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21 July 2002 - BGS onboard

RRS James Clark Ross Diary

Position at 1200: 59° 01.1'N, 015° 41.9'W (105 miles North West of Rockall)
Distance steamed since Grimsby (10/09/01): 55229 Nautical Miles
Air temperature: 13.9°C; Sea temperature: 12.8°C


A week off Rockall (Banging away....on behalf of BGS)

After the previous month you could say that the last week has been awfully quiet, then again we only have a science party of seven. That is six from BGS (British Geological Survey) and Mick Mackey from the Coastal Resources Centre at University College Cork, he's a Seabirds and Cetacean observer so spends his time on the monkey island, in all weathers, whenever the visibility gives him a reasonable view. (Cetacean's are marine mammals i.e. whales and dolphins; according to the dictionary anyway).

Well it's generally quiet, that is if you cannot make out the distant boom of the airguns firing every six seconds. The array is being towed astern and it's quite strange how you can hear the guns in the most unlikely spots and places where you would expect to hear them, there is nothing. The airguns are one of the tools being used by BGS for the geological survey that we are presently conducting to the west of the northern tip of Scotland. The cruise started just west of the Outer Hebrides. We are now beginning to work a series of survey lines to the north and west of Rockall. Rockall is a rock pinnacle sitting out in the Atlantic and so extends the United Kingdom territorial claim of the seabed further away from the UK mainland.

During the survey the vessel steams along at about four and a half knots, which keeps the bridge watchkeepers busy as they constantly monitor the speed and direction to ensure the correct course is achieved. Apart from "driving" the ship in the right direction, the cruise has fairly little impact on the ship as once mobilised we just feed the personnel and the airguns with food and compressed air and they get on with it. We'll leave it up to you to decide who gets what! The timing of all this is good as it means we can get some of the sorting out done ready for the refit, which starts a week on Thursday in Portsmouth.

The weather, so far, has been quite kind to us, having threatened to blow up a bit it hasn't lasted for long before abating. Long may it continue.


The Science bit in the middle

Geological Background of Survey BGS02/02, North Rockall Trough and Hatton Bank
by Dave McInroy & Dave Smith (PSO)

As part of a continuing programme to understand the geology of the region west of Scotland, The British Geological Survey (BGS) Continental shelf and Margins Programme are mapping the geology of two areas on this cruise, the North Rockall Trough and Hatton Bank. The Rockall Trough is an intra-continental basin and lies off northwest Britain and Ireland (you can see the trough on BBC weather maps). In the northern area we are currently surveying, the water depth ranges from a couple of hundred meters to over a kilometre. Further south, the water depth in the trough reaches 4 km. The second area we are mapping is Hatton Bank, a submerged piece of continental crust, which lies approximately 600km northwest of Britain. Beyond Hatton Bank lies the oceanic crust of the North Atlantic Ocean. More on Hatton Bank next week.

The Rockall Trough can be thought of as the 'Atlantic Ocean that never opened'. The Rockall area experienced continental rifting in the Late Mesozoic and Early Cenozoic (150 to 55 million years ago) as the North Atlantic Ocean tried to open. This essentially formed a trough into which sediments have since been deposited. The early attempts at opening were accompanied by extensive volcanic activity, and much of the Atlantic margin off northwest Britain is covered in thick basalt. The Atlantic Ocean failed to open through the Rockall Trough, and instead opened further west 53 million years ago, leaving a 'failed rift' preserved in the continental crust. By mapping the sediments preserved in the Rockall Trough, we hope to better understand what has happened to the trough during the last 53 million years since the Atlantic Ocean opened.

We know that the Rockall Trough hasn't simply sat undisturbed during this time. It has experienced periods of subsidence, uplift and compression, events that will have had an impact on how the sediments were deposited in the trough. We aim to identify known unconformities, which are boundaries between rock layers that represent gaps in sedimentation, periods of erosion or both. An unconformity would form when, for example, the Rockall Trough subsided rapidly and the surrounding continental shelf became submerged. This would have effectively halted the trough's supply of sediment. Also, the deeper water in the trough would have allowed bottom currents to circulate, eroding the existing sediments in places. This may have happened in the Rockall Trough in the Late Eocene (about 38 million years ago). Another effect of regional submergence is to reduce the effectiveness of ridges on the sea floor as barriers to ocean currents. If such a barrier can be bypassed by ocean currents, sediments can be transported to previously new areas. This may have happened in the Rockall Trough in the Early Miocene (about 17 million years ago) when the Greenland-Scotland ridge was bypassed, providing a link from the Arctic Ocean to the North Atlantic Ocean. A more detailed description of the Rockall Trough and its sediments during the last 55 million years is given in: M. S. STOKER, T. C. E. VAN WEERING & T. SVAERDBORG 2001. A Mid- to Late Cenozoic tectonostratigraphic framework for the Rockall Trough. In: SHANNON, P. M., HAUGHTON, P. D. W. & CORCORAN, D. V. (eds) 2001. The Petroleum Exploration of Ireland's Offshore Basins. Geological Society, London, Special Publications, 188.

This is how they do it

How does the British Geological Survey carry out marine surveys and collect the data? We run a series of lines at approx. 5 knots and simultaneously collect data from a variety of marine geophysical systems. These are: two types of seismic reflection, Airgun and Sparker sources. Each of these makes a bang in the water and we listen to the reflections from the rock layers beneath the seabed using hydrophones, (underwater microphones).

A bubble pulse rises towards the float from the airguns. Click to enlarge Davie Baxter with a serviced airgun. Click to enlarge


Above: Left: A bubble pulse rises towards the float from the airguns. Right: Davie Baxter with a serviced airgun. Click on the images to enlarge them.

A picture is built up of the rocks below the seabed as we travel along; this is a bit like cutting a slice through a cake with many layers. Being a lower frequency sound source, the Airgun record will penetrate deeper (up to 1500 m below seabed) than the Sparker (500 m) but has less resolution. The Airgun operates by suddenly discharging high-pressure air into the sea, making a bang.

Dave Wallis, watch keeping on the seismic records. Click to enlarge A seismic record, showing a slice through the earth like a slice through a cake. Click to enlarge Dave McInroy and Dave Smith (PSO) looking over the line recordings. Click to enlarge


Above: Left: Dave Wallis, watch keeping on the seismic records. Middle: A seismic record, showing a slice through the earth like a slice through a cake. Right: Dave McInroy and Dave Smith (PSO) looking over the line recordings. Click on the images to enlarge them.

The Sparker operates by discharging high voltage (3000 V) electrical energy (2200 Joules) into the sea through 135 electrodes. This discharge creates small bubbles, which collapse making a bang. Simultaneously with the seismic reflection profiles, we run a magnetometer that measures the earth's magnetic field, this instrument is also towed behind the vessel.

The Sparker on deck to be given a 'hair cut' by Mike Wilson. Click to enlarge Sparker data. Click to enlarge


Above: Left: The Sparker on deck to be given a 'hair cut' by Mike Wilson. Right: Sparker Data.

A gravity meter installed within the ship measures the small variations in the earth's gravitational field, and finally we like to know how deep the water is and for this we use the vessels excellent Swath Bathymetry system, which has been explained in previous weekly updates.

A gravity meter
A gravity meter. click to enlarge

Using the data from all these instruments the geologists can, through interpretation, produce maps and models to gain a better understanding of the rocks below the seabed, as described above. However it is only by sampling below the seabed with corers that these models can be calibrated. This is carried out by drilling the seabed and retrieving a physical sample. Regular readers to these web pages will have seen some of the BGS coring techniques in action most recently JR63 May 2001, JR64 July 2001 and JR71 February/March 2002.

The Sparker 'lighting' up the sea at night
The Sparker 'lighting' up the sea at night


Department of the Week

The Engineering department at work? Having showcased the Bridge watchkeepers a couple of weeks ago we have moved onto the Engineers and hopefully show them in some of their normal working environments. It's obviously a fact of life these days that the paperwork has taken over.

Dave Cutting, the Chief Engineer, in front of his computer Bill Kerswell, working once again on the fridge machinery. Click to enlarge


Above: Left: Dave Cutting, the Chief Engineer, in his usual pose i.e. in front of a computer. Right: Bill Kerswell, working once again on the fridge machinery. Click on the images to enlarge them.

Charlie Smith and Mark Robinshaw (Motormen)in their second home - the gash room. Click to enlarge Steve Eadie (4th Eng) writing the daily log up and Gerry Armour (3rd Eng) keeping his records


Above:Left: Charlie Smith and Mark Robinshaw (Motormen) in their second home - the gash room. Right: Steve Eadie (4th Eng) writing the daily log up and Gerry Armour (3rd Eng) keeping his records. Click on the images to enlarge them.

Simon Wright (Deck Engineer) in the workshop. Click to enlarge Norman Thomas (Electrician) check the voltage supplies to the Labs. Click to enlarge


Above: Left: Simon Wright (Deck Engineer) in the workshop. Right: Norman Thomas (Electrician) checks the voltage supplies to the Labs. Click on the images to enlarge them.


A Bit of Fun

Mark and Charlie our Motormen didn't want their "friends" from the gash room to miss out on starring on the web. We are starting to worry about those two sometimes; maybe they've been at sea too long?

Mark and Charlie's 'friends' in the gash room
Click to enlarge
Mark and Charlie's 'friends' in the gash room. click to enlarge


Photographic Credits

This week thanks go to Dave Smith and Pat Cooper for allowing me to use their images.