Teacher Logbook:
Thursday, July 6, 2000 1900 hrs.

Staring from the aloha deck toward the bow of the ship you can look around and see about 270ø of horizon. Everywhere I look, water. I had to remind myself that we're sitting over the 3km x 8 km caldera of an active volcano. Axial is no Mt. Everest, rising only 700 meters above the mean level of the Juan de Fuca Ridge crest. However, it holds the interest of many scientists who wish to reveal its hidden mysteries.

The entire ocean floor was hidden until sonar (SOund Navigation and Ranging) mapping systems were used to indirectly "see" the relief on the bottom. To better understand the extent and volume of the '98 eruption at Axial, we are using an Imagenex "pencil beam" sonar mapper to produce high resolution bathymetry. The sonar is mounted on ROPOS, which flies 25 meters above the seafloor with the instrument, while traveling at 0.5-1.0 knots. The sonar head points down and sweeps 120ø from side to side as it sends out pings at 675 kHz. Since sound travels at 1500 meters per second in sea water, the time it takes the ping to reach the seafloor and return to the instrument is used to calculate the bottom depth. The swath width is only about 80 meters so each trackline maps a long skinny patch of the floor. ROPOS and the ship then turn around and head in the opposite direction, moving adjacent to the previous trackline. Repeating this process 8 times, we will "mow the lawn" and map a portion of the southern part of the 1998 lava flow. This long and tedious mapping method will take 14 hours.

Meanwhile, most of the scientists are preparing bio samples, sorting rocks or catching up on some much needed rest. At the science meeting today chief scientist Bob Embley shared that overall, the dives were going well. The long lists of dive tasks for ROPOS were being completed in a timely fashion. However, I did hear some extremely bad news that, I hope, won't force us back into harbor early. We're running low on whole bean coffee.

Jeff

Imagenex system used on ROPOS for high-resolution mapping of the seafloor.

Imagenex map from 1999 of the 1998 new lava flow. (Click for larger image.)