We are now about half way through our expedition. We are closing in on four weeks out at sea and have just under four weeks remaining before we return to terra firma. We recently reached the western-most point in our journey and turned around to return east across the study area. Now it’s time to switch gears in our data collection: moving from deploying and recovering OBSs to towing a multi-channel seismic (MCS) streamer behind the boat. But let’s take one step back and look at how this all works.
We begin this process by generating a sound in the water. This sound (or pressure wave) travels down through the water, into the Earth, and will transmit through and reflect off of layers in the Earth based on changes in density and velocity (the speed of sound in specific materials). When the pressure waves return, we can record them on instruments (we are using both the OBSs and the MCS streamer to record these returns) and process them into images or models of what the Earth looks like beneath the surface. This practice fits into a field of science called seismology.
To generate the sound, we tow airguns behind the ship. In our case, we have 40 airguns, 36 active and 4 spares, which hang from massive floats in the water. They shoot in tandem to release a large volume of air (6600 cu. in.) into the water, which creates a sound. While we are ‘shooting’ the guns release every 37.5 m as the boat moves across the transect with a dominant frequency of about 50 Hz. You may be thinking, ‘don’t you have to be careful of animals in the water when you do this?’ The answer is yes- more on that in an upcoming post!
The MCS streamer is a cable we tow behind the boat containing listening instruments called hydrophones. Hydrophones are specifically designed to detect the returning pressure waves in water. We are using a 12 kilometer-long streamer, which allows for recording seismic arrivals from deep within the crust. We drive in preplanned straight lines over areas that we want to image. Once we finish one ‘line’ we are all busy inside the boat processing the data as we move on to the next line.
We were lucky enough to get to assist with the task of deploying the streamer, which was very exciting! Being able to participate in the data collection is a very special experience. Even when there are no issues, getting the streamer out in the water is a time-consuming and physical job. The ship’s awesome science crew worked day and night in cool weather and rain to resolve problems and get the streamer deployed. Barring any issues, we will continue shooting and collecting MCS data for 12 days as we travel back east across our study area. – Justin and Bobby