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Cruise Report for NBP 95-07

SeaBeam 2112 Multi-Beam Bathymetry Data

SeaBeam 2112 multi-beam bathymetry data were collected during almost the entire time the ship was underway. Collection and recording of the multi-beam data occurred twenty- four hours a day, seven days a week including the transits to and from the study areas. Continuous recording produced hundreds of megabytes of computer data. The collected data was partitioned into files representing approximately one hour surveying tracts that were sometimes greater than ten megabytes in size depending on the water depth. In shallower water, distances from the ship sensors to ocean bottom features are smaller. Under these conditions the sonar signal returns quicker and each beam covers a smaller area. Here, the SeaBeam system must ping more often to cover the same along track length and, thus, generate more data.

The SeaBeam data processing software is currently under development and as such noise and anomaly filtering of the collected data is somewhat less than desirable. Until these facilities are more completely developed, hand editing is the only approach to clean up the data. This requires inspecting each of the individual scan lines and manually removing spikes, noisy sections, and artifacts. To keep up with the volume of data we were collecting, almost everyone on the ship was employed to assist with hand editing of the data. Although this helped us keep up with the volume of data, inconsistencies in editing between people actually created more work and some of the files had to be re-edited.

Along with the editing problems, we ran into other complications with the data. The inversion scheme used to generate the bathymetry images from the reflected sonar data uses water column velocity profiles that are calculated from the XBT temperature profiles. Water temperature varies with depth and with the component waters that mix in the area. Using an incorrect water velocity profile causes the inversion to indicate incorrect travel times for the sonar returns and thus indicate inaccurate distances for the traveling sonar waves. This is exaggerated in the outer beams and the individual scan lines can have 'mustache' curls where the ends curl upwards or downwards. In an image, these can appear as small furrows or ridges that parallel the ship track at a distance.

To assist with visualization of the surveyed features, I set about creating some three- dimensional images of the ocean bottom features. A large volcano in the Central Bransfield Basin is approximately three kilometers across the mouth and 800 meters high. When displayed in three dimensions and animated, the contrast between the taller, steeper back side and the shorter, shallower front side become obvious. In the 3-D image one can almost see where the lava might have flowed down the front of the volcano. In the Three Ridges image the furrows in the sedimented bottom west of the ridges and curving around the front of the ridges are the result of an incorrect water velocity profile as described previously. Here the furrows are overlapping outer beams of successive survey tracks.

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