Time-Distance Helioseismology: How The Inversion Results Depend On The Approximation Used

During the last decade, time-distance helioseismology has provided important new insight into the solar sub-photospheric structure and dynamics of sunspots, active regions, supergranular cells, and large-scale flows. These results were based either on the ray-path or on the Fresnel-zone approximations. We present inversion results of travel-time perturbations of wavepackets propagating inside the Sun, using both ray-path and Fresnel-zone kernels for real and artificial data. The ray approximation was the first approximation in time-distance helioseismology for deriving the travel-times. However new types of kernels are being developed to take into account the finite-wavelength effects of the wavepackets (such as Fresnel-zone kernels), and thus improve the resolution and accuracy of the inversions. Since many results have been obtained with the ray-path approximation, it is important to compare them with the new Fresnel-zone inversions to quantify their accuracy.