1. A method for reducing or eliminating noise in an electrical or magnetic signal produced by an organ in the body, the signal having a highest spatial frequency component of X cycles per unit measure of distance, the method comprising: providing, on or outside the exterior surface of the body, an array of sensors, each of said sensors for independently measuring the signal, wherein the sensors are spaced apart from one another by at least a minimum spacing of ½•X −1 ; sampling the signal, by each sensor, at a plurality of different times, so as to produce, for each sensor, a set of time-dependent measurements of the signal; identifying a first variance in the measurements corresponding to a selected sensor that is common to the measurements corresponding to all of the other sensors; identifying a remaining variance in the measurements corresponding to the selected sensor; and removing from the measurements corresponding to the selected sensor a second variance that is determined, at least in part, by said remaining variance, so as to reduce or eliminate noise that is unique to the measurements of the selected sensor.