Day 3 contd...

Real Data Break… So how do we collect this kind of data…
We have two surveys going on side by side, an acoustic and biological or ground-truthing survey.
In order to work out what kind of experimental transects we carry out. The multibeam echosounder is the first tool employed. These are also known as Swathe and were developed in the 1970s by the US Navy in order to map large swaths of the ocean floor to assist the underwater navigation of its submarine force. As technology improved until the 1990s, higher frequency systems suitable for high-resolution mapping in shallow water were developed in support of navigational charting. It works by sending an array of sound signals in a fan shape from a transducer at the bottom of the ship. The sound travels through the water, reflects off the seafloor and returns to the transducer, where the time taken for the return trip is measured (see diagram). A simple formula can work out the depth and as the ship moves (approx speed of 3-4 knts) the depths are continuously recorded.
We use the backscatter from the multibeam in order to infer the roughness of the seabed. The level of blackness component of the multibeam represents the level of reflectivity of the seabed, where white represents hard ground and black soft ground. A lot of analysis post-deployment takes place where the signals are cleaned in order to remove anomalous results, e.g., a signal arising from a passing fish.
We employ deep tow side scan sonar in the deeper areas. It provides high resolution images at depths that surpass those of analog systems.
Before deep tow, we deploy the SVP. This is the Sound Velocity Profile. The SVP reveals some common structure to the ocean. The largest variation is the speed of sound in water, it occurs with changes in depth, the SVP is the tool used for predicting how sound will travel.
For those who want to know more see http://www.fas.org/man/dod-101/navy/docs/es310/SNR_PROP/snr_prop.htm
Transects or lines are planned on the basis of results from the multibeam and or deep tow. We need to get data from a range of depths, habitat boundaries, ridges, flat areas…
The images are captured by a video and camera specially designed for the deep, all encased in a frame that is also a lander. Attached here is also a Conductivity Temperature Depth profiler (CTD). The boat will move into a position at the start of the line and deep vid-cam system lowered and will follow the line as much as possible. There will be currents and internal waves at work in thewater column, so manoeuvrability can be difficult. The video is continuously recording and is kept as close to the seabed as possible. The system is landed and photographs are taken every minute as a statistical image. However, photographs are to be taken when needed to document everything occurring down there.