Acoustic Impedance and the Reflection Coefficient

Reference

Seismic reflections are born at contrasts in acoustic impedance, the product of rock density and velocity.

The equation

R = (Z2 – Z1) / (Z2 + Z1), Z = ρV
  • Z1, Z2 are impedances above and below the interface
  • R is the normal incidence reflection coefficient
  • Positive R means impedance increases downward

Worked example

Shale with density 2.50 and velocity 2000 over sandstone with density 2.65 and velocity 2400 gives impedances of 5.00 and 6.36 million units, so R = 0.12. About one eighth of the wave amplitude reflects, a bright but ordinary reflector.

When to use it

The building block of synthetic seismograms and inversion. Remember it is amplitude, not energy, and it applies at normal incidence: with offset comes AVO, which carries its own information.

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