Below the lithosphere the upper mantle becomes more plastic in its rheology. In some regions below the lithosphere, the seismic velocity is reduced. This low velocity zone extends down to a depth of several hundred km.
The lower boundary separating the upper and lower mantle - is a transitional zone.
The transition zone is located in the depth interval 400 - 650 kilometers from the Earth's surface. The transition zone is an area of great complexity; it physically separates the upper and lower mantle.
Physical and chemical processes occurring in Earth's upper mantle, have a great and often decisive influence on many geodynamic, geophysical, geochemical phenomena observed on the surface of the Earth.
For example plate tectonics. In the last two decades, global plate tectonics (mobilism) has been widely recognized by the scientific community.
And it can be argued that this theory is the paradigm of our time.
From the standpoint of mobilism can explain many geodynamic, geological, geophysical, geochemical phenomena, as well as to resolve the issues of earthquake prediction and the distribution of mineral resources.
Plate tectonics itself requires a robust justification.
The cornerstone of plate tectonics is the explanation for the global movement of tectonic plates.
The source of energy for the movement of plates and the substrate on which the plates "float" are in the upper mantle, according to modern concepts.
Therefore, the study of the upper mantle - it is a very urgent task.
Direct penetration to great depths within the earth - a difficult task.
The level of complexity of the problem can be understood, if we compare the achievements in space exploration and in the study of the deep Earth. Space exploration task the next few years are manned flights to Mars and Venus. In the depths of Earth exploration of humanity through their first ten kilometers. The Earth's upper mantle lies much deeper.
Therefore, the main information providers about the upper mantle internal structure is geophysical research methods such as seismology, gravimetry, geothermal, geomagnetism. Information provided by geophysical methods, is indirect. That is, it can not pick up as an example of the breed, and knock on it with a hammer. The data of all geophysical methods should be studied together.
Geophysical data to compare, to combine with petrological, mineralogical, geochemical and other geological data.
Only with such an integrated approach to the study of Earth's upper mantle is possible to obtain reliable results. Structural seismology is an indispensable method of geophysical exploration of the Earth's upper mantle. One of the main reasons is that for a seismic data developed well-proven technology to extract from observed seismological data the information about internal structure of the Earth's upper mantle.