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Error Distribution in Surface Mapping for Seismic Survey Operations in Logistically Constrained Terrain -A Case Study from upper Assam Basin
Y.P. Singh, R.K. Pathak, G.R. Saini and K.K. Nath
Geophysics Department ,
Oil India Limited,
Duliajan-786602.
E-mail: ypsingh@oil.asm.nic.in
Abstract
Seismic method is the most widely used tool for hydrocarbon exploration. This method is based on measurement of earth response (reflected energy) from an induced sound source located near surface. Depending upon the geometry of surface observation points and source locations, the survey is called 2D or 3D seismic. In 2D seismic survey it is ideally expected that the source and receiver locations are located at fixed interval in a straight line. However, in practice the accuracy of spacing of such surface observation points depends largely on availability of control points and accessibility along projected line. Often in absence of adequate control points along the profile it is required to carry out surface survey from far off points and distribute the measurement errors between the points. Here we present a case study of such a case in Upper Assam basin.
In 1998-99 field season, Oil India Limited planned to shoot a seismic profile across the mighty river Brahmaputra. There were two GPS points near the planned profile, one on each bank of the river and the horizontal distance between these points was 18 km. No other control point was available in between to cross check the accuracy of the survey. The reverine terrain of mighty Brahmaputra prevented to go straight for fixing observation points at fixed interval on the profile. Therefore, we planned control point survey and line setting out survey simultaneously. Due to long distance between GPS points, as expected the error in horizontal positioning was 234 mtr and the error in elevation measurement was 19 mtr. The coordinate adjustment, angular adjustment and elevation adjustment methods for error distribution could distribute error on control points. However, the error had to be distributed along the setting out points to get accurate coordinates of source and receiver locations for seismic survey. Therefore, the available method was modified for error distribution on setting out points both for horizontal error and vertical error. The error distribution on setting out points both for horizontal and elevation measurements, has given the accuracy of the order of 1 cm.
The method presented in this paper is an economical, accurate and quick technique for surveying computations specially in logistically difficult terrains where laying out of straight profiles is not possible due to obstacles on ground.
Introduction
Its ability to convey the information in spatial dimensions through maps makes cartography to play a central role in the collection, annotation and interpretation of all types of exploration data. The exploration activities are 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 measurement locations. This makes the navigation or survey measurement requirements 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.
Geophysical Exploration
There are various type of geophysical methods used 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. The surface measurements are superimposed on the gridded topographic map of the area and then downward continued to the target depths with respect to the surface positions to generate new map depicting features having commercial viability for exploration of the resources. Each geophysical method results in variety of maps and the processing and interpretation of these maps differ from each other. Utility of these maps also differs depending upon the measured parameters relating with rock properties. In hydrocarbon exploration, seismic method is the most extensively used technique.
In seismic survey operations there are three stages i.e. data acquisition, processing and interpretation. In examining the seismic data, the survey events are not just locations but include a combination of the position of energy source(s), receiver groups and even the positions of individual receivers in the array making up each group. The processing and interpretation of each seismic record requires knowledge of the field geometry relating to the source and receiver positions as well as the measurements and methods that incorporate with the external system of reference defining the global reference for occurrence of the event vis - a - vis local reference. When a particular prospect is looked in the context of other prospects within the basin or anywhere in the world, the external system of reference is very important.
Position Specifications in Geophysical Survey
A geophysical survey whether conducted on land or at sea, is planned and specified within certain tolerance limits for the positioning effort. The tolerance limits accommodate the requirement of accuracy in the measurements for data processing as well as location positioning of the drilling locations delineated on the basis of interpretation of the geophysical data. The relative as well as absolute accuracy of survey parameters is defined separately for horizontal position and elevation measurements on the basis of the geophysical parameters of a particular area. Spradley (1985) has defined the desired accuracy for horizontal positions and elevations as follows.
Relative accuracy of horizontal position
- Receiver position - The standard error in down profile distance between any two seismic receivers in a record should not exceed ¼ of the common depth point (CDP) interval or bin size for stacking the data. If there are errors greater than ½ of the CDP interval, traces will not be placed in their proper stacking combination.
- Source position – A seismic source location should be determined with an accuracy of ¼ the CDP interval or bin interval relative to the position of the receiver groups.
However, within a seismic prospect, the accuracy of relative positions of events should be maintained within a tolerance of order of the CDP interval. This is a function of both the resolution at the target horizons and the dip angles at points of the ties and well data interpretation.
Relative accuracy of elevation
The relative elevation of receiver groups as well as source points within a seismic spread should be positioned with a standard error of less than velocity/ (72XFrequencymax). For a given recording interval, any elevation error in the prospect divided by the velocity of surface material should not create a two way travel time in excess of sample interval.
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