Seeing Below the Surface
Seismic refraction is one of the oldest and most reliable geophysical methods for mapping subsurface structure. A seismic source (hammer blow, explosive charge, or vibrator) generates waves that travel through the ground. When these waves encounter an interface where the velocity increases, they refract according to Snell's law. At the critical angle, the refracted wave travels horizontally along the interface and continuously re-radiates energy upward — the 'head wave' — which is recorded by a line of surface geophones.
Travel-Time Curves
Plotting arrival time versus distance reveals two distinct branches: the direct wave (a straight line through the origin with slope 1/V₁) and the refracted arrival (a straight line with slope 1/V₂ and a positive time intercept). The slopes directly give the layer velocities, while the time intercept yields the layer depth through t_i = 2h×cos(θ_c)/V₁. The crossover distance where the two lines intersect marks the offset beyond which the head wave arrives first.
Critical Angle and Head Waves
The critical angle θ_c = arcsin(V₁/V₂) is the geometric key to refraction. A ray hitting the interface at exactly this angle refracts to travel horizontally in the faster medium. As it propagates along the interface at velocity V₂, it continuously generates secondary waves that return to the surface at the critical angle. This head wave has a conical wavefront and always arrives with apparent velocity V₂ along the surface, regardless of the angle.
Field Applications
Refraction surveys are standard practice in engineering geology (mapping bedrock depth for foundations), hydrogeology (finding water table depth), and crustal seismology (determining Moho depth). Forward and reverse shooting — placing sources at both ends of the geophone spread — resolves dipping interfaces. Modern tomographic refraction methods relax the flat-layer assumption and can image complex velocity gradients and lateral heterogeneity.