Effectiveness of Wide Marine Seismic Source Arrays

Abstract The use of wide source arrays in marine seismic surveys has become a topic of interest in the seismic industry. Although the primary motivation for wide arrays may have been to get more guns in a source array without increasing the inline array dimension, wide arrays can also provide the added benefit of suppressing side-scattered energy. Comparisons of common midpoint (CMP) stacks of data acquired offshore Washington with wide and conventional-width source arrays, however, show only small and sometimes inconsistent differences. These data were acquired in an area where side-scattered energy is a problem. Comparisons of unstacked data, however, show dramatic differences between the wide and conventional source array data. The disparity between the stacked and unstacked data is explained by analyzing the effective suppression of back-scattered energy by CMP stacking. Energy reflected from scatterer positions broadside to a given CMP location has a lower stacking velocity than that of the primary reflection events. Thus, CMP stacking greatly attenuates the side scattered energy. In this survey area, the action of CMF stacking was so powerful in suppressing the broadside energy that the additional action of the wide array was inconsequential in the final stacked sections. In other areas, where the scattering velocity is comparable to the primary stacking velocity, wide arrays may provide some advantage. Because they can degrade cross-dipping primary events, wide arrays cannot be used indiscriminately. They should be considered a special purpose tool for attacking severe source-generated noise from backscattered waves in areas where the action of CMP stacking is insufficient. Introduction The use of crossline (wide) source arrays in Marine seismic acquisition has been advocated (Tree, et al, 1982) to help suppress side-scattered coherent waves. Whereas the action of long inline arrays in the field can be simulated by array forming in the computer, that of wide arrays cannot, unless data are acquired along finely spaced parallel lines. Therefore, if larger array widths do indeed provide noticeable suppression of noise on common midpoint (CMP) stacked sections, there is little practical alternative to employing wide arrays in the field. To test the effectiveness of a wide source array in suppressing back-scattered waves, a field experiment was conducted in April 1982. This paper analyzes the results from four seismic lines from offshore Washington, surveyed using both a wide and compact source array. Comparisons of the conventionally processed stacked sections for the wide and compact arrays show only minor differences, perhaps marginally favoring the results for the wide array, but sometimes inconsistent. The wide array did, nevertheless, perform as could be expected, but obvious differences in the treatment of side-scattered noise are evident only on unstacked data and velocity analyses. We will show that in this area CMP stacking effectively suppressed the same energy that the wide array was designed to attack. This understanding of the action of CMP stacking will lead us to some generalizations concerning the applicability of wide arrays.