Above: Interpretation of OBS data (Thank-you to Prof Christine Peirce and Matt Funnell) This is from OBS number 7 line L of our seismic surveys on JC132. So very up to date information – hot off the press!!
When the Ocean Bottom Seismic Instruments come to the surface scientists are able to get a computer print out of the information recorded. The data looks like the picture above.
The water wave is easily identifiable but will not tell scientists anything about the rock. Instead, it is the horizontal lines (or close to horizontal) that tell the scientists about the velocity and therefore the density of the rock. Horizontal lines have a velocity of 6km/sec (lower oceanic crust). Specific rocks have specific velocities and densities. The further away the shot the deeper the seismic waves will travel to the OBS instrument – the deeper rock waves will travel faster so will reach the instrument first.
Each vertical line is one airgun shot and the whole diagram is one profile from one end of a track to the other. Each OBS is crossed at least twice at 90 degrees (on different tracks)so that the data can pinpoint where exactly on the seafloor the OBS has landed. The water waves are vital for this as they have well known velocity of 1.5km/sec +/- 3% ….the speed of sound in water.
Below: Diagram showing RRS James Cook tracks. The location of the OBS instruments are on the seafloor where lines intersect (not every intersection!!) . Tracks are N-S or E-W.
Above: Is a picture of the Ocean Core Complex we are studying taken on a previous research expedition. (Thank- you to Prof Tim Reston)
The OCC looks very different to the surrounding rock. It is a smooth dome with distinct scratches.
Superimposed upon the picture is a fault scarp from the Andes – similar in scale. You can see the flat valley floor and the detachment fault rising from beneath the surface. The scratches (striations) are on the footwall which is the part that rises. The valley is part of the hanging wall.
Nigel in his house