Guided (channel) waves and normal-mode propagation
In Figure 13.3a the first arrival () has traveled at the velocity 2.7 km/s; find the water depth.
A wave guide is a layer in which a wave can propagate with little loss of energy. In a water layer nearly total reflection occurs at both boundaries, at the surface because of the very large impedance contrast and at the bottom reflection beyond the critical angle. The phase is inverted at the surface because the reflectivity is nearly , but not at the seafloor (beyond the critical angle) until the angle becomes very large.
Figure 13.3b(i) shows waves bouncing back and forth in a wave guide. For certain incident angles and frequencies , they interfere constructively. In Figure 13.3b(ii), is a wavefront traveling upward at the angle . The previous cycle of a parallel wavefront that passed through the position earlier followed paths such as EFGH and BDA and thus coincides with . Clearly . , , so . Taking into account the phase reversal at the surface the condition for constructive interference is
Writing for the phase velocity, the frequencies that are reinforced are
In addition to the upgoing waves parallel to and in Figure 13.3b(iii), a second downgoing set and will combine with the set to build up the energy traveling in the direction along the wave guide. Energy travels from to in the time that wavefront moves to , so the phase velocity of the energy traveling along , , is
Since both and are functions of , is dispersive with a group velocity given by equation (2.7a):
The derivative is always negative for a water channel wave (Figure 13.3c), so .
If the wave-guide effect had been due to reflection beyond the critical angle at both boundaries, as with a low-velocity coal seam, the phase would not have been reversed at either boundary and equations (13.3a,b) would reduce to
Equation (13.3c) gives
and we use equations (13.3b,e) with to get :
What frequency is reinforced when ?
We use equation (13.3b) with , , :
|Equally inclined orthogonal geophones
|Vertical seismic profiling
Also in this chapter
- S-wave conversion in marine surveys
- Equally inclined orthogonal geophones
- Guided (channel) waves and normal-mode propagation
- Vertical seismic profiling
- Effect of velocity change on VSP traveltime
- Mapping the vertical flank of a salt dome
- Poission’s ratio from P- and S-wave traveltimes