What Is the Seismic Refraction Two-Layer Traveltime Calculator?
Seismic refraction surveying is an active geophysical exploration technique used in geotechnical engineering and hydrogeology.
It maps soil thickness and bedrock depth by measuring the arrival times of head waves refracted along layer boundaries.
How Does the Seismic Refraction Two-Layer Traveltime Calculator Work?
Computes the Snell critical refraction angle between layer 1 and layer 2.
Inverts the crossover distance where refracted wave first-arrivals overtake direct waves.
Determines bedrock interface depth z1 and time intercept ti.
Seismic Refraction Two-Layer Traveltime Calculator Formula & Variables
The core mathematical equation utilized by this calculator is expressed as:
Snell refraction critical angle geometry mapping crossover offset to bedrock interface depth.
How to Use the Seismic Refraction Two-Layer Traveltime Calculator
- Enter P-wave velocity of the surface soil (V1) in m/s.
- Enter bedrock P-wave velocity (V2) in m/s (must exceed V1).
- Input the observed crossover offset distance in meters from the geophone spread.
Step-by-Step Example Calculation
Geotechnical Soil-over-Bedrock Survey
Input Values:
Understanding Your Result
Refractor depth gives the thickness of the soil layer above solid rock.
Critical distance defines the closest geophone offset where refracted energy can physically return to the surface.
Factors That Affect the Result
- Velocity contrast: Greater contrast (V2 >> V1) brings crossover distance closer to the shotpoint.
- Water table: Saturated sediments can elevate V1 to ~1500 m/s regardless of soil density.
When Should You Use This Calculator?
- Foundation engineering to determine excavation depth for building footings.
- Quarry and mining overburden volume estimation.
Assumptions & Limitations
- Assumes planar, horizontal, non-dipping layers.
- Requires V2 > V1; cannot detect hidden velocity inversions (low-velocity layers).
Frequently Asked Questions
Calculation Accuracy & Reference Note
Calculations are exact for planar homogeneous isotropic media.