Figure 3.15: Graphic demonstrating sound propagation in a water layer over a layered seafloor. A large part of the sound travels downwards. Sound that hits the sea bottom at an angle greater than the critical angle (θ) will be totally reflected. This sound is then trapped within the sea layer, which in turn channels or guides it. This phenomenon is known as waveguide or normal mode propagation.
the water wave merge, at which point they form a single wave called the Airy phase. At this abrupt end of the wave train, energy has been transported in the water layer waveguide with the minimum group velocity. Te onset of the water wave is sometimes used in marine
refraction work to determine the source-receiver range (since the water speed is well known). We conclude that with increasing horizontal distance from
airguns, the signal decreases in strength but increases in time duration during the guiding of the sound. Te initially short airgun array signal, some 10 milliseconds in length, can become quite long. In the water wave, higher frequencies arrive before lower frequencies. Tis geometrical dispersion effect will be sensed as a frequency modulated tone or ‘hooting’ by anyone listening down there in the water column.
Figure 3.16: Seismic signal recorded in the water layer at a distance of 13 km away from a source vessel. The water depth is 70m. The initial source signal some 10 milliseconds long is significantly broadened in time since the signal frequencies travel with different velocities.