The Diminishing Returns of Low-Frequency Vibroseis and How Only Physics Will Get Us out of the Rut

Abstract

All land seismic sources are inefficient in the conversion of their energy/force, generated at the surface, into useful seismic signals. It is not commonly known that less than 5% of the source’s energy is converted into energy of the propagating wave. Until now, one of the objectives of the designers of land seismic sources has been to increase the maximum force (or energy) output at the surface to increase the amplitude of the seismic signal. Today’s vibroseis sources, at about 40 tonnes, are at their maximum size and weight limits for many road infrastructures limiting their areas of use, as well as being beyond the budget of most seismic operators. As force output is related to vehicle weight (hold down force), the vibroseis source is effectively at its maximum output (and therefore maximum seismic signal) under its current configuration. For this reason, to achieve further increases in the amplitude and frequency content of the propagating seismic signals, the emphasis in future research and development must be in understanding the many factors that are involved in the conversion of the force/energy generated by the surface source into useful seismic signals. This has been missing in the past; it is where only physicists can play a role. In this article, we discuss the physics of impulsive and vibroseis sources and their interactions with the ground to show that improving the amplitude/frequency content and efficiency of the vibrator, especially at low frequencies, cannot be fixed by ‘mere engineering’.