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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
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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
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.
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.
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:
Tolerance limit and Error observed
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.
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.
There are two options available for elevation adjustment.
We deployed SET-B type of Total Station and SDR-31 data logger unit of Sokkia make. Due to logistics of the area the total observation points were as large as 2673. The available SDR-31 does not have enough memory to store such large volume data at a time. The data was therefore transferred in a PC. However, there is a limitation with available software with SDR-31 that it makes error adjustment for the point on traverse and not to the point set out from the corresponding traverse point. As per our requirement, the error distribution should be done on the set out point instead of the traversed point, as discussed in Figure-2. In the presence of obstacles set out points are on the arm of the traverse points, therefore, the error adjustment is done on the matched points (when traverse is along the profile) and not on all set out points. Therefore, computation was made for the error adjustment on the remaining points. The profile before error adjustment (solid line) and after error adjustment (dashed line) are shown in Figure-4.
Figure - 2 Traversing cum Line setting out
Figure - 3 Relative position of control point, bench
Figure - 4. Error adjustment of the profile
Conclusion
The accurate error adjustment within the limits of accuracy of the measurements in a seismic survey work for hydrocarbon exploration is very important for accurate positioning of subsurface features. The method explained in this paper is an economical, accurate and quick technique for surveying computations specially in logistically difficult areas where straight profiles without obstacles are not possible to be laid out on ground and resurveying is costly and difficult proposition.
Acknowledgement
The authors are thankful to the management of OIL INDIA LIMITED for granting permission to present this paper.
References:
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.
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.
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:
- Normal Traversing
- Traversing cum line setting out
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.
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.
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|
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.
There are two options available for elevation adjustment.
- Weighted - Any misclosure in the elevation is distributed in proportion to the length of the traverse line leading to the point (Like the compass rule discussed above).
- Linear - Any misclosure in the elevation is distributed evenly in each log of the traverse route. We used the weighted method for adjusting the error in elevation.
We deployed SET-B type of Total Station and SDR-31 data logger unit of Sokkia make. Due to logistics of the area the total observation points were as large as 2673. The available SDR-31 does not have enough memory to store such large volume data at a time. The data was therefore transferred in a PC. However, there is a limitation with available software with SDR-31 that it makes error adjustment for the point on traverse and not to the point set out from the corresponding traverse point. As per our requirement, the error distribution should be done on the set out point instead of the traversed point, as discussed in Figure-2. In the presence of obstacles set out points are on the arm of the traverse points, therefore, the error adjustment is done on the matched points (when traverse is along the profile) and not on all set out points. Therefore, computation was made for the error adjustment on the remaining points. The profile before error adjustment (solid line) and after error adjustment (dashed line) are shown in Figure-4.
Figure - 2 Traversing cum Line setting out
Figure - 3 Relative position of control point, bench
Figure - 4. Error adjustment of the profile
Conclusion
The accurate error adjustment within the limits of accuracy of the measurements in a seismic survey work for hydrocarbon exploration is very important for accurate positioning of subsurface features. The method explained in this paper is an economical, accurate and quick technique for surveying computations specially in logistically difficult areas where straight profiles without obstacles are not possible to be laid out on ground and resurveying is costly and difficult proposition.
Acknowledgement
The authors are thankful to the management of OIL INDIA LIMITED for granting permission to present this paper.
References:
- Spradley L.H. 1985, " Surveying and navigation for Geophysical Exploration". D Reidel Publishing Company.
- SOKKIA SDR-31 Operational manual