This month’s Focus is related to land data acquisition. In the early days of recording 3D on land the general recording geometry was very similar no matter where in the World the survey was being acquired. Although we recorded data with both explosives and vibroseis and the geometries were slightly different for each, most surveys with the same source were recorded in a similar manner. In recent years we have seen a wide divergence in all aspects of land 3D acquisition with different geometries, recording system technologies and trace densities between different regions. In this May Focus issue, we have three articles that will cover the scope from optimizing the survey design from a geometry perspective, through a cost analysis of two types of geometry that have been used in Canada, to a study in the WCSB of a technique that is quite popular in the Middle East.
The first article by Keith Millis, Case Caulfield, and Andrea Crook takes a new look at ways to improve the positioning of sources and receivers when surface obstructions and permit problems limit our ability to record a regular 3D survey grid. The commonest method of specifying source and receiver offsets is to use a set of guidelines that skid the locations in increments of the nominal source and receiver intervals. Frequently, these guidelines are designed with the goal of not recording duplicate source to receiver raypaths or unnecessary offsets, but in some regions this is not always done and data quality suffers. The new method described by Keith uses subsurface illumination, spatial sampling and imaging criteria to optimize the re-positioning of both sources and receivers simultaneously.
The second article is written by Dale Harger and Jason Schweigert, who review the relative merits of the Mega-Bin geometry with the more conventional Orthogonal line geometry after normalizing the costs of each survey to be the same. They not only review the trace statistics (offset, azimuth, fold and trace density, etc.) but also look at the required noise attenuation and imaging capabilities in view of the differences between the old, now rarely-used, post-stack migration algorithms and much more commonly used pre-stack migration algorithms. Many of these pre-stack imaging methods routinely have 5-dimensional noise attenuation and regularization applied prior to the imaging.
Focus article number three, by Paul Thacker, Dale Harger and Doug Iverson, presents two case histories that were designed to evaluate the use of some vibroseis technologies that, although frequently used in the Middle-East, have not seen common usage in the Western Canadian Sedimentary Basin. They begin with a nice overview of the different techniques and then show one case history where test data were recorded at the end of a conventional survey to provide data with no risk. In the second case history, also from Eastern Alberta, the complete survey was recorded using single small (15,000 lbs. hold down) vibrators and single sweeps. Their excellent data examples demonstrate clearly the potential benefit that some of these methodologies can provide in terms of imaging, operational efficiency and cost.
About the Author(s)
R. Malcolm Lansley received a B.Sc./A.R.C.S. in physics/mathematics/geophysics from Imperial College of Science and Technology in London in 1969. Nine years ago Malcolm joined Sercel as VP of Geophysics, where he advises on the geophysical usage of all Sercel equipment including survey design, data collection, data processing, and interpretability of the resultant data. Prior to joining Sercel he had worked for more than 35 years with Geophysical Service, Inc., Halliburton Geophysical Services, Western Geophysical and PGS in all areas of the world, both onshore and offshore. Although he started working with 2D in 1969, his first involvement with 3D seismic exploration was in 1972, and since that time he has been concerned with all aspects of 2D and 3D seismic exploration from survey design, through data acquisition and data processing, to interpretation. Malcolm has authored more than 60 papers on seismic exploration, data processing and interpretation. His research interests include 3D technology (both land and marine), marine data acquisition, wavelet processing, multicomponent recording and vibrator theory and usage. He also teaches a variety of courses on vibrator theory and usage, 3D survey design, data acquisition and data processing for SEG. He is a member of SEG, EAGE, CSEG, ASEG, RMAG, SBGf and the Geophysical Society of Houston.