Rapid survey of many minor archaeological sites gives a more balanced picture of the past in a given region. The development of accurate portable magnetometers now allows mapping of such buried sites in many types of soils. New differential instruments are more convenient, eliminating most external disturbances. The major remaining problem is the separation of weak archaeological magnetic `signals' from `noise' due to minor geological irregularities. Digital filtering, nonlinear processing and a system of data display which makes use of the eye's ability to see faint shapes in confused backgrounds have been developed for use with a large computer. Optimum filter techniques can be worked out from computed theoretical model studies of signals and noise.