From reading our previous posts you might think that life at sea is super action packed and that everyone is having fun working on deck all the time. However, the reality is that we spend most of our time in the science lab waiting. Waiting. And more waiting. Waiting for the instruments to rise up off the seafloor that is. Everyday all day long the scenario unfolds as follows. The ship drives up to the waypoint where we deployed a receiver and the science team dials in the acoustic transponder frequency for that receiver. We start pinging on that instrument to establish communication with it. Pinging here means that the acoustic transceiver system in the science lab generates an acoustic ping signal at the specific frequency of that receiver’s acoustic transponder, which is somewhere in the 9.0 to 13.5 kHz range. That signal is sent to the acoustic transducer permanently mounted on the ship’s hull, and that transducer generatesƒtg the actual sound pulse that is sent out into the sea. The MT receiver sitting on the seafloor will hear this ping after it propagates from the ship to the receiver. The time it takes for this signal to travel from the ship to the receiver depends on the range (distance) to the receiver and the 1500 m/s velocity of sound in the ocean. So for example, if the ship is directly over a receiver at 4500 m depth, the sound takes 3 s to reach it. The receiver will then reply to the ship with a short ping at 12 kHz (all our units respond at 12 kHz). Once we have established communications with a receiver (sometimes we need to drive the ship to a certain side of the receiver to get it to hear the signal, depending on if there are canyon walls nearby for example), we then send it a coded signal that tells it to fire off its burnwire release. If the receiver hears this code correctly, it replies with 7 short pings as confirmation. We then wait for 4 minutes. During that time, the receiver’s acoustic unit puts a 14 V load on a small section of wire (the “burnwire”) that is exposed to seawater with a seawater ground return path. This wire electrolyzes away and after about four minutes it gets thin enough to snap. When it snaps, a spring loaded gate is opened and that lets go of the cable that attaches the receiver to its 345 lb concrete anchor. At that point the instrument is free to rise back to the surface.
The instruments rise fairly slowly due to the drag of the long yellow electrode arms, with the ascent rate being 20 m/minute. In shallow water they will rise to the surface in a few minutes or less, but in 2000 m of water we have to wait 100 minutes, and in 6000 m of water it takes 300 minutes. In deep water (greater than about 1000 m) what we usually do is to pop (release) a few of them in a row, so that there will be three or four of them rising up at a time, with the plan being to have one reach the surface every 30 to 45 minutes. Then we drive down the line pulling them out of the ocean as they surface. This way we can recover them a few times faster than if we released them one at a time. The ships cost a lot of money to operate, so everything we can do to save time and gain efficiency is worthwhile.
As promised here is the action packed instrument release video. Get some popcorn and prepare to be blown away by the stunning visual imagery we get to watch all day long:
And now for the interpretation: This video shows the acoustic release system in action. Once you get past the fact that we are using a 25+ year old dot matrix printer to convert the acoustic signals into a printed form, you can concentrate on what the image represents. We send a ping out from the ship every second, and the instrument responds with its own ping. The wide of the sweep (left to right side of the printed region) is a 1 s sweep. The left side of the papers shows the outgoing pings from the ship (the dark band that prints at the start of each sweep). The instrument responds some time later. In this case that’s the dark band about 1/3 of the way from the left side. If you look up near the top of the paper, you can see the 7 short reply pings that are the confirmation of the release command being heard by the receiver. Later in the video you see the exciting moment of the receiver releasing from the seafloor. Since it is then getting closer to the ship, its ping gets closer to the outgoing ping (meaning it moves to the left in the image) as the instrument ascends through the water column. There is also a bottom bounce that appears at the same time as the seafloor. This is the ping that goes from the receiver, bounces off the seafloor and is then heard by the ship; this one only happens when the receiver is still close to the seafloor so that the bounce is strong enough to be heard.