Data

For the studies described here, two different sources of data are used. The emphasis is on seismograms of the Yellowknife array (YKA), which are used to study the structure of the upper mantle discontinuities with P$^d$P phases. In addition, the temporary Tw~st array is used to study the origin of the Lehmann discontinuity by a change of anisotropic structure by the study of SS precursors.
The two arrays and the events used are described in the following subsections.