Case Study
A case study from OIL's operational area in Upper Assam basin is presented for traversing cum line setting out. OIL’s operational are is divided into two parts by the mighty river Brahmaputra. The area lying in southern side of the river is called South bank and that on the northern side is called North bank. The south bank area is well explored and has major producing fields of OIL. North bank area due to their difficult topography and logistics, is less explored. Due to lack of substantial data over and around the area covering river Brahmaputra, the geological horizons in the form of formation boundaries of the two areas were not well correlated. Therefore, to provide such a correlation framework between South bank and North bank, in 1998-99 field season OIL planned to acquire seismic data across the river Brahmaputra. Accordingly, a seismic profile, about 34 km long, connecting a producing well in South bank and a proposed location in North bank of river Brahmaputra was planned and recorded. Along this profile the river bed is about 18-20 km. Within this length the mighty Brahmaputra flows in the form of many tributaries. The river channels change their course very frequently. Therefore, it was not possible to fix the reference points within the river bed zone and no bench mark was available in the area. In the near vicinity of the profile, there were only two GPS points fixed by OIL, one in South bank and other in North bank. The areal distance between two points was 28 km. The survey work with other constraints commenced with GPS point in South bank as starting reference point. The planned profile crosses various river channels, villages, jungles and other obstacles. Prior to line setting out, no loop was closed by traversing because with little more effort traversing cum line setting out was possible. There were two options available to sort out this problem:
- If the error is more than the tolerance limit, resurvey may be done.
- Readjustment of the traverse error. However, readjustment of the large survey error may result in shifting of the profile little bit from the planned position.
As resurveying would be a time consuming and costly effort with a chance of error again, it was decided to adopt the second option which is economical and feasible option under the circumstances. It was kept in mind that the length of profile is about 34 km and there is no reference point along the profile for cross checking, so mis-tie was expected when the traverse was closed on the North bank reference point. It took about 55 days of field work to complete the survey on this profile. When the traverse was closed on the North bank reference point, the error in horizontal position was found to be in the range of ±6.89 m/km towards east and the error in elevation was ±0.676 m/km. In our case both source and receiver group interval were 40m and CDP interval was 20m. The tolerance limit and error observed are given in table below.
Tolerance limit and Error observed
| Position | Tolerance limit (±) | Error per point |
| Horizontal | 5.0 m | 0.276 m |
| Elevation | 10.0 cm | 2.70 cm |
It is clear from the table that the errors are small in comparison with tolerance limits.
Traverse Adjustment
On re-inspection of survey data no erroneous measurement was identified. Therefore, it was decided to distribute the error on all the points on the traverse as well as set out points. The desired traverse adjustment procedures are as follows.
Coordinate Adjustment
There are two coordinate adjustment procedures.
- Compass rule – The compass rule distributes the coordinate error in proportion to the length of the traverse line. The formula is
Northing adjustment = (L*Closure North)/TL
Easting adjustment = (L*Closure East)/TL
Where L – length of traverse segment to the point & TL-sum of the traverse segment length
- Transit rule - the transit rule distributes the coordinate error in proportion to the northing and easting of each traverse line. The formula is
Northing adjustment = (DN * Closure North) / å | DN|
Easting adjustment = (DE * Closure East) / å| DE|
Where
DN – change in northing for the traverse segment,
DE- change in easting for the traverse segment,
å | DN| - sum of the absolute value of the changes in northing of all the traverse segments,
å | DE| - sum of the absolute value of the changes in northing of all the traverse segments.
We have used the compass rule for adjusting the coordinate error.
Angular Adjustment
There are two options available for angular adjustments.
-
Weighted - Any angular misclosure is distributed among the angles of the traverse route based on the sum of the inverse of the forward and back traverse line lengths at each angle. The back sight and forsight lines are considered to have infinite lengths for the purposes of weighting computation.
Ð adjustment =((1/to dist) + (1/from dist)) / (å{(1/to dist) + (1/from dist)}) * Ð closure
- Linear – Any angular misclosure is distributed evenly among the angles of the traverse route.
We used weighted method for adjusting the angular error.