Monday, September 1, 2014

Day 2

Monday 1st September, 2014

Following a 10-hour transit all the way from the Port of Catania we finally made it to our destination – our first site of investigation out five. Most of the day the scientific crew spent their time in the laboratory, located on the lower decks of the vessel. The laboratory is in constant communication with the bridge (from where the captain and officers steer the ship). This is important to ensure that the ship is traveling along the planned route as it scans the sea bottom of the study area.

During this shift we collected our first data from the Multibeam Echo Sounder (on board it is referred to as multibeam for short).  The multibeam records data on the seafloor bathymetry and acoustic backscatter. In principle it sends a series of acoustic pulses in a fan from underneath the ship's hull. This sends a suite of soundings to the seafloor which are then reflected back to the receiver on board. This is used to get an idea of the depth and the sea bottom topography. The backscatter gives information about the reflectivity of the seabed, which is related to the type of sediment (sand and gravel give a much stronger echo than soft mud, for example).

Ship using multibeam echosounder to map a swath of seafloor.
Ship using multibeam echosounder to map a swath of seafloor. Credit: Fisheries and Oceans Canada.

Matthew and Daniela analyzing results from the multibeam sonar to produce a DTM (digital terrain model) of the Malta Escarpment

Prior to this, the multibeam first has to be  calibrated using an SVP (Sound Velocity Profile). The SVP sensor is deployed from the boat and records the sound velocity in the water as it is lowered through the water column. We will need this information to convert the times of the acoustic echos into real depths underneath the ship, making sure that our data is producing an accurate representation of the seafloor.

 The SVP device is being lowered into the water to calibrate the multibeam system

On the same principle an independent system of pulses are transmitted to establish the layering of the sediments below the seabed. This sub-bottom profiler, also known as CHIRP,  obtains seismic reflections up to the first 50 meters into the sediments. The two systems, multibeam and CHIRP, are working simultaneously and they do not interfere with one another because of the different frequencies they use.

 Joshu anaylsing the seismic reflections from the CHIRP

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