The multibeam sonar is taken out of the water for re-calibration at a specific time each day. Click image for larger view and image credit.
OSU Graduate Student Morgan Erhardt prepares the CTD for a cast. Click image for larger view and image credit.
Surveying the Seafloor with Multibeam Sonar
The first leg of this expedition has been dedicated to mapping the San Andreas Fault and surrounding seafloor via multibeam sonar. Multibeam sonar sends out a series of ‘pings’ to the sea floor and uses the time it takes from when the ‘pings’ were sent out to when their echo off the sea-floor returns to a sensor on the sonar to determine how deep the water is. Sonar can also indicate where ridges, rocks and other features of higher elevations are, as they will have a shorter return than the rest of the seafloor around them.
To process the multibeam data, periodic casts must be made with a CTD (Conductivity [salinity], Temperature and Depth) sampling device. This data will be used to calibrate sound velocity in water, which is needed in conjunction with the time it takes the ping to return to determine seafloor depth.
Due to wave action and the movement of the ship, there are variations and outliers in data. To fix these, we enter the raw data into a computer program, such as Caris, and then edit out the outliers and abnormal returns to create a profile of the seafloor along the survey lines.
Multibeam surveys are run in a systematic patter, which is often referred to as “mowing the lawn.” Survey lines run parallel, with the ends overlapping to cover gaps in the data. To keep the boat on the correct survey line, members of the science crew keep a close watch on the heading and steer the boat either through a pulley system attached to the wheel, or at the wheel itself.
Thus, through running a consistent survey and processing the multibeam data onsite, the research team is able to visualize where the San Andreas Fault is running under the sea and the surrounding seafloor terrain.
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