Velocity Models From Frequency-Domain Waveform Tomography in Controlled Source Seismology

Recently, a number of leading research groups have presented examples of the success of the frequency-domain approach to waveform inversion. Apart from the use of the frequency-domain, a key common factor in all of these studies is the tomographic use of large-offset, transmitted, refracted arrivals. In this respect we refer to such work as waveform tomography. Large offset data are very sensitive to velocities; waveform tomography provides a way to unravel the complexities of the refracted arrivals in order to yield well constrained velocity models. Waveform tomography is an extension of waveform inversion methods first developed in the early 1980's (e.g., Lailly, 1983 and Tarontola, 1984). The frequency-domain version of waveform inversion developed in the 1990's (e.g., Pratt and Worthington, 1990) has now emerged as an efficient imaging tool, capable of being used on a production basis for large scale 2D problems. The examples of frequency-domain waveform tomography herein image the Earth's interior successfully on a wide range of scales. The results indicate the importance of the refracted wavefield and the equal importance of low frequencies; ultimately surveys should be specifically designed for waveform tomography. Major challenges remain to move the 2D implementations into the 3D world, to properly account for shear waves, mode conversions and multi-component data, and to incorporate the critical second order effects of anisotropy and attenuation.