Helioseismic tomography

''Helioseismic tomography'' is a method using observations to construct slices of the Sun's internal structure. It is based on a reduction of observations to time-distance surfaces and hypersurfaces. We present a procedure for measuring time-distance surfaces and hypersurfaces, and thereby a method of studying localized inhomogeneities in the interior of the Sun, such as abnormalities in the sound speed (e.g., a thermal shadow, Parker 1987a), or local subsurface flows, or magnetic fields. We also present a simulation of measuring time-distance surfaces and illustrate how to measure the size of an inhomogeneity, its location in depth, and the deviation of its sound speed compared to its local surroundings.