During an earthquake, two distinct mechanical body waves—primary (P) waves and secondary (S) waves—are produced at the focus. A research team sets up an underground laboratory equipped with the following instruments:
• Array A: Three orthogonally oriented geophones that register ground displacement components along the x, y, and z axes.
• Array B: A mechanical grid consisting of narrowly spaced vertical steel bars placed in front of an identical set of geophones situated 50m farther from the focus. The grid can be rotated about the propagation axis.
Using simultaneous recordings from Array A and Array B, the team wishes to determine which arriving signals correspond to P waves and which correspond to S waves.
a. Using physical principles, classify P waves and S waves according to the orientation of particle motion relative to propagation direction.
b. For each wave type, state whether it can be filtered or modified by a mechanical polarizing grid of the type described and explain why.
c. Design a procedure—making explicit reference to time-of-arrival data and any changes in amplitude at Array B as the grid is rotated—that the researchers could use to distinguish unambiguously between P and S waves in their recordings.
d. Discuss one potential limitation of using mechanical polarization methods in seismology and propose a way to mitigate this limitation in future experiments.