HELIOSEISMIC TOMOGRAPHY

''Helioseismic tomography'' is a method using observations to construc t slices of the Sun's internal structure. It is based on a reduction o f observations to time-distance surfaces and hypersurfaces. We present a procedure for measuring time-distance surfaces and hypersurfaces, a nd thereby a method of studying localized inhomogeneities in the inter ior of the Sun, such as abnormalities in the sound speed (e.g., a ther mal shadow, Parker 1987a), or local subsurface flows, or magnetic fiel ds. We also present a simulation of measuring time-distance surfaces a nd illustrate how to measure the size of an inhomogeneity, its locatio n in depth, and the deviation of its sound speed compared to its local surroundings.