Acoustic Impedance Calculator: Velocity Times Density for Seismic Reflections

Calculate acoustic impedance from velocity and density, build a layered impedance profile with depth, or solve for whichever of velocity, density, or impedance you are missing. Everything updates live.

Every seismic reflection traces back to a single rock property that most interpreters never calculate directly, acoustic impedance. It is nothing more than velocity multiplied by density, yet that one number, and how sharply it changes from one layer to the next, controls essentially all of the amplitude behavior seen on a seismic section. This tool builds that number directly from log or estimated rock properties.

Start with a single velocity and density to get impedance in seconds. Build a layered profile with depth to see how impedance steps from one rock unit to the next, using presets for common lithology pairs. Or solve backward, if you know two of the three quantities, velocity, density, or impedance, and need the third.

What Acoustic Impedance Actually Is

Acoustic impedance is defined as velocity multiplied by density, a rock property with no direct physical meaning of its own but enormous practical importance, since it is impedance contrast, not velocity or density alone, that generates a seismic reflection. Two rock units can have very different velocities and very different densities individually and still produce almost no reflection if their impedances happen to be close, while two units with only a modest velocity difference can produce a strong reflection if their densities pull the impedance contrast apart.

Impedance is typically reported in mega Rayl, where a Rayl is one kilogram per square meter per second, the natural SI unit that falls out of multiplying velocity in meters per second by density in kilograms per cubic meter. Most rocks fall somewhere between about four and eighteen mega Rayl, with unconsolidated sediments and gas charged sands at the low end and tight carbonates at the high end.

Building an Impedance Profile With Depth

A single impedance number rarely tells the full story, what matters for seismic interpretation is how impedance changes across a boundary. Building a simple layered profile, even from estimated velocities and densities rather than full logs, makes it straightforward to see where the largest impedance contrasts sit and therefore where the strongest reflections should appear.

A shale over sandstone contact typically produces a moderate positive impedance step, since sandstone usually carries both higher velocity and comparable or higher density than the shale above it. A shale over limestone contact tends to produce a much larger step, since carbonates carry substantially higher velocity even at similar density. A gas charged sand sitting beneath a wet sand is a classic case where density and velocity both drop together, producing a sharp negative impedance step and the bright amplitude anomaly interpreters associate with a direct hydrocarbon indicator.

Solving for a Missing Property

Well log data does not always include every property needed directly. A sonic log gives velocity, a density log gives density, but sometimes only one of the two is available alongside a target impedance value estimated from a well tie or seismic inversion. Since impedance is a simple product, any one of velocity, density, or impedance can be solved directly from the other two without any iteration.

This tool lets you toggle which property is unknown and keeps the other two editable, solving the missing one instantly as the known values change, useful for quick log editing checks or for sanity checking a seismic inversion result against expected rock properties.

Common Mistakes

Assuming a strong reflection always means a large velocity contrast, when a large density contrast alone, or the combination of two moderate contrasts, can produce the same result.

Comparing impedance values calculated in different unit systems without converting first, since a raw velocity times density product only equals mega Rayl when both inputs are already in consistent SI-derived units.

Treating impedance as a fixed rock property rather than one that depends on the specific velocity and density measured at a given depth and fluid content, since gas saturation alone can shift impedance substantially.

Skipping the impedance step entirely and trying to reason about reflection strength from velocity or density individually.

Who Uses This

Petrophysicists and log analysts building impedance curves from velocity and density logs, or checking one property against a target impedance.

Seismic interpreters relating amplitude anomalies back to the rock property changes that actually generate them.

Geophysicists performing simple forward modeling or QC on seismic inversion results against expected lithology impedances.

Students and early career geoscientists building intuition for how velocity and density combine into the property that controls seismic reflectivity.

Try It Above

Use the quick convert tab for a single velocity and density check, move to the layered model to build a depth profile with lithology presets, or use the solve missing property tab when you have two of the three quantities and need the third.

Related Calculators and References

Continue building your workflow with the NMO Correction Calculator. For background on the formulas used above, see SEG Wiki: Acoustic impedance.