ALIA Expedition
On Dredging Techniques

Dredging is one of the ways in which scientists can gather information (in this case, rocks) about the ocean and what lies below it. Compared to grabbing them individually, with a submarine, dredging is practically stone-age technology. Dredging is basically dragging a metal bucket along the bottom of the ocean. Of course it is not dragging just any metal bucket, or even just any part of the ocean. Dredging, although it is a very simplistic technique, has intricacies and complexities that make it seem more like an art and less of a science.

First, the basics. The dredge is a metal box with no bottom that weighs on the order of 100-200 lbs. Attached to this metal square is a chain bag, lined with a mesh that holds the rocks. This weighs another couple of hundred pounds, bringing the total to about a quarter of a ton. On the opposite side of the chain bag is a chain bridle that attaches the dredge to a steel cable. Inside this bridle is a clever series of weak links that break first, so that if worse comes to worst, the cable doesn't break. 300 ft up the cable is a pinger that, when it works, tells the crew where the dredge is with respect to the ocean floor. The cable that attaches the dredge to the ship is a 9/16 inch cable that is damaged permanently at 24,000 lbs of pressure, and breaks completely above 27,000 lbs. This cable is expensive: to break it is an approximately $30,000 mistake. This is why the weak links are installed, so that you lose the dredge before you lose the cable.

This seems like the end of the story, because dragging a bucket on the bottom of the ocean is deceptively hard. The visual image, at least in my mind, is a somewhat glorified version of what a toddler would do in a sand pit: pull around a bucket to get stuff into it. The problem with that scenario is that the toddler's arm would have to be 5000 ft long, and have zero dexterity. The ship is capable of moving at about 2.5 mph, while dredging, and spooling the cable in and out at no more than 60 meters per minute. With this information, the toddler is only allowed to touch the bucket via a mile-long yo-yo string, and is not allowed to walk. This much is obvious: if he pulls it off, that's one clever toddler.

Back to business: there are two ways of pulling this dredging thing off. The first school of thought, which seems to work better in shallow water, is to drop the dredge to the bottom, and then start moving forward at 2 knots (2.6 mph). The other school, which works better in deep water, is to let wire out while steaming, letting the dredge hit the bottom. The reason for the difference has to do with the dynamics of a cable, and the water that the cable is moving through.

In shallow water, where the first technique works best, water is less of a concern. The water's resistance, while significant, is not enough to keep the wire from pulling the dredge forward. In deep water, though, the cable hugs the surface because of the resistance of moving forward. This is why it is essential to let out wire as you are steaming, so that for the beginning portion, the dredging works as though the water was shallow, because the dredge has not hit bottom yet. When it does hit bottom, the wire has already sunk from the surface, and is ready to tow the dredge forward across the bottom.

It is important to know where the dredge is and what it is doing. To this end there are two tools that we use: pingers and tensiometers. The pinger we have talked about before. It hangs above the dredge and lets us know how far above the bottom it is. The way it does this is very similar to the way SONAR works. The pinger emits a tone which the ship can hear. The tone goes out in all directions, and so we hear the ping directly. The sound also bounces off of the bottom of the ocean, and we hear that too. Since it takes more time to make the longer second trip, we thus hear two sounds. We can time the tones and see how far apart they are. By seeing how far apart they are, we can tell how far the pinger is above the ground. In addition, as we pull the dredge forward, tensiometers tell us how much the dredge is pulling back. This is important because when the dredge is stuck, the tension goes up, so we can tell to a certain degree what the dredge is doing. It is also important, because if the tension goes above 14,000 lbs, we know that stuff may begin to break (weak links first, then the cable).

The process to dredging is pretty formulaic. First, the dredge is planned, based on wind direction, speed, and desired location. Then the boat moves into location a few miles before the target, and begins to lower the dredge and steam forward. When the dredge hits the ground, tension, both on the cable and in the control room, increases. We have to pay close attention to the few inputs that we have, and proceed carefully, because nothing in the dredge is better than losing it. Once the dredging is deemed finished, the dredge is reeled in and examined.

All things considered, dredging is a simple way of getting rocks from the bottom of the ocean. Biblical powers aside, it is the simplest and cheapest way of getting to the bottom of the sea. Simple and cheap is good, when you can swing it, and so dredging remains after many decades of successful scientific expeditions and other technical advancements. Of course, it is not perfect for everything, and so more expensive and complicated methods have been devised and executed (submarines, ROVs, ocean deep drilling, etc.). Since they are expensive, though, only in the distant future will they replace the tried-and-true staple of dredging.