Similar to light waves, seismic waves can reflect, refract, and become diffracted at Earth's structural discontinuities. Seismic wavefield has one component that propagates through the Earth and along its boundaries in two ways: body waves and surface waves, respectively. The other component is stationary and a result of interference among intersecting waves, and is commonly known as normal modes or free oscillations of the Earth. Here, body waves and free oscillations sensitive to the IC are discussed in detail. Theoretical background is given to enhance understanding of these two critical types of data for seismological studies of the IC. Data analysis examples of both IC sensitive body waves and normal modes are given. Main observational and measurement principles are presented and backed by examples. Theoretical background to understand basic principles of array seismology is given.