Abstract What the geoscientist might really like to do is cut the Earth along a line, down some thousands of metres, and have a look at the rocks, and their pore fluids, in place. While this rather invasive technique is not yet popular, analogous methods are. Seismic tomography is a means of making a picture of a slice of the earth using seismic data (the word tomography being derived from the Greek τoμoσ which means section or slice). Seismic techniques have long been used to create subsurface pictures. The medical community has also been greatly assisted by computed tomography (CAT or CT scanning) for some time (e.g., Kak, 1979; Coulam et al., 1981; Seeram, 1982). However, it is only recently that seismic tomography (ST) has been developed for hydrocarbon exploration. Although some of the methods used in exploration geophysics for a number of years can be classified as tomographic, ST provides a general unifying concept of geophysical parameter estimation and imaging. More recent acquisition geometries (e.g., crosswell) and diverse data sets (well log, borehole seismic and surface seismic) can also be processed and integrated via ST methods. The current interest in and promise of ST in geophysical exploration is made possible by a number of factors including cross-fertilization from other sciences, advances in seismic field data acquisition, imaging theory and computing speed. Tomographyi s a type of inversep roblem. That is, measurements are first made of some energy which has propagated through a medium. The