Low-frequency bottom loss

The shallow refracted path through sea floor sediments plays a significant role in the transmission of acoustic energy at low frequencies. For bottom grazing angles of 90° to 25°, low-frequency acoustic energy was observed to come from reflected paths. For bottom grazing angles of 25° to 10° the dominant source of low-frequency acoustic energy is from shallow refracted paths through the sediments. At angles less than 10°, low-frequency acoustic energy is received from both the refracted and the reflected paths. The refracted path is possible because of the positive gradient within the sediment. The sudden emergence of the refracted arrival is related to the overall sound path length in the sediment and sediment absorption of sound. Since sediment absorption is directly proportional to frequency, only low-frequency energy is transmitted via this path. The refracted path may well exist where unconsolidated sediments of at least a few hundred feet are present.