Revisiting the Vibroseis wavelet

Abstract

The well-established Vibroseis source has been heavily used since its development in the 1950s (Crawford, et. al., 1960). Typically, a swept-frequency signal (or sweep) of length 2 - 10s is injected into the earth. The raw reflection record is complicated and uninterpretable. However, crosscorrelation with the theoretical reference sweep, or with a measured base-plate reference, simplifies the recording. The output is assumed to be the earth response convolved with the autocorrelation of the sweep (referred to as the Klauder wavelet). Since Vibroseis was first developed a number of variations to the standard linear sweep have been investigated, with the general aim of optimising the character of the correlated Vibroseis wavelet. These include Vari and encoded sweeps (e.g. Edelmann and Werner, 1982), non-linear sweeps (e.g. Goupillaud, 1976), and pseudo-random Vibroseis (Cunningham, 1979). Often, decisions regarding sweep parameters are made in the field, based on unprocessed field records. There has been some research into the effects of phase (Cambios, 2000), and the influence of such processes as deconvolution (Gibson and Larner, 1984). However, this research does not seem to be widely considered in pragmatic Vibroseis sweep evaluation. Hydrocarbon exploration is now targeting increasingly subtle traps, relying on incremental improvements in seismic technology. At the same time, for reasons of economics, Vibroseis technology is being deployed in new target areas (e.g. Coal Seam Gas (CSG) and coal exploration). For these reasons, it is timely to revisit the fundamental importance of the Vibroseis wavelet in seismic interpretation, and the many factors which control its character. A primary purpose of this paper is to give an example of how Vibroseis wavelet evaluation can extend beyond a simplistic comparison of sweep autocorrelations to include the influence of earth attenuation, phase distortion and processing. We also comment briefly on the choice of reference signal for correlation.