What does a seismic refraction survey primarily determine?

• A. Determine subsurface layer velocities and depths
• B. Map surface topography
• C. Assess groundwater chemistry
• D. Detect magnetic anomalies

Answer: (A)

Seismic refraction surveys determine subsurface layer velocities and depths. In this method, seismic waves are generated at the surface and travel down to layer boundaries where velocity changes, then refract along those boundaries back to the surface. The first-arriving energy at receivers depends on the velocity contrast and the geometry of the layers, so travel times collected at multiple offsets form a pattern that reveals the velocity of the upper layer and the depth to the next boundary. By analyzing these travel-time plots, you can infer both the velocities of successive layers and the depths to their interfaces. This information directly characterizes the subsurface structure in terms of layer speeds and thicknesses. It isn’t used for mapping surface topography, assessing groundwater chemistry, or detecting magnetic anomalies, which require different methods or data types.