Week 3: Water Column Data/End of cruise
- In my earlier posts, I have described how how mutlibeam mapping data are acquired. These data still need some processing to be presented in images like I shared with you earlier. There are three main types of information that we can glean from the multibeam sonar mapping data:
- Seafloor depth (or bathymetry) is calculated by using the time it took for the sound waves to travel from the sonar to the seafloor and back (two way travel time) and an estimate of the sound speed in water.
- Seafloor backscatter information uses the amount of the acoustic energy that a particular seafloor reflects back towards the sonar. This information allows scientists to differentiate between low reflective seafloors (e.g. mud and sand) and high reflective seafloor (e.g. rocks).
- There is one more way the multibeam data can be used from the changes seen in the water column. As transmitted sound waves pass through the water columnn on their way to the seafloor, any targets in the water column reflect energy back to the sonar. Possible water column targets may include strong water temperature and salinity changes gradients, schools of fish, marine mammals, gas seeps or planktonic organisms. When there are such objects in the water column, the multibeam computer screen shows a change in image color. One commonly seen color pattern are large stratified diffuse masses that vertically ascend and descend in the water column at night. This could be interpreted as phytoplankton or zooplankton making their evening migration in the water column. To compensate for changes in water temperature and salinity that might alter the multibeam data, regular XBT casts are launched to record a water column profile that allows the computers to adjust the raw data.
We are ending our trip now and have acquired and processed a lot of useful data for the scientific community and general public.
Wednesday evening the NOAA Ship Okeanos Explorer will port in Norfolk, VA and await the next crew. Here are some final ship pictures. I will miss my time at sea. Thank you for following along and sending me your questions. Thank you also to the people who have helped me be part of this, my advisor Dr. Wade Jeffrey, Tracy Ippolito for the website and the crew of NOAA Ship Okeanos Explorer who made my stay on the ship a memorable experience for me.
|Okeanos' Remotely Operated Vehicle (ROV) - Little Hercules|
|The ROV wench aboard Okeanos Explorer|
|VSAT satellite terminal|
|Oil rig in the Gulf of Mexico|
|Okeanos' small boat|