Laterolog and Induction
Learning objectives
- Contrast the galvanic laterolog and the electromagnetic induction tool
- Explain why the mud type sets the choice of tool
- State the resistivity range each tool suits
- Pick the right resistivity tool for a given environment
Two Ways to Measure Resistivity
Two very different tools measure formation resistivity. The laterolog is galvanic: electrodes drive a real current from the tool, through the borehole, and out into the rock, then measure the voltage it takes. The induction tool is electromagnetic: coils set up an alternating field that induces eddy currents in the formation, and a receiver reads the field those currents produce. One pushes current through the mud; the other needs no contact at all.
The Mud Decides
That difference makes the mud the deciding factor. The laterolog must drive its current through the borehole, so it needs a conductive (salty) mud; in oil-based or very fresh mud the current has nowhere to go and the laterolog fails. The induction tool needs no conductive path, so it is the only choice in oil-based or fresh mud, and in salty mud the borehole signal overwhelms it. Salty mud points to the laterolog; resistive mud points to the induction.
And the Resistivity
The formation resistivity tips the balance the rest of the way. The laterolog stays sharp in very high-resistivity beds, where it is the better reader. The induction is at its best in low-to-moderate resistivity; above a few hundred ohm-metres its signal fades and it loses accuracy. So the decision map has the induction in the upper-left, fresh mud and modest Rt, and the laterolog in the lower band of salty mud, all the way out to the highest resistivities. Choose the tool to the mud and the formation, and the resistivity it returns can be trusted as Rt.
References
- Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.
- Ellis, D. and Singer, J. (2007). Well Logging for Earth Scientists, 2nd ed. Springer.