Early in the 1970s, seismologists realized that a full three-dimensional (3D) interpretation was needed to satisfy variations in the observed seismic travel times. The starting point of modern seismic tomography (from σεισµός = quake and τόµος = slice) is probably the 1974 AGU presentation by MIT's Keiti Aki (Aki et al.) in which arrival times of P-waves were for the first time formally interpreted in terms of an ‘image’ as opposed to a simple one-dimensional graph of seismic velocity versus depth. That Aki's co-authors came from NORSAR – the Norwegian array to monitor nuclear test ban treaties – was caused by a quirk of history: Aki had originally planned a sabbatical in Chile, but when a military coup d'état brought the Allende government down in 1973 he changed plans and accepted an invitation from his former MIT student Eystein Husebye for a short sabbatical at NORSAR, which was equipped with a state-of-the-art digital seismic network and computing facilities. Even so, in the twenty-first century it is easy to underestimate the difficulties faced by early tomographers, who had to invert matrices of size 256 × 256 using a CPU with 512 Kbyte of memory. The collaboration between Aki, Husebye and Christoffersson was continued in 1975 at Lincoln Labs in Massachussets (Aki et al.).

The name that was later given to the new imaging technique is more than an accidental reference to medical tomography, because the earliest radiologic tomograms also attempted to get a scan of the body that focuses on a plane of interest, albeit using X-rays rather than seismic waves.