How Logs Measure

Part 2, Chapter 2: The Logging Environment and Invasion

Learning objectives

Name the four physical principles behind the log suite
Match each measurement to the rock property it gives
Explain the tradeoff between depth of investigation and vertical resolution
Place the measurements on a reach-versus-resolution map

Four Families of Physics

For all the curves in a modern suite, there are really only four kinds of physics underneath. Nuclear: the gamma ray passively counts the natural radioactivity of clay, while the density and neutron tools actively scatter gamma rays and slow neutrons. Acoustic: the sonic times a sound pulse through the rock. Electrical: the resistivity drives currents and reads conductivity. Knowing which physics a curve comes from tells you what it can and cannot see.

Reach Versus Resolution

Every measurement makes the same trade. A tool that reaches deep into the formation, like the deep resistivity, averages over a large volume and so has coarse vertical resolution. A tool that reads a thin slab at the wall, like the density pad, has fine resolution but sees only the shallow, possibly invaded rock. There is no single best tool, which is exactly why a suite is run: the curves are chosen to cover the map, deep and shallow, fine and coarse, together.

One Property Each, and What Comes Next

Each family is tuned to a different unknown. The gamma ray gives shale volume; density, neutron, and sonic give porosity (and between them, lithology and gas); resistivity gives water saturation. That is the whole interpretation chain again, now grounded in physics. With the environment, invasion, and the measurements understood, the core log suite is next, beginning with the gamma ray in Chapter 3.

References

Ellis, D. and Singer, J. (2007). Well Logging for Earth Scientists, 2nd ed. Springer.
Rider, M. and Kennedy, M. (2011). The Geological Interpretation of Well Logs, 3rd ed. Rider-French.