Interference Pattern Anomaly of an Acoustic Field Induced by Bottom Elasticity in Shallow Water

This paper reports the interference pattern of an acoustic field in the 100–300 Hz frequency band observed in a shallow water experiment. Assuming there is a conventional fluid sea bottom, the observed interference pattern is first demonstrated to potentially be abnormal, as a measured waveguide invariant of 1.03 corresponding to the 200–300 Hz frequency band could indicate that the bottom sound speed is relatively high, which is inconsistent with another observation from the interference pattern that the sound field is dominated by only one mode if the frequency is lower than 150 Hz. The numerical analysis shows that, owing to the high bottom sound speed, the third mode can still strongly interfere with the first mode at a frequency of 150 Hz, at least. Accordingly, a dual-layer sea bottom model consisting of a fluid sediment layer and an underlying half-space elastic substrate was proposed to interpret the observed anomaly. The results reveal that the acoustic leakage attenuation due to the elastic substrate plays a major role in the observed dominance of one mode in the sound field occurring below a frequency of 150 Hz. Therefore, the deep bottom’s elasticity may have a significant impact on the interference characteristics of low-frequency acoustic fields in shallow water under certain conditions.