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.
