Although the amplitude of the signal itself is an inverse function of distance, it is easier to discriminate between an underground explosion and an earthquake at a great distanre from the source than closer at hand. Two types of elastic waves are used–the longitudinalP-waves(detected by a seismometer of about 1 sec period) and the surface RayleighR-waves(detected by a 20 sec seismometer). An account is given of the principles of short-period array systems and their diagnostic performance, and of the independent diagnosis afforded by observations of theR-waves. The relevance of the experimental facilities and of the analytical results to more general geophysical and geological problems is briefly mentioned.