Wednesday, July 31, 2013

Binning is the name of the game

As we come to the end of our cruise I thought that now would be a good time to talk about the way in which both seismic and multi-beam sonar data are quantified (basically nerd out). In both cases we "bin" the data into grid cells, which are predefined based on the resolution that we expect to achieve given the ideal data density of individual cells within the grid. 

A screen capture from the multi-beam sonar Seafloor Information System (SIS).
The image on the left shows swath coverage. The image on the right shows an active ping through the water column.
Multi-beam sonar (swath seafloor mapping) data are collected, gridded (binned) to the predefined cell size, and output in two flavors. Bathymetric grids, which are essentially 3D topographic maps, and Backscatter grids, which display the reflectivity of the seafloor. The reflectivity varies due to both incidence angle of the respective beams and the density of the surface (e.g. hard rock, sediment etc). As the ship moves along at a given velocity, the multi-beam sonar sends a "ping" from the transducers (transmitters) to the seafloor and then waits until the receipt of the last return to ping again. The ping rate (Hz or 1/seconds) is a function of the depth of the ocean as well as the sound speed through water (XBT's are useful!). The swath width also scales as a function of depth. Our average depth is ~4800m (2.98 miles), which allows for an achievable swath width of ~20km (12.43 miles!).

Swath coverage display of the backscatter (reflectivity of the seafloor) collected across a swath.
In order to gain insight on the density of the multi-channel seismic (MCS) data that we are collecting we use the Spectra software package. Spectra tracks the position of the ship, streamers, and air guns in real time using GPS and an acoustic network, and then bins the data accordingly within the predefined grid. The goal is to get an equal amount of seismic traces (reflected seismic waves) in each bin. The traces can then be stacked (combined), which increases the signal to noise ratio. Stacked traces within a bin are called "fold" and ideally represent traces from all offsets along the streamer in respect to the source.

A screen capture of the Spectra display. The image on the left shows active binning of the MCS data.
The image on the right shows the bins being infilled (filling holes).
We are getting to the end of the "No Mores," which means we are finished on Friday!! Stay tuned for a word from our Chief Scientist along with a look at the MCS data (and our cruise pic). 

James Gibson