Tuesday, March 25, 2014

Long lines and lots of instruments

If you want to image the Earth’s crust and upper mantle with seismic data, you need to record the arrival of seismic waves that have propagated down to, in our case, depths of up to ~30 km.  These deep-diving phases travel quickly through the denser, higher velocity rocks of the lower crust and upper mantle, and they arrive back at the surface ahead of shallower phases at long source-receiver offsets (see video below).  


video


To record these lower-crustal and upper-mantle phases as “first arrivals”, where they are not obscured by the arrival of energy from shallow paths, we use long lines.  Long lines mean lots of receivers and lots of driving to deploy and recover these instruments.  We could have used lots of sources instead, but the blasts we used to get seismic energy into the lower crust and upper mantle in this experiment take a lot of time and money to setup.  Receivers are much cheaper, so we used a lot of them.  (For similar wide-angle/long-offset work at sea, airgun sources are cheaper than putting seismometers on the seafloor, so we use many shots and a smaller number of receivers out there.)


This time-lapse video shows Team 13 of 14 recovering 89 of the 1200 total short-period seismograph stations from where our line crossed Fort Benning, near the northwestern end of the line.



Nathan Miller, LDEO