Numerical wavefield modeling, based on time domain finite difference solutions to the full elastic wave equation, is used to quantitatively predict the sensitivity of monostatic backscatter to variations in geological properties of the seabed. This article addresses the hypothesis that observed backscatter signals from the seafloor are produced by a combination of seafloor (interface) and subseafloor (volume) scattering from structures having variations at scale lengths comparable to the wavelength of the insonifying acoustic field. Wavelength-scale seafloor roughness and subseafloor volume heterogeneity parameters are defined using stochastic spatial probability functions having Gaussian autocorrelations. The range of the variations in these parameters is constrained to realistic values based on estimates derived from seafloor bathymetry and other geologic data. Modeling results show that backscattering from rough, basaltic (hard) bottoms, in the absence of large-scale seafloor slope, is dominated by rough surface scattering with little contribution from volume scattering. Contrary to this, sediment (soft) bottoms with subseafloor volume heterogeneity, with or without seafloor roughness, produce significant backscattering signals compared to a homogeneous sediment bottom.