Editing Well logging (section)

===Porosity logs===
[[Porosity]] logs measure the fraction or percentage of pore volume in a volume of rock. Most porosity logs use either [[acoustics|acoustic]] or [[Nuclear physics|nuclear]] technology.  Acoustic logs measure characteristics of sound waves propagated through the well-bore environment. Nuclear logs utilize nuclear reactions that take place in the downhole logging instrument or in the formation. Nuclear logs include density logs and neutron logs, as well as gamma ray logs which are used for correlation.
<ref>{{cite book
  | last = Sengel
  | first = E.W. "Bill"
  | title = Handbook on well logging
  | publisher = Institute for Energy Development
  | year = 1981
  | location = Oklahoma City, Oklahoma
  | isbn = 0-89419-112-8
  | page = 168 p }}</ref> The basic principle behind the use of nuclear technology is that a neutron source placed near the formation whose porosity is being measured will result in neutrons being scattered by the hydrogen atoms, largely those present in the formation fluid. Since there is little difference in the neutrons scattered by hydrocarbons or water, the porosity measured gives a figure close to the true physical porosity whereas the figure obtained from electrical resistivity measurements is that due to the conductive formation fluid. The difference between neutron porosity and electrical porosity measurements therefore indicates the presence of hydrocarbons in the formation fluid.

====Density====
{{See also|Density logging}}
The density log measures the [[bulk density]] of a formation by bombarding it with a radioactive source and measuring the resulting gamma ray count after the effects of [[Compton Scattering]] and [[Photoelectric effect|Photoelectric absorption]].  This bulk density can then be used to determine porosity.

====Neutron porosity====
{{See also|Formation evaluation neutron porosity}}
The neutron porosity log works by bombarding a formation with high energy [[Neutron temperature|epithermal neutrons]] that lose energy through [[elastic scattering]] to near thermal levels before being absorbed by the [[Atomic nucleus|nuclei]] of the formation atoms.  Depending on the particular type of neutron logging tool, either the [[gamma ray]] of capture, scattered thermal neutrons or scattered, higher energy epithermal neutrons are detected.<ref>{{Cite web|url=http://www.glossary.oilfield.slb.com/Display.cfm?Term=epithermal%20neutron%20porosity%20measurement|title=Schlumberger Oilfield Glossary}}</ref>  The neutron porosity log is predominantly sensitive to the quantity of [[hydrogen atom]]s in a particular formation, which generally corresponds to rock porosity.

[[Boron]] is known to cause anomalously low neutron tool count rates due to it having a high capture cross section for thermal neutron absorption.<ref>
{{cite book
  | last = Etnyre
  | first = L.M.
  | title = Finding Oil and Gas from Well Logs
  | publisher = Kluwer Academic Publishers 
  | year = 1989
  | isbn = 978-0442223090
  | page = 249 p }}</ref>   An increase in hydrogen concentration in clay minerals has a similar effect on the count rate.

====Sonic====
{{See also|Sonic logging}}
A sonic log provides a formation interval transit time, which is typically a function of lithology and rock texture but particularly porosity.  The logging tool consists of at least one piezoelectric transmitter and two or more receivers.  The time it takes for the sound wave to travel the fixed distance between two receivers is recorded as an ''interval transit time''.