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Error Propagation in Positioning of Seismic Profiles in Undulating Terrain – A Case Study From Himalayan Foothills
Case Study A case study from OIL's operational area (figure-1) in foothills of Himalayas is presented. The detailed map of the area with elevation contours and seismic lines recorded during the field season 2001-2002 is shown in figure-2. The elevation contours in the area indicate 525 m variation (from 225 m to 750 m) in source / receiver elevations as we proceed towards north depicting highly undulating terrain in northern part of the study area. Straight lines shown by black colour (figure –2), are planned seismic profiles. For seismic survey along these planned seismic profiles, the line stacking survey was done with the help of latest technology like DGPS, Total stations, data loggers etc.  Figure - 1: Location Map of Study Area To study the effect of undulating topography on fixing of source / receiver points at constant interval we have selected one 11.5 km long seismic profile (shown by green colour line in figure - 2) in the northern part of the study area. Figure - 3 shows the graph of elevation -vs- ground stations fixed along the profile, which indicate that elevation changes rapidly from point to point. The undulating topography depicted from figure - 3 indicates that fixing of ground stations (source / receiver points) at constant interval is a challenging task in such type of terrain even with the help of latest technology. However, it is the basic requirement of seismic data processing and interpretation for accurate subsurface mapping. At the time of designing of geophysical parameters the source /receiver interval was decided to be 50 m which is shown by the straight line in figure - 4. This planned source / receiver interval when plotted should be a straight line, of the length equal to the sum of all intervals, parallel to the axis of source / receivers which have been plotted in figure - 4 and is shown by black colour. The actual source / receiver interval against source / receiver stations are shown by red curve in figure - 4 which deviates a lot from the planned straight line. This deviation against source /receivers is plotted in figure – 5. When we analyze figure – 3 and figure – 4 together, it is very clear that at the places of drastic elevation changes we could not maintain fixed 50 m source / receiver interval between consecutive shot / receiver locations. It rather varied from 29 m to 52 m. The variation in source / receiver interval will create an error in positioning of common depth points generated at the time of seismic data processing.  Figure-2: Map Showing Survey Lines with elevation contours  Figure – 3: Elevation Along Seismic Profile Studied In the study area we have used latest state-of-the art technology for surveying to minimize the survey error. The supplied information is the line direction, source / receiver point interval and the elevation of each source / receiver point. As has been said earlier, in ideal situation the source / receiver interval is kept fixed and subsequent data processing and interpretation is based on this assumption. However, as we have observed from figure – 4 and figure – 5, the source / receiver interval deviates from the planned value. The seismic processing methodology for 2-D straight profile does not take variable source / receiver interval into consideration properly for subsurface mapping. Therefore, to find out the actual error in position of sources / receivers, the seismic profile was loaded by assuming constant source / receiver interval and also with measured source receiver interval. The error in the position of sources /receivers was calculated. Figure – 6 is the plot of the error against source / receiver points. This error is near to zero in plane area. As we proceed towards hilly terrain, where elevation changes drastically, the error increases continuously. Furthermore, analysis of error plot (figure – 6) vis – a – vis elevation plot (figure – 3) indicates that error is constant in the zone of plane areas (source /receiver points: 90-140; 260-290). Figure – 6 also indicates that the cumulative error in the positioning of last source /receiver points will be about 800 m for a seismic profile of 11.5 km if the data is processed using constant source / receiver point interval.  Figure – 4: Plot of the Planned and Actual Source / Receiver Intervals  Figure – 5: Plot of the Deviation in Horizontal distance from the desired value  Figure – 6: Error in Positioning between planned locations and actual and measured locations Conclusion 2-D seismic survey in highly undulating terrain may result in erroneous subsurface mapping if accurate and realistic surface measurements are not considered for data processing and interpretation. The present study indicates a possible mis-positioning of the subsurface CDP points 800 m in 11.5 km long profile due to undulating surface terrain. Acknowledgement The authors are thankful to the management of OIL INDIA LIMITED for granting permission to present this paper. References:
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