Data streaming in…

During the second phase of our cruise, we are collecting seismic imaging data using a long cable (6 to 9 km) filled with pressure sensors that we tow behind the ship, traveling very slowly.  The streamer records sound waves emitted by our seismic source that have bounced off of layers in the Earth. The close spacing of many pressure sensors and the length of the cable allow us to record returning seismic waves in great detail, which we can use to reconstruct detailed pictures of geology below the seafloor, including sedimentary layering, faults and other features.  It’s very powerful and cool data. What’s more, we can create preliminary images very soon after the data are collected to get a first peak, so we have been busy doing that as the data roll in (thus a gap in my blog entries…).

The upper panel shows a cartoon of a subduction zone, where two tectonic plates converge and one is thrust beneath the other one. The plate boundary fault separating these two plates produces very large earthquakes. We use sound waves to create images of this fault zone and other structures; the sound waves (represented by red arrows) are generated by our seismic source, bounce off of geological features below the seafloor, and are recorded by our seismic streamer. The lower panel shows an example of a recording on the streamer. We can see many sound wave reflections, including from the plate boundary fault. Orange and black colors represent positive and negative wiggles (zoom of data from blue box shown in inset), which provide information on the physical properties of the fault zone.
Seismic streamer stored on gigantic reel on R/V Langsetb

The streamer is stored on huge reels on the stern of the Langseth. To deploy it, the science technical team on the Langseth unspools it into the ocean, which takes about ~12 hours. Ideally, we would like to tow the streamer 9 meters below the sea surface, straight behind the ship along our collection profile. We can control the depth using ‘birds’ attached to the streamer. However, we have almost no control on its position; it often gets pushed around by currents. Yesterday, as we passed through a particularly strong current that was nearly perpendicular to our data collection line, the seismic streamer had a ~90º bend – ouch! 

We have collected over half of our planned dataset so far, and the data look excellent!

Navigation display of streamer location (purple line and text), ship (white circle/blue line and text) and our data collection line (yellow). The streamer has moved north away from the data collection line due to currents.

Donna Shillington, NAU

Go back to Aleutians Field Blog