WSGF — Time for change part 2: Forces and energies acting on the seismic source and earth systems

Abstract

In land geophysical exploration an impulsive or vibratory source, operating at the earth’s surface, is used to generate the seismic signal during the field acquisition. Comparisons of the manufacturer’s output specifications will often be a factor in selecting a seismic source for ‘good S/N’ (signal to noise ratio) in a particular survey area. For impulsive sources, such as dynamite, weight drop, and accelerated weight drop (AWD), the source specifications are given in energy units (Joules, foot-pounds) whereas vibroseis sources are rated in force units (Newtons, pounds-force). For either type of source, the manufacturer’s specifications are the output ‘power’ of the source mechanism and do not take into account any inefficiencies/losses in energy in the source mechanism or the interacting earth volume when generating the seismic signal. In this paper, I examine some of the factors that determine the transfer of energy from the source mechanism to the seismic signal and show, for AWD sources, the energy transfer during the impulse can be greatly improved by optimization of the source parameters. For vibrators, the energy transfer to the ground may be a better indicator of the propagating wave than the current WSGF estimates.