Vibroseis evolution: May the ground force be with you

Although the precision of positioning and fidelity of recording of the sensors in land seismic exploration has increased significantly over the last three decades, the same cannot be said about the accuracy in the determination of the time varying signal generated from land seismic sources. At best, the current methods of deter¬mining the source signature from vibrator sources used on land are approximate. A selective examination of representative past and present models and concepts in the modelling of the source/earth interactions suggests that current trends in land seismic acquisition (towards point source, point receiver surveying, increased emphasis on seismic attributes, and the use of distributed simultaneous and overlapping source events) are forcing a renewed focus on more accurate measurement and/or prediction of land seismic source signatures. New models are required that better describe the dynamically interactive nature of the source-earth relationship. ‘All models are wrong, but some are useful’. This aphorism by the statistician Box (Box and Draper, 1987) is meant to emphasize that any mechanistic model should have the following advantages: It contributes to our scientific understanding of the phenomenon under study. It usually provides a better basis for extrapolation (at least to conditions worthy of further experimental investigation if not through the entire range of all input variables). It tends to be parsimonious in the use of parameters and to provide better estimates of the response Paraphrasing the above, the most that can be expected from any model is that it can supply a useful approximation to reality. From this, a pragmatic geophysicist (some would say a cynical one) might ask how wrong do models have to be in order not to be useful.