X marks the spot

We’re cruising along picking up the receivers deployed along our second profile the past day, with 19 on deck, 20 more to go, and an expected completion tomorrow night. Three receivers are currently in the middle of their four-hour journey floating back up to the sea surface. They rise at only about 20 meters per minute, so we stagger their releases about half-hour apart since that’s about how long it takes to bring them back aboard the ship and then drive the 4 km to the next station. Yesterday we released six of them in a row, timed to be about three hours between the first and last one reaching the surface, which may be a new record (or tie) for most number of instruments we’ve had in the water column at the same time.  Ship time is expensive and we only get one shot at this experiment, so we’re always trying to be as efficient as possible so that there’s more time for getting more data.

To navigate and release the instruments, we have three acoustic ranging systems at our disposal, as shown in the photo below. The yellow ORE deck box on the left sends the frequency modulated acoustic codes for the transponder units on each receiver by making acoustic pings with the transducer mounted on the ship’s hull. The ORE transponders on the seafloor receivers are always listening for their own special frequency code pings to do some action (enable, disable or release). After using the ORE box to enable the seafloor transponder unit, we then perform a brief navigation survey using the Benthos digital ranging system (blue box on the right). Technically we could do this with the newer yellow ORE box, but we haven’t had a chance to update our ranging software for that system, so for now we stick with the Benthos unit.  We have the ship drive in a cross pattern over the deployment location for the receiver and we collect acoustic travel time ranges, which are the time it takes for a ping to travel from the ship to the receiver, and then back to the ship. We also collect  the ship’s position and heading data for each ping. Since we know the speed the ping travels in seawater (about 1500 m/s), we can then use the range measurements and ship’s position data to triangulate the receiver’s position and depth on the seafloor.

Three acoustic systems: ORE (yellow), Scripps (gray) and Benthos (blue).

Here’s an example in the figure below. The bottom plot has the acoustic ranges we measured as the ship drove in a pattern over the receiver. The y-axis is the two-way travel time for the ping to go from the ship to the receiver and back.  The horizontal axis is time of day the ping was sent. The upper plot shows the ship’s track and is color coded by the two-way travel time for the range measurements at each location.  When the ship is directly over the receiver its at the closest range and that’s when the ranges are colored red below, whereas the blue dots show when the ship was farther away. So x marks the spot in this example.  However, we’ve seen the receivers drift up to a few hundred meters from their drop locations, so sometimes the shortest ranges are offset from the locus of the navigation survey’s X pattern.

Acoustic ranging data for navigating the position of the seafloor EM receiver deployed at station 307.

After we’ve collected enough navigation data we switch back to the ORE box and send a release code to the seafloor receiver. Once confirmed, the receiver will electrolyze a small wire holding a spring gate on its release, a process that takes about four minutes. When the release gate opens, the anchor strap is released and slips through loops on the concrete anchor, allowing the receiver to begin floating up to the sea surface.   We confirm liftoff by using the Scripps analog ranging system attached to an old school dot-matrix printer. In the video below you can “see” the ping for the receiver at a constant distance from the left side, which corresponds to its acoustic range. When it starts to lift off from the seafloor, we get two pings, one from the receiver and one from the ping bouncing off the seafloor, which you can see in the section printed in the video below. That’s the quickest way to confirm liftoff. Alternatively we could collect digital ranges with the ORE unit and wait around until the ranges are shallower than the water depth, but that could take 10 minutes or more, whereas the more detailed analog print possible with the Scripps system gives you that nice seafloor bounce echo to confirm liftoff immediately. Again, we’re always looking for ways to save time.

A video and some photos from yesterday:

Christine nails the grapnel throw!
Bern trained Janine how to use the ship’s air tugger winch and here she is helping recover a receiver. The air tugger pulls in the line that goes through a block on the crane and then down to the recovery hook attached to the EM receiver being recovered (see one of our previous photos for a more complete instrument recovery scene).
Nice sunset tonight