Illustration of the fluid pressure perturbation interacting with the underlying solid medium. The blue arrows represent a moving horizontal pressure field, which can be generated by either acoustic waves (Section 2) or horizontal-moving fluid waves, such as winds or ocean waves (Sections 3 and 4). For the subsurface, a homogeneous medium implies that n = 0 (Section 2), while a 1-D medium implies that n is a positive integer (Sections 3 and 4). The red triangle represents a seismometer on the medium surface.