In the 1960s seismologists learned how to apply time-series optimization theory to seismic data--see FGDP for that. Eventually time series reached the point of diminishing returns because its approach to spatial relations was oversimplified. In the 1970s seismologists learned to apply the wave equation. That's what this book has been about. You can see that the job of applying the wave equation is not yet complete, but we have come a long way. Perhaps we have solved most of our ``first-order'' problems, and the problems that remain are mainly ``second order.'' For second-order effects to be significant, all the first-order phenomena must be reasonably accounted for.
Some first-order effects that this book has touched on only lightly relate to obvious, as well as subtle, imperfections in seismic data.