This model is loosely based on the geological concept of velocity profiles. Seismologists use the travel times of seismic waves in combination with computer modeling software to determine these profiles within the earth's crust and mantle. In some instances geologists will purposefully set off explosions on the earth's surface in combination with sensors on the ground to time the speed of the explosion's shock wave through the ground. The principle behind this technique is that shock waves travel fast through hard rock and slow through soft rock. Additionally, waves that hit the mantle or some other change in rock type at an oblique enough angle will reflect, and waves that hit at an acute angle will diffract.
For detailed instructions on using the program, take a look at the info window from the Sonar model.
Click on one of the pictures to see a quicktime movie of the model: