BASIC CONCEPT
There are some new aspects to the survey design in three dimensions relative to 2-D surveys. The 2-D surveys are as linear as the terrain allows. Source and receiver are normally in-line with each other. Arrays may be multidimensional but most often are also in the line of survey. For 3-D surveys this is seldom the case.
The 3-D survey also includes multiple source lines as well as multiple receiver lines, and it is possible to record two source lines. The arrays also may respond in less predictable manner as they are not necessarily in line with either the source or receiver locations. If the survey is planned to acquire a good directional range of offsets, the arrays will see oncoming wavefront from a number of angles. This will require a more sophisticated analysis of the array effect. Use of the patterns and other multi-azimuth array patterns is sometimes practiced.
The accent of 2-D lines is on the fold coverage and offset range. For 3-D survey, the fold may be less but the azimuth range is added to the offset range as a parameter. If structure is complex, the good azimuthal range become important. In fig-1. There are some arbitrary number of seismic traces in a bin. For velocity analysis the bin needs to contain a range of offsets. The range of azimuth in a bin is also a consideration. The concept of azimuths also is a new factor in 3-D survey.
The azimuthal property is not significant when the geology features only gentle dips and lateral consistency. The effect of dip is to increase apparent velocity. Thus the velocity analysis must have an azimuthal property. The imaging of complex structure is also improved by surveys with a good range of azimuths.
There are some new aspects to the survey design in three dimensions relative to 2-D surveys. The 2-D surveys are as linear as the terrain allows. Source and receiver are normally in-line with each other. Arrays may be multidimensional but most often are also in the line of survey. For 3-D surveys this is seldom the case.
The 3-D survey also includes multiple source lines as well as multiple receiver lines, and it is possible to record two source lines. The arrays also may respond in less predictable manner as they are not necessarily in line with either the source or receiver locations. If the survey is planned to acquire a good directional range of offsets, the arrays will see oncoming wavefront from a number of angles. This will require a more sophisticated analysis of the array effect. Use of the patterns and other multi-azimuth array patterns is sometimes practiced.
The accent of 2-D lines is on the fold coverage and offset range. For 3-D survey, the fold may be less but the azimuth range is added to the offset range as a parameter. If structure is complex, the good azimuthal range become important. In fig-1. There are some arbitrary number of seismic traces in a bin. For velocity analysis the bin needs to contain a range of offsets. The range of azimuth in a bin is also a consideration. The concept of azimuths also is a new factor in 3-D survey.
The azimuthal property is not significant when the geology features only gentle dips and lateral consistency. The effect of dip is to increase apparent velocity. Thus the velocity analysis must have an azimuthal property. The imaging of complex structure is also improved by surveys with a good range of azimuths.
The Fresnel zone takes on some new characteristics in 3-D. Generally, even in 2-D, this important concept is given a small amount of attention. As the target sizes historically decrease in the size the zone become more important.
Essentially, the theoretical point source expands as it propagates in depth, “illuminating” a circular area at vertical incidence. In the seismic context this is the reflecting surface constructively contributing to the reflection. A good approximation to the radius of the zone is :
R = (Z/F)1/2
which show that the zone increases in radius with depth but decrease with higher frequency wavefronts. Migration serves to reduce the zone to some minimal size when accurately done and the data fits the assumption.
Fig-2. Fresnel
Zone
PRELIMINARY BASIC PARAMETERS
There are some parameters that need to be estimated as an input to designing 3D survey. The physics and concepts are somewhat independent of whether the survey is to have two or three dimensions or just involve some modification to their calculation.
1. Offset
The imaging of shallow, target, and deep horizons still requires certain offsets of source and receiver. The calculation and direction may be different but the rule is :
Offset = depth of target horizon
2. Fold
The fold required for noise compression is a function of the local S/N conditions. This translates in 3D to the number of trace in bin. Because of the extra focusing by migration and the flexibility of binning, fold can be less than required 2-D survey.
3. Frequency
The temporal frequency required Is not much different from that for 2-D survey. The rule for the resolution of layer of given thicknesses are best determined by modeling. The general rule is that the resolution of thin bed requires it to be sampled twice with a quarter wavelength of the highest frequency.
4. Migration
At this point it is relevant to remember that when dipping beds are in the preliminary model of the survey the extent of the survey must be increased. The tangent of the angle of maximum dip modifies the areal extent of the survey.
A 3D DESIGN SEQUENCES
There are so many ways to begin and complete a survey design. The specific sequence of steps that follow are general guidelines. The information gathering, modeling, and dimensionally independent parameters, such as offset range, are presumed to have been computed as previously describe. We must determine the offset ranges, temporal frequency, required fold, bin size, and available field equipment capacity. Less direct variables such as survey size adjustment for migration and any azimuthal requirements are also presumed to be ready. Another assumption is that the 3D software can cope with the template you have in mind. A summary of the proposed sequence for developing in design script is :
Step-1
Determine the subsurface bin size. Twice the chosen bin size is source and receiver station spacing.
Step-2
Compute the number of source stations per square kilometer required to achieve fold with the available equipment. The number of stations per square kilometer allow computation of source line spacing.
Step-3
Compute a receiver line spacing.
Step-4
Find the number of receiver lines allow by field equipment constrained by the required offset ranges. The result is the template.
Step-5
Decide on the x and y roll along.
Step-6
Allow for obstacles and run analyses of the script for offset distribution in the bins, ranges of offset in the bins, and azimuthal properties of the bin.
Step-7
Estimate time and costs of the script and iterate until attributes, costs and time are satisfied. Write the shooting script. Prepare to make more adjustments when the survey is begun. Often conditions found in the field requires changes in the shooting script.
PRELIMINARY BASIC PARAMETERS
There are some parameters that need to be estimated as an input to designing 3D survey. The physics and concepts are somewhat independent of whether the survey is to have two or three dimensions or just involve some modification to their calculation.
1. Offset
The imaging of shallow, target, and deep horizons still requires certain offsets of source and receiver. The calculation and direction may be different but the rule is :
Offset = depth of target horizon
2. Fold
The fold required for noise compression is a function of the local S/N conditions. This translates in 3D to the number of trace in bin. Because of the extra focusing by migration and the flexibility of binning, fold can be less than required 2-D survey.
3. Frequency
The temporal frequency required Is not much different from that for 2-D survey. The rule for the resolution of layer of given thicknesses are best determined by modeling. The general rule is that the resolution of thin bed requires it to be sampled twice with a quarter wavelength of the highest frequency.
4. Migration
At this point it is relevant to remember that when dipping beds are in the preliminary model of the survey the extent of the survey must be increased. The tangent of the angle of maximum dip modifies the areal extent of the survey.
A 3D DESIGN SEQUENCES
There are so many ways to begin and complete a survey design. The specific sequence of steps that follow are general guidelines. The information gathering, modeling, and dimensionally independent parameters, such as offset range, are presumed to have been computed as previously describe. We must determine the offset ranges, temporal frequency, required fold, bin size, and available field equipment capacity. Less direct variables such as survey size adjustment for migration and any azimuthal requirements are also presumed to be ready. Another assumption is that the 3D software can cope with the template you have in mind. A summary of the proposed sequence for developing in design script is :
Step-1
Determine the subsurface bin size. Twice the chosen bin size is source and receiver station spacing.
Step-2
Compute the number of source stations per square kilometer required to achieve fold with the available equipment. The number of stations per square kilometer allow computation of source line spacing.
Step-3
Compute a receiver line spacing.
Step-4
Find the number of receiver lines allow by field equipment constrained by the required offset ranges. The result is the template.
Step-5
Decide on the x and y roll along.
Step-6
Allow for obstacles and run analyses of the script for offset distribution in the bins, ranges of offset in the bins, and azimuthal properties of the bin.
Step-7
Estimate time and costs of the script and iterate until attributes, costs and time are satisfied. Write the shooting script. Prepare to make more adjustments when the survey is begun. Often conditions found in the field requires changes in the shooting script.
Fig-3. A Schematic of
3D Design
Below is the example of final template 3-D design for orthogonal and brick patterns. The very detail recommendation including the cost estimation are shown below.
Below is the example of final template 3-D design for orthogonal and brick patterns. The very detail recommendation including the cost estimation are shown below.
Fig-4. Table shows the
final recommendation of 3D parameters
FINAL DELIVERY
At the end of the 3-D design, we will provide our client with a complete report and CD, this final delivery will include :
1. Final Report
This document reports the recommendation of some acquisition parameters with a flexibility for client to choose some options of template. Some consideration and data analysis are also included in this report.
2. SPS Files
Beside the final report, we also provide our client a CD containing : RPS (Receiver), SPS (Shot Point) and XPS (Recording geometry) such that it will make easier for client to "replay" in Mesa software or directly uploaded to surveying software and recording instrument. If the RPS and SPS are included with real co-ordinate, we can create 3-D seismic lines overlay to topographic map.
We are very grateful to assist you acquiring the best 3-D seismic data acquisition with high precision, high resolution in acceptable cost.
FINAL DELIVERY
At the end of the 3-D design, we will provide our client with a complete report and CD, this final delivery will include :
1. Final Report
This document reports the recommendation of some acquisition parameters with a flexibility for client to choose some options of template. Some consideration and data analysis are also included in this report.
2. SPS Files
Beside the final report, we also provide our client a CD containing : RPS (Receiver), SPS (Shot Point) and XPS (Recording geometry) such that it will make easier for client to "replay" in Mesa software or directly uploaded to surveying software and recording instrument. If the RPS and SPS are included with real co-ordinate, we can create 3-D seismic lines overlay to topographic map.
We are very grateful to assist you acquiring the best 3-D seismic data acquisition with high precision, high resolution in acceptable cost.
