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Getting the Big Picture of the Seafloor
From: Woods Hole Oceanographic Institution
| By:
Hanumant Singh |
EDITOR'S INTRODUCTION |
As scientists slowly begin to explore the mysteries of the ocean floor, new technologies are quickly developed to overcome the obstacles of deep-water expeditions. One such problem that scientists from the Deep Submergence Laboratory at the Woods Hole Oceanographic Institute (WHOI) are solving is how to capture images of large objects from the seafloor. Deep underwater, where the use of light to capture distant objects is difficult, a process called photomosaicing is now being used to photograph submerged objects. In this feature, WHOI's Hanumant Singh discusses the process of photomosaicing and how it is being used to relay images to the surface of everything from ancient ships and hydrothermal vents to whale carcasses and plane crashes. |
 ne of the fundamental problems with working underwater is that the electromagnetic spectrum (including visible light) attenuates extremely rapidly and nonlinearly. From a practical imaging standpoint, this means that large objects cannot be framed within a video or other optical camera's field of view. Thus obtaining a global perspective of a site of interest on the seafloor--an archeological find, a geological channel, a hydrothermal vent, the debris field associated with a plane crash or the remains of a whale carcass--requires piecing together a series of images in a process called photomosaicing. This involves running a carefully planned survey over the site, collecting a series of overlapping images, identifying common features in the overlapping imagery, and then merging the images to form strips, which are then assembled into larger mosaics. |
Several factors make this a hard problem. There may be constraints on the way underwater vehicles can perform surveys due to insufficient accuracy in small-area navigation and a lack of mechanisms to automatically control the vehicle. Physical constraints on the distance separating cameras and lights as well as constraints on the energy available
The process of photomosaicing.
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for operating the lights also constitute major impediments. Finally, and most importantly, the unstructured nature of the underwater terrain introduces incremental distortions into the photomosaic as successive images are added in. |
One objective of scientists, engineers, and graduate students at the Deep Submergence Laboratory is to develop a key set of capabilities for mosaicing using a variety of platforms. Those we work with include the WHOI-operated, human-occupied submersible Alvin, remotely operated vehicle Jason, and autonomous underwater vehicle ABE, as well as the US Navy's NR-1 submarine. Our funding sources include the Office of Naval Research, the National Science Foundation, and WHOI Assistant Scientist support from the Penzance Endowed Fund, the John P. Chase Memorial Endowed Fund in Support of Scientific Staff, and David G. Mugar. |
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We are focusing on bounding the errors in the photomosaics to enable quantitative measurements (for example, the size of amphorae).
Photomosaics from the Skerki bank area off Sicily in the Mediterranean Sea.
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Two important issues here are the roles of navigation and of multisensor information. Traditional acoustic navigation can be used to limit the error in the photomosaics while the image registration inherent in photomosaicing can be used to complement and thus yield more accurate navigation estimates of the vehicle. The fusion of multisensor information, in our case high-resolution bathymetric measurements, provides three-dimensional information that can be used to rid photomosaicing of the primary source of error, terrain effects. Eventually we hope to build seamless quantitative photomosaics of sites repeatedly over time to enable us to detect spatial and temporal changes in the context of deep ocean mapping exercises. |
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