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PHYS2403 Simulation Lab

Experiment 4: Seismic Refraction & Travel-Time Curves

Seismic refraction uses critical refraction (Snell's Law) to determine depth to bedrock, water tables, and soil thickness. By plotting first arrival times versus offset, we determine layer velocities and boundary depths. Run the simulator to map the ray paths and plot the travel-time curves.

Parameters

Layer 1 Velocity (v_1)500 m/s
Layer 2 Velocity (v_2)2500 m/s
Layer Thickness (h)20 m
Survey Presets
Receiver Distance (x)120 m

Subsurface Seismic Ray PathsNormal refraction

Refractor v_2 = 2500 m/sLayer 1 v_1 = 500 m/sθc = 11.5°Soil Layer (h = 20m)

Travel-Time Curve (t-x)t_i = 78.4 ms

slope = 1/v1t_i = 78msxc = 49m0125m250m0
Current offset: 120 mFirst arrival: 126.4 ms (refracted)

Refraction Calculations

Depth from Intercept Time

The refracted wave line intercepts the time axis at tit_i. This corresponds to the total time spent traveling vertically down and up through Layer 1 at the critical angle. We compute layer depth hh as:

h=ti1000⋅v12cos⁡θch = \frac{t_i}{1000} \cdot \frac{v_1}{2 \cos \theta_c}
Crossover Distance

The distance xcx_c where the direct wave and refracted wave travel times are equal. Close to the source, the direct wave arrives first. Farther away, the refracted wave (traveling in the faster Layer 2) catches up and arrives first.

xc=2h⋅v2+v1v2−v1x_c = 2h \cdot \sqrt{\frac{v_2 + v_1}{v_2 - v_1}}