Results of receiver function stacking for each of the circles.

The columns are:

clon - longitude of the circle's center
clat - latitude of the circle's center
d410 - depth of the d410
sig4 - standard deviation of the depth of the d410
d660 - depth of the d660
sig6 - standard deviation of the depth of the d660
MTZ  - Mantle Transition Zone thickness
sigm - standard deviation of MTZ thickness
a4   - stacking amplitude of the d410
a6   - stacking amplitude of the d660
NRFs - Number of receiver functions in the circle

Note that a value of zero is given to the depth if the Pds arrival is
not well-defined.