Editing Formation evaluation (section)

===Porosity logs===
The two main nuclear porosity logs are the Density and the Neutron log.

Density logging tools contain a [[caesium-137]] [[gamma ray]] source which irradiates the formation with 662&nbsp;[[Electronvolt|keV]] gamma rays. These gamma rays interact with electrons in the formation through [[Compton scattering]] and lose energy. Once the energy of the gamma ray has fallen below 100&nbsp;keV, photoelectric absorption dominates: gamma rays are eventually absorbed by the formation. The amount of energy loss by Compton scattering is related to the number electrons per unit volume of formation. Since for most elements of interest (below Z = 20) the ratio of atomic weight, A, to atomic number, Z, is close to 2, gamma ray energy loss is related to the amount of matter per unit volume, i.e., formation density.

A gamma ray detector located some distance from the source, detects surviving gamma rays and sorts them into several energy windows. The number of high-energy gamma rays is controlled by compton scattering, hence by formation density. The number of low-energy gamma rays is controlled by photoelectric absorption, which is directly related to the average atomic number, Z, of the formation, hence to [[lithology]]. Modern density logging tools include two or three detectors, which allow compensation for some borehole effects, in particular for the presence of mud cake between the tool and the formation.

Since there is a large contrast between the density of the minerals in the formation and the density of pore fluids, porosity can easily be derived from measured formation bulk density if both mineral and fluid densities are known.

Neutron porosity logging tools contain an [[americium]]-[[beryllium]] [[neutron]] source, which irradiates the formation with neutrons. These neutrons lose energy through elastic collisions with nuclei in the formation. Once their energy has decreased to thermal level, they diffuse randomly away from the source and are ultimately absorbed by a nucleus. Hydrogen atoms have essentially the same mass as the neutron; therefore hydrogen is the main contributor to the slowing down of neutrons. A detector at some distance from the source records the number of neutron reaching this point. Neutrons that have been slowed down to thermal level have a high probability of being absorbed by the formation before reaching the detector. The neutron counting rate is therefore inversely related to the amount of hydrogen in the formation. Since hydrogen is mostly present in pore fluids (water, hydrocarbons) the count rate can be converted into apparent porosity. Modern neutron logging tools usually include two detectors to compensate for some borehole effects. Porosity is derived from the ratio of count rates at these two detectors rather than from count rates at a single detector.

The combination of neutron and density logs takes advantage of the fact that lithology has opposite effects on these two porosity measurements. The average of neutron and density porosity values is usually close to the true porosity, regardless of lithology. Another advantage of this combination is the "gas effect."  Gas, being less dense than liquids, translates into a density-derived porosity that is too high. Gas, on the other hand, has much less hydrogen per unit volume than liquids: neutron-derived porosity, which is based on the amount of hydrogen, is too low. If both logs are displayed on compatible scales, they overlay each other in liquid-filled clean formations and are widely separated in gas-filled formations.

Sonic logs use a pinger and microphone arrangement to measure the velocity of sound in the formation from one end of the sonde to the other. For a given type of rock, acoustic velocity varies indirectly with porosity. If the velocity of sound through solid rock is taken as a measurement of 0% porosity, a slower velocity is an indication of a higher porosity that is usually filled with formation water with a slower sonic velocity.

Both sonic and density-neutron logs give porosity as their primary information. Sonic logs read farther away from the borehole so they are more useful where sections of the borehole are caved. Because they read deeper, they also tend to average more formation than the density-neutron logs do. Modern sonic configurations with pingers and microphones at both ends of the log, combined with computer analysis, minimize the averaging somewhat. Averaging is an advantage when the formation is being evaluated for seismic parameters, a different area of formation evaluation. A special log, the Long Spaced Sonic, is sometimes used for this purpose. Seismic signals (a single undulation of a sound wave in the earth) average together tens to hundreds of feet of formation, so an averaged sonic log is more directly comparable to a seismic waveform.

Density-neutron logs read the formation within about four to {{convert|7|in|mm|spell=in}}  of the borehole wall. This is an advantage in resolving thin beds. It is a disadvantage when the hole is badly caved. Corrections can be made automatically if the cave is no more than a few inches deep. A caliper arm on the sonde measures the profile of the borehole and a correction is calculated and incorporated in the porosity reading. However, if the cave is much more than four inches deep, the density-neutron log is reading little more than drilling mud.