Seismic refraction surveys are based on which principle?

• A. The differential seismic velocities
• B. Seismic wave reflection at layer interfaces
• C. Magnetic susceptibility contrasts
• D. Thermal conductivity

Answer: (A)

Seismic refraction surveys rely on velocity contrasts between subsurface layers and the way seismic waves bend and travel along a boundary when they hit at angles governed by Snell’s law. A wave moving through a slower near-surface layer reaching an interface with a faster layer refracts into that faster layer and, because the faster layer can guide energy, travels along the boundary as a head wave. The energy returns to the surface, and the first-arrival times at increasing distances form a pattern that is controlled by the velocity of the deeper, faster layer. From that travel-time vs distance behavior, you can determine the deeper-layer velocity and the depth to the interface. This dependence on differential seismic velocities is exactly what makes refraction work. Other phenomena, like reflection at interfaces, magnetic properties, or thermal conductivity, are not the basis for seismic refraction.