primary method to quantify ocean productivity on synoptic scales. Major ocean color sensors are discussed, including CZCS, SeaWiFS, MODIS, and MERIS, along with several recent and unique sensors including HICO, GOCI, and VIIRS. Atmospheric correction and ocean color retrieval algorithms are compared and summarized, and calibration and validation methods are discussed.
Since visible light provides the only means for penetrating the ocean from space, active sensing of the vertical structures of the ocean using light can provide much needed information in ocean sensing and monitoring. This is primarily performed by ocean lidar, which is the topic of Chapter 6. Principles and sensing techniques are discussed that cover depth sounding, temperature, chlorophyll, CDOM, as well as recent studies involving fisheries and subsurface layers.
Unlike visible bands, microwave remote sensing is not influenced by clouds and is capable of providing 24/7 sensing of the ocean from space, despite its inability to penetrate water. The sensing of sea surface height, salinity, temperature, sea ice, wind, and roughness is covered briefly in this chapter. Active sensing with imaging radar and synthetic aperture radar, which provides very detailed information of the ocean surface, is also discussed in Chapter 7. Active sensing helps in deriving subsurface features, such as internal waves, as well as in detecting surface materials such as oil slicks.
Chapter 8 focuses on infrared remote sensing of the ocean, namely, sea surface temperature. This is currently one of the most widely used ocean sensing and monitoring methods, affecting weather forecasting, hurricane predictions, fisheries, and long-term climate change, to list a few. Major sensors are discussed, including AVHRR, MODIS, and VIIRS, along with corresponding retrieval algorithms.
Ocean sensing platforms and major instruments are the topics of Chapter 9. Platforms are key to ocean research, ranging from traditional ocean research vessels to unmanned underwater vehicles, unmanned aerial vehicles, floats, buoys, drifters, and large-scale moorings and observatories. Several in situ sampling instruments are briefly mentioned along the lines of functionality and type; the brevity of their coverage in this book is due to the myriad of specialized sensors for various applications that are in existence, in addition to the new types and designs which come online with ever-increasing design-to-deployment rate.
The last chapter discusses integrated solutions for addressing ocean sensing needs, using an example of Tactical Ocean Data Systems (TODS), where in situ sampling, remote sensing, and ocean models are fused to provide an optical ocean forecasting product. Future goals and requirements are discussed in terms of scales of study, automated data processing, and sensor planning. The long-term outlook discussed in this chapter reflects the author's opinions on sustainable growth provided by the ocean.
This book reflects the author's preferences, and is meant to encourage the reader toward more advanced topics. As a result, several key areas have been intentionally ignored or unrepresented.
Without the support from my family, this book would not be possible. I want to thank my wife and also my best friend, Chunzi Du, for her encouragement. I also want to thank my sons, Joshua and Wilson, for asking the right questions and putting up with me ignoring their play invitations. I would also like to acknowledge the support from the Naval Research Laboratory, as well as the help from anonymous reviewers, particularly editors Dara Burrows and Teresa Wiley Forsyth.