A multiphasic scan, or multiphasic sweep, was a method whereby a vessel's sensors could be reconfigured to penetrate areas of intense interference where conventional scanning methods would prove impractical.
In 2371, Kathryn Janeway, captain of the USS Voyager, attempted to reconfigure ship's sensors to a multiphasic bandwidth in an effort to penetrate the strong electromagnetic radiation being emitted by a dark matter nebula in the Delta Quadrant, while attempting to track a vessel which had attacked one of their shuttlecraft. Tuvok later also used a multiphasic scan, especially the alpha k band to track the ion trail. (VOY: "Cathexis")
The next year, Captain Janeway ordered a multiphasic scan performed on a group of attacking ships controlled by the Bothans, in an attempt to discern any defensive weaknesses. The scans, however, revealed no useful information which the Voyager crew could use to defend itself. (VOY: "Persistence of Vision")
The same year, Lieutenants Tom Paris and B'Elanna Torres ran a multiphasic sweep in an attempt to locate the Cardassian ATR-4107 which the Voyager crew had discovered in the Delta Quadrant, after the missile masked its ion trail with a randomized EM field. (VOY: "Dreadnought")
In 2373, upon encountering a derelict Borg cube in the Nekrit Expanse, Janeway ordered Ensign Harry Kim to run a multiphasic scan on the vessel. Kim's scans showed no active energy signatures, and no life signs. (VOY: "Unity")