While our initial data processing efforts focused on improving bathymetric processing, it became increasingly clear that there is also a great need for improved processing of backscatter data (both from Multibeam Sonars and Sidescan Sonars). With this in mind, we began, in 2005, a new effort aimed at improving the suite of backscatter processing tools available. The aim was two-fold: to develop easy to use tools that will generate "pretty" images of Sidescan sonar or Multibeam backscatter that will be suitable for small object detection, as well as geologic and habitat interpretation, and to develop tools that allow for the quantitative analysis of backscatter data in support of seafloor characterization and small object identification.

A lab-wide effort was started to develop a new suite of backscatter processing tools in an effort to meet these two objectives. The effort was led by Luciano Fonseca with input from many others. The goal was to create an integrated suite of tools that would allow us to import backscatter or sidescan data from a number of sensors, in various forms and formats, convert these data to an internal GSF format, correct these data (where possible) for source levels, beam patterns, gains, area ensonified, attenuation and local slope, and then either analyze and/or display these data in a georeferenced mosaic. The result of this effort is GeoCoder, a C++ mosaicing tool that reads multibeam or sidescan sonar data in GSF, XTF or a range of native formats and applies a series of radiometric and geometric corrections to the data including corrections for beam pattern effects.

Normally, the empirical beam-pattern correction is calculated as the residual necessary to flatten the angular response registered by the sonar system; i.e., to normalize the backscatter at 45 degrees, sometimes adding a Lambertian correction. The approach used by GeoCoder calculates the beam pattern as the residual to the modeled angular response of the ensonified seafloor that then reveals the actual nonlinearity of the transducer angular response. Data are then georeferenced (or geocoded – thus the origin of the name) in a projected coordinate system using an interpolation scheme that emulates the acquisition geometry.

A feathering algorithm smooths the transition between overlapping lines and an anti-aliasing algorithm makes it possible to produce a lower resolution mosaic that is not degraded by aliasing. Slant range is corrected using actual bathymetry, and a trend-adaptive angle-varying gain helps remove artifacts that appear when different bottom types are found along a single swath. Lines can be removed or remosaicked, and the overlap area between parallel lines can be controlled by filter parameters.

GeoCoder also supports a statistical package that identifies patterns in the backscatter response that can be used in support of seafloor characterization (see below). Statistics calculated for backscatter bins include: mean, mode, range, minimum, maximum, standard deviation, variance, percentiles, quartile range, skewness, kurtosis, moments of any order, and also parameters extracted from a gray-level co-occurrence matrix (contrast, homogeneity, dissimilarity, entropy and energy). Taking advantage of the corrections made to the backscatter, GeoCoder also serves as the front end for a new and exciting approach to using multibeam backscatter data for seafloor characterization called ARA (Angular Range Analysis—formerly known as AVO). The ARA tool will be reported on in the seafloor characterization section.

Since its development, GeoCoder has become a simple-to-use tool for generating a high quality sidescan-sonar or backscatter “mosaic” that has been greeted with much excitement in the community. There has been tremendous interest in this software from NOAA, our industrial partners, and academic institutions which has led to a number of licensing requests and requests for training.

Given the high demand for use of GeoCoder, the list of systems that it supports (and the list of users) is quickly growing. The complete list of systems and formats supported is now:

• Kongsberg/Simrad multibeam .all (beam time series and beam average)
• Simrad Sidescan
• Reson (.xtf, .s7k), snippets, beam average and sidescan
• Klein sidescan, sdf, sdf2
• XTF sidescan (various sonars)
• GSF multibeam (various sonars, beam average and snippets)
• HSX sidescan (various sonars)
• Seabeam (beam average and sidescan)
• Geoswath (.rdf)
• C3D (.xtf)

In further support of our backscatter (and other) processing efforts, Brian Calder has developed and licensed (to industrial partners SAIC and GeoAcoustics) software to convert GeoAcoustics data to GSF format; a prototype to convert the native GeoSwath format (RDF) into GSF has also been developed.

The value of GeoCoder is also demonstrated by the growing interest from our industrial sponsors; licenses for GeoCoder have been issued to:

• Caris
• Reson
• Fugro
• Triton
• Hypack
• IVS 3D
• Chesapeake Technology

Additionally, a number of NOAA programs and academic partners are actively using GeoCoder, these include:

• NOAA SANCTUARIES
• NOAA Alaska Fisheries
• NOAA Pacific Coral Reef Program
• NOAA Ship Thomas Jefferson
• NOAA Ship Fairweather
• NOAA/JIMAR Coral Reef Ecosystem Division
• Jacobs University Bremen, School of Engineering and Science
• University of Galway
• University of Ulster, Northern Ireland
• Oregon State University
• University of Saint Andrews
• Geological Survey of Canada
• CIDCO-Le Centre Interdisciplinaire de Développement en Cartographie des Océans
• Stockholm University, Department of Geology and Geochemistry
•  Alaska Department of Fish and Game
• University of Illinois at Urbana-Champaign, Departments of Geology, Geography and Civil Engineering

With the departure of Luciano Fonseca for the UNESCO IOC office in the spring of 2009, support for GeoCoder transferred to Dr. Yuri Rzhanov who has focused on repackaging GeoCoder so that it is easier to track changes and fix bugs. Additionally, the code has been modified to accept the latest Kongsberg datagram format (revision M—version 12) and the XTF reader has been upgraded.