← Back to Tutorials

How to Digitize Well Logs (Gamma Ray, Resistivity, Sonic)

⚡ Quick Answer

Yes. Import the scanned log into Engauge Digitizer, set the depth axis (depth increases downward — place axis points accordingly), set each track’s value axis (logarithmic for resistivity), and trace each curve per track. For very long logs, work in overlapping depth sections and merge on the depth column.

Why Digitize Paper Well Logs

Decades of wells were logged on paper: gamma ray, resistivity, sonic, and density curves sitting in filing cabinets as raster scans. To correlate them with modern digital wells — or to load them into interpretation software — those curves need to become numbers. Engauge Digitizer does this for free; commercial alternatives like Neuralog exist, but the core job (trace a curve against calibrated axes) is identical.

Step-by-Step: Digitizing a Well Log

  1. Scan or capture at high resolution: 300 DPI minimum. Thin log curves blur into nothing at low resolution, and depth precision lives or dies on pixel count.
  2. Set the depth axis first: In the Axis Wizard, define the Y axis as depth. Place one axis point on a shallow depth gridline near the top and one on a deeper gridline near the bottom — Engauge handles the downward-increasing direction correctly as long as your values are honest.
  3. Set each track’s value axis: Logs have multiple tracks (GR, resistivity, sonic…), each with its own scale. Digitize one track at a time: resistivity tracks are usually logarithmic (see our log axes guide), GR and sonic are usually linear.
  4. Trace each curve: Add one curve per log curve in the Curves List, name them (e.g. “GR”, “Deep Resistivity”), and trace with Segment Fill or manually. Distinct curve colors per track prevent mix-ups.
  5. Export and depth-match: File > Export to CSV. If you worked in sections, merge the CSVs on the depth column in a spreadsheet before loading into your interpretation software.

💡 Tip: Note the track boundaries and scale headers printed at the top of the log image — screenshot them into your notes. Six months later, “Track 2 = 0.2–2000 ohm-m, log scale” is the difference between usable data and a mystery CSV.

Common Pitfalls

  • Log vs linear mix-up — the #1 error. Resistivity is almost always logarithmic; double-check every track’s header.
  • Depth direction — if your exported depths decrease downward, you swapped the axis points; swap them back.
  • Grid-heavy scans — old logs have dense grids; zoom in and use color filtering so the tracer follows the curve, not the grid.
  • Long-log drift — on very long images, calibrate axes per section rather than once for the whole log; scans stretch slightly.

⚠️ Note: Digitized logs are excellent for correlation and re-interpretation, but check your company’s data policy before using them for reserve estimates — some workflows require the original LAS or certified digitization.

Related Guides

How to Digitize Logarithmic Axes

Log-scale setup — essential for resistivity tracks.

How to Batch-Digitize Multiple Graphs

When you have a whole archive of logs.

Extract Data from a Box Plot Image

Another niche chart type, same manual-reading skills.

Frequently Asked Questions

Can Engauge Digitizer digitize well logs?

Yes. Set the depth axis (note: depth increases downward on most logs — place axis points accordingly), set each track’s value axis (logarithmic for resistivity tracks), and trace each curve. It is free and works, though dedicated commercial log digitizers add depth-matching conveniences.

How do I handle the reversed depth axis on a well log?

Just place your axis points honestly: click the shallow depth value at the top and the deeper value at the bottom. Engauge calibrates from the values you give it, so a downward-increasing axis works fine — the exported depths will be correct.

What about the logarithmic resistivity tracks?

Set those track axes to logarithmic in the Axis Wizard and place axis points on labeled decades, exactly as in our logarithmic-axes tutorial. Mixing up a log track with a linear one is the classic well-log digitizing mistake.

My log is very long and thin. How do I digitize it?

Work in overlapping sections: digitize the log in 2–3 depth windows, export each, and merge on the depth column in your spreadsheet. Overlap the windows by a small depth interval so you can verify the merge.

Ready to try it yourself?

Download Engauge Digitizer free and follow along with this guide.

Download Engauge Digitizer