A full-spectrum chirp sonar system [LeBlanc et al., J. Acoust. Soc. Am. 91(1) (1992)] was used to invert three-dimensional density and sound-speed structure of the subbottom at the SWARM-95 (shallow water acoustics in random media) experimental site on the New Jersey Shelf. Biot's theory was used in a forward model to calculate normal-incident reflection seismograms for layered marine sediments. Efficient inversion algorithms based on global optimization techniques were used to survey large areas for characterization of sediment properties and their spatial distributions. Chirp sonar inversion results were compared with ground-truth core data at the AMCOR-6010 site and good agreement was observed. Three-dimensional images of sediment density and sound-speed structure were constructed from several parallel runs along the SWARM acoustic track 1003 at the outer-shelf sediment wedge area. Corresponding 3-D wave-number spectra of density and sound-speed variability were also calculated.