Subcritical Acoustic Penetration into Sandy Sediments with Negligible Interface Roughness

Recent observations of anomalous subcritical penetration in sandy sediments has renewed interest in the physics of acoustic sediment interaction. This paper presents measurements conducted at subcritical grazing angles over the frequency range of 1-12 kHz and examines the effects of multipath arrivals on the bottom penetration. Broadband signals, over a frequency range of 500 Hz to 12 kHz, were transmitted from a fixed source tower into the sediment and were received on a buried hydrophone array. The array was repositioned sequentially in range to acquire signals with grazing angles of 28, 15,8,5,4, 3, and 2 degrees. The direct-path penetration ratios measured here compared favorably with previous studies. For multipath propagation, it is shown that additional arrivals create significant variability in penetration ratio. The frequency regime associated with the evanescent wave was well defined at even the lowest grazing angles. Seismo-acoustic modeling using OASES 2.2 showed reasonable agreement out to about 7000Hz for grazing angles of 28, 15, and 8 degrees. The penetration ratio using a second (buried) reference hydrophone yielded much improved model-data comparisons.