Dedicated to …..

Over the last few days we have started to record a different type of seismic data to that recorded by our instruments on the seabed – it is called reflection seismic data. Essentially a towed cable full of pressure sensors – called hydrophones – is towed behind the vessel which listens for echos of our seismic sound waves coming back from the various rock layers in the subsurface.

The Head of the Ocean-Bottom Instrument Group, in her “day job”, is actually a scientist and so we have asked her to explain it – it’s over to you Boss.

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Firstly before I start this blog I should mention that the equipment we are using to do the reflection survey at 13N is new and this is its first use in anger for a science project.

So today marks a landmark day for UK academic science – it has, after a very long time, regained the technology in the National Marine Equipment Pool to do this kind of surveying.

There are many people to thank who have helped us get to this landmark day, but there is one in particular who knows who he is but would be very embarrassed if I named him here. So – a very, very big thank you – and this post is dedicated to you.

So what’s it all about then? Well hopefully this cartoon explains it.

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As you can see from the above cartoon, we are towing a set of airguns – called an array – which sends sound waves out in all directions. Some of this energy goes downwards and bounces off the seabed (reflects), some of it travels through the seabed (transmits) and continues on to deeper layers where it, in turn, reflects and transmits.

How much energy reflects and how much transmits is dependent on how big the change in density is across the boundary (interface) between rock layers of different types (lithologies).

The streamer records the returning (reflected) signals from the various interfaces between the various rock layers and we can take those recordings and turn them (by processing the data in clever ways) into a picture of the subsurface as if we have sliced through it.

However, there are some critical things that we have to make sure the various parts of the equipment are doing, so that we can generate the very best subsurface picture (image) that we can.

The first of these is that we must make sure that the streamer is towing flat and horizontal at the same depth all the way along it. So as part of the set-up we add or remove weight to the streamer to balance it for the sea conditions that we will be working in.

As you can see from the picture below, each of the depth control devices (marked by the green bars in the top half) that we also attach at 150m along the streamer, is used to drive it to tow level at the target depth. In this case 10m below the surface. Remember the green bars reflect a streamer that is 3km long.

Each of these depth control devices (or birds) also has a compass so that we can map its sideways (lateral) position. In this example (lower half) we are just coming round a turn to starboard and the streamer is following us round, but is not yet straight behind the vessel.

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The second thing that is vital is to design the seismic sound source well – it needs to create a short, sharp pulse and we need to monitor it. The picture below shows the output of each of the 12 airguns in the array. The combined output of these 12 airguns is the seismic sound source.

Here all the airguns are firing and we know they are in good condition by the shape and size of the signals shown below. Monitoring these allows us to spot failures as they happen and plan when we need to do repairs or maintenance. In the same way it is important to control the tow of the streamer, it is also important to control the tow depth of the airguns and tow them consistently at the same depth.

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So, having got a good seismic source and be towing it and the streamer both at constant, but different, depths, we then have to continuously monitor the data and quality control (QC) check it as it comes in. We have set up a control centre to do this where each screen shows us the airguns, the streamer, the data (in three forms) and a map to show us where we have been and where we are going in the survey pan.

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One of the data forms is called the near channel display. This is what the first hydrophone down the streamer is measuring and it gives us an outline look of what the subsurface looks like. The left-hand side of the image below shows we have a rounded, lumpy seabed.

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We then apply a basic (or brute) processing to the data from all the hydrophones down the streamer and convert it into a better look at the subsurface. The image below shows what the seabed looks like with the surveying approach, and you can see it is actually very lumpy at 13N.

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To process this data and create the final image will take many months and will be done when we get back to the lab. But for now, the above kind of image is good enough to tell us that our data is good and our design of the survey equipment configuration and specifications are also appropriate for the task in hand.

So really the secret to success is thinking the problem through. Determining what you need in terms of equipment. Planning and testing how you configure it to “see” what you need to below the surface and then monitor it in use so that maintenance and repair is done as soon as needed.

The second secret to success – have a plan and think about the turns.

When towing a 3km long cable you have to think about and plan how you turn the vessel between lines as irreparable damage can be done to the equipment. Here is the plan for the current survey and all of the turns have been carefully worked out so that we do not wrap the front of the streamer around any part of the airgun array. We give the start and end points of these lines – called way points – to the vessel’s Bridge officers who steer the vessel around this pattern maintaining the specific survey speed and the specified rate of turning – 4.9 kn and not more than 2 degrees a minute in this case.

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We then follow our progress as we go – using a mapping tool created by one of the OBS team – even the position of our OBSs are marked, and we track the end of the streamer behind us (yellow dot), the location of each shot (red dots) and so on.

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And the bonus is that the instruments still on the seabed also hear the shots we are firing and so it adds to the refraction data we have already acquired, an example of which is shown below. The OBS is at 0 km in the x-direction and the signals it records from each shot are plotted up the y-direction (time). A trained refraction seismologist can look at one of these plots – called a record section – and tell you exactly where the signals have travelled, how deep below the surface they have reached and which rocks they have travelled through. There will be more on this in a later blog once we get the 58 instruments currently sitting on the seabed back.

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