Editing Wireline (cabling) (section)

===Resistivity tools===
In any matrix which has some porosity, the pore spaces will be filled with a fluid of oil, gas (either hydrocarbon or otherwise) or formation water (sometimes referred to as connate water). This fluid will saturate the rock and change its electrical properties. A wireline resistivity tool direct injects current (lateralog-type tools for conductive water-based muds) or induces (induction-type tools for resistive or oil-based muds) an electric current into the surrounding rock and determines the resistivity via Ohm's law. The resistivity of the formation is used primarily to identify pay zones containing highly resistive hydrocarbons as opposed to those containing water, which is generally more conductive. It is also useful for determining the location of the [[water contact|oil-water contact]] in a reservoir. Most wireline tools are able to measure the resistivity at several depths of investigation into the bore hole wall, allowing log analysts to accurately predict the level of fluid invasion from the drilling mud, and thus determine a qualitative measurement of permeability.

Some resistivity tools have many electrodes mounted on several articulated pads, allowing for multiple micro-resistivity measurements. These micro-resistivities have a very shallow depth of investigation, typically in the range of 0.1 to 0.8 inches, making them suitable for borehole imaging. Resistivity imagers are available which operate using induction methods for resistive mud systems (oil base), and direct current methods for conductive mud systems (water based).