GUIDE

How to calibrate a log curve scale

Depth calibration and curve scales for digitizing

In short

Calibration tells the software how picture position maps to depth and to curve values.
Each curve needs its left and right track values and a linear or logarithmic scale.
Use a backup scale where a curve wraps around the track.

Depth calibration

Enter two known depths from the depth scale and pick their horizontal marks on the scan. Choose feet or metres. Use marks that are far apart; a short baseline magnifies reading errors. Depth ties and breaks are available for logs where the depth scale is interrupted.

Curve calibration

For each curve, enter the value printed at the left edge of the track and at the right edge, then pick those edges on the scan. A gamma ray track might run 0 to 150 API from left to right, while a density track might run 1.95 to 2.95 g/cm³.

Linear or logarithmic?

Look at the grid. Evenly spaced lines mean a linear scale. Lines that bunch together toward the right usually mean a logarithmic scale, which is typical for resistivity (for example 0.2 to 2000 ohm·m). Choose the same type as the printed track, since the wrong choice distorts every value.

Backup scales

When a curve runs off the track and wraps back from the other side, mark that interval and give it a second scale (for example 0 to 150 and then 150 to 300). The values in that interval are converted with the backup scale.

Check your calibration

Trace a short interval and compare the values with the printed scale.
Use the data checks to find values outside the printed scale.
If values look compressed, re-check that the left and right values are the right way round.

Common questions

Do all curves in a track share a calibration?

Each curve has its own calibration, and you can copy one from another curve when they share a track and depth.