Device for investigation of the porosity of geological formations

A device for neutron well logging is described in which errors due to caked drilling mud on the walls of the hole are compensated for. This is achieved by using two neutron sources and two detectors. One of the neutron sources emits neutrons with so high energy, about 3 or 4 MeV, that their slowing down length is much greater than the thickness of the drilling mud, while the other emits neutrons with an energy of about 240 KeV (lithium-plutonium) or 25 KeV (antimony - beryllium), ie they have a very high probability of interacting with the material in the drilling mud. The detectors are adjusted to react selectively to neutrons of epithermal energy, and the difference in the signals represents the porosity, or hydrocarbon content of the geological formation. (JIW)