The estimation of subsurface properties of the earth by use of observed geophysical data is identified as "geophysical inversion", and can be defined to include nearly everything that geophysicists do. As we approach the new millennium, it is interesting to contrast past and present inversion methods with possibilities for the future. Although the earliest inversions were done entirely within the geophysicist's brain, inversion became more quantitative and computational with the advent of high performance computing. Geophysicists have recently invoked global optimization through the use of exhaustive search techniques. Three-dimensional elastic models have been included as inversion drivers. Constraints and Bayesian statistics are used in the appraisal of inversion solutions. Although inversion progress has advanced with concomitant progress in computing, automated inversion, without a human arbiter, is still a fantasy.
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