As part of the research experiment at 13N we will be imaging the seabed using sound sources. The same sound sources can be used to do this imaging in two different seismic ways. One approach is called refraction seismology and the other is called reflection seismology.
The seabed instruments that we will deploy will be used for refraction seismic imaging which measures the thickness of the individual rock layers and the speed at which the seismic signals propagate through them. Speed (or velocity as its called in this approach) is directly related to rock density, and rock density can tell us about rock lithology or type. So we can tell the difference between a sediment and an igneous rock, and even between different types of igneous rocks such as the basalt and gabbro that comprise the upper and lower parts of the oceanic crust respectively, which is what we will be looking at.
Refraction seismology basically makes use of signals travelling laterally. Reflection seismology, on the other hand, makes use of sound signals travelling near vertically and, as the name implies, bouncing or reflecting off the interfaces between different rocks types in the subsurface.
Reflection seismology measures the time it takes to travel down to each layer and then travel back to the surface detector. These times are then converted into depths using the velocities measured using the refraction approach. The result of reflection seismology is a cross-sectional view through the crust showing the relationships between individual layers.
The two methods are often carried out in parallel as, with careful planning, the same sound sources can be used for both methods at the same time.
In contrast to refraction seismology, the detectors used for reflection seismology are towed behind the vessel in a long snake-like array called a streamer. Currently we are testing the individual components of the streamer before connecting them all together and winding them onto a winch. The images below show the component parts of the streamer currently being tested, and the computer-based systems into the lab that do the data recording and allow the data to be quality controlled as we acquired it.
We will be towing this reflection seismic data acquisition system behind the Royal Research Ship James Cook which is currently berthed in Southampton being prepared for the cruise prior to ours.
The RRS James Cook is about 100m long and can accommodate up to 52 scientists, technicians and crew. It has a maximum research cruise duration of 52 days, and once outside the 12nm limit will make its own power, water and treat its own sewerage. In other words, it is totally self-contained.