Homeward Bound

5 weeks away from home and we’re finally sailing back to port as JC149 comes to an end. As much as I am very excited to get back to London for some home comforts and see my friends and family, I can’t really believe it’s all over so quickly.

The last week has been a mixed bag of events to say the least. We were able to shoot a bonus 7th line with the towed streamer parallel to our previous survey lines across the fracture zones. Tim got 30 mins of ‘experimental’ seismic at the end of the line with some varying shot times too. We had a final weekend BBQ on Saturday which was much less wet and windy than our last attempt over the Bank Holiday, and felt like a real celebration as we near the end of this cruise. As we transit back to Pointe a Pitre, watch keepers are gradually transitioning to more sociable working hours, which means I’m finally seeing some sunshine as my night shifts come to an end!

As I sat excited (and a little nervous) at Heathrow all those weeks ago I had no idea what I was about to join, and so as we wrap up here, I’ve put together a bit of a summary of my time aboard and try and shed some light on what life at sea has really been like.

The science
Only 8 weeks ago, I was sitting some of my final university exams, including a module in exploration seismology. I spent weeks over Easter revising the main principles of acquisition geometries and survey fold, the hydrophone spacing and the shot interval and yet here on back deck really is a 13-gun array and 3km of towed streamers. As with many things in geology and geophysics, it’s one thing to learn in a lecture theatre but it’s another thing being able to see it for yourself. I’ve come to appreciate the huge scale of marine seismic acquisition. It’s all very well having the science party in the lab eagerly awaiting a seismic section to interpret, but getting to that stage requires a fleet of people working all hours on deck to manoeuvre the equipment in and out of the sea – deck crew, petty officers, cadets, airgun technicians, OBS teams. And of course the team that know the vessel better than anyone, the officers on the bridge. I can’t imagine we’d have much data to play with if it had just been myself, Jenny and a team of scientists trying to throw the grapple hooks to bring in the OBS and operating cranes.


It’s been amazing to see the scale of the operation. Here is the RRS James Cook in port at the end of JC149. See the size of the cars and people next to the ship

I was also able to see the magnetometer deployed and recovered on a couple of occasions, which provided a nice conclusion to my MSci work and my degree, having spent 4 months working with the magnetic data from last year’s cruise. I have come to appreciate that even transiting at 10 knots with the maggie out, the amount of ground covered is pretty small. My data set contained 120 cruises and I can now see the huge scale of operation to collect that much data.

It’s not all plain sailing
Pardon the pun, but I have learnt that in marine data acquisition things do go wrong sometimes. We lost 2 OBS, we had a night time OBS recovery where the lights didn’t work, we’ve even had exploding seismometers, and at one stage in firing we had airgun failure and had to loop back and restart. These all pale into insignificance, however, compared to the remarkable successes of the cruise. In just 34 days we have deployed and recovered 150 OBS stations, shot active source seismic across all 6 planned surveys and an additional 7th line in our contingency time at the end. We have collected swath bathymetry data, magnetic measurements and successfully dredged to collect samples directly from the oceanic crust. From the first reaction on Tim and Jenny’s faces to the first printed seismic line, to the final sail back to land, it’s fair to say JC149 Leg 3 has been a resounding success.

Ship life
I never really imagined what it would be like working 24hrs a day on a ship. It’s true, with no weekends you do tend to lose a sense of days and find yourself guessing what day it is based on what’s for dinner that night. Working midnight until 4am, you can never be really sure how to greet people either. Good morning? Goodnight? But there is a funny sense of comradery on the night shift as you tuck into a meal at 2am that you’re not sure whether to call breakfast or dinner. This is the first time I’ve ever been at sea, I haven’t so much as crossed the English Channel by ferry, and so I was struck hard with sea sickness in the first 2-3 days. Rumour has it that I’ve now got the land sickness to look forward to, and I shouldn’t be surprised if I find myself walking in very wobbly lines for a few days. Although it’s easy to moan about all the things you miss back at home – considering we’re several hundred miles from land I have been treated very well. Cooked buffet meals 3 times a day, fresh bed sheets delivered to your door every week, a shower more powerful than the one in my own London flat, wifi access (mostly), and of course the Caribbean sunshine. The Purser and the Galley team really make sure that if you’re away from home for so long, at least it’s comfortable and enjoyable.


The Leg 3 science team on the foredeck on the last (very windy) day.

The end result
Data collection and research science on this scale is a huge project but it’s immensely rewarding. There are some really satisfying short term goals whilst you’re on board. After the first seismic line that was printed, geoscientists flocked with colouring pencils to give it our best interpretation. And the dredging days were particularly exciting for hands on science. But the data acquired on this cruise fits into the much broader goals of the VoiLA project. Several years lie ahead now to understand what we have recorded in our time here. Rob has some data at last from leg 1 to push on with his PhD, we have dredged samples from the sea floor to be passed onto the petrologists in the research group, and some of the team will be working with the highest resolution seismic imaging technique, full waveform inversion (FWI) to understand the results from leg 3. I have been very lucky to have even played a small role in this fantastic research group.

And now?
With the exception of 2 cadets there is a whole crew change over, as the RRS James Cook sets sail to the Canary Islands for another project. For those that have been on since April 17th including Jenny, Tim and Rob, they are rewarded with a precious few days relaxing on land before flying home.


The beach at Le Gosier, Guadeloupe treating us to one last sunset before our long journeys home.

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“Heavy duty shopping baskets” and finally some real rocks

With less than a week to go until we dock, spirits are high here aboard the RRS James Cook. Despite the rain in recent days (yes, rain, it hardly seems fair…) there have also been whispers of a final weekend barbecue. Some very efficient work from the OBS teams means we’ve finished the planned survey lines a little ahead of schedule and we’re hoping to squeeze in one more bonus seismic line to fire before heading back to port. With this contingency time, we’ve also has chance in the last week to do something a little different – sea floor dredging. VoiLA is an integrated research group, and although the seismic, gravity and magnetic data we have collected will aid piecing together the big structure, nothing beats a hand specimen of the rocks you’re really interested in.

The science behind the acquisition on this one is pretty straight forward, but there’s a little more explanation required behind the geology and what we’re actually looking for. We chose to target an area along the side of a ridge; a little bit of literature research and our own swath bathymetry survey in transit between survey lines confirmed our target site. A steep cliff face on the southern edge approximately 700m in height, exposing a large section of the oceanic crust.

First let’s consider the scale of this operation. This cliff face is located 5km underneath the sea. Imagine running that distance – it might take Mo Farah only 15 minutes to run, but let’s say it would take you half an hour? Now imagine being in a boat in the Atlantic and dangling a wire all the way down that distance in order to hit a cliff face. We lower into the sea, as perfectly described by 2nd Officer, Malcolm, what is essentially a heavy duty shopping basket which we then drag up the cliff face, breaking rocks and collecting them to bring back to the surface. It’s a very precise operation as 3 technicians and 2 scientists sit around one monitor carefully noting down any spikes in tension in the cable. It takes a few hours to winch the line back in, but the results were worth waiting for. Despite being hardy geophysicists, spending much of our days arguing with GMT scripts and unravelling seismic, the team of scientists couldn’t help but get involved with these fresh hand specimens we had required.


The dredging basket comes in, and we get our first hands on sorting out our haul!

A side note on igneous rocks

After sifting through our new collection, it became apparent we had 4 very distinct types of rocks and they give a nice window into how the oceanic crust was formed. The rocks formed at mid-ocean ridges like that in the Atlantic are made due to what’s known as “decompression melting”. As two plates move apart and the pressure on the rocks below reduces, their melting temperature also reduces. We can model and imagine the melt ascending to the surface and cooling in different ways, creating different rock types. We classify them in a neat model, called the Penrose model, typical of how we believe the majority of oceanic rocks are made.

The source of the melt is a mantle rock called peridotite – it is ultramafic which means it has a low silica content. We can classify igneous rocks on their percentage composition of silica, varying from “felsic” (~70% high silica content) to “ultramafic” (~40% low silica). Generally speaking, the colour of the rock can be a good indicator and mafic rocks tend to be darker in colour than felsic rocks. The minerals that make up peridotite rock don’t melt evenly, and the melt tends to be mafic rather than ultramafic.

A typical model

Starting at the top then, we have basalts. These are formed as melt reaches the surface. The melt cools very quickly as it hits the sea water, and so it is made of very small crystals of minerals. Imagine dissolving salt into water, and then boiling a saucepan with the mixture in, and setting some aside on a sunny windowsill. In the saucepan you’ll only be left with very fine salt, whereas in the dish on the windowsill you’ll see very large crystals of salt after a few days. The same principle applies here – the smaller the temperature difference and so the longer the cooling time, the larger the crystals are allowed to grow and solidify. The melt is extruded and cooled very quickly, and it solidifies in curved mounds like pillows – hence their name pillow basalts. They make up the very top of most oceanic crust, only to be covered with sediments on the sea floor.

Next in the sequence we have dolerite. It has the same composition as basalt but has a medium crystal size, as it does not breach the surface and cools a little more slowly. It forms in what we call “sheeted dykes” – that is, as plates move apart, melt will tend to progress upwards along the strike of the partition and cool in thin bands or sheets.


A sketch model of how mid ocean ridges make oceanic crust along with our samples.

The third rock type in the sequence is gabbro. This again has the same composition but cools the slowest and as such is characterised by large crystals. These large crystals tend to settle in layers and it is thought this occurs in a magma chamber which feeds the ridge with melt. Finally beneath this, the fourth layer is the depleted peridotite, which many geologists consider to be part of the mantle.

At mid ocean ridges this sequence is continually made and as plates move apart they get carried away, just like a conveyer belt. We were hoping to see parts of this section on our exposed cliff face, alongside some potentially serpentinised or altered rocks where we anticipate water has entered the system, as mentioned before.

The big haul

So what did we find? With no resident igneous petrologists on board, we can’t be certain, but we have an idea.


Our first interpretation of what we’ve collected. The engineers have even been able to cut open and polish some fresh surfaces on the basalt for us to have a closer inspection.

First of all, we could all readily identify the basalts – really dark in colour and very fine grain. The engineers on board have even managed to cut a few sections open and polish the surface to give us a better look.

Next the dolerite – a little harder to tell. It seems a little bit too light in colour, but the grain size is certainly different to that of the basalt. Many of the pieces tend to be fractured columns, just like you might see at the Giant’s Causeway, and is an indicative of the cooling time.

The gabbros are a little easier to decide upon as they have large crystals, easy to see even without a hand lens.

And the fourth rock type? We were optimistic for serpentinite but we’re not so sure. Let’s not forget this cliff face is part of a fracture zone which has been faulted and offset a long way from where it was formed. It’s possible this is something called a “fault breccia” – a rock that has been severely broken and altered due to the force of a faulting system.


Jenny and Rob looking very pleased with our findings as they arrive on back deck.

These samples will get taken home, and examined by people that really know what they’re talking about. We can make thin sections, only a few mm thick, and look at the minerals in different lights under a microscope to determine exactly what we have found. Despite our best guess interpretation, this feels like a very appropriate way to wrap up JC149, with a lab full of the rocks we have been trying to indirectly image for the last 11 weeks. Next stop? Home.

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Waves, waves, waves – but what do they tell us?

25 days ago when I first stepped aboard the RRS James Cook, the third and final leg looked like a long way to go, but here we are airguns firing on survey line 3. With just under 2 weeks to go, I’m a little reflective on my time so far as we sail our final seismic line. This is the longest I’ve ever been abroad for, and I’ve learned an awful lot (both science and Trivial Pursuit knowledge). I turned another year older last week (boat birthdays with night shifts are weird) and my tan isn’t as good as I’d hoped it would be by now. I’m excited for what survey line three has to offer as we venture further south to another fracture zone and conduct some of our first sea floor dredging.


The catering team very kindly made me a birthday cake to celebrate at the end of last week!

The last week or so hasn’t been without its troubles though. During survey line 1 we lost pressure in one of the airguns midway through firing, but we carried on regardless. Just before 1am on Thursday night, however, a burst pipe and a cooling system failure caused all the gas compressors to stop. Duty engineer that evening, Gavin, was quickly alerted and within minutes had located the problem and was working with technicians Stephan, Will and Ben to map out a plan for the next few hours. Also in the main lab for this early morning meeting was Captain, John, and PSO Jenny as they calmly took charge of the situation and decided to complete a gentle loop back to where we lost pressure and resume firing after a marine mammal watch at first light. It delayed us around 10 hours, but that’s what contingency days are for. All the team members involved that night worked calmly and efficiently, especially those who’d received a very early wakeup call (or late, depending on which way you look at it).

Why do we have 2 different types of recording instruments?
On each survey line we’ve gone up and down firing and in each case we have been recording the data with two different types of receivers – the OBS we have been deploying on the sea floor and hydrophones towed behind the ship for 3km. But what is the difference and why do we have two different measures?

When we’re firing the airguns, the compressed gas is released in bubbles which collapse. Huge pressure waves travel down through the sea and into the seabed. Pressure waves, like sound waves, can then travel through the rocks in the subsurface. When the incoming wave (the incident wave) reaches a boundary, some of the wave will be reflected back and some will carry on into the Earth but travelling at a different angle – this is refraction. Our two types of receivers look to measure the wave in these two different scenarios: reflection and refraction. A boundary is defined where there is a change in the acoustic property of the rocks. This may be due to a density change, the presence of a fluid, or anything that causes the speed of sound within a material to increase or decrease.


The figure shows a simple sketch of reflection and refraction. In the case of refraction, there are 2 scenarios shown, A and B showing typical and critical refraction.

Our towed streamer hydrophones are looking to detect these simple reflections. The reflection is controlled by Snell’s Law which says the wave will reflect back at the same angle at which it came in.


Our OBS are looking to measure how the waves refract and the figure shows 2 scenarios. In scenario A, where the speed of the wave is faster in layer 2 than layer 1 (V1 < V2) the wave will bend and deflect closer to the boundary, so the angle r is greater than the angle i. In scenario B the incoming wave hits the boundary at what is known as the ‘critical angle’, ic. At the critical angle the wave travels parallel along the boundary with a speed of V2. Some of the energy from this head wave will return to the surface, also leaving at an angle of ic. By recording the waves that arrive back at the surface and using simple equations like Snell’s law and speed = distance/time we can get an idea of what the individual layers of rock beneath the sea bed look like. The figure shows a simplified model and real geology has far more complexities but seismic techniques like this still stand as a fundamental tool in geophysics, to look at the Earth in cross section.

Why do we use both reflection and refraction data?
Although the reflection data has to ability to show us in very high resolution the shapes of rock layers and boundaries in the subsurface, reflections are highly sensitive to the velocities of the materials the waves are passing through. The arrival of refracted waves brings accurate velocity information about particular boundaries, thanks to the head wave. We can tie these in to adjust our reflection data, and make sure all the boundaries plot at the correct relative position, and they are not skewed by very high or low velocities. The OBS do record some reflection data, but they are primarily useful for this information they can provide about layer velocities.


The very last of the OBS being deployed. Even to the very end, the teams are all still working 24hr shifts to get instruments out and data collected.

So what are we looking for in the 3 seismic survey lines?
The first line we shot was targeted over the forebulge as described previously, and we were hoping to see something like one of the subduction sketches in previous posts. An incoming plate, being forced to bend down into the Earth and a portion of the sediments on the top, being scraped off like a snow plough to form an ‘accretionary wedge’. Survey lines 2 and 3 lie end to end and traverse 2 of these fracture zones that extend from the Mid-Atlantic. A seismic line gives us a cross section that cuts perpendicular to these fracture zones as well as the ridges which lie parallel. How were the American plates squeezed to make these ridges? We hope to understand better the structure of these ridges, as they are going to be subducted beneath the Caribbean, and have been before now. Any particularly high or low velocity zones we have may indicate the presence of a fluid or a bulk material or mineral change.

What’s up next?
This is our third time round now, deploying OBS, firing the airguns, recovering OBS so we’ve got the process well refined and very slick. It takes about a week in all, so I’ll be sure to report back next week as we do some final data collection on (dare, I say it…) our way back to dry land!

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24hrs in the life of a science party watch keeper

Whilst there is lots of incredible science going on around me 24hrs a day, it’s not all work here aboard the James Cook. Before I joined this cruise, I had no idea what to expect from my time and so I thought I’d use this post to dispel some of the misconceptions about research cruises and give you my best possible guided tour of the ship.

11am – My days have a particularly late start, no such thing as 9-5 office hours for the science party. I am on the 12-4 shift and have adapted to something of a nocturnal lifestyle. I wake up and venture to the galley for breakfast. Whilst everyone else is tucking into big hot dishes, I stick to my cereal and fruit for breakfast.

Midday – I head down to the main lab where I’ll be on watch for the next 4hrs. Depending on what part of the survey we’re at the main tasks might vary. At the moment we are recovering OBS from the second survey line. Once the technicians have had successful communications with the OBS and they’re released we have a pretty accurate estimated time that they will rise to the surface. 10 minutes or so before the surface ETA I head up to the bridge to be ready and waiting for when the instrument pops up. There is a very competitive league going on at the moment for who can spot the OBS first, but seemingly no one can keep up with eagle-eyes Tim!

No matter what is going on with the seismic acquisition, there is a 16 screen set up where we are regularly monitoring the acoustic instruments as well as the gravimeter and magnetometer. Acoustic profiling is looking to map out the topography of the sea floor and when going across some parts of this system, for instance, the ridges, the water depth can change quite substantially, so it’s important to check the instruments are measuring in a suitable depth range.


All the instruments and monitors we need to keep an eye on during watch.

4pm – My shift is over for the afternoon so it’s time to soak up the last of the sunshine and head outside. The front deck is free reign during the day time and crew are often spotted taking a quick shut eye in the resident hammock on board, during rest hours.

5:30pm – Every evening there is a huge buffet style dinner, prepared by Head Chef, Darren, and his team. 3 courses, side salads, the works! He’s also very good at catering for the vegetarians amongst us, as I get a specially prepared veggie main every day! After 4 and a half weeks of three course dinners, cooked for me, and dishes cleaned for me – going back home is going to be a shock to the system!

6pm – The galley often decants into the bar next door after dinner is over. There is a strict two beer maximum daily on board, and as many people still have a night shift ahead of them a can of coke and a packet of crisps is often the order of the day. Today I’m feeling particularly productive so I take myself off for an hour or so to put some of the blog together and work on my poster for my MSci project at university.

7pm – Trivial pursuit: a favourite pastime for many of the crew in the evenings. The question cards are from the early 1980s and so many of us born in the mid-90s tend to spend much of the game baffled by the questions, but all the same I’ve learned a lot of pub-quiz knowledge on this cruise!

9pm – Time to hit the gym. With no weekends, being awake strange hours in the day, and having access to a whole array of snacks in between meals it can be easy to get complacent and spend a lot of time napping between shifts. I head to the gym every day, and at this time I’m normally on my own so I can put whatever I want on the sound system. The gym is equipped with a treadmill, exercise bike, rowing machine and weights. Safe to say the added challenge of the pitch and roll of a ship makes the treadmill an interesting experience and nothing faster than a 5 minute kilometre is advised!


The gym and sauna on board.

10:30pm – I’ve got an hour and a half until I’m back on shift again. Fortunately, on the mezzanine deck there is a cinema room with surround sound and more DVDs than I have ever seen. Starting to feel sleepy and as the ship seems eerily quiet, I opt for another mid-noughties rom-com that requires minimal brain power, and sip away at my coffee to stay awake until next shift.

Midnight – I’m back on shift for the next 4hrs. Shortly before starting I’ll take a mish-mash meal of fruit and toast in the galley along with other technicians and deck crew who are also preparing to start their night shifts. Although the sleeping pattern was hard to overcome to start with, I have grown to love the nightshifts. You feel like part of a funny little club who are still ticking away working at 3am and the deck crew and officers never seem to run out of great stories from previous cruises. The night time shift is quite quiet whilst we are recovering OBS as they take approximately 2 hours to rise to the sea surface, so this is a good time to catch up on some work for university.

4am – Bed time at last! Just as a whole new batch of crew start their early morning shifts it’s time for me to hit the hay. As much as we like to keep to schedule, I have no idea what lies ahead in my shift tomorrow morning but that’s half the adventure really!


My home for the 34 days out of dock! Not uncommon to see OBS floating outside window during recovery as the starboard deck is just above my room.

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Bank Holiday Antics

It seemed like a small miracle that even when sailing in international waters, hundreds of miles offshore this ship has WiFi, email access and telephone access. Sadly on Friday the dream was over and the internet satellite gave up the ghost. Unable to email and instant message the outside world, or even desperately refresh BBC News for some more reading material, it looked set to be a bleak few days. Thankfully the Catering team stepped up and laid on a huge BBQ on the back deck for everyone. It wouldn’t be a Bank Holiday Weekend BBQ without some wind and rain (yes, it does rain here in the Caribbean), but we’re British and we persevered through it. This is also the last of a back log of blog posts since the internet crash, originally from Bank Holiday Monday 29th May.


Not fazed by the spitting rain and threatening skies, we celebrated the end of survey line one with a Bank Holiday weekend barbecue.

In my last post, the magnetometer got all the limelight, yet we have more tools at our disposal on the RRS James Cook to help understand the geology here in the Caribbean.

Magnetic properties may be particularly unique to certain rock types, but all sediments have a mass and a density and we can exploit this when measuring the local gravity field. Typically, on the surface of the Earth, if an object is dropped then the rate at which it will accelerate towards the Earth is 9.8m/s². Variations in the material underneath your feet, however, will vary ever so slightly what the acceleration due to gravity is.

We can therefore exploit this property to locate where there is surplus or missing mass, relative to an average mass and gravity pull in an area. Additional mass may suggest the presence of a very dense material and will leave us with a “gravity high”, whereas a mass deficit and possibly a lower density will show as a “gravity low”. One of the possible conduits for water into the system that we have discussed is these fracture zones from the Mid-Atlantic Ridge and the precarious triple junction where the North American, South American and African Plates meet. Serpentine group minerals that we might expect in these fracture zones, are less dense and so we are on the look out for diagnostic gravity lows. The Atlantic has also been squeezed from the North and South as well as the East and West and along these faults we see high topography, dense ridges, and so we’re looking for gravity highs along these features. Gravity measurements along with the magnetics become important considerations to tie in with the active source seismic data when we interpret it.


Where we may expect to see gravity variations in a subduction zone

Actually recording gravity is a very sensitive process and one raw measurement may need many corrections applied to account for exactly where the recording was taken. On Earth for example, as the planet spins on its axis, it becomes ever so slightly squashed at the north and south pole, so it’s not a perfect sphere but oblate. This means the radius of the Earth at the equator is ever so slightly more than at the poles. This also means if you were to stand at the equator you are further away from the centre of the Earth than at the poles. If we know gravity is “stronger” the closer you are to the Earth’s surface then gravity at the equator is “weaker” than we might expect. This difference is tiny, around 0.5%, but it’s measurable and we can correct for it with a “latitude correction”. The ship is also moving significantly whilst we record the data and so we must account for motion corrections too. These are called Eotvos corrections and accurately account for the balancing forces required due to the Earth’s spin.

The ship is fitted with a gravimeter on board, in its own special temperature controlled room. It is mounted in a frame surrounded on all sides by damping springs which act to keep the machine steady even as the ship pitches and rolls over the waves. The gravimeter is very sensitive so the room is kept at a constant temperature to ensure none of the springs or internal mechanisms expand or contract with temperature, and therefore stiffen or loosen.


Our gravimeter on board is located at the very centre of the ship in its own temperature controlled room.

Whenever the James Cook comes into port, the instrumentation team take an independent portable land gravimeter out to record the local reading. This is reading we refer to as a base station and we use it to tie in with our readings we have made at sea. It is normally measured at the dockside and the team carefully measure the height difference between the two gravimeters to ensure they are measuring from the same point. The local gravity in one place can vary from the beginning to the end of the day even by slight changes due to the moon and tidal forces, or temperature change of the equipment mechanisms – this is called drift. We account for drift and also try to limit the errors by recording 3 times and taking an average.

The seismic equipment joins the ship for limited periods and comes bundled with its own team of technicians, whereas the gravimeter is looked after by the Scientific Ship Systems team. These are technicians who work at NERC and are responsible for other instruments that are permanently fixed on board like the acoustic sounding and magnetometer. Headed up by Andy on this cruise, without his expertise we would have a lot less data to work with.

Update since internet crash: Andy is also the saviour who fixed the internet satellite on Monday evening along with Chris, so this blog post is an all-round appreciation post for the most popular guys on the ship at the moment.

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