Examination of a broadband sweep with and without Force Control
This article proposes that this known significant discrepancy between the Weighted Sum Ground Force (WSGF) prediction of the vibrator output (the Sallas Approximation; see Sallas, 1984) and downhole measurements of the output is not so much a ‘lie’, as it is simply the natural consequence of the limitations and inaccuracies of the WSGF model. The authors believe that these model deficiencies arise from a woefully incomplete understanding of the complex interaction of the vibrator mechanism with the ground. If this is correct, than an improved understanding of the interactions of the vibrator mechanism and the ground during a sweep should lead to a more complete model of the vibrator/earth interactions, with significant improvements in both the actual energy content of the propagating wavelet as well as more ‘truthful’ quality control reporting thereof. Modern vibrator control systems use both phase and force control systems in controlling the output motions of the reaction mass (RM) and baseplate (BP). Feedback from various sensors on the vibrator mechanism is used to adjust the input hydraulic controls so that the WSGF ‘mimics’ the desired reference sweep. In this paper we examine the response of a downhole vertical geophone to a broadband sweep generated on the surface, initially with both force and phase control operating, and then with force control disabled, to observe what effects force control has on the vibrator mechanism and the propagating seismic wavelet, as well as to develop an improved understanding of the interaction of the vibrator mechanism with the ground during a sweep.