May 10, 2003 -- Making Maps with ABE

Slideshow

Mike Jacuba

		April-May Sun Mon Tue Wed Thu Fri Sat 20 21 22 23 24 25 26 27 28 29 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Today's Question Q. How does someone become an Alvin pilot? (From Mrs. Heidenreichs' Class, Chinook Middle School, Lacey, Wa) A. Pat Hickey, Alvin Expedition Leader, answers: The pilots and technicians who operate and maintain ALVIN are a diverse lot..Some are degree'd engineers while some, like myself, have an enormous amount of experience working in offshore applications such as the Navy and oil field. When we hire new people, we typically look for someone who has formal training, university or college, in mechanics, electronics or electrical systems. A base knowledge of computer systems is also a plus. But all the education in the world can not prepare someone for the extended time we all spend at sea. Typically, we spend 4 months assigned to the ship and then take a 2 month vacation. Not bad, hey, 4 months off every year? But you have to remember that you spend your time at sea with on average, 55 other people, share a room and a bathroom, eat with the same people day after day, and when at sea, can only walk for 275ft in any one direction. Things can get "cozy" at times. This is the primary reason that new people don't stay past their first 4 months at sea. On the plus side, the food we eat is excellent, we are in port on average every 3 weeks and so can get off the ship for a few days and, during the times when we are on vacation, can live anywhere in the world we like when not on the ship.

As part of the ABE group, it is my task to process the multibeam sonar collected by ABE on its nightly sojourns into the deep into maps of the bathymetry around and including the Lost City site.

ABE has been recently equipped with an SM2000 multibeam sonar unit that provides exceptionally detailed maps of the rough topography at the summit of the Atlantis Massif.

A dedicated PC104 stack in the main electronics housing on ABE records the raw signals (in-phase and quadrature) from each of 80 receiving elements within the SM2000 unit onto a hard disk. After ABE returns to the surface following completion of its mission, I download this data onto my computer and begin the processing of this raw data into bathymetry.

The ABE being deployed July 2002. An earlier installation of the SM2000 multibeam sonar is visible on the underside of the vehicle.

Saving the raw signals on ABE allows for tremen-dous flexibility in processing. First the raw signals are beam-formed. The beam-formed data then passes through a bottom detection algorithm which returns a range-to-bottom on every beam. These ranges, combined with the known angle of each beam are then combined with vehicle position and attitude data from some of ABE's other sensors and the result registers each bottom return in a global coordinate frame. At this point in the process, the bathymetry exists as a 'point cloud.' Following gridding, which smoothes the data to some extent, the bathymetry can be plotted as a map.

The resolution of the maps created in this way have revealed many details about the Lost City site and surrounding terrain that are impossibly obscured in acoustic bathymetry collected from the ship, but that are at a much larger scale than are visible out of Alvin's viewports. Maps generated by ABE taken down by Alvin operators and the scientists on every dive help to navigate the vent field.

A closeup of the the Lost City vent field. Bathymetry generated by ABE.

Because ABE flies low over the bottom (about 40 m) and follows the terrain, each sonar beam only covers a small portion of the bottom and a precise (~0.5 m) range-to-bottom can be collected that represents about a square meter of the ocean floor. This is in contrast to typical ship mounted sonar systems, where each beam might insonify 10s to 100s of square meters of the bottom in deep waters.

The large number of dive opportunities on this cruise has afforded us the opportunity to try some new mapping techniques. As mounted on ABE, the SM2000 can be swiveled to point up to 60 degrees to one side. This arraingment is courtesy of Rod Catanach and has allowed us to map the steep cliff faces surrounding the Lost City site from the side and resolve their texture in detail.

An area map of bathymetry generated by ABE, cumulative sonar from dives 80 through 84.

Processing the sonar can be a frustrating process. There are a great many variables, some of which must be determined from the data itself, making the process iterative to some extent. But when a map comes together and reveals features like 100 sheer cliffs, 60 m tall spires and a pancake flat summit, I've found the work is very satisfying.