Seismic Impedance and Reflection Coefficient Calculator

Acoustic impedance Z is the product of a material's density and its seismic wave velocity, Z = rho x V. It quantifies how much a material resists the passage of a seismic wave, and the contrast in impedance between two adjacent layers is what determines how strongly a seismic wave reflects at their boundary.

At normal (zero-degree) incidence, R = (Z2 - Z1) / (Z2 + Z1), where Z1 and Z2 are the acoustic impedances of the upper and lower layers. R ranges from -1 to +1, with its magnitude showing how much wave amplitude reflects and its sign showing the reflection's polarity.

A negative R means the layer below has lower impedance than the layer above (Z2 < Z1), producing a reflected wave that is phase-reversed (polarity flipped) relative to the incident wave. This 'polarity reversal' is a well-known seismic signature, notably associated with gas-charged sands in exploration geophysics.

The transmission coefficient (amplitude) is T = 2*Z1 / (Z1 + Z2). In this amplitude convention, R and T satisfy 1 + R = T exactly, reflecting how the incident wave's amplitude splits between the reflected and transmitted waves at the boundary.

Reflection strength depends on impedance contrast (density times velocity), not density alone. If a lower-density layer has a proportionally higher velocity (or vice versa), the two effects can partly cancel, producing similar impedance values and a weak reflection despite the density difference.

No, this calculator uses the simplified normal-incidence (straight-down) reflection formula. Real seismic surveys record reflections across a range of incidence angles, requiring the full Zoeppritz equations, which also account for shear-wave velocity and angle-dependent amplitude variation (AVO analysis).

Most subsurface rock boundaries produce reflection coefficients with magnitude well under 0.3, since adjacent sedimentary layers often have only moderate impedance contrast. Strong reflectors, like a hard limestone over soft shale or a gas sand, can produce magnitudes of 0.3 or higher.

Anomalously strong reflection coefficients, especially with a negative polarity ('bright spots' with phase reversal), can indicate gas-charged porous rock, since gas dramatically lowers both density and P-wave velocity, sharply reducing impedance relative to the surrounding rock. This calculator's polarity flag highlights exactly that signature.

Density is entered in kilograms per cubic meter (kg/m3) and velocity in meters per second (m/s), so their product (impedance) comes out in kg/(m2*s), which this calculator displays in the more convenient MRayl unit (10^6 kg/(m2*s)). Reflection and transmission coefficients are dimensionless.

Yes, the underlying physics (impedance mismatch driving wave reflection) is identical in medical ultrasound and other acoustic engineering fields. Just substitute the relevant material densities and sound velocities, the same R = (Z2-Z1)/(Z2+Z1) formula applies.