Seismic Wave Velocity in Various Rock Types
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The seismic waves produced by the detonation of an explosive charge or any other system are transmitted through the Earth in the form of vibrations or waves that travel at different speeds. These speeds are influenced by several factors, including lithology, depth, porosity of the material, compaction, lithification, and fluid content, among others. Furthermore, these speeds depend on the elastic modulus and density. These parameters are essential for velocity analysis in the interpretation of seismic data. Below is a table showing the propagation speeds of seismic waves; these values specifically refer to longitudinal waves (P-waves).
| Material | Velocity (V) [m/s] |
|---|---|
| Weathering layer | 300–900 |
| Modern alluvium | 350–1,500 |
| Clays | 1,000–2,000 |
| Marls | 1,800–3,200 |
| Sandstone | 1,400–4,500 |
| Conglomerates | 2,500–5,000 |
| Limestone (Calizas) | 4,000–6,000 |
| Dolomite (Dolomias) | 5,000–6,000 |
| Salt (Sal) | 4,500–6,500 |
| Gypsum (Yeso) | 3,000–4,000 |
| Anhydrite (Anhidrita) | 3,000–6,000 |
| Gneiss (Gneises) | 3,100–5,400 |
| Quartzite (Cuarzitas) | 5,100–6,100 |
| Granite (Granitos) | 4,000–6,000 |
| Gabbro (Gabros) | 6,700–7,300 |
| Dunite (Dunitas) | 7,900–8,400 |
| Diabase | 5,800–7,100 |
Analysis of Seismic Propagation Speeds
An analysis of these data shows that igneous rocks generally exhibit higher speeds than sedimentary rocks. These speeds vary within the indicated limits depending on the depth and the elastic constants. In general, for the same rock type, the speed increases with geological age; similarly, for a rock of the same age, the speed increases with depth.
Internal Structure of the Earth (According to Gutenberg)
In the seismic charts, the drop in P-wave velocity at approximately 2,900 km represents the limit of the Earth's core. Since shear waves (S-waves) do not appear in this region, it acts as a liquid. The discontinuity located at about 50 km or less below the soil surface, known as the Mohorovičić (Moho) discontinuity, represents the base of the crust.
The Mantle and Core Boundaries
The region between the crust and the core is called the Mantle. In its outer part, the "P" wave has a speed of 8.1 km/s, which increases with small discontinuities toward the core. At the limit of the core, the "P" wave velocity drops while the "S" wave velocity increases to the base of the mantle. Within the nucleus, Vp (P-wave velocity) increases rapidly with depth toward the center of the Earth.
Geophysical Modeling and Material Identification
Thus, it is possible to obtain curves showing the variation of elastic constants—such as compressibility and stiffness—with depth. Based on these velocity variations, researchers can identify the materials inside the Earth and determine their physical state. Other researchers have designed models, such as the Geodynamic and Geostatic models, to explain the internal structure of the Earth, but they all point to the same basic layout for the Earth's interior.