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!**

Fissure eruptions

As well as in the form of lava domes and seamounts (see earlier post), there is a third way magma can erupt onto the seafloor: as a fissure eruption. Fissure eruptions generally conjure up images of lava fountains erupting from long rifts in Iceland, and an underwater version of this eruption occurs 3000m deep at the Costa Rica Rift.

A fissure eruption at Krafla, Iceland

From our swath bathymetry data, we can identify a few large craters up to 2km wide dotting the seafloor, and their formation can be linked to these fissure eruptions. These craters (known as calderas) were probably once lava domes, formed by small eruptions from a shallow magma chamber when enough pressure had built up. 

Schematic diagram of a fissure eruption occurring along a fault, emptying a magma chamber that was previously feeding a lava dome, and forming a crater or caldera in its place

Occasionally the stress of the stretching crust pulling apart at the ridge will cause a fracture to open up perpendicular to the spreading direction. This fault will propagate through the crust laterally until the stress (known as tensile stress) has been dissipated. These fractures can sometimes propagate into the subsurface magma chambers, diverting the pressurised magma away from its usual eruption site at the lava dome. The magma will travel along this fracture, forming a dyke, and erupt in a long fissure eruption the length of the fault. These eruptions can often empty the magma chamber which reduces the pressure in the surrounding crust. A similar process to this occurs on land when water is extracted from an underground aquifer, resulting in subsidence. Here, the rapid exodus of magma causes a zone of decompressed crust to occur beneath the lava dome. The above crust will then collapse into this space compacting the magma chamber and resulting in a caldera forming at the surface, where once stood a 100m high lava dome.