P to S converted receiver functions recorded at VBB FDSN California stations are studied in the frequency range of 0.1 to 1 Hz. Microseismic noise is maximum in this frequency range, but signal processing by the wavelet transform enables us: (1) to enhance seismic phases associated with seismic velocity gradients and discontinuities at the base of the upper mantle, (2) to extract accurate arrival times, and (3) to obtain an accurate insight into the frequency content. In the case of California, one seismic discontinuity and two zones of high gradients in the depth range of 625 to 720km are recurrently observed for two different data sets.