Error Distribution in Surface Mapping for Seismic Survey Operations in Logistically Constrained Terrain -A Case Study from upper Assam Basin
Planning of Survey Work
After defining the geophysical parameters, survey parameters and the tolerance limits, the plan map is prepared generally on toposheet with marking boundary of the proposed area and positions of the seismic profiles (Figure-1). 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.  Figure-1 Plan Map Showing Topographical features Traversing and line Setting out Methods In seismic industry, now days, the survey work is conducted with the help of the total stations (Electronic Distance Measurements Theodolite also called as EDM Theodolite) with attached data logger unit. In seismic survey work the traversing is of two types:
In normal traversing the control points are fixed randomly to cover the whole area at accessible locations which help in future for controlling the survey error and ties. Traversing cum line setting out For the ease of processing and interpretation, seismic data should be acquired along straight profiles. However, due to geographical barriers like prohibited areas (military zone), rivers, lakes, villages, railway tracks buildings etc. it is not always possible to set out the profile as straight as planned on map while going on the ground. The source point and receiver point interval is of 25-50 m range. The line setting out procedure is run along the profile as far as possible. However, when these points fall within the obstacles mentioned above or behind the obstacles that are not directly visible from the previous point, the traversing adjacent to the profile is carried out in the vicinity of the obstacles. The points within and near the obstacles are set out by these traverse points. This procedure is called as traversing cum line setting out and illustrated in Figure-2. A profile AB has to be set out. This profile has various obstacles between A and B. From A to A' we go along the profile. Due to obstacle 1 further movement along the profile is not possible, therefore, a deviation from point A' along the traverse A'A"A'" P is taken and the near by points of obstacles are set out from the points A", A'" etc. Then again we go along the straight profile from point P to P'. The same procedure is repeated for all the obstacles falling along the profile. Computing ties to Control In order to minimize the amount of effort in establishing ties between the traverse and control points, it is useful to make initial rough estimates to those coordinates not given at the beginning of the traverse. Consider a traverse originating at triangulation station at A (Figure-3), for which the surveyor has horizontal coordinates but no exact elevation. However, near the profile, but not on it, at C there is a benchmark having a known sea level elevation. The seismic profile ends at D. The other nearest control points are a triangulation station at E and a benchmark at F. For the computation purpose let us assign an arbitrary elevation, such as 1000 mtrs to the starting point A, then run the traverse computation completely upto its end at F. This one set of calculations provides three essential pieces of information required for further processing. The horizontal misclosure between triangulation stations indicates the accuracy level of the horizontal survey over the distance traversed, including the entire seismic profile. The vertical error at station C can be interpolated as the measure of correction to the original, arbitrary estimate of elevation. The relative elevation computed from the traverse can be compared to the relative elevations indicated for benchmarks C and F, giving a value for the vertical misclosure over that portion of the traverse. In practice, the existence and location of control points are much more complex than the example just described above. The distances of the control stations from the prospect may exceed the length of the profiles within the prospect. To circumvent this problem a close traverse should be run from the control point to the prospect and then back to the control point. The reverse of the problem of inadequate survey control is that in which the survey has an abundance of control points along profile or within the prospect. In such a case tie should be made from the traverse to each control point available. There are cases in which the survey methods may have a high order internal precision than the order of the control network, yet it requires a tie to the geodetic frame of reference represented by the control points. |