Environmentally sensitive 3D land seismic survey design; a case study

Designing a land 3D survey involves the consideration of a number of geophysical, financial, operational and environmental factors. In this case study of a 3D seismic survey design environmental considerations were of paramount importance as one of the main aims of the design process to come up with a 3D survey design with minimal environmental impact. The main geophysical parameters, maximum offset, minimum offset, size of the survey (migration aperture) and required fold were determined, where possible, by a combination of theoretical and empirical methods. That is, previously recorded 2D data are reprocessed to simulate various folds and offsets in order to gauge the effects of various parameters. In this way we hoped to arrive at a more accurate set of geophysical criteria for the future 3D acquisition.

Once these requirements were established source and receiver line geometries were investigated that would satisfy these requirements and minimise the environmental impact. Four different shooting methodologies were investigated including two orthogonal geometries and two cross-swath geometries. It was found that the two orthogonal geometries did not achieve the fold needed in the survey and were environmentally unsuitable as they required excessive vibrator travel on receiver lines. Environmental and geophysical factors led to the investigation of cross-swath geometries that result in geophysically desirable symmetric sampling over the survey area and reduce travel on receiver lines. The geometry selected for the survey was a modified cross-swath geometry in which receiver line spacing is reduced to increase the nominal fold. Although we pay a penalty in that we must duplicate most source positions the modified cross-swath geometry significantly reduces vibrator and line laying vehicle traffic when compared to ‘standard’ orthogonal shooting. This means that less intensive line preparation such as rolling and brush cutting is required which reduces the environmental impact.