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.
Email id : syeshpal@yahoo.com
Abstract
Seismic survey is one of the most widely used geophysical methods for hydrocarbon exploration. This method is based on measurement of earth response (reflected energy) to induced sound energy from sources located near surface. Depending upon the geometry of surface observation points (receiver locations) and source locations, the survey is called two dimensional (2-D) or three dimensional (3-D) seismic survey. In 2-D seismic survey it is ideally expected that the source and receiver locations are located at fixed interval in a straight line. However, the accuracy of spacing of such surface observation points depends largely on availability of control points, accessibility along projected line and surface topography. In practice, while surveying for a 2-D straight line the geophone and shot point locations are placed at regular intervals along the surface irrespective of the surface geometry. This results in variable shot / receiver interval in the undulating terrain.
While processing the data acquired in highly undulating terrain, special care has to be taken to accurately image the subsurface. In this paper an attempt is made to identify the effect of surface undulations on seismic profile layout. In such cases it is impossible to keep the source / receiver interval constant between adjacent sources / receivers due to variable slope and lack of line of sight between sources / receivers which in turn violate the basic assumption of seismic data processing of a straight line geometry for subsurface imaging. In a case study pertaining to data acquired in Himalayan foothills area of OIL’s operations, the cumulative error estimated in actual and planned source / receiver interval was about 800 m in a 11.5 km long profile. This error if not accounted properly in data processing, may result in erroneous subsurface image.