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.
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.
Donna Shillington, NAU