The marine vibrator source

Abstract

After the introduction of thc Vibroseis principle in seismic exploration in the early 1960s, considerable effort has been put in the development of a marine version of the land vibrator. Figure 1 shows one of the latest versions of the marine vibrator. The incentive for the development of thc marine vibrator was the feeling that further refinement of available marine sources was insufficient tor future explorarion needs. The marine vibrator's ability to fill this gap lies mainly in the perfect control that one has, at least in principle, over the emitted source signal. Further the marine vibrator can be used in shallow water areas, where conventional seismic sources are difficult to deploy. Our aim is to investigate some gcophysical issues related to the performance of the marine vibrator. For this purpose, we start with a description of the Vibroseis principle. Since the Vibroseis method has been and is being used extensively on land, problems and important aspects of the marine vibrator performance can he deduced from more than 20 years of experience with land vibrators. Also, differences in performance between land and marine vibrators will be discussed. A particular issue that deserves attention is the choice of the signal that should be monitored on the vibrator for phase and amplitude control. This choice is intimately related to the way the wavefield behaves in the far field. Also, the power output of the vibrator is of great practical importance and is investigated here. To analyse these problems we develop a model of the marine vibrator and its surrounding medium. The model contains three elements: (1) a description of the propagation of seismic waves in the medium, (2) a formulation of the boundary conditions at the surface of the vibrator, and (3) a description of the mechanical properties of the vibrator. Finally, some real data examples are shown, and a comparison is made between the model results and the measurements.