The Operation

The survey sites were chosen based on previous data collected in the water column. Hydrothermal plumes produce a temperature anomaly in the water that can be detected down to less than 1/10 of a degree. Also, since they are spitting out water that is laden with particles and metals, the “cloudiness” of the water can also be detected and indicate that a hydrothermal vent is nearby. Chemical sensors can also detect the chemical anomalies in the water. There are various instruments that are attached to tow cables to detect these anomalies in the water column and give us an idea of where to look for the hydrothermal vents. Since these methods are much faster and can cover a large area, they are usually the first means of figuring out roughly where the vents are located. This kind of data was previously collected all over the survey area to give us an idea of where to look in more detail. The ideal order of operations for us is as follows:

1) Place two transponders on the seafloor in the vicinity of a known water column anomaly. These transponders tell the ship where the AUV is and also tells the AUV where it is, so the data can be properly located on a map. The ship uses GPS to determine its position, but GPS does not work under the ocean, so the transponders are the only way for the AUV to know where it is located and survey the desired area. This process takes ~6 hours.

2) Deploy the AUV. There is a special launching structure that was bolted to the rear deck of the ship that lifts the AUV over the edge and drops it in the water. Deploying takes 30 min to an hour if all goes smoothly.

3) AUV mapping. The AUV then goes down to the survey area and maps the area on its own, then returns to the surface and sends the boat a signal to let us know to pick it up. The AUV mapping takes ~6-12+ hours depending on whether it stays down as long as it's supposed to.

4) Place next two transponders. Ideally, while the AUV is surveying the first area we will place transponders at the next area so it is all ready to go once the AUV gets to the surface.

5) Camera Tow. Ideally, we should have the data from the first AUV survey and then would tow the camera over this same area to get a better idea of the materials that are in the survey area. This step can be interchanged with the next step, dredging, if the AUV map is not quite ready.

6) Dredging. Hopefully, we have seen some hydrothermal chimneys in the AUV data and confirmed their existence with the camera tows. Then we dredge the area to collect some samples. While lava rocks are fine for the geochemists and are useful to tell us about the nature of the area, the Nautilus people are by far most interested in hydrothermally-derived rocks that contain economically valuable metals. We have not recovered any of these rocks from the first four dredges, but we undoubtedly will once we can actually target the dredges at actual known hydrothermal vent sites. The first four dredges have been mostly “blind” since we have not had the AUV data to help us know where to dredge. Hopefully, for the fifth dredge and beyond, we will have better luck.

While the above procedure is the ideal order of operations, it still has not happened according to plan once. We have had a number of issues, including the winch being wound up improperly, the AUV having difficulty contacting the transponders and aborting as soon as it got in the water, and the line used to retrieve the AUV getting caught in the propeller. There have been some technical difficulties with the TowCam and the AUV as well, but it seems that we have pretty much gotten those ironed out. One problem with the schedule being so unpredictable is that it screws up everyone’s sleep schedules, since we can’t really rely on things happening when they are supposed to. We have all had some very long days because of this unpredictability, but hopefully in the future we will be able to get on a somewhat regular schedule.

Post rock-sawing photo. The sawing was very messy for most of the cruise, and then we found out that it was because the blade was spinning the wrong way and blasting all of the water and rock bits up at us instead of down. Probably should have figured that one out sooner.