Time Depth Converter: Well to Seismic Tie from Checkshot Data

Geophysics

Enter checkshot survey stations, depth in meters and one way time in milliseconds. Leave unused rows blank.

Depth (m)One way time (ms)

Time Depth Conversion Summary

Generated by calculatgeek.com Time Depth Converter

Diagram showing a well track tied to a seismic section using a checkshot time depth curve, converting depth to two way time
A checkshot survey records one way time at known depths in the well. Interpolating along that curve lets you convert any depth to two way time, or any seismic time back to depth.

Tying a well to seismic starts with a simple question. If a marker sits at 1800 meters in the well, where does it show up on the seismic section in two way time? The checkshot survey, a set of depth and travel time measurements recorded by a geophone lowered down the well, gives you the answer. Enter your own checkshot stations in the calculator above, or load the example, to build a time depth curve and convert between depth and time in either direction.

What a Checkshot Survey Gives You

A checkshot survey records the one way travel time of a seismic pulse from the surface down to a geophone at a known depth in the well. Repeating this at several depths builds a table of depth and time pairs. From that table you can compute two useful velocities. Interval velocity is the speed of sound through the specific rock layer between two consecutive stations. Average velocity is the overall speed from the surface down to a single depth. Both come from the same table, just divided differently.

The Conversion

Average velocity is simply depth divided by one way time. Two way time, the value that actually appears on a seismic section, is just double the one way time, since the recorded seismic wave travels down to the reflector and back up again:

Vavg = depth / one way time, TWT = 2 x one way time

To convert a depth that falls between two checkshot stations, the calculator interpolates linearly along the time depth curve rather than assuming a single constant velocity for the whole well. This matters because velocity almost always increases with depth as rock compacts, so a single average number would misplace anything away from the exact stations you measured.

Worked Example

The example checkshot table in the calculator runs from the surface to 2500 meters, with interval velocities that climb from about 1563 m/s near the surface to 2500 m/s at the base, a realistic compaction trend. Converting a marker depth of 1800 meters lands at a one way time of 1008 ms, or 2016 ms two way time, with an average velocity of about 1786 m/s at that depth. Switch the mode to time to depth and enter a two way time instead to run the conversion in reverse.

Why This Matters for Interpretation

  • Placing formation tops and fault picks from well logs onto the correct seismic reflector
  • Building an accurate synthetic seismogram for a well to seismic tie
  • Converting a depth structure map into time, or a time horizon into depth, across a field
  • Spotting checkshot busts or bad velocity picks where the curve suddenly reverses or flattens

A good time depth curve is one of the most used, and most often taken for granted, tools in well to seismic integration. Getting the interpolation right, rather than relying on one blended average velocity, is what keeps deeper picks tying correctly.

Related Calculator and Reference

For converting between interval, average, and RMS velocities directly, see the Seismic Velocity Converter. The checkshot method and time depth interpolation approach used here follow the description in the SEG Wiki entry on well ties.