SACLANT UNDERSEA RESEARCH CENTRE LA SPEZIA (ITALY) LA SPEZIA Italy
Sonar performance predictions of reverberation in shallow water rely upon good estimates of the bottom scattering strength and reverberation statistics. This report describes a new technique for measuring bottom scattering in shallow water. The technique employs a vertical receive array. with either broadband incoherent sources lightbulbs or coherent sources. Five coherent sources were designed with a spacing to minimize multipaths at 600. 1200 Hz. 1800 Hz and 3600 Hz. The technique is not only capable of producing scattering strengths both monostatic and vertically bi-static but can also provide direct evidence of the bottom scatter mechanism current modeling techniques have to rely on sometimes tenuous clues for discerning the bottom scatter mechanism. This feature of the measurement technique is particularly important, inasmuch as determining the physical scattering mechanism is crucial for eventual database development in support of low frequency active sonar performance predictions. There is also mounting evidence that the second-order reverberation statistics i.e., clutter can be linked to the bottom scatter mechanism. Bottom scatter measurements and interpretations are provided at two shallow water sites in the Ligurian Sea. The results show that the measurement technique is viable. and shows the surprising result that sub-bottom scattering from as deep as 25 m in the sediments can contribute to the scattering at 1800 Hz.