Error Propagation in Positioning of Seismic Profiles in Undulating Terrain – A Case Study From Himalayan Foothills
Y. P. Singh & G. R. Saini
Geophysics Department, Oil Inida Limited, Duliajan, Assam- INDIA, 786 602.
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There are various geophysical methods for mapping subsurface resources of the earth. In all these methods the physical properties measured on the earth surface are interpreted to locate exact position of various geologic features within subsurface which have accumulation of resources like hydrocarbons and other minerals. These surface measurements are processed with respect to the gridded topographic map of the area and then downward continued to the target depths to generate new map, depicting features with commercial viability for exploration of the resources. Each geophysical method results in variety of maps useful for exploration of different mineral resources. For Hydrocarbon exploration, seismic method is the most extensively used technique and helps in generation of various maps for identification and estimation of hydrocarbon reserves.
Through the spatial information contained in maps, cartography plays a key role in the collection, annotation and interpretation of all types of exploration data. In exploration activities conducted globally over all types of terrain, whether it is on land or at sea or over transition zones, for assigning the surface position to the subsurface features of interest, the survey measurements on ground and their subsequent processing is very important and needs high degree of accuracy. Hydrocarbon exploration, in particular, requires a very accurate positioning of the subsurface locations (Spradley, 1985). This makes the navigation or survey measurements stringent and the maps prepared based on these stringent measurements are only used for further analysis and positioning of the subsurface reservoir locations on ground. Now a days with the availability of DGPS system and other equipments like Total Stations, and data loggers (SOKKIA Total Station /SDR-33 operational manual), the surveying has become very fast and accurate.
Planning of Survey Work for Seismic Exploration
In planning a survey, it is customary for a prospect area to be subdivided into a grid of seismic profiles based on the geologic, topographic and previous geophysical work, which provide the desired density of sampling to meet program objectives. With the help of the plan map and Differential Global Positioning System (DGPS), some reference points are fixed on ground. Seismic profiles are positioned with respect to the known reference points and prominent features on ground in the area. At the same time, these reference points are used for fixing more control points in the area to further maintain the accuracy of the survey. After fixing the reference points, there are various survey methods for profile location marking on the ground. Out of these methods, the traversing and line setting out are the two main methods, which are used throughout the survey work.
The requirement for seismic data processing for accurate subsurface mapping survey is that the profile direction, receiver point interval and shot point interval should be constant for a seismic profile. Even with available latest technology for surveying it is a tedious task in logistically difficult areas where line of sight between consecutive points is not clear and slope varies from point to point. The direction of the profile can be maintained within tolerance limit (Spradley, 1985) for seismic data processing. Normally, 2-D data for a straight line is processed by assuming constant source /receiver interval. However, this assumption can give various processing pitfalls and the resultant positioning of common depth points (CDPs) will be erroneous and may lead to misjudge the subsurface feature location. In this paper we have tried to quantify the error due to variable source / receiver point interval in an undulating terrain. We have selected one seismic profile from the operational area of Ganga Valley Project, Oil India Limited.