A new method to estimate seafloor roughness spectra applied the surface backscatter model of Jackson et al. [1986> to multi-beam sonar backscatter data. The use of principal components analysis, following spectral estimation, derives a single mappable index from correlated spectral parameters. This index can be mapped as a continuous field corresponding to geological observations in the area. This method allows for quantitative delineation of the neovolcanic zone and other zones of volcanic and tectonic activity from hull-mounted sonar systems. ¿ American Geophysical Union 1993