The solar dynamo is thought to be generated at the tachocline and the deepest layers of the convection zone. Yet much about these layers or how this mechanism works remains unknown. In this work we present the first of a series of steps in order to apply helioseismic holography to the study of the tachocline. Traditionally a plane-parallel approximation has been used for the calculation of the pupil for earthside seismic holography. In this work we have used a spherically symmetric model for the pupil to map an active region in the frontside of the Sun to test its potential. We also present the theoretical background to use a spherical polar expansion to calculate the Green's functions instead of the usual acoustic ray path approximation. These new Green's functions will allow to reach the deepest layers of the convection zone with more accuracy.