ROVs to the Rescue
Guillermo "Memo" de Anda has explored about 120 cenotes in Mexico's Yucatán State since he became coordinator of the Universidad Autónoma de Yucatán's underwater archaeology department in 1996. It's a considerable achievement for a full-time professor, but a drop in the bucket when you realize that there are 2,500 cenotes in Yucatán State alone. Part of the problem stems from the remote location and difficult access of many of the sites: some cenotes require three days to reach, while others require a 60-foot rappel just to reach the water--and then there's the matter of getting back up.
The other factor that has limited cenote exploration is the nature of the research. Archaeological cenote diving blurs the line between cavern diving, in which divers always remain within sight of the sinkhole entrance, and cave diving, a more technical and dangerous undertaking in which divers penetrate deep into an underwater cave system. Because of the considerable investment in time and money it takes to become a bona fide cenote diver, to say nothing of the physical dangers involved, de Anda's university diving-support team for now consists of only two cave-diving-certified graduate students, with two more working on their certification. Seventeen students have completed underwater archaeological training in the university program to date.
One solution is to allow students to explore the cenotes without even getting their feet wet through the use of remotely operated vehicles (ROVs), robots outfitted with cameras and various tracking and sensing systems that are able to explore underwater at greater depths and for much longer periods of time than humans can.
In winter 2004, a team from the underwater research organization Promare (www.Promare.org) brought an ROV down to Yucatán to get a better understanding of the unique demands of cenote environments, and to evaluate the feasibility of developing an affordable and efficient ROV program for the university's underwater archaeology program. The ROV, provided by Deep Ocean Engineering, was a larger, 68-pound, open-frame model called Triggerfish. For this preliminary tour, Triggerfish was outfitted with a videocamera and a five-megapixel digital camera.
Triggerfish's first stop was Sabak Ha, a site south of Merída commonly referred to as a "monster" or "bottomless" cenote. Sabak Ha is Maya for "Dark Water," but from the surface this mature cenote resembles a quiet pond or small lake. Human divers have reportedly reached depths of 450 feet at Sabak Ha without seeing the bottom, while the top of the debris pile (formed by the intial collapse of the sinkhole tens or hundreds of thousands of years ago) starts around 200 feet.
||The ROV is controled from a topside base, which in this case has been set up in the back of a van. Triggerfish's "pilot" monitors its depth and direction from the monitor and "flys" it from the console. All communication and information is transmitted through the 500-odd feet of thick tether that connects the ROV to its base. From the console, the pilot can control the ROVs horizontal and vertical thrusters, manipluate its lights, and take pictures with its five-megapixel digital camera.|
It was thought that a deep, wide-open cenote like Sabka Ha would be an easy site to take Triggerfish for a test "fly." As it turned out, the vast amount of junk that has accumulated in the cenote over the years--including entire trees, endless yards of fishing line and plastic-bottle fishing bobbers that invariably got sucked into the ROV's vertical thrusters--proved to be annoying obstacles. Then there was the considerable amount of organic sediment that covered the debris mound.
Brett Phaneuf, Promare director, and Dirk Rosen, Deep Ocean vp of special projects, flew Triggerfish around Sabak Ha's debris mound for an afternoon to a depth of about 350 feet. "I have no doubt that there are many artifacts present," Brett concludes, "but most are obscured by sediment." For a huge cenote like Sabak Ha, he suggests outfitting the ROV with additional tether (they had 550 feet on hand) and more complex acoustic instrumentation, "i.e. a scanning sonar system, and an acoustic tracking system to provide us data about where we have been throughout the survey. Also, larger more powerful lights. We might also attach a small sub-bottom profiler for studying the debris mound."
Click here to see ROV footage from cenote Santa Maria.
The Promare team had a very different experience at a cenote they were brought to by the Yucatán State department of ecology. "Cenote Santa Maria was quite small and confined compared to Sabak Ha," Brett recalls. While no significant archaeological discoveries were made in the 125-foot-deep cenote, the ROV team recognized that smaller sinkholes have their own technological requirements. "Since the cavern is not as expansive as Sabha Ka, we would not necessarily need an acoustic tracking system [to keep track of it]," he observes. "A tracking system may not even work in the cavern as there would be considerable problems with what is known as multipath error--basically the sound ricocheting all over the place, making it difficult to get accurate readings. We would possibly have the same trouble with scanning sonar. Also, given the confined nature of similar cenotes, we would need a smaller vehicle ensure the safety of artifacts located...less power, slower moving, small tether, easy to maneuver so that we don't disturb sediment on the bottom or the top of the cavern causing us to lose visibility, or hit artifacts while moving in a confined space, or drag the tether across the artifacts unknowingly. In cenote Santa Maria, we reached several spots where we were almost literally jammed into the available space with the Triggerfish but could have explored more with a smaller vehicle."
It's a good 30-40 feet to the water's surface, and the 68-pound. ROV was carefully lowered down the side of the cenote. (Jorge Pérez de Lara)
Meanwhile, some village kids decided to take a stab at flying Triggerfish. (Paul Wilder)
The biggest excitement occurred one afternoon when Memo decided to get some shots of the ROV in action. He chose to dive a cenote located on the outskirts of the home village of Dionisio Orozco, a Maya guide who has worked with Memo for the past eight years. It's not every day that scuba divers--much less underwater robots--show up to plumb the depths of the village cenote, and a crowd appropriate to the spectacle soon gathered.
|Dionisio seemed the most affected. He quietly admitted that after eight years of working with the archaeologist, of hauling dive gear, filling air tanks, and collecting tips on new sites to dive, this was the first time he had actually seen Memo underwater. (Jorge Pérez de Lara)
Click here for ROV footage of the dive.
Although Promare only had a short time in the Yucatán to test out Triggerfish, they learned that different types of cenotes would require different ROV systems and ancillary equipment. "There is no 'perfect' ROV for this work and it will require a broad suite of ROVs, ROV tools, and imaging and positioning acoustics," Brett concludes. "However, I think the value in using an ROV, particularly a small system on sites like cenote Santa Maria is that we can explore the site quickly, safely and with good results prior to divers entering an unexplored cave system. Working in Sabhak Ha will require a
considerable technical and financial commitment that, in my opinion, will yield far fewer results than focusing on the cenotes with more restricted openings."
Memo agrees in focusing future efforts on smaller ROV systems. "They could be extremely useful, especially in situations that would allow us to check out the conditions inside a cenote and the existence of archaeological material before we undertake the very complicated logistics of transporting cave diving equipment to a remote site. "
The initial exploratory trip was a success and the Universidad Autónoma de Yucatán and Promare are talking about future projects. But perhaps the most unanticipated benefit of the ROV technology was that for a few hours, the residents of one small Maya village had the opportunity to go beneath the surface of their local cenote and explore in real-time the world that for their ancestors was the dwelling of Chac, the lord of life-giving rain, and the boundary where the living crossed over into the dead.