Adjoint processing defined

``Data processing'' in earth soundings analysis could mean anything anybody does to seismic data. A narrower definition is those processes that are routinely applied in industry, such as those described in Oz Yilmaz's book, Seismic Data Processing. As we will see in chapter of this book, much of echo soundings analysis can be interpreted as the adjoint of seismogram modeling. Here we use the word ``adjoint'' in the mathematical sense to mean the complex conjugate of the matrix transpose. Not all processes can be accurately characterized as the adjoint to seismogram modeling, but many can, including normal moveout, stacking, migration, dip moveout, and more. Since these are the heavyweights of the industry, the simple word ``processing'' can almost be understood to stand for ``processing by adjoint modeling.'' As we will see, such processing applied to perfect data generally gives an imperfect result. This imperfection leads thoughtful people to the concept of inversion.