The XSonar and ShowImage software programs, developed by the USGS, Woods Hole Science Center (Danforth, 1997), were designed to process sidescan-sonar data within the X Window systems environment running under Linux/UNIX and now MAC OS X systems. These programs enable the user, through a Graphical User Interface, to correct geometric and radiometric distortions inherent in sidescan-sonar records, and map and/or mosaic the processed sidescan-sonar records.
The four basic processing steps used in XSonar and WHIPS (see below) are as follows:
- Demultiplexing - median filter is applied to the raw sidescan-sonar data reducing the data by a defined factor (generally 6 or 12), and resulting in the removal of speckle noise (high intensity returns not representative of sea floor characteristics)
- Slant-Range - corrects slant-range distances to true ground distance, and removes water column offset
- Destriping - corrects for low amplitude returns along individual pings due either to vertical motion (pitch) of the tow-fish or system inconsistencies
- Beam Pattern - corrects for decreases in beam intensity with range due to decreasing grazing angles and signal attenuation.
Woods Hole Image Processing System (WHIPS)
The WHIPS software, also developed at the USGS, Woods Hole Science Center (Paskevich, 1992a, 1992b), was designed to run on UNIX/Linux operating systems using the Unidata NetCDF software interface. WHIPS represents a series of command line programs, the basics of which correct radiometric (beam pattern and destripe) and geographic (slant-range) distortions inherent in the sidescan-sonar records (see above). The WHIPS software is comprised of additional utilities that enable high- and low-pass filtering of the data that can greatly reduce noise. This software can also map and/or mosaick the sidescan-sonar records in a variety of projections.
PCI Geomatics is a developer of commercial software packages offering a variety of tools and processes in remote sensing, photogrammetry, spatial analysis, and cartography. The Sea-floor Mapping Group utilizes GCPWorks to digitally mosaic sidescan-sonar records. Sonar data are imported from either the XSonar/Showimage or WHIPS software into the PCI software as fully processed, geographically oriented data files.
|Screen Shot of the PCI
graphical user interface.
(Click image to enlarge)
The steps involved in digitally mosaicking sidescan-sonar records are as follows:
- Ground Control Points - common targets (geologic features) on adjacent sidescan-sonar records are chosen as ground control points (GCPs); essentially used to "tie down" sidescan-sonar records at numerous locations throughout the image domain, and to correct for navigational errors, or feature offset due to undefined layback of the tow vehicle.
- Cutting and Pasting - a stencil line is drawn around an area of an individual sidescan-sonar record (swath) that will be "pasted" to an adjacent sidescan-sonar swath.
This procedure is followed for each sidescan-sonar swath, resulting in the formation of a composite digital mosaic.
Additional software tools frequently utilized are tone matching and reprojection. Tone matching equilibrates the sidescan-sonar records by normalizing the digital number (DN) values (0 - 255; black - white) between adjacent swaths or defined areas within a mosaic. Reprojection utilities transform mosaics from one defined projection, or datum, to another. Light areas indicate areas of high backscatter and dark areas, low backscatter.
For examples of processed data, please navigate to the sonar equipment page and scroll to the last section.
Danforth, W.W., 1997, Xsonar/ShowImage: A Complete System for Rapid Sidescan-Sonar Processing and Display, U.S. Geological Survey Open-File Report, 97-686, pp. 99
Paskevich, V., 1992a, Woods Hole image processing system software implementation using NetCDF as a software interface for image processing, U.S. Geological Survey, Open-File Report 92-25, pp. 72.
Paskevich V., 1992b, Digital mapping of side-scan sonar data with the Woods Hole Image Processing System Software, U.S. Geological Survey, Open-File Report, 92-536, pp. 87.