Diameter of Investigation

The diameter of investigation is the distance from the centre of a logging tool out to the point in the formation where the tool stops being able to "see." Different log tools see different distances: a shallow tool might only read 5 to 15 centimetres into the rock, while a deep induction resistivity tool can read out past one metre. The number matters because every well has a damaged zone where drilling fluid has invaded the formation around the borehole, and a shallow log mostly measures that damaged zone rather than the undisturbed rock further out. Picking the right log for the job means picking a tool whose diameter of investigation reaches past the invasion and into the rock you actually want to know about.

Key Takeaways

Diameter of investigation describes how far a logging tool reads into the formation, measured from the tool axis or borehole wall outward. Tools with shallow investigation depths see mostly the damaged zone near the wellbore. Tools with deep investigation depths see the undisturbed formation beyond the damage.
Common shallow tools include the microresistivity log (5 to 10 centimetres), the density log (about 10 to 15 centimetres), and the neutron log (about 25 centimetres). Common deep tools include induction resistivity (1 to 2 metres) and laterolog deep (1 to 3 metres depending on tool spacing).
The mismatch between shallow and deep readings is the basis for invasion analysis. A shallow tool reads the resistivity of the invaded zone (filled with mud filtrate). A deep tool reads the resistivity of the virgin rock. Comparing the two readings tells the petrophysicist about the invasion profile and helps identify hydrocarbon-bearing zones.
The diameter of investigation depends on the tool's physics. Resistivity tools depend on coil spacing or electrode geometry. Nuclear tools depend on the spacing between source and detector and on the energy of the radiation. Acoustic tools depend on transmitter-receiver spacing and on the formation velocity.
Modern array tools (induction arrays, laterolog arrays, NMR with multiple wait times) collect readings at several depths of investigation in a single logging pass. Inversion software then builds a radial profile of formation properties, mapping how resistivity, porosity, or fluid saturation changes outward from the borehole.

Fast Facts

The deepest-reading common openhole logging tool is the array induction or array laterolog with the longest electrode spacing, which can read out to between 2 and 3 metres beyond the borehole. Even that is a tiny fraction of the lateral extent of a reservoir. Every well log measures only the immediate vicinity of the wellbore, and every reservoir model fills in the much larger volume between wells with statistical interpolation. The truthful version of "we know the reservoir from the logs" is "we know the reservoir within 2 metres of every well, and we are guessing about everything in between."

What Diameter of Investigation Means in Practice

Picture shining a flashlight at a wall in the dark. A small flashlight lights up a small spot on the wall. A bigger flashlight, or one held closer, lights up a bigger spot. A logging tool works similarly, except instead of light, it sends out an electrical current, a radioactive particle, or a sound wave, and the "spot" it can see is in the rock around the borehole rather than on a wall. The diameter of investigation is the size of that spot.

The size of the spot matters because every well has a problem: the rock right at the borehole is not the same as the rock a metre away. When the well was drilled, mud filtrate (the liquid part of the drilling fluid) pushed into the formation, displacing some of the original pore fluid. This invasion creates a zone immediately around the wellbore where the rock looks different to the log than the undisturbed rock further out. A shallow tool reads mostly the invaded zone. A deep tool reads mostly the undisturbed rock.

How Petrophysicists Use the Difference

The classic application is in resistivity-based fluid identification. In an oil zone with water-base mud, the mud filtrate is fresh and conductive. The invaded zone shows low resistivity. The deeper zone (oil-bearing) shows high resistivity. Comparing the shallow and deep readings produces a clear separation curve on the log that highlights the oil zone.

In a water zone, both the invaded zone and the deeper zone contain conductive water, so both readings are low. The shallow and deep readings track each other closely. The lack of separation on the log is the signal that the zone is wet, not oil-bearing.

Modern array tools take this further by collecting four to six readings at different investigation depths in one pass. Software then inverts the readings to build a complete radial profile of resistivity from the borehole wall out to several metres into the formation. The profile reveals not just whether invasion occurred, but how deep, and whether the formation contains a transition zone or a sharp oil-water contact.

Diameter of investigation is sometimes called depth of investigation, radial range, or investigation depth. Strictly speaking, "diameter" measures across the borehole and "depth" measures from the borehole wall outward, but the terms are often used interchangeably in practice. Related terms include invasion (the displacement of formation fluid by drilling mud filtrate around the borehole during drilling; the reason logs at different investigation depths can read different values for the same rock), resistivity log (a family of logging tools that measure formation electrical resistivity at different investigation depths; the most common application of investigation-depth analysis), density log (a shallow nuclear log that reads bulk density of the rock at about 10 to 15 centimetres into the formation; commonly paired with a neutron log of similar investigation depth for porosity calculation), induction log (a deep-reading resistivity tool that uses electromagnetic induction rather than direct electrode contact; investigation depth depends on coil spacing and is typically 1 to 2 metres for the deep curve), and array tool (a logging tool that measures multiple investigation depths simultaneously through several detector spacings; allows radial profiling of formation properties from a single logging pass).

Why Two Resistivity Readings Tell You What the Rock Is Hiding

A petrophysicist on a Permian Basin horizontal well in west Texas reviews the openhole log run from a vertical pilot hole drilled before the lateral. The log shows a 12-metre interval where the shallow microresistivity reads 1.2 ohm-metres while the deep induction reads 28 ohm-metres. The wide separation between the two curves is the signal she is looking for.

The shallow tool is reading mostly the invaded zone, where mud filtrate has displaced formation fluid. The deep tool is reading past the invasion into the undisturbed rock. The high deep resistivity tells her the formation contains oil rather than salt water (which would have read low). The low shallow resistivity confirms the mud was conductive enough to invade and to be visible to the shallow tool. The combination is a textbook hydrocarbon zone.

If the two curves had tracked each other closely at any value, she would have concluded either that no invasion occurred (rare, and indicates very low permeability) or that the formation is wet. Either way, the team would not have planned a horizontal drain through that interval. Instead, the lateral is drilled across the 12-metre zone the next month, and the well comes on at 480 barrels of oil per day. The information that mattered most was the gap between the two readings, not either reading on its own.