differential spectrum

1. n. [Formation Evaluation]
A technique in nuclear magnetic resonance (NMR) logging that is based on the difference between the T2 distributions, or spectra, acquired at different polarization times. The technique often is used to detect gas or light oil. These fluids have long T1 that exceed 1 s. A measurement made with a long polarization time will polarize much of these fluids and give significant signal at the appropriate T2. A measurement made with a short polarization time will polarize little of these fluids and will give a much smaller signal. Other fluids, with shorter T1, will be polarized in both cases, so that a difference in signal at the appropriate T2 identifies gas or light oil.