Seismic Reflection Two-Way Travel Time Calculator
Convert between reflector depth and two-way travel time in milliseconds using an average seismic velocity.
🌍 What is Seismic Reflection Two-Way Travel Time?
Seismic reflection two-way travel time (TWT) is the time a seismic pulse needs to travel down from the surface to a reflecting layer boundary and back up to the receiver. It is the native vertical axis of every seismic reflection section, measured in milliseconds, because time is what the recording instruments actually observe. Depth is something geophysicists infer afterward by combining that time with a velocity.
Exploration geophysicists, petroleum engineers, and geotechnical surveyors use TWT constantly. An interpreter picks a horizon on a seismic section at, say, 1450 ms and needs to know how deep that is before planning a well. A marine surveyor reads a seabed reflection at 120 ms and wants the water depth. A student checks how survey design affects the time window that must be recorded to image a target at 3 km. In every case the work is a conversion between two-way time and depth through an average velocity.
A common misconception is that the number on the time axis is the time to the reflector. It is not, it is the round trip, so the one-way time is exactly half. Another is that a single velocity applies everywhere. Velocity usually increases with depth as rocks compact, so a constant-velocity conversion is an approximation that works best for shallow targets and uniform geology. Finally, the simple relation here assumes a flat reflector and zero offset, with the source and receiver at the same surface point.
This calculator works in both directions. Give it a depth and an average velocity and it returns the two-way time in milliseconds, or give it a picked time and it returns the depth in meters and feet. It also plots TWT against depth for your velocity, with slower and faster velocity lines, so you can see how much your answer depends on the velocity you chose.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Shallow Sandstone Target
Reflector at h=1000 m with V=2500 m/s
Example 2 — Deep Carbonate Reservoir
Reflector at h=3200 m with V=3800 m/s
Example 3 — Picked Horizon on a Seismic Section
Horizon picked at 1450 ms with V=3000 m/s
Example 4 — Seabed Reflection in Water
Seabed picked at 120 ms with V=1500 m/s
❓ Frequently Asked Questions
🔗 Related Calculators
What is two-way travel time in seismology?
Two-way travel time (TWT) is the total time a seismic wave takes to travel from the source down to a reflecting boundary and back up to the receiver. It is the vertical axis of a seismic section and is normally measured in milliseconds.
How do you calculate two-way travel time from depth?
TWT = 2h / V, where h is the reflector depth in meters and V is the average velocity in meters per second. The result is in seconds, so multiply by 1000 for milliseconds. For h = 1000 m and V = 2500 m/s, TWT = 0.8 s, or 800 ms.
How do you convert two-way time to depth?
Depth = V x TWT / 2, with TWT in seconds. For a picked horizon at 1450 ms and an average velocity of 3000 m/s, depth = 3000 x 1.450 / 2 = 2175 m. The division by two removes the return leg of the trip.
Why do we divide by two when converting TWT to depth?
The recorded time covers the downgoing and upgoing paths, which are the same length for a normal-incidence reflection. Only one of those legs equals the depth of the reflector, so the depth is half of velocity multiplied by TWT.
What is the difference between one-way time and two-way time?
One-way time is the time for the wave to travel from the surface to the reflector. Two-way time is the round trip, so it is twice the one-way time. Check shots and VSPs measure one-way time, while surface reflection data is displayed in two-way time.
What velocity should I use to convert TWT to depth?
Use the average velocity from the surface to the reflector. Typical values are about 1500 m/s for water, 1600 to 2500 m/s for unconsolidated sediments, 2500 to 4500 m/s for sandstone and shale, and 4500 to 6500 m/s for carbonates and salt.
What is the difference between average velocity and interval velocity?
Average velocity is the total depth divided by the total one-way time from the surface to a reflector. Interval velocity is the velocity of a single layer between two reflectors. This calculator uses the average velocity, which is the correct input for a single time to depth conversion.
How many meters of depth does 1 ms of two-way time represent?
One millisecond of TWT corresponds to V / 2000 meters of depth. At 2000 m/s that is 1 m per ms, at 3000 m/s it is 1.5 m per ms, and at 4000 m/s it is 2 m per ms. Faster rocks therefore stretch more depth into each millisecond.
Is a seismic section plotted in time or depth?
Most seismic sections are plotted in two-way time because that is what the survey actually records. Depth sections exist, but they require a velocity model and a depth conversion or depth migration step, which carries its own uncertainty.
How accurate is a constant velocity time to depth conversion?
It is accurate only when velocity is nearly uniform. Because velocity normally increases with depth from compaction, a single average velocity can be off by 5 to 15 percent for deep targets. Use well data or a layered velocity model for drilling decisions.
Does this calculator account for dipping reflectors or offset?
No. It assumes normal incidence, meaning the source and receiver are at the same point above a flat reflector, which is the zero-offset case. For non-zero offset you need normal moveout correction, and for dipping reflectors you need migration.