Techniques for improving Vibroseis data

Abstract

An ongoing program of field experiments to analyze vibrator‐earth response has verified two major problems in Vibroseis® data acquisition: amplitude and phase control of vibrator output, and suppression of vibrator‐generated harmonic distortion. This study confirms that these problems are more severe on rigid ground (i.e., frozen tundra) than on soil. The test results demonstrate that a large‐amplitude vibrator resonance at 40 Hz and a higher frequency (equal to or greater than 100 Hz) earth‐baseplate resonance dominate the vibrator‐frozen ground response. Adaptive, closed‐loop amplitude control of ground force virtually eliminates baseplate‐ground decoupling at the resonant frequencies while maximizing force output throughout a sweep. Analysis of far‐field downhole signals indicates that phase locking to ground force, along with amplitude control, provides a more stable downgoing wavelet than phase locking to baseplate or reaction mass acceleration. Quantitative spectral analysis of downhole and vibrator accelerometer signals shows severe harmonic distortion and demonstrates the suppression of all even harmonics and certain deleterious odd harmonics after rotating the phase of successive sweeps by a phase increment of less than 180 degrees. ®Trade and service mark of Conoco Inc.