Land 3-D Survey Design by Computer

Today’s 3-D surveys can be large or small, laid in straight lines across deserts or tundra or along winding roads through forest and jungles. Because 3-D acquisition requires careful planning it is essential to analyse the complexities which naturally arise. Computer programs are now available to assist the explorationist with this task.

The fundamentals of 3-D design start with the interpreter who must analyse the likely target geology and establish the desired fold (hence likely signal/noise) and bin size (spatial resolution and maximum non-aliased frequency on dipping events). Ray paths to formations will determine minimum and maximum acceptable shot-receiver offsets.

The basic equations governing all 3-D surveys are:

NS = F/(NC.b.b)

SLI = 1/(NS.b.2)

where NC = number of recording channels, b = bin size in metres, and NS is the number of shots per square metre needed to create the fold F, and SLI = shot line interval. Receiver line interval depends only on the minimum acceptable offset and is a critical part of determining the layout strategy – straight lines, bricks, zigzags, buttons to name a few. Increasing receiver line interval can, of course, reduce clearing costs. Each strategy has pros and cons from the processor’s and the field crew’s perspective and must be analysed. How will the data resulting from such a survey respond to velocity analysis, static corrections, DMO, muting, and migration? Will any noise in the data be cancelled by the stacking process (stack-array effect for linear noise, or enough offsets to attenuate multiples)? A comprehensive analysis of the fold and the offset and azimuth distribution in each CDP bin is essential.

Finally the field crew’s concerns must be addressed. Equipment is expensive to place and expensive to move. Different designs can save dollars – roll-on versus. roll-off for example. The computer program must allow easy movement of shots to undershoot lakes, rivers, pipelines, buildings or to fill in ‘holes’ created by shooting or recording along non straight lines.

Changes at each stage of the design process can be made and costed quickly leading to a successful, efficient 3-D seismic survey.