Summary
Devon Energy and Dawson Geophysical Company evaluated a land 3D seismic survey recorded with two diverse acquisition techniques. The purpose of the effort was to directly compare data characteristics and quality from conventionally sourced vibrator units with the units coded to produce "impulses" or "thumps". The comparison data sets were recorded by repeating the sources over a common surface area of 23 square miles located in the Permian Basin in Southeast New Mexico. A conventional sweep data volume was acquired with 2 – 18 second sweeps per source point with bandwidth from 4-96 hertz. The impulsive data set was acquired with the "Galcode" methodology of a 13 bit (8191 points) code with 2 milli-seconds between the code points. The Galcode theory allows a series of closely spaced vibrator thumps, like small hammer hits, to be coded in such a way that during the decode process a composite big hammer hit is produced. The impulsive wavelet from the hammer hit carries the potential of a broadband wavelet that could be valuable especially on the low end frequencies of the seismic source spectrum. The Galcode theory also allows the vibrator sets to be programmed with different codes with implication to simultaneous sourcing and the potential economic or data quality uplift that can be leveraged. The "Impulsive" or "Thump" technique was described by Thomas, Chandler, and Osten (2010) and by Thomas, Jurick, and Osten (2012). After the decode process for the Galcode source records and standard correlation for the conventional field data the volumes were identically processed. The vibrator sourced seismic data volume recorded with the Galcode method upon visual and analytical scrutiny favorably compared with the conventional acquired sweep data set.
Introduction
The numerical codes introduced by Currie (1990, 1991) can be utilized with modern day vibrator source controllers and equipment to produce a series of closely spaced impulse or thumps. Twenty years later the adapted methodology coined Galcode has become a viable seismic acquisition vibrator source technology demonstrated through field trials by Thomas et al. (2010, 2012). Figure 1, illustrates the raw thump code with the little hammer hits on the left and the big composite hammer hit or impulse wavelet on the right. A theoretical spectral comparison of a coded sourced impulse wavelet compared to a conventional correlated Vibroseis sweep pilot wavelet is shown in figure 2. Note the impulse wavelet has a greater range of high and low frequencies and reduced side lobe energy. The Galcode application works well with single vibrator set deployment or in a simultaneous source recording environment. The primary advantage of the coded thump mode could come from the broad band nature of the impulse or with the coded simultaneous source technique. The simultaneous technology can be leveraged for increased fold, finer bin size or reduced cost of p-wave surveys. Nine Component (9C) surveys will certainly become more economical with the simultaneous recording of the P, Si and Sx source modes.
