Acoustic propagation anomalies caused by thin geoacoustic layers

The effect of a thin sedimentary layer on the low frequency and long-range propagation is discussed from a theoretical and experimental point of view. It is demonstrated that there are at least two prominent effects. When the top layer has some rigidity, but is relatively soft and is overlying a hard rock subbottom, a shear wave resonance effect may occur, resulting in abnormal high transmission losses for a number of very narrow frequency bands. The second effect is high reflection loss resulting from generation of interface waves at the sediment-substrate boundary. Some experimental results from broad band acoustic measurements are presented. Seismo-acoustic models were derived from geophysical surveys and used as input to a numerical acoustic model. In general the agreement between the modeled and measured values of the transmission loss were quite good. For certain frequencies and ranges the measured propagation loss exhibited anomalies that can be explained by thin-layer effects.