  We present a new method for helioseismic diagnostics of the three-dimensional
structure of sound speed, magnetic fields and flow velocities in the convection
zone by inversion of acoustic travel-time data. The data are measurements of
the time for acoustic waves to travel between points on the solar surface and
surrounding annuli obtained from continuous observations at the South Pole in
1991 and from high-resolution observations from the Solar and Heliospheric
Observatory (SOHO) in 1996. The travel time of the waves depends primarily on
the sound speed perturbations and the velocity of flow along the ray paths. The
effects of the sound speed perturbations and flows can be separated by
measuring the travel time of waves propagating in opposite directions along the
same ray paths. Magnetic fields result in anisotropy of the wave speed. A 3D
inversion method based on Fermat's Principle and a regularized least-squares
technique have been applied to infer the properties of convection in the quiet
Sun and in active regions.
