A Marine Digital Hydraulic Vibrator: Part I-Concepts And Coding Techniques
Authors: Bouyoucos J.V. , Nelson D.E. 
Published: October 2008

A new marine vibrator using digital hydraulic switching technology is described. The digital hydraulic technology increases the efficiency of power conversion, and enables transmission codes to be used that maximize acoustic power generation for a given source depth. All power conversion is done at the source, so that it can be towed at substantial distances from the survey ship by means of an electrical umbilical. Digital signal trains with low harmonic ghosting have been developed to drive an array of such sources over a broad frequency spectrum for deep seismic, high resolution exploration. Orthogonal codes are also available for 3D exploration.

1.0 The Digital Marine Vibrator System Concept

The MULTIPULSE ® system is a marine vibrator system with the distinctive property that it transmits rectangular waves, or pulses, with varying periods. All of the pulses have the same height, so that only two signal levels are transmitted. This description fits a binary transmission system wherein only "one's" and "zero's" are transmitted. An example of the transmitted signal is shown in Figure 1.

The MULTIPULSE system generally includes a number of MULTIPULSE sources, each of which includes radiating pistons which are accelerated synchronously either inward or outward, each direction at a single specified constant rate. Since sound pressure for a small wavelength source is proportional to volume acceleration, the above-mentioned binary levels correspond to whether the radiators are being accelerated one direction or the other.

It has been found to be especially important that the acceleration rates inward and outward be unequal. The outward acceleration is, in fact, chosen to be twice the inward acceleration. Since cavitation of the radiator-water boundary occurs on the inward, or "sucking" stroke, and as this part of the stroke has the lower acceleration level, the source can produce more power at the cavitation limit than one transmitting sine waves - in fact four times the power.

The control which is exercised over the vibrator to define the transmitted signal spectrum, or its autocorrelation function, is the choice of switching times when the direction of acceleration is reversed. The MULTIPULSE source is designed to be highly accurate in this regard, the switching time accuracy being held to within a small fraction of a millisecond.

The intentional transmission of a signal which is obviously rich in harmonics seems to violate twenty years of experience in using linear vibrators. This paper will explain why and how this was done.

Not only has the technique been proven effective, but the transmitted signal is sufficiently low in harmonic ghosting such that continous transmission is employed. The source is never turned off. As soon as one sweep reaches completion, the next begins. There is no need for a listen period dictated by harmonic ghosting. Accordingly, greater total energies can be transmitted within a given record interval. Clearly, for this feature to be useful, the receiver system must possess sufficient dynamic range to see signal reflections through the continuous transmissions.