The interface between two different homogeneous sediment layers, each composed of uniform grain sizes, is a region of heterogeneity comprising a thin layer with a thickness typically much smaller than a seismic wavelength. Seismic waves propagating parallel to the interface experience a reduction in both their amplitude and frequency content but the effect on group velocity is unresolvable. In the case of a single homogeneous layer bounded on both sides by homogeneous halfspaces, the ability to spatially resolve the layer thickness from amplitude or frequency information is limited by diffraction when the layer thickness is smaller than a seismic wavelength. However, the presence of a subwavelength interface or layer is always marked by a decrease in amplitude and in some cases by a decrease in frequency. ¿ 1999 American Geophysical Union