Chain gang ……

….. at least the sun is out and the sea is calm.

One of the developments we’ve been working on is an array of sensors that is deployed vertically in the water column from the seabed upwards, that measures changes in the pressure due to the passage of sound signals.

Active-source seismologists – those that make their own sound waves for sub-seabed imaging – like to know what is going into the sub-surface so that when they measure those signals coming back having interacted with the individual rock layers, they can work out what the rocks are from features that might result from changes in the signals the scientists have generated.

These source generation changes can occur for a whole variety of reasons, even as simple as changes in ship’s speed as the sound source is towed.

The idea then is if we know what’s gone in, we can distinguish changes in that for every source fire that occurs, and tease out the changes in the rocks that are being imaged that are geology related and not simply a function of how they have been imaged.

That’s the idea – building something to do it is a bit more tricky and requires a thought process that includes how do we deal with sound waves that bounce off the seabed, sea surface etc that are also included in what we record.

The simple answer is that we are helped by the speed of sound in sea water being one of the most consistent things in the geophysical world – at 1500 m/s =/- only 3% anywhere on the planet.

As we know that, we can work out how long sound waves take to travel through water of various depths, and similarly how long it takes for the bounces off the sea surface and seabed to travel.

And then it all becomes as simply as making the string the sensors on, the right length – as simple as a piece of string.

The tricky part is actually getting that sensor string to stay on the seabed and stream vertically upwards, and come back to the surface when we instruct it to do so, and not get tangled up as its doing it.

Item 1 – get a large ballast weight to hold it down.

A cheap and simple solution to this problem is a clump of chain believe it or not.

And whilst deploying that it is important that we don’t follow it to the seabed, and so we have to tie ourselves onto the vessel via a harness.

This ballast weight effectively also counters that large amount of flotation that is put on the top of the sensor string to pull it straight and upright in the water.

Item 2 – we need a release device to jettison the ballast weight when we want to recover the sensor string.

And we don’t just use one, and they cost a fortune.

Release units on moorings are used in pairs and only one of them needs to open on command to release the ballast – its a belt and braces approach because on large-scale oceanographic moorings the instruments that are suspended from them are very expensive indeed – but arguably the data they collect is priceless.

Item 3 – something to record the sensor outputs.

Suspended in multiple places along the sensor cable are datalogger tubes (white, in metal cages in the image below), these record the signals coming out of the pressure sensors which you can in this image by the seaman’s (blue shirt) left knee.

This whole sensor string is many 100s of metres long and getting it into the sea and out again, more importantly, without getting the whole lot knotted up is – an art form.

And where have we put it?

Somewhere in the image above, along the side of one of the largest and deepest seabed canyon on Earth where it will sit and listen for large sediment avalanches passing by. We can’t say exactly where as this is pirate country.

Why have we put it there you ask?

Well running offshore these days are many large deep sea cables that route power and the internet around the globe.

A large scale sediment avalanche can sweep these away and destroy them – so scientists need to better understand what causes these avalanches and how destructive they can be, so that we know where the least risky places to run the cables are.

There is nothing more annoying than the buffering symbol when waiting for a web page to load, and web pages that won’t load, and emails that won’t send or receive etc, ……..if there is no cables along which to send them ……. and these days each cable carries a huge amount of data, phone calls, emails etc etc.

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