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The next class of well logs to be considered is generally referred to as porosity logs. Although each produces a porosity value from basic measurements, none actually measures porosity directly. Two such logs, the density and neutron, are nuclear measurements. A third log, the sonic, uses acoustic measurements, and the fourth and newest log senses the magnetic resonance of formation nuclei. When used individually, each of the first three has a response to lithology which must be accounted for, but when used in concert, two or three at a time, lithology can be estimated and a more accurate porosity derived.

Nuclear magnetic resonance (NMR) logging was first introduced by Schlumberger in 1978 (Maute, 1992) but was not initially widely used because of operational limitations. With the commercial introduction of the Magnetic Resonance Imaging Log (MRIL) by NUMAR Corporation (now part of Halliburton) in 1980 (Halliburton, 1999) and the release of the Combinable Magnetic Resonance Tool (CMR) by Schlumberger, the technique is steadily gaining acceptance.

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