Editing Free-air gravity anomaly (section)

== Significance ==
Over the ocean where gravity is measured from ships near sea level, there is no or little free-air correction. In marine gravity surveys, it was observed that the free-air anomaly is positive but very small over the [[Mid-ocean ridge|Mid-Ocean Ridges]] in spite of the fact that these features rise several kilometers above the surrounding seafloor.<ref>{{Cite journal|last1=Cochran|first1=James R.|last2=Talwani|first2=Manik|date=1977-09-01|title=Free-air gravity anomalies in the world's oceans and their relationship to residual elevation|journal=Geophysical Journal International|volume=50|issue=3|pages=495–552|doi=10.1111/j.1365-246X.1977.tb01334.x|bibcode=1977GeoJ...50..495C|issn=0956-540X|doi-access=free}}</ref> The small anomaly is explained by the lower density crust and mantle below the ridges resulting from [[seafloor spreading]].  This lower density is an apparent offset to the extra height of the ridge indicating that Mid-Ocean Ridges are in isostatic equilibrium.