This study presents an improved approach to common-conversion point
stacking of converted body waves that incorporates scattering kernels,
accurate and efficient measurement of stack uncertainties, and an
alternative method for estimating free surface seismic velocities. To
better separate waveforms into the P and SV components to calculate
receiver functions, we developed an alternative method to measure near
surface compressional and shear wave velocities from particle motions.
To more accurately reflect converted phase scattering kernels in the
common-conversion point stack, we defined new weighting functions to
project receiver function amplitudes only to locations where
sensitivities to horizontal discontinuities are high. To better quantify
stack uncertainties, we derived an expression for the standard deviation
of the stack amplitude that is more efficient than bootstrapping and can
be used for any problem requiring the standard deviation of a weighted
average. We tested these improved methods on Sp phase data from the
Anatolian region, using multiple bandpass filters to image velocity
gradients of varying depth extents. Common conversion point stacks of
23,787 Sp receiver functions demonstrate that the new weighting
functions produce clearer and more continuous mantle phases, compared to
previous approaches. The stacks reveal a positive velocity gradient at
80-150 km depth that is consistent with the base of an asthenospheric
low velocity layer. This feature is particularly strong in stacks of
longer period data, indicating it represents a gradual velocity
gradient. At shorter periods, a lithosphere-asthenosphere boundary phase
is observed at 60-90 km depth, marking the top of the low velocity
layer.