Final appearances

The output of an impulse-response program always looks awful. The main reason is the large area of $( \omega , k_x )$-space which is near the Nyquist frequency or above the evanescent cutoff. Since we rarely sample data in time as coarsely as the Nyquist criterion permits, the program below defaults to final filtering with the filter (1+Z)/(1-.8Z). This filter still passes a lot of energy outside the usual bandwidth of seismic data. Since all land data and most marine data do not have the zero frequency component, the program contains an option to filter further with (1-Z)/(1-.8Z). I haven't displayed anything with this extra filter because I wanted this book to show all the artifacts you might encounter. Furthermore, I deliberately enhanced the visibility of artifacts on wiggle-trace, variable-area plots by plotting with the nonlinear $\gamma =1/2$ gain. (Perspective hidden-line drawings always have linear gain). Since my plots are necessarily about 10 cm square in this book and in practice you will look at plots of about a hundred times the area, I plotted only one second of travel time.