The present invention relates to a control system for a vibrator system capable of generating and applying a reciprocating force to an object, such as the underlying ground or water. The control system is generally provided with a means for outputting a drive signal having a fundamental frequency to the vibrator system to activate the reciprocating of the base plate of the vibrating system and a means for receiving a vibrator output signal indicative of the force applied to the object. The vibrator output signal includes at least one harmonic frequency signal and is delayed with respect to the phase of the initial drive signal. To increase the signal-to-noise ratio of the vibrator system, the control system includes a harmonic distortion reduction means receiving the vibrator output signal for outputting at least one harmonic drive signal to the vibrator system. The harmonic drive signal has a proper phase, amplitude, polarity, and frequency which are adjusted to minimize at least one of the harmonic frequency signals being produced by the vibrator system.
This invention relates in general to vibroseis and, more specifically, but not by way of limitation, to the enhancement and/or signal strength optimization of low frequency content of seismic signals for use in surveying boreholes and/or subsurface earth formations. In embodiments of the present invention, physical properties of a seismic vibrator may be analyzed and used to provide for determination of a driving force necessary to drive a reaction mass to produce a sweep signal with enhanced low frequency content for injection into the ground for vibroseis. In certain aspects, the physical properties may be considered independent of any geophysical properties related to operation of the seismic vibrator.
A process for analyzing, decomposing, synthesizing and extracting seismic signal components, such as the fundamental of a pilot sweep or its harmonics, from seismic data uses a set of basis functions. The basis functions may be any of the harmonics or other components derived from the pilot sweep. These basis functions are modulated with the data being analyzed. The resulting data output from the modulation are then low-pass filtered so that the instantaneous amplitude and differential phase of the particular signal component to be synthesized may be computed. Each targeted component is then synthesized and the result subtracted from the remaining data.
The force exerted by a reciprocating actuator is estimated as a weighted sum of three quantities: a reaction-mass acceleration, a baseplate acceleration, and a relative acceleration of the reaction-mass relative to the baseplate. In one embodiment, acceleration sensors measure the reaction-mass acceleration and the baseplate acceleration; and a displacement transducer measures a relative displacement of the reaction-mass relative to the baseplate. A double-differentiator generates the value of the relative acceleration from the second derivative of the relative displacement. In the sum, the relative acceleration is weighted by factor representative of the ratio of the difference between the masses of the reaction-mass and the baseplate to the sum of the masses of the reaction-mass and the baseplate.
