Singing to the seismometers

Seismic source elements on the ship before deployment.

During this cruise, we are using sound waves to image geological structures deep below the seafloor.  Determining the structure of Earth’s interior is one of the main types of seismology, and seismologists use a wide range of techniques and types of seismic waves to examine different depth intervals and resolutions.  Many of these approaches use distant or local earthquakes as the source of sound waves. The special thing about the type of seismology we are employing on this expedition (‘active source seismology’) is that we create our own sound waves, so we can control their positions, timing and character.  This allows us to obtain much higher resolution images of the upper part (~60 km) of the Earth than is possible with other methods.

We are towing a seismic source array behind the Langseth that creates sound waves by emitting pulses of highly compressed air.  By using a combination of different source elements with different sizes, we can create a nearly ideal seismic source signal: a sharp, simple burst that is short in duration and contains a range of frequencies.

Seismic source array towed behind the ship (right side of picture) near Atka Island. The seismic source elements are hanging 9 m below the black floats that can be seen at the surface. Each float has a GPS to record its location.

We have spent the last 2.5 days steaming along our first transect VERY slowly (~5-5.5 miles/hour) emitting sound waves to be recorded by the ocean bottom seismometers (OBS).  The OBS are autonomous instruments that are recording data to internal disks, and we will not know what they recorded until we retrieve them in a couple of days.  However, we were delighted to learn that seismometers deployed on the Aleutian Islands by the Alaska Volcano Observatory for volcano monitoring recorded our signals! Unlike our ocean bottom seismometers, data from seismometers on land can be retrieved remotely and monitored in real time; these instruments are very important for monitoring earthquake and volcanic activity. The image below show the amplitude of signals at different frequencies that were recorded over a one-hour time period when the ship was near the seismic station. If a strong signal is recorded at some frequency, it shows up in lighter colors. The regular high amplitude blips on this recording are the Langseth’s seismic source! Encouraging! Many thanks to Matt Haney (USGS/AVO) for sharing these plots with us.

The top panel shows an example of recordings of our seismic source on an AVO seismic station on Kanaga volcano onshore. One hour of data are displayed from September 10, when the ship was close to Kanaga. Our source shows up as regular, light-colored blips in this spectrogram. The lower panel show a map of the islands, volcanoes (red triangles), AVO seismic stations onshore (orange squares) and ocean bottom seismometers (dark blue circles). The ship track is shown with a blue line, with our location every hour labeled with a circle and text. The dark red line shows where the ship was during the hour of recording shown in the upper panel.

Donna Shillington, NAU

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