Seismic signals from a single event usually contain a number of components (phases) which have travelled by different propagation paths, or with a different mode of propagation. These may be superimposed and obscure signal components of interest. Seismic background noise may also be of sufficient amplitude to obscure the signal onset, which is often of relatively small amplitude, or even obscure the whole signal.An array of seismometers, spaced over a distance comparable to the signal wavelength, can be used as a filter to separate and help identify signal components on the basis of azimuth and apparent velocity at the Earth's surface. A signal/noise ratio improvement is also obtained for the first arrival, improving the accuracy of locating the hypocentres using triangulation methods from several stations.Linear cross arrays have been operated during recent years by the United Kingdom Atomic Energy Authority and many events analysed, an example of which is shown. The theoretical performances of symmetrical cross and L-shaped arrays are given in the form of directivity patterns, and their method of use described. Correlation methods are shown to be necessary for obtaining a good azimuth or velocity response, and their advantages and limitations considered