The waves that help scientists to understand the internal structure of the Earth are

Scientists understand the Earth's internal structure primarily by analyzing seismic waves, which are categorized into body waves and surface waves. Body waves, consisting of Primary (P) and Secondary (S) waves, are the most important source for this purpose because they travel through the Earth's interior [4]. While both P and S waves provide data, S-waves (secondary waves) are uniquely critical for determining the state of the Earth's layers. S-waves are transverse waves that can only travel through solid materials and cannot pass through liquids [2]. By observing 'shadow zones' where S-waves are absent, scientists concluded that the Earth's outer core is liquid. Although P-waves (longitudinal) also travel through the interior, the specific property of S-waves to be blocked by non-solid layers makes them the definitive tool for mapping the internal boundaries and physical states of the Earth's core and mantle [4].

Sources

1. [4] https://www.britannica.com/video/rock-vibrations-Earth-earthquake-waves-P-surface/-218347
2. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 4: Earths Interior > Seismic Waves > p. 58