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Cruise Report for NBP 95-07

SeaBeam 2112 Side-Scan Data

The SeaBeam 2112 data we recorded included binary side-scan data along with ASCII bathymetry data. Side-scan data characterize the spectral reflectance of an incident surface to the transmitted sonar signal. This type of data can be used to distinguish between smoothly sedimented terrains and rougher, rocky terrains of the deeper ocean floor. In the field, such information might be useful when selecting dredging and coring sites. To this end, a test area was chosen to develop a processing methodology for side- scan data. The area chosen corresponded to a six hour track where single-channel seismic data were also collected. The idea being that the accuracy of the results of a sediment detection algorithm for the side-scan data could be corroborated by comparison with the seismic profile.

The MB-System routines (current release 4.2F) are a collection of commands created by a group at the Lamont-Doherty Earth Observatory of Columbia University specifically for the processing of oceanographic data. A general methodology for processing side-scan data utilizes a selection of the MB-System routines. The sequence involves the following steps: 1) using 'mbcopy' to copy out data within the region of interest to another file for processing (although this is not necessary, it is done to help reduce the total amount of data and minimize overall processing time), 2) using 'mbanglecorrect' to correct for beam grazing angle and ocean bottom bathymetry, 3) passing the data through 'mbfilter' to apply high and low pass filters to enhance or smooth the features of interest, 4) applying 'mbcut' to remove the natural noise that occurs in side-scan data for beams directly below the ship, and 5) finally, using 'mbgrid' to fit the data to a regular grid and dump ASCII format data suitable for use with other visualization programs such as the University of Hawaii's Generic Mapping Tools (GMT) or Math Works' MatLab.

Parts of seven of the SeaBeam data files covered the test area. Most of the steps outlined above proceeded rapidly but some required up to twenty minutes per file to complete processing. Also, a few small discrepancies were discovered between the current release of the MB-System software and the accompanying documentation. The goal of using the side-scan data for sediment detection was not accomplished within the scope of the cruise. However, the methodology appears promising and with enough time and processing power should be obtainable to at least some degree.

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