Direct, reflected and head-wave travel-time models
For source and receiver on a flat surface a horizontal distance x apart above a uniform layer of depth h and speed v₁, direct travel time is x/v₁ and reflection from its horizontal base has time 2√(h²+x²/4)/v₁.
Key knowledge
For source and receiver on a flat surface a horizontal distance x apart above a uniform layer of depth h and speed v₁, direct travel time is x/v₁ and reflection from its horizontal base has time 2√(h²+x²/4)/v₁. If the lower medium has v₂>v₁, the ideal critically refracted head-wave branch has t=x/v₂+2h√(v₁⁻²−v₂⁻²), where its geometric path exists. Observed first arrivals are the minimum among available detectable branches, not a single universal straight line.
Values and equations
t_direct=x/v₁; t_reflect=2√(h²+x²/4)/v₁; t_head=x/v₂+2h√(v₁⁻²−v₂⁻²)
- t_direct,t_reflect,t_head
- travel times for the specified ray branches s
- x,h
- surface separation and layer thickness m
- v₁,v₂
- upper/lower branch speeds m s⁻¹
Conditions and scope
- h>0,x≥0 and constant speeds, planar stationary layers, geometric rays and same surface source/receiver depth. For the head path x≥2h tanθ_c with sinθ_c=v₁/v₂; earlier arrival than the direct branch is a separate crossover condition.
- Finite frequency, attenuation, weak arrivals, dipping/curved interfaces and lateral variability alter interpretation. Different parameterized structures can fit finite travel-time data; the formula does not supply a unique general seismic inversion.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Disclosed geometry derivation from inspected Snell/critical-ray model: reflection has two equal hypotenuses; head path has two h/cosθ_c legs and lower-interface length x−2h tanθ_c, giving the stated intercept. Source printed17–19 supports the ray construction, not a claim that these layered formulas are printed there. Excluded later incorrect chord/critical formulas and all waveform/amplitude inference.
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Entry details
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- uni-earth-seismic-layer-times
- Revision
- core.1
- Structured reference record
- University core expansion