Recent evidence concerning the structure of the upper mantle from the dispersion of long period surface waves

Most of our present detailed knowledge concerning the earth's deep interior has come to \nus from the study of body-wave travel-time data, supplemented by the semi-empirical use of \namplitude information. The use of travel-time information alone, even in areas where it can be \napplied, has severe limitations. Velocity reversals lead to fundamental theoretical difficulties, \nand discontinuities in velocity or velocity gradient lead to formidable practical difficulties. \nHowever, it is just these features of the earth that are most interesting and important in discussions \nof earth history, mantle composition, phase changes, and convection. The long debate \nover the low-velocity zone and the so-called 20° discontinuity illustrates the nature of the difficulties.