(Please start at the bottom of the page to follow our adventures from the beginning.)
The objective of PICTURES is to generate images of faults and other structures beneath the seafloor. Although we have to wait until the end of the cruise to recover the Ocean Bottom Seismometers and get our hands (and computers) on the data, we get the multichannel seismic (MCS) reflection data in real time and have been having a wonderful time processing the data to generate pictures. These data are recorded on a long (up to 12.5 km!) tube that is towed behind the ship and contains thousands of hydrophones. The hydrophones record minute pressure changes generated as sound waves from our seismic source bounce off the seafloor and structures beneath it. The cartoon below shows ray paths of reflected waves recorded on the hydrophone streamer and refracted waves recorded at distances beyond the streamer by OBSs or sonobuoys (expendible floating buoys that contain a hydrophone and send data by radio back to the ship). For this cruise we are using OBSs rather than sonobuoys because they stay at a fixed place on the seafloor and can record data to much greater distance.
The seismic reflection data are rearranged in the computer to generate images like the one shown below, which shows the oceanic crust of the Nazca plate being thrust beneath the continental margin of Chile. Although it looks like the underthrust (i.e. subducted) crust is flat beneath the accretionary prism, this is an illusion. The vertical axis in this image is the time it takes for the acoustic energy to travel from the energy source to the reflector and back to the streamer. To convert this axis to depth, information on the speed of sound is needed, and sound travels more quickly in the accretionary prism than in the ocean. The speed of sound in water is approximately 1500 m/s. A rough guess of the speed of sound in the accretionary prism is 3500 m/s. We therefore estimate that the Nazca plate crust is at about 7 km depth at the trench and at about 9.5 km depth at the eastern edge of this image. The OBS data will provide more detailed information on the speed of sound beneath the seafloor that will be used to improve the imaging.
And for a contrasting view, here is an image we just got this morning showing a large seamount or ridge entering the subduction zone.
We are scrambling to process the data as fast as it comes in to get a first look at the data. Check back again for more pictures from PICTURES.
– Anne Tréhu, November 2016