variegated seismological conditions has been accumulated. The method has been developed further in two main directions: - the perfection of the techniques and equipment of three-component field studies incorporating a continuous control of the identity of equipment both in deep boreholes and on the surface; - the creation of efficient and straightforward software for the automatic processing of PM data. Whereas the early stages of development of the method have been conspicuous chiefly by the use of polarization for the discrimination of the wave field, in recent years the emphasis shifts to the use of polarization parameters to obtain additional information about the Earth. This is especially important, as the polarization parameters are very sensitive to inhomogeneities in cross-section and enable those parameters of the Earth to be studies which cannot be obtained from kinematics. Here one should especially mention the polarization of transverse waves, which can act as one of the most reliable parameters for identifying and studying anisotropic properties of the Earth. At present various modifications of PM are being developed applicable both to uphole - (PM RW = reflected waves method, PM CDP = common-depth-point method, PM refracted-wave method, PM of regional studies utilizing earthquakes) and to downhole observations (PM VSP = vertical seismic profiling). Not all the PM modifications are developing at the same rate. The method which has up to now been developed most extenSively is PM VSP.
