[seismology] In reflection seismology, a polarity reversal or phase change is a local amplitude seismic attribute anomaly that can indicate the presence of hydrocarbons and is therefore known as a direct hydrocarbon indicator. It primarily results from the change in polarity of the seismic response when a shale (with a lower acoustic impeda... Found on http://en.wikipedia.org/wiki/Polarity_reversal_(seismology)
(from the article `Earth`) An important characteristic of Earth`s magnetic field is polarity reversal. In this process the direction of the dipole component reversesi.e., the ... Found on http://www.britannica.com/eb/a-z/p/86