When all traces on a seismic-reflection record show about the same deflection at about the same time, the line-up is marked and called a seismic reflection. An important fact is forgotten. The fact is that the reflection seen on the record is almost invariably a composite of the various reflections caused by a set of closely spaced reflecting layers. When the arrangement of the layers in the set changes, the various reflections add together in a different way, and the character of the composite reflection seen on the record changes.

A series of artificial seismic records has been made to show this composition of reflections. The records were made by connecting a standard reflection seismograph to an acoustic model. The model was a 300-foot length of steel pipe with input and output transducers at one end. Records were made for a wedge, a pinch-out, a complex of thin layers, a sand bar, layers corresponding with well resistivity logs, and a regular layer system.

End_of_Article - Last_Page 432------------