The Borehole Image
Learning objectives
- Explain how a borehole image is made, by resistivity pads or acoustics
- Read the image as the unrolled borehole wall, azimuth by depth
- Recognize a dipping bed as a sine wave
- Contrast resistivity pad coverage with full acoustic coverage
Painting the Borehole Wall
A borehole image tool paints the wall of the hole. A resistivity microimager presses pads of tiny button electrodes against the rock and reads a fine-scale resistivity map; an acoustic imager rotates a transducer and records the amplitude and travel time of the reflected pulse. Either way the curved wall is unrolled into a flat picture, 360 degrees of azimuth across and depth down, shaded dark for conductive and light for resistive.
Beds Become Sine Waves
The defining fact of the chapter is geometric. A flat plane cutting an inclined cylinder intersects it in an ellipse, and that ellipse, unrolled, is a sine wave. Its peak-to-peak height is
so a flat bed is a straight line, a steep bed a tall wave, and the low point of the wave points downdip. Every dip read in this chapter is that one relationship run backwards.
Coverage and Resolution
The two tools trade coverage against resolution. The resistivity pad tool reads millimeter detail but leaves blind stripes between its pads, covering perhaps sixty to eighty percent of the wall. The acoustic tool sees the full 360 degrees but at coarser resolution, and it reads the hole shape as much as the rock. The image is the raw material the next sections turn into dip, fractures, and texture.
References
- Luthi, S. M. (2001). Geological Well Logs: Their Use in Reservoir Modeling. Springer.
- Rider, M. and Kennedy, M. (2011). The Geological Interpretation of Well Logs, 3rd ed. Rider-French.