Many young people seem to enjoy tackling tough theoretical problems, but when the time comes for application they are often disappointed to find that the theory is in some ways irrelevant or inadequate to the problem at hand. At first this causes a diminished interest in practical problems. But eventually many come to see the real problems as more interesting than the original mathematical models. Why is this?

Maybe life is like a computer game.
I have noticed that the games students like best are not
those with a predetermined, intricate logical structure.
They like the games that allow them to gradually uncover the rules
as they play.
It is really fun when a period of frustration with a game is ended
by some application of a personal idea.
But to be fun, a game must
*have*
rules, and you must
*be*
*able*
to uncover them with a reasonable amount of effort.
Luckily, reflection seismology, together with modern computers,
provides us with a similar environment.
Sometimes a game can be too frustrating, and you need a hint to
get you over some obstacle into a new and deeper level.
Reading this book is not like playing the game.
It is more like being given a collection of hints,
a bag of tricks, to help you to the deeper level.

These tricks are mostly new, many being less than ten years old. They have been selected because they really work, not always, but often enough. I have repressed the urge to include many promising tricks that have not been sufficiently tested.

Practical problems are not only deeper than theoretical problems,
but ultimately they yield more interesting theory.
For example, in freshman physics laboratory I learned to deduce
Newton's laws of motion from simple experiments.
I should have found experimentally that force equals mass times acceleration.
Of course I didn't find exactly that.
The experiment didn't seem to work out too well because of friction.
*Friction,*
now there's a really interesting subject for you.
Physicists, chemists, metallurgists, earth scientists,
they all know Newton's laws but wish they understood friction!

The theoretical book you are now holding wouldn't have been written except that two earlier theoretical approaches, (1) the theory of mathematical physics in stratified media and (2) time series analysis couldn't touch some of the most interesting aspects of our data. Some people thought we just had dirty data! Reflection seismic data are repeatable. Most of our problems really arise from the theory, not the data.

10/31/1997