Guided (channel) waves and normal-mode propagation

From SEG Wiki
Jump to navigation Jump to search
ADVERTISEMENT

Problem 13.3a

In Figure 13.3a the first arrival () has traveled at the velocity 2.7 km/s; find the water depth.

Background

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


(13.3a)

Writing for the phase velocity, the frequencies that are reinforced are


(13.3b)

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


(13.3c)
Figure 13.3a.  Wave-layer channel wave, source at 4 km (after Clay and Medwin, 1977).
Figure 13.3b.  (i) Raypaths in a wave guide; (ii) showing reinforcement of reflection wavefronts; (iii) relation between phase and group velocities.

Since both and are functions of , is dispersive with a group velocity given by equation (2.7a):


(13.3d)

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


(13.3e)

Solution

Equation (13.3c) gives

and we use equations (13.3b,e) with to get :

Figure 13.3c.  Phase and group velocities versus normalized frequency where , , , and (from Ewing, Jardetsky, and Press, 1957).

Problem 13.3b

What frequency is reinforced when ?

Solution

We use equation (13.3b) with , , :

Continue reading

Previous section Next section
Equally inclined orthogonal geophones Vertical seismic profiling
Previous chapter Next chapter
3D methods Specialized applications

Table of Contents (book)

Also in this chapter

External links

find literature about
Guided (channel) waves and normal-mode propagation
SEG button search.png Datapages button.png GeoScienceWorld button.png OnePetro button.png Schlumberger button.png Google button.png AGI button.png