The author has previously reported [J. Acoust. Soc. Am.88, 324–334 (1990)] on a model for acoustic backscatter from the volume beneath a smooth sediment seabed. In the present paper, the model is generalized to an in‐plane scattering model such that the source and receiver need not be colocated but must lie in the same vertical plane. In the model, scattering is caused by fluctuations in sediment porosity. The model allows for penetration of the incident wave into the bottom at subcritical grazing, and retransmission of scattered spherical waves through the (planar) interface. The frequency and grazing angle dependence of the acoustic scatter are determined primarily by the correlation function of the porosity fluctuations, whereas the magnitude of the scatter is controlled by the porosity variance. Results are obtained from the model over the frequency band 5–200 kHz, for a variety of incident and scattered grazing angles, using two sample porosity correlation functions. The first is an exponentially decaying correlation function and the second is a point scatterer correlation function. The results obtained illustrate the significance of the correlation function in determining the scattering strength of the bottom volume.