Multi-Path Interference: Path-lengths.

The configuration of the acoustical system at 3-Mile Slough is shown in the diagram. Identical source and receiver transducers are positioned approximately 2 m under the water, separated by a direct path length of 220 m. The reflection from the free surface of the water travels a path about 5 wavelengths longer than the direct ray; the bottom reflection travels about 30 wavelengths farther. The channel has a fairly uniform depth of about 7 m, though fairly large-amplitude sand waves propagate slowly down the channel.

The model which we have adopted for the interference is based on the following assumptions:
  1. The bottom reflection is lossy enough that its effect can be neglected, along with higher-order reflections.
  2. The interference term between direct ray and surface reflection will be multiplied by an "overlap factor" which accounts for the fact that the delay of the reflected pulse causes part of each bit of the m-sequence to interfere with the following bit, which has a random relative phase shift of zero or 180 degrees, which give cancelling interference terms.