The vibroseis system is an engineered system that commonly uses hydraulically operated vibrators to send continuous acoustic signals (instead of shock-waves) through the earth. The vibroseis signal is normally a swept-frequency sinusoid (called a “sweep”). In general, the sweep is from 3 to 24 seconds, but any length is possible. A vibroseis field recording is then converted into a “conventional” seismogram by the cross-correlation of each trace of the vibroseis field record with the transmitted sweep. The principle of vibroseis is well known in the art and is presented, for example, in W. E. Liang, Appendix B: History and Early Development of the Vibroseis System of Seismic Exploration, VIBROSEIS, 749-765, GEOPHYSICS REPRINT SERIES NO. 11 (1989), incorporated herein by reference.
When vibroseis was first released, it was thought to be the means of perfect control over the spectrum of the seismic wavelet. But today, even though it has proven to be very successful, vibroseis could hardly claim to give perfect control over the source, or even to know exactly what the power spectrum is at the source. The basic conventional compressional wave vibrator in current practice is illustrated in FIG. 1. A swept frequency drive force is applied between the baseplate B and a reaction mass M from a generator f1, which may be an electrodynamic or hydraulic driver. As illustrated in FIG. 2, a hold down mass M2 may be used to hold the baseplate B in contact with the ground.
The function and design of various conventional vibrators are well known in the art. For further discussions on vibrators and vibroseis, readers are directed to W. E. Lerwill, The Amplitude and Phase Response of a Seismic Vibrator, GEOPHYSICAL PROSPECTING, Vol. 29, 503-528 (1981); F. S. Kramer, R. A. Peterson & W. C. Walter, Seismic Energy Sources 1968 Handbook, 38th ANNUAL MEETING OF THE SEG (1968); Guido Baeten, Jacob Fokkema & Anton Ziolkowski, The Marine Vibrator Source, FIRST BREAK, Vol. 6, No. 9 (September 1988); R. W. Time, A. M. Young, and J. E. Blue, Transducer Needs for Low Frequency Sonar, PROCEEDINGS OF THE SECOND INTERNATIONAL WORKSHOP ON POWER TRANSDUCERS FOR SONICS AND ULTRASONICS (1990); U.S. Pat. No. 5,959,939; all incorporated herein by reference.
Both conventional hydraulic and electrodynamic vibrator designs have a fundamental problem related to the mechanical impedance of the system. At low frequencies, the spring (hydraulic fluid in the hydraulic version or suspension spring in the electrodynamic version) has a large value compared to the impedance from the reaction mass. At high frequencies, the system becomes more influenced by the impedance from the reaction mass. This is easy to understand from the formula below.
R is the damping and describes how much energy can be transmitted into the ground. f is the frequency, m is the mass and k is the spring constant for the system. As will occur to   Z  =      R    +          2      ⁢                           ⁢      π      ⁢                           ⁢              f        ·        m        ·        j              -                  k                  2          ⁢                                           ⁢          π          ⁢                                           ⁢          f                    ·      j      those of ordinary skill in the art, when f is low, the system will be dominated by the spring, and when f is high, the system will be dominated by the mass. In practice, this means that the vibrator works well when the ground conditions are such that a good impedance match between the vibrator and the ground can be achieved. This is seldom the case. When the vibrator is poorly matched with the ground, the system has low efficiency for parts of the frequency spectrum—usually, the lower frequencies. The low efficiency reduces the control over the vibrator and generates harmonic distortion. Thus, there is a long felt need for a vibrator with increased efficiency, improved control over the frequency spectrum of the source, and matched impedance with the earth.