Use this seismology calculation to reproduce a stated convention, screen observations, or compare scenarios whose units, coordinate conventions, catalog selection, and model assumptions are known.

z₁ (m)

Required number in m. Upper receiver depth.

z₂ (m)

Required number in m. Lower receiver depth.

t₁ (s)

Required number in s. First arrival at z₁.

t₂ (s)

Required number in s. First arrival at z₂.

The cited overview of Seismology supplies background for the terminology and domain context used by this tool.^[1]

Interval and average shear velocities from downhole first-arrival times at two depths. Inputs are interpreted exactly in the displayed units and the calculation returns the following fields without presentation rounding.

V_int (m/s)

Returned number in m/s. Velocity between the receivers.

V_avg (m/s)

Returned number in m/s. Average velocity from surface to z₂.

• Earth structure, source geometry, path effects, site response, instrument response, catalog completeness, uncertainty, and the fitted range can dominate the accuracy of simplified seismic calculations.
• z₁ must be at least 0.
• z₂ must be at least 0.
• t₁ must be at least 0.
• t₂ must be at least 0.
• Use finite inputs in the displayed units, preserve source measurements and assumptions, and independently verify consequential decisions.

For analysis or hazard decisions, retain uncertainty and provenance and compare the result with waveform processing, calibrated regional models, and guidance from qualified seismologists or the responsible agency.

• RMS Velocity Dix Equation Interval Velocity Calculator

  Dix equation interval velocity between two horizons from RMS velocities and zero-offset times.

• Blind Zone Hidden Layer Depth Checker

  Flag layers that cannot produce a critically refracted first arrival because their velocity does not exceed every overlying layer velocity.

• Normal Incidence Two Way Time Thickness Calculator

  Layer thickness from normal-incidence two-way time and interval velocity.