A theoretical and experimental investigation into the penetration of parametric acoustic beams into sediment is conducted. The theory is based on the assumption that the interface completely truncates the array. With this assumption, the asymtotic farfield of the secondary radiation in sediment is developed and discussed. When the primary field is truncated in its nearfield, the secondary farfield is found to be due to two apertures, one coincident with the transducer and one with the truncation. At subcritical angles of incidence the field is similar to that produced by a conventional beam. At post-critical angles of beam incidence the presence of the truncation aperture results in a steeper and deeper penetration of the beam than in the conventional case. As the trucation moves into the farfield of the primaries, the effect of the truncation aperture is reduced until the parametric beam behaves qualitatively similar to a conventional beam. Experimental measurements of the secondary field throughout two vertical planes in the sediment are made, and are in good agreement with the theoretical predictions. The wide bandwidth of the parametric array is exploited to distinguish in time Snell's law and evanescent arrivals in the sediment. These latter are seen to be important close to the interface. It is concluded that the postcritical penetration of parametric sound beams into sediment is due to virtual secondary sources close to the interface and suitably phased.
|Date of Award||1984|