JC114 in pictures and numbers

A flock of pelicans flying past the ship whilst at anchor in the Gulf of Panama with a fantastic view of Panama City and the Sonne.

The Cook has just sailed under the Bridge of the Americas and safely docked in Balboa, drawing to a close three hugely successful cruises for the OSCAR project.

 

A view over the aft deck on the last day of seismic activities, showing the streamer, the airguns in the water, and the magnetometer cable on the far right

On JC114 alone we've built up some impressive statistics, with:

5280 nautical miles sailed,

83 OBS's deployed and recovered (plus two vertical arrays),

95,500 airguns shots fired,

11 billion cubic inches of air pumped through our airguns,

1830 swath files processed,

80 (at least) red-footed boobies adorning the ship at one time,

234 whales and dolphins sighted, and

2 sea creatures boarding the ship via OBSs (a starfish and a crab) .

For the whole project we've completed three cruises on two ships for a total of 112 days at sea, and are incredibly grateful for the help we've received from all the scientists, technicians, crew and captains on board the James Cook and Sonne.

 

A OBS surfing the waves before recovery.

Although it's sad that the cruise has come to an end- this is just the beginning, as soon we'll be heading back to our respective institutions to begin analysing our fantastic data! 

 

A local fishing boat that brought us two huge dorado in exchange for our technicians fixing their bilge pump.

Finally, a thank you to our cruise blog contributors:

Gavin Haughton, Matt Funnell, Jowan Barnes, Richard Hobbs, Rob Harris, Christine Peirce, Dean Wilson, Emma Gregory and all contibutors of photographs.

 

Scientists on the bridge trying to spot the tailbuoy at the end of the streamer, 4.5 km behind the ship.

Thank you all for reading, we hope you've enjoyed hearing about our science at sea!

Two boobies perching on the A-frame with the Sonne in the background.

504B

By Richard Hobbs, OSCAR PI

Most scientists that study the ocean crust will have heard of the 504B. I heard about the site during my PhD when on a 6-month visit to Lamont-Doherty Earth Observatory, as it is now called, a research institute just north of New York in the United States. The drilling at site 504B started during my first Post-doctoral post as part of the British Institutions Reflection Profiling Syndicate in Cambridge, and here I am at the twilight of my career doing a geophysical survey over site 504B. 

So what is 504B? 504B is one of the deepest borehole ever drilled into ocean-crust and it is situated in the Panama Basin where we are working. The hole was started in 1988 as part of Leg-111 of the Ocean Drilling Project (ODP), an internationally funded project to explore the deep ocean by direct sampling. Before ODP, and its forerunner DSDP, most people that studied ocean crust worked on ophiolites – deformed fragments of ocean crust that have been pushed up to the surface by tectonic forces. The drill ship, JOIDES Resolution, was capable of sampling rocks under the oceans in water depths of over 8000 m so it was now possible to sample ocean rocks in-situ and answer first-order questions about their age, composition, and properties of the ocean crust. One of the holes, 504B, was selected as the site to attempt to drill to the level at which the ocean-crust had originally formed at the axial magma chamber. The site is 230 km south of the present-day ridge axis and drilled into crust that is 5.9 Ma (million years old). The hole reached a depth of 2111 m below the ocean floor over a series of expeditions from 1986, with two in 1991 and the last in 1993.

Cartoon of well 504B, showing the crustal structure (after Carlson 2011).

So why is 504B important to the OSCAR project? From a geophysical perspective, I use remote sensing to learn about the earth on which we live. Some of these methods have been explained in previous posts. Though I can map the physical properties to the Earth, I need calibration points like 504B so I my measurements can be directly related to actual rock. Also, this hole provides important evidence of alteration of the basalt rocks by the passage of high-temperature hydrothermal fluids shortly after its formation some 5.9 million years ago. In fact there is still some residual water circulation in the cracks and fractures at this present-day even though the ocean crust is now sealed from the ocean by over 250 m of sediment that acts like a lid.

Determining the thickness of this sediment 'lid' is a major objective for the OSCAR project as this  controls the type of heatflow from the crust into the ocean. The best method to remotely map the thickness of layers of sedimentary rock is seismic reflection. An acoustic pulse is generated by a sound source behind the vessel. When the pulse reaches the sea-bed a fraction of the energy is reflected back to the surface, the rest continues to propagate deeper into the Earth and at every deeper change in the sediment generates another reflected echo. At the surface we tow an array of highly sensitive hydrophone detectors that record the returning echoes which we can turn in to pictures that give a cross-section through the sediments.

Seismic reflection section of the crust at hole 504B, showing the sediment layers in high resolution overlaying the ocean crust

SCIENCE with RV Sonne

For five days we had something new to look at on the horizon - sehr schön. The new German Science vessel, RV Sonne, joined us as we led a merry dance from drill site 504B to the Costa Rica Rift and back, then up to the Rift one more time. Following at a distance of ~9 km, the German ship rolled out their airgun source (G-guns) and opened up.

The new German RV Sonne, joining the Cook in seismic operations for 5 days

The sensors of the OBSs were tingling as the ground moved (see previous post) in tune to a total of 19,170 shots fired in repeat pattern: GI guns, Bolt guns and G-guns. Also caught in the firing line was the multichannel streamer. Still towed by the Cook (see post on reflection data), the 4.5 km hydrophone serpent felt waves of seismic energy wash over it from both ends.

The use of multiple seismic sources firing to one streamer in this layout is known as synthetic aperture profiling. This layout allows us to record energy arriving from greater distances without towing a longer streamer - wünderbar. The energy from the Cook's guns arrives at source-to-receiver offsets of 0 to 4.5 km. Additionally, the energy from the Sonne's guns arrives at source-to-receiver offsets of 4.5 to 9.0 km.

All that's left to do is some clever 'wiggle knitting' - alles klar! The arrivals from each source are separated and stitched back together by matching the common reflection points and accounting for the different directions the energy has travelled.

Data from the Cook (blue) and Sonne (red) stiched together into a synthetic aperture gather, giving us source-receiver offsets of up to 9 km.

In the end we have a dataset that bridges the gap between the standard MCS reflection data and the OBS refraction data. The extended source-to-receiver offsets gained by synthetic aperture profiling can tell us a great deal about the velocity structure of the sediments and upper crustal rocks.

Seismic refraction: imaging the lower crust and upper mantle: PART 2

***This blog post follows on from the previous post (Part 1), so if anything sounds unfamiliar, check that one out too!***

We have been using three different sources of seismic energy during the cruise, all of which our ocean-bottom seismographs (OBSs) have recorded:

i) a GI-gun array to image the sediment layers and the upper part of the crystalline oceanic crust at high resolution;

ii) a Bolt-airgun array to propagate signals laterally through the mid-to-lower crust; and

iii) a G-gun array on the RV Sonne to generate the long distance, deep travelling signals that reach the lower crust and the mantle below. 

 

A view over the aft A-frame of the Cook. The Bolt airgun array is on the right and the GI-gun array on the left, with the air bubbles for both sources just breaking the surface by their towing floats.

In our work area the oceanic crust is ~10 km thick so the crust-mantle boundary – or Moho as it is named after the eminent seismologist Mohorovicic – is ~13 km below the sea surface (10 km of crust plus 3 km of water). To image this boundary we need to propagate seismic signals to more than 13 km below the surface and to at least 50 km laterally to see these signals returning from depth where they have travelled through the mantle, to our instruments located on the seabed. (Have a look at the diagram in Part 1 to see how an OBS further from the source will record signals that have penetrated deeper in the Earth.)

 

An example G-gun source refraction data plot from an OBS, showing signals which have travelled through the sediment cover, through oceanic igneous crust, through the mantle, and direct to the OBS through the water.

The OBSs also record earthquakes travelling through the work area, and we have recorded several of these from as close as Panama to as far away as Japan. The arrival of the magnitude 6 Panama earthquake of the 31^st January is shown below superimposed on top of some of the RRS Cook’s airgun array seismic arrivals at an OBS located within the northern ridge-axis grid.

OBS data showing Cook airgun arrivals, and the rather larger arrivals from the Panama earthquake

We've come to the end of our seismic activities now, after shooting seismic lines over the Costa Rica Rift, around borehole 504B, and also out to the west over the Ecuador Fracture Zone and Ecuador Rift spreading centre. We're just recovering the last of our OBSs deployed in the south of our study zone, before heading off to map some interesting areas of the seafloor for a few days.

Navigation plots of JC114 (up to date on the 25th Feb), showing our seismic tracks, path from Puerto Caldera, and the locations of the nearest global earthquakes to our work area which our OBSs may have recorded, including the Panama earthquake shown above.

Seismic refraction: imaging the lower crust and upper mantle: PART 1

Throughout the cruise we’ve been deploying ocean-bottom seismographs (OBSs) onto the seabed.  We have now finished a total of 81 deployments and, to date, we have successfully recovered 48 of those, each with its own multi-component dataset from three geophone sensors used to measure three-dimensional ground motion, and a hydrophone which measures pressure waves in the water column.

 

An OBS being deployed into the ocean. Each instrument has a variety of different parts: the four sensors to make the measurements, the data logger to record these and keep time, an acoustic release we signal from the ship to bring the OBS back to the surface, an anchor and float to sink and raise the instrument, a flag, light and radio to enable location of the OBS once on the surface, and a strayline to help recovering the OBS back onto the ship.

Twenty-seven OBSs are currently awaiting recovery from the seabed once the final phase of airgun shooting is complete, together with a further four that were deployed on the Sandra Ridge to the north of the work area to record local earthquakes, that will be recovered during our transit to Balboa for the end of cruise.

So why do we record airgun seismic signals using seabed instruments?

The multi-channel streamer towed behind the vessel measures signals that travel near-vertically down into the sub-seabed and reflect from the boundaries between individual rocks layers due to their difference in density. The resulting images are in two-way travel time of the recorded reflections, and give a cross-sectional-like view of the sub-surface but contain no information that allows them to be converted into true depth, so we cannot answer the question “how deep is this layer beneath the seabed?” or “how thick are these sediments?”

To answer these questions we need to know the speed, or velocity, at which each seismic signal travels through each layer, including the water layer. The water layer is a relatively easy velocity to measure using a sound velocity tool suspended from a wire lowered to near the seabed and back again. The velocities of rock layers are not so easy to measure. However, with these velocities we can convert the measured reflection times into distance much as you would use the speed limits on roads and the distances between two points to work out the time it would take to travel between A and B.

Diagram of marine seismic acquisition, showing the acoustic source (airguns), and the two types of receivers we are using: the multi-channel streamer towed behind the ship, and the OBSs on the seafloor.

 This is where an ocean-bottom seismograph (or 35 of them- which is the maximum we have had deployed along any seismic line during this cruise at any one time) comes in handy and we use the seismic refraction approach. By synchronizing their internal clocks with the same clock used to time the airgun shots (our acoustic pulses), we can measure the time it takes for signals to travel from the airgun array to each OBS on the seabed, and if we know their distance away from the shots we can work out the speed the signals travel through each sub-surface layer. We use GPS for this purpose as it can equally well provide an accurate time source as it can tell you where you are at any point.

The figure above shows how the method works and how it can be used in conjunction with reflection surveying killing two birds with one stone and making cost-effective use of the expensive ship time that we have been awarded for this project.

**Tune back in tomorrow for Part 2 of this post!**