Day 2. Sailing to Puna Ridge

September 27, 1998. Studying an active volcano adds a lot of excitement to our voyage. Today, we saw some of this activity. As we sailed up the southeast coast of the Big Island, giant white plumes containing water vapor and hydrochloric acid came into view. Lava erupting from Pu’u ’O’o along the East Rift Zone flows downhill for 12 kilometers and into the sea, generating these large clouds of steam.

The lava from the Pu’u ‘O’o eruption is not currently flowing above ground. Instead the lava flows underground through a network of lava tubes. These tubes insulate the lava so that it is still running hot and fast when it pours into the ocean. Most of these tubes are several meters underground. Some, however, are quite close to the surface. Those people who decide to walk on the thin crust above these tubes do so at their own peril.

The seas were delightfully calm when we reached Puna Ridge this afternoon, but there is still a tedious job ahead of us. Once an instrument such as the DSL 120 plunges underwater, we won’t know where it is. So before we start our mapping, we must deploy 18 transponders at four-kilometer intervals down the spine of Puna Ridge. These transponders will help us locate the position of the underwater vehicles.

To understand the function of the transponders, imagine that you have been sent into a forest to gather information about a specific area. You have no map and you don’t know where you are. You may begin gathering data, but you don’t know whether you are studying the right area. Furthermore, any data you do bring back will be useless because you don’t know which part of the forest relates to the information gathered. Now imagine that there are markers in the forest. You have a map showing you where these markers are. You can use them to pinpoint where you are and find your study site.

The transponders act as markers for the towed instruments. Here is how they work. When a vehicle such as the DSL 120 is sent down, it will start sending signals to the transponders. The transponders that receive the signals will then send signals to the ship. By measuring the time it takes for at least two different transponders to receive the signals, we will be able to determine the exact distance the signals had to travel. From this information, we can pinpoint the location of the vehicle.

The placement of the transponders is critical. For example, they would be of little use if they were deployed next to objects that would block the signals. Here Tom Crook, Jennifer Reynolds, and Debbie Smith plot the location where each transponder will be deployed.

The transponders themselves look like giant yellow beach balls. Each is attached to a 100 or 200 meter cable. When the bridge signals that the ship is in the right place, the engineers drop the transponder overboard and let out the cable. Since the transponders are buoyant, they bob at the surface. The engineers then attach an anchor to the other end of the cable and drop that in. The anchor pulls the transponder to the bottom of the ocean.

The engineers will work into the early hours of the morning deploying the transponders. Tomorrow will be the real test. Did the transponders land where they were supposed to land.