The Handbook of Sidescan Sonar

Knowledge of the Earth and its evolving environment is proving increasingly crucial. Scientific, economic, political, and social decisions all depend at some time or another on this knowledge, and we like to think that we know all there is to know about our planet. One may be justified in doing so today, in the 21st century, by looking back at those maps with white unexplored regions that were still prevalent at the beginning of the 20th century. Yet, in many respects, we know more about the solid surface of other planets than about our own Earth. Rovers driving on Mars for years on end, landers on far-away Titan, and now the international missions to the Moon cannot mask the fact that ocean bottom landscapes only a few kilometers from our shores are still completely unknown.

Acoustics is the best and often the only means to investigate the water column and seabed efficiently and accurately. There is a large variety of instruments now available, and sonar mapping systems can be roughly divided into three categories: single-beam echo-sounders, multibeam echo-sounders, and sidescan sonars (Figure 2.1).

The previous chapter (Chapter 2) presented basic acoustic theory, from the generation of a sound wave to its scattering on/in the seabed and its reception by the sonar platform. Acoustic pressures are used to quantify the response of the seabed at the particular frequency used. These backscatter measurements are combined across-track to create a swath, and successive swaths are combined to form an image of backscatter variations. If the sonar is calibrated, these variations can provide generic measurements, called backscatter strengths, comparable with results from other surveys (with the same sonar at the same settings, or not). These processes are detailed in Section 3.2. Sidescan sonar imagery can be usefully compared and merged with bathymetry measurements. The newest generation of sidescan sonars can now produce imagery and bathymetry at the same time, using interferometry (explained in Section 3.3). These measurements of the seafloor topography are at the same resolution and co-registered with the measurements of the acoustic response of the seafloor. The large majority of sidescan sonars currently in use do not have interferometric capabilities though, so bathymetry is derived from other sources, like multibeam sonars. These measurements usually do not have the same ground resolution (e.g., 100m multibeam bathymetry and 6 m sonar imagery), and rarely benefit from the same exact geographic registration requiring the use of techniques like rubbersheeting (presented in Chapter 4). The problem is compounded by the fact that new multibeam systems also provide some forms of imagery, calculated by analysing each beam or only portions centered on the irst seabed return.

As they follow each other, backscatter profiles build up a fuller picture of the seabed. As time of reception (i.e., range from the sonar) increases, each profile will show typical variations (Figure 4.1, see color section). First, there are very low returns, due to background noise and reverberation as the signal still propagates in the water column. The first seabed returns (from small rubble, in this example) mark a strong difference, as the transmission loss is minimal. As it propagates with time, the acoustic signal will then image areas farther away from the sonar track, and subject to increasingly higher transmission losses. Using the terrain of Figure 4.1 as an example, the first topographic change (fault scarp close to the sonar track) is marked by a sharp peak in reflectivity, from slopes directly facing the imaging sonar. The small volcano will be clearly visible, as its sonar-facing slope will be more reflective, the caldera at the top will show lower backscatter, and its slope facing away from the sonar will be less reflective. The fissure on the ground will also be marked by a strong decrease in reflectivity, especially if it is deep enough and large enough that some portions of it are in the acoustic shadow zone. Backscatter levels return to normal on the other side of the fissure. Far-range fault scarps will still be associated with increased reflectivities, but subdued because of the higher ranges (and attenuation).

Morphologically, mid-ocean ridges are the most complex and geologically active terrains on the seabed. They are now recognized as the places where the Earth’s oceanic crust is constructed (starting with the seminal paper of Vine and Matthews, 1963). Characterized by high topographic relief, the mid-ocean ridges are dominated by volcanic and tectonic processes. Globally, over 60,000 km of mid-ocean ridges produce ∼35 km³ of new volcanic crust every year. This new crust is welded to the retreating edges of the older crust as the Earth’s plates move apart. Some of these plates will collide, one plate sliding below another (a process known as subduction). This oceanic crust is destroyed in deep-ocean trenches, characterized by extreme topographic relief and dominated by tectonic and sedimentary processes. Spreading and subduction are the two extreme stages of the evolution of oceanic crust, corresponding to its creation and its recycling. Since the recognition of plate tectonics in the late 1960s, there has been much theoretical and practical work on their respective mechanisms. This chapter aims at presenting the main structures that can be observed on the seabed, and how they can be related to specific volcanic, tectonic, or sedimentary processes.

Continental margins mark the transition between the oceans and the continents, and are traditionally defined as the region between the upper limit of the tidal range and the base of the continental slope. They extend from the coastal zone (presented in Chapter 8, “Shallow-water environments”) to the abyssal plains and basins (presented in Chapter 6, “Abyssal plains and polar seas”), and they are roughly divided into three regions: continental shelf, continental slope, and continental rise (Figure 7.1). Continental margins are the region on Earth where most of the sediments are deposited (as much as 90% of the sediment generated by erosion on land) (McCave, 2002). It is important, however, to recognize the long-term processes that led to their formation: in the past 1 million years, sea level has only been as high as now for less than 5% of the time, and for the past 7,000 years only (e.g., Thomsen et al., 2002). For the most part, continental shelves were developed sub-aerially by fluvial processes at a lower sea level (as much as 130m lower than now), and their features were smoothed off by wave action during the next sea level rises. For much of the last million years, sediments were fed more directly into the ocean basins, and these variations explain the wide variety of facies observed around the world (see Richards et al, 1998 or McCave, 2002 for more complete descriptions).

Shallow-water environments are even more essential to our everyday life than continental margins (Chapter 7). These environments directly shape and are shaped by our commercial, ecological, and leisure activities. Most of the world’s fishing is still drawn from coastal waters, particularly in developing countries, and the pressure on these resources has clearly reached the point of non-sustainable return. As fish stocks dwindle across the world, the pressure on shallow-water environments is amplified by the construction of infrastructure (harbors, dikes, coastal defenses, etc.). Modification of sediment redistribution along shores can now be clearly attributed to specific projects (e.g., along the southern British coastline, where some beaches are more eroded after others have been protected from accelerating erosion). In the past, this had led to the silting of estuaries, or the abandonment or modification of once prospering harbors (e.g., Brugge in Belgium, or antique Greek harbors now lying 5 kilometers inland). The effective monitoring and sustainable management of shallow-water environments and their habitats relies heavily on the mapping of the seabed and water column, and sidescan sonar is the tool of choice, because of its relative cost and higher ease of deployment as well as because of the customer base it has developed among many types of end-users.

The previous chapters showed the variety of natural features and processes observed underwater with sidescan sonars. But, increasingly, surveys and maps show the presence of man-made structures all over the world. Apart from shipwrecks and a handful of communications cables, human activity has increased over the last half-century to cover all depths with a staggering variety of structures. This chapter will divide them into “planned structures” (Section 9.2) and “accidental structures” (Section 9.3). The former are intentional manifestations, and include pipelines and cables (Section 9.2.1), dump sites (Section 9.2.2), harbors and their approaches (Section 9.2.3), and the exploitation of seabed resources (Section 9.2.4). The latter presents structures that have just come to attention, such as the effects of fishing (Section 9.3.1) or were never intended, like shipwrecks (Section 9.3.2.) and marine pollution (Section 9.3.3). Section 9.4 combines marine waste, although some of it is already presented in Section 9.2.2, and military mines (which, although originally laid with intention, are usually not desired now). Section 9.5 sums up the current state of research on the use of sidescan sonars in underwater archeology, and what the next steps will be. Because of the amount of different structures it shows, this chapter should be of interest to all users of near-shore environments, all developers of underwater sites, and all managers of offshore facilities.

The previous chapters have shown how sidescan sonar data were acquired, how they were (or should be) processed and interpreted, and examples have been shown at all depths and in all underwater environments so far studied with sidescan sonar. Sidescan sonar imagery, like any data, is rarely devoid of anomalies and artifacts. They may be easy to spot or mistake for real features, and they may be difficult to interpret or remedy. The present chapter aims at showing most sources of errors and artifacts, how they can be avoided during processing, and how to recognize and interpret them when they do occur. This will be demonstrated by drawing both on the most recent theoretical studies on the subject, and on real-world examples from a variety of applications. The reader may also find it profitable to look at a short (but now somewhat dated) publication from EG&G Marine Instruments (Fish and Carr, 1990)¹ which presents sidescan sonar operations in very shallow water, mainly for the detection of man-made structures. The different sections of this chapter follow the acoustic wave from transmission to reception and processing. This includes propagation through the water column, backscattering toward the sonar platform, processing, and final interpretation. All these stages are prone to errors and artifacts; some of them are unavoidable, but all of them should be recognizable.

The interpretation of sonar images, and more generally of remote-sensing images, has traditionally been performed visually by trained interpreters. This presents the distinct advantage of using the skill of the interpreter to limits which are often unattainable by computers. But there are also many disadvantages to a purely visual interpretation. First of all, it is subjective: two interpreters with different experience, or different skills, are likely to get different interpretations for some features, depending on their experience of the sonar used or of the environment studied. Visual interpretation is also time-consuming, and a longer amount of time spent on analysis does not ensure higher objectivity. Structural geologists all know that some morphologic trends will be highlighted, unwillingly and unconsciously, when the time spent on interpretation is too long. The other important disadvantage of visual interpretation is that it is qualitative. Objects are outlined, trends and patterns are shown. But their quantitative assessment requires either the interpretation to take place directly on a numeric support (with all the associated problems of screen size and limited range of scales available) or to scan and quantify the interpretation made on physical supports (paper maps, photographs, etc.). The last decade has seen many new useful tools for visualization of data; for example, in 3-D with the ubiquitous Fledermaus software (http://www.ivs3d.com/) and the use of immersive environments (based on virtual reality advances) like the ones used in seismic prospecting (e.g., Lin et al., 2000; Shell et al., 2006). Because of their cost and the necessary investment in hardware and software, these systems are still not within the reach of all sonar users, and they will still not replace the need for a full computer-assisted interpretation.

The Earth is a small planet, and modern communication tools remind us every minute of this simple fact. Browsing through Google Earth or one of its Internet equivalents, we can zoom in on any point in the world, and gather all the information we want about it. But the oceans are still largely hidden from view. We know more about other planets, or asteroids passing in the sky, than we know about the underwater domains just a few kilometers away. Yet, learning about our planet, monitoring its health and its evolution, managing its resources in as sustainable a way as possible cannot happen unless we know what is happening under water. And for this, the only tool is acoustics. Animals developed and have used sonar for a long time: dolphins can detect and identify objects from a few centimeters to a few hundreds of meters in very opaque waters; whales can communicate from one side of the ocean to the other. But we have only just begun to master the construction and use of sonars. Along with the development of computer-aided techniques for processing the signals and interpreting the images, these technologies have greatly advanced our knowledge of the deep seas.