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How It Works
Seismic refraction surveys line geophones along the ground surface and record how long it takes a shock wave (from a hammer strike or small explosive) to reach each sensor. When waves cross from a slow layer into a faster layer, they refract along the interface. The intersection of the direct and refracted travel-time curves gives the data this calculator uses to find layer depth, intersection distance, or layer velocities.
Example Problem
A refraction survey finds the crossover distance (d) is 30 m. The upper-layer velocity (v₁) is 500 m/s and the bedrock velocity (v₂) is 2,000 m/s. What is the overburden depth?
- Velocity ratio: (2000 − 500)/(2000 + 500) = 0.6
- √0.6 = 0.7746
- t = (30/2) × 0.7746 = 11.62 m
The overburden (soil/weathered rock above bedrock) is approximately 11.6 m deep.
Frequently Asked Questions
What is seismic refraction used for?
It is used to determine depth to bedrock, map the water table, estimate overburden thickness for mining, and characterize subsurface conditions before construction. The method is fast and inexpensive compared to drilling.
How deep can a seismic refraction survey measure?
The maximum depth is roughly one-third to one-fifth of the total geophone spread length. A typical 120 m spread can resolve layers to about 25-40 m depth. Deeper investigations require longer spreads and a larger energy source.
What is the crossover distance in seismic refraction?
The crossover (intersection) distance is where the direct wave and the refracted wave arrive at a geophone at the same time. Beyond this point, the faster refracted wave arrives first. This distance, combined with the two layer velocities, is used to calculate overburden depth.
What are typical seismic velocities for soil and rock?
Loose dry soil: 200-500 m/s. Saturated soil or dense clay: 500-1,500 m/s. Weathered rock: 1,500-3,000 m/s. Sound bedrock (granite, limestone): 3,000-6,000 m/s. The lower layer must be faster than the upper layer for refraction to work.
Related Calculators
- Seismometer Calculator -- calculate earthquake magnitude, seismic moment, and energy.
- Soil Resistivity Calculator -- measure soil resistivity using the Wenner four-pin method.
- Geotextile Calculator -- compute permittivity and transmissivity for drainage fabrics.
- Sound Wave Calculator -- calculate wave speed, frequency, and wavelength relationships.
- Length Unit Converter -- convert depth and distance units for geophysical surveys.