Review of vibroseis data acquisition and processing for better amplitudes: adjusting the sweep and deconvolving for the time‐derivative of the true groundforce

Abstract

ABSTRACTThe goal of vibroseis data acquisition and processing is to produce seismic reflection data with a known spatially‐invariant wavelet, preferably zero phase, such that any variations in the data can be attributed to variations in geology. In current practice the vibrator control system is designed to make the estimated groundforce equal to the sweep and the resulting particle velocity data are cross‐correlated with the sweep. Since the downgoing far‐field particle velocity signal is proportional to the time‐derivative of the groundforce, it makes more sense to cross‐correlate with the time‐derivative of the sweep. It also follows that the ideal amplitude spectrum of the groundforce should be inversely proportional to frequency. Because of non‐linearities in the vibrator, bending of the baseplate and variable coupling of the baseplate to the ground, the true groundforce is not equal to the pre‐determined sweep and varies not only from vibrator point to vibrator point but also from sweep to sweep at each vibrator point. To achieve the goal of a spatially‐invariant wavelet, these variations should be removed by signature deconvolution, converting the wavelet to a much shorter zero‐phase wavelet but with the same bandwidth and signal‐to‐noise ratio as the original data. This can be done only if the true groundforce is known. The principle may be applied to an array of vibrators by employing pulse coding techniques and separating responses to individual vibrators in the frequency domain. Various approaches to improve the estimate of the true groundforce have been proposed or are under development; current methods are at best approximate.