Photographs into Models: Documenting the World Trade Center Ship
By Carrie Fulton If you attend any archaeology conference or glance through recent issues of…
Autonomous Underwater Vehicles (AUVs) are built for a variety of purposes and come in many shapes and sizes with near limitless combinations of sensors and payloads. Some are built solely for oceanographic uses, collecting water column data salinity, dissolved oxygen content, etc., while commercial survey AUVs are designed to collect geophysical (e.g. side scan sonar or seismic, ect.) or hydrographic data. Camera systems are a relatively new addition to deep AUV systems. Currently, there are only a few companies, institutions, or government agencies that operate AUVs equipped with digital still cameras capable of survey to 1,000 meters or deeper.
I am writing here primarily about C & C Technologies’ C-Surveyor AUVs, because I have the most access to these systems (a HUGIN 1,000, two 3,000, and a 4,500 meter systems). Although the sensor payload of each of these AUVs may be slightly different, the basic payloads include an EM 2000 multibeam bathymetry system, Chip Edgtech subbottom profiler system, and duel frequency side scan sonar (120 kHz or 230 kHz dynamically focused and 410 kHz, or synthetic aperture). C & C’s has equipped three of these AUVs with digital still cameras (George 2009a).
In 2001, C & C began using the first commercial deep-water AUV in the Gulf of Mexico. C & C surveyed the first of several shipwrecks with their AUV in January 2001 when the AUV passed the SS Robert E. Lee during a pipeline survey for BP and Shell. The SS Robert E. Lee was a passenger freighter sunk by the German submarine, U-166 during World War II. A continuation of the project led to the startling discovery of the U-166 in March of 2001. During the course of the survey two other historic shipwrecks, the Mica Wreck and the later designated Mardi Gras Wreck were imaged with sonar as well as four of SS Robert E. Lee’s lifeboats. Between January 2001 and January 2012, C & C collected over 246,000 line kilometers of deep-water AUV data, enough to circle the earth more than six times at the equator. These have included surveys of over 30 deep-water shipwrecks many of which are historically significant.
In 2009, C & C began integrating digital cameras into their AUV fleet. The AUV photography system provides black and white still photographs of the seafloor while the vehicle travels at a speed of 3.7 knots. An image is taken approximately every 1.75 seconds which equates to one photo every 3.5 meters of travel at normal survey speeds (George 2009b). The length of the camera footprint is equal to 0.75 times the AUV with an aspect ratio of 4:3. The AUV is typically flown at 6 to 10 meters altitude during camera surveys with a typical tracklines spacing of 5 meter or less allowing for overlap of photos.
The first shipwreck imaged with the C & C AUV camera was the Ewing Banks Wreck in 2,000 feet of water. The near immediate success of the camera provided archaeologists with another tool to quickly assess and ground truth potential archaeological sites in deep-water. Soon other wrecks were imaged with the AUV camera including the Mardi Gras Wreck in 4,000 feet of water and the U-166, in 4,800 feet of water.
Three of advantages of the AUV camera system are a) the ability to take the collected images and efficiently mosaic the photos into larger geo-referenced images; b) the ability to combine those images with the other geophysical data to aid in interpretation and site analysis; and c) the ability to quickly ground truth targets detected with the geophysical sensors.
Several hundred photographs are collected during a typical camera survey and it is important to know what portion of the seafloor each photo represent. C & C developed a software application to sync the photos with the AUV navigation/positioning system and convert each photograph to a geo-referenced image. In addition, a post processing routine was developed to equalize the repetitive flash pattern produced on each photograph, adjust for spherical light spreading, linear attenuation, and flash scattering resulting from water column particulates. The result of this processing is nice evenly lighted geo-referenced images that can then be more easily mosaiced and imported into a GIS system.
Having the photo mosaic and geophysical data (e.g. side scan sonar, multibeam bathymetry, and subbottom profiler) collected simultaneously allows all the site data to be analyzed in conjunction. The photo mosaic can also be draped over the swath bathymetry to provide a three-dimensional photographic perspective of the site. Although individual photographs and ROV investigation may be required for detailed analyses of specific areas or features of a wreck site, being able to quickly see the bigger picture along with the geophysical data offers a larger perspective of a site for assessing site formation, artifact distribution, and other aspects of the site.
The AUV camera is also an excellent tool for ground truthing unidentified targets. Often potentially significant targets are detected with side scan sonar during an archaeological survey and a recommendation has to be made based solely on the geophysical data. Having the option to collect photos over select targets, helps remove most of the ambiguity in the interpretation.
AUV cameras are advantageous to both the survey industry and the advancement of deep-water marine archaeology. Since the introduction of digital still camera systems into survey class AUVs, the technology has repeatedly proven its value, efficiency, and effectiveness. Although the technology is still in its relative infancy, it has immediately demonstrated its benefit for deep water AUV surveys in ground truthing unidentified targets, inspecting previously known sites, and creating geo-referenced photo mosaics to analyze historic shipwreck sites.
What other potential archaeological uses or advantages are there for this type of technology?
All images courtesy of C & C Technologies, Inc.