A theoretical investigation of the farfield secondary radiation present in sediment due to a parametric array incident on a water–sediment boundary is conducted, based on the assumption that the interface completely truncates the parametric array in the nearfield of its primary radiation. The field is found to be due to two apertures, one coincident with the transducer, the second with the truncation. At subcritical angles of primary beam incidence the field is similar to that produced by a conventional beam. At postcritical angles of primary beam incidence the presence of the truncation aperture results in a steeper and deeper penetration of the sediment than in the conventional case. It is concluded that the presence of virtual sources near the interface is responsible for the postcritical penetration of sediment by a parametric beam.