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Error Distribution in Surface Mapping for Seismic Survey Operations in Logistically Constrained Terrain -A Case Study from upper Assam Basin



Elevation Adjustment
There are two options available for elevation adjustment.
  • Weighted - Any misclosure in the elevation is distributed in proportion to the length of the traverse line leading to the point (Like the compass rule discussed above).
  • Linear - Any misclosure in the elevation is distributed evenly in each log of the traverse route. We used the weighted method for adjusting the error in elevation.
Error Adjustment for Traversing Cum Line Setting out
We deployed SET-B type of Total Station and SDR-31 data logger unit of Sokkia make. Due to logistics of the area the total observation points were as large as 2673. The available SDR-31 does not have enough memory to store such large volume data at a time. The data was therefore transferred in a PC. However, there is a limitation with available software with SDR-31 that it makes error adjustment for the point on traverse and not to the point set out from the corresponding traverse point. As per our requirement, the error distribution should be done on the set out point instead of the traversed point, as discussed in Figure-2. In the presence of obstacles set out points are on the arm of the traverse points, therefore, the error adjustment is done on the matched points (when traverse is along the profile) and not on all set out points. Therefore, computation was made for the error adjustment on the remaining points. The profile before error adjustment (solid line) and after error adjustment (dashed line) are shown in Figure-4.


Figure - 2 Traversing cum Line setting out



Figure - 3 Relative position of control point, bench



Figure - 4. Error adjustment of the profile

Conclusion
The accurate error adjustment within the limits of accuracy of the measurements in a seismic survey work for hydrocarbon exploration is very important for accurate positioning of subsurface features. The method explained in this paper is an economical, accurate and quick technique for surveying computations specially in logistically difficult areas where straight profiles without obstacles are not possible to be laid out on ground and resurveying is costly and difficult proposition.

Acknowledgement
The authors are thankful to the management of OIL INDIA LIMITED for granting permission to present this paper.

References:
  • Spradley L.H. 1985, " Surveying and navigation for Geophysical Exploration". D Reidel Publishing Company.
  • SOKKIA SDR-31 Operational manual
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