Survey Planning & Illumination with NORSAR-3D

Introduction Overview This short presentation gives some applications of NORSAR-3D ray tracing. Survey aperture Fold and Amplitude, Planned v Modelled Survey offset and azimuth Migration aperture Effect of Overburden S.R.M.E. Aperture This presentation is intended for seismic acquisition geophysicists who may not have considered the application of ray tracing to survey design questions.

Simple Slope Model Introduction A simple model was used to demonstrate the technique. The model contains a variable velocity field overlying a reflector with a cosine dip region in the center. A simple model was used to demonstrate the technique. The model contains a variable velocity field overlying a reflector with a cosine dip region in the center.

Aperture Study, All Azimuth Survey Introduction Aperture Study, All Azimuth Survey The survey used here is a “Wide Azimuth” survey. In the illustration, the closely spaced shot lines are shown, together with the receiver boundary square for the shot location marked with the cross. The receiver pattern moves as the shot location moves. Shot Pattern The survey used here is a “Wide Azimuth” survey. In the illustration, the closely spaced shot lines are shown, together with the receiver boundary square for the shot location marked with the cross. The receiver pattern moves as the shot location moves. The parameters were: 150m x 150m shot point spacing 150m x 150m receiver spacing, 12km x 12km shot centered box. Receiver boundary for indicated shot

Aperture Study: Area of Interest Introduction Aperture Study: Area of Interest Having ray traced the survey, the Hit Count attribute is plotted in the reflection point domain. This shows where the reflection points are and the density. The display shows the attribute after masking it to a 4km x 2km area of interest. Having ray traced the survey, the Hit Count attribute is plotted in the reflection point domain. This shows where the reflection points are and the density. The display shows the attribute after masking it to a 4km x 2km area of interest.

Aperture Study: Shots Contributing Introduction Aperture Study: Shots Contributing Down dip extension An alternative domain for display is the shot domain. This display shows the shots that contributed only to the area of interest on the target. Clearly, the survey needs to be extended in the down-dip direction. An alternative domain for display is the shot domain. This shows where the shot points are that generated the reflections, and the number of reflections. In this display, the shots are displayed after the reflection points have been masked, so that it shows the shots that contributed only to the area of interest on the target. Clearly, the survey needs to be extended in the down-dip direction. In more complicated models, the required survey boundary can be less obvious.

Introduction Overview This short presentation gives some applications of NORSAR-3D ray tracing. Survey aperture Fold and Amplitude, Planned v Modelled Survey offset and azimuth Migration aperture Effect of Overburden S.R.M.E. Aperture This presentation is intended for seismic acquisition geophysicists who may not have considered the application of ray tracing to survey design questions.

Reflection Points, 0 deg Dip Introduction Reflection Points, 0 deg Dip Shots This diagram relates the reflection point positions to the streamer locations assuming a flat, horizontally layered Earth. The reflection points and shot locations for two shots on adjacent shot lines are shown, but only the streamers for the left shot are drawn for clarity. The reflection point positions for the outer streamers are identical to the CMP positions. Outer Reflection points This diagram relates the reflection point positions to the streamer locations assuming a flat, horizontally layered Earth. The reflection points and shot locations for two shots on adjacent shot lines are shown, but only the streamers for the left shot are drawn for clarity. The reflection point positions for the outer streamers are identical to the CMP positions. Streamers associated with left shot.

Reflection points, 10 deg dip south Introduction Reflection points, 10 deg dip south This diagram is equivalent to the previous diagram except that a 10° dip to the south has been included in the reflector. This asymmetry skews the reflection points away from the CMP points as depicted. This diagram is equivalent to the previous diagram except that a 10° dip to the south has been included in the reflector. This asymmetry skews the reflection points away from the CMP points as depicted.

Hit Map, E to W Direction Introduction Hit map for E-W The central part of the map, the attribute is striped. This is caused by the dip of the reflector. The simple model with the cosine dipping reflector, shown in the insert, was used again for this example. A conventional multi-streamer 3D survey in the east-west direction was used. The sailing direction was east to west for all sail lines. The reflection point count is mapped in the reflection point domain. At the eastern and western extremities of the map, corresponding to the horizontal parts of the reflector, the reflection count is constant. This agrees with the constant CMP fold used to design the survey. However, in the central part of the map, the attribute is striped. This is caused by the dip of the reflector.

Hit Map, Alternating Directions Introduction Hit Map, Alternating Directions This map was made using the same model and survey parameters as the previous map except that the sail line directions alternate. This alters the hit count pattern This map was made using the same model and survey parameters as the previous map except that the sail line directions alternate. This alters the hit count pattern

SMA 20Hz, E to W Directions Introduction Would this appear in migrated seismic data? Plot the migration amplitude using SMA The stripes persist. One may say that stripes in the hit maps would not appear in migrated seismic data because of the smoothing that is implicit. However, in this map the Simulated Migration Amplitude attribute is plotted for the east to west survey direction. The stripes persist.

SMA 20Hz, Alternating Directions Introduction SMA 20Hz, Alternating Directions This shows the Simulated Migration Amplitude llumination for the survey shot in alternating directions. This shows the Simulated Migration Amplitude illumination for the survey shot in alternating directions.

Introduction Overview This short presentation gives some applications of NORSAR-3D ray tracing. Survey aperture Fold and Amplitude, Planned v Modelled Survey offset and azimuth Migration aperture Effect of Overburden S.R.M.E. Aperture This presentation is intended for seismic acquisition geophysicists who may not have considered the application of ray tracing to survey design questions.

Illumination in SEG Salt Model Introduction Illumination in SEG Salt Model The SEG/EAGE salt model was used to illustrate this topic. The salt model consists of a salt body in a sedimentary velocity field. A plane, horizontal target has been added. The SEG/EAGE salt model was used to illustrate this topic. The salt model consists of a salt body in a sedimentary velocity field. A plane, horizontal target has been added.

Hit Map, E-W Survey, 6km Streamer Introduction Hit Map, E-W Survey, 6km Streamer This is the hit count attribute plotted for an east-west survey using a 6km streamer array. Away from the salt, the reflection count is continuous. Underneath the salt, the reflection count varies because of the focusing effects of the salt. Under the southwest corner of the salt there is an illumination hole. This is the hit count attribute plotted in the reflection domain for an east-west survey using a 6km streamer array. Away from the salt, the reflection count is continuous. Underneath the salt, the reflection count varies because of the focusing effects of the salt. Under the southwest corner of the salt there is an illumination hole.

Hit Map, N-S Survey, 6km Streamer Introduction Hit Map, N-S Survey, 6km Streamer This is the hit count attribute plotted for a north-south survey using a 6km streamer array. Under the southwest corner of the salt, the illumination hole persists but the details are different. If one had a particular prospect location in mind, a choice might be made between the east-west and north-south surveys. This is the hit count attribute plotted in the reflection domain for an north-south survey using a 6km streamer array. Under the southwest corner of the salt, the illumination hole persists. The salt focusing patterns further north and east similar in character to the east-west survey, but the details are different. If one had a particular prospect location in mind, a choice might be made between the east-west and north-south surveys.

Hit Map, N-S Survey, 10km Streamer Introduction Hit Map, N-S Survey, 10km Streamer This is the hit count attribute plotted for a north-south survey using a 10km streamer array. There is very little difference between this map and the one made with the 6km streamer array. The 10km streamer is probably not worthwhile. This is the hit count attribute plotted in the reflection domain for an north-south survey using a 10km streamer array. There is very little difference between this map and the one made with the 6km streamer array. Under the southwest corner of the salt the illumination hole persists. The 10km streamer is probably not worthwhile.

Maximum Offset, 10km Streamer Introduction Maximum Offset, 10km Streamer For each image point, this attribute shows the maximum offset that contributed to that image point. It confirms that for most of the target, 7km streamers are enough. Only in a few places below the salt would a longer streamer contribute to the image. This map is similar to the previous maps in that it was made with an east-west survey using a 10km streamer, but in this case, the maximum offset attribute is plotted. For each image point, this attribute shows the maximum offset that contributed to that image point. It confirms that for most of the target, 7km streamers are enough. Only in a few places below the salt would a longer streamer contribute to the image.

Shot point placed in the shadow zone Introduction Shooting from target In the previous SEG/EAGE salt model examples, non of the acquisition geometries filled the illumination hole. An efficient alternative analysis for these local trouble spots is the flower plot display. A shot is placed in the illumination hole at the target. One-way rays are propagated from the shot up to an array of receivers on the surface. Dense set of receivers. In the previous SEG/EAGE salt model examples, non of the acquisition geometries filled the illumination hole. An efficient alternative analysis for these local trouble spots is the flower plot display. A shot is placed in the illumination hole at the target. One-way rays are propagated from the shot up to an array of receivers on the surface. Shot point placed in the shadow zone

Shooting from target: Flower Plot Introduction Shooting from target: Flower Plot Rays with equal departure inclination and opposite azimuth are paired and plotted The figure shows that the east-west and north-south azimuths would not produce any ray pairs. However, the northwest-southeast direction produces ray pairs at long offsets. Azimuth Offset Best acquisition direction The rays are then searched to find pairs with equal incident angle and opposite azimuth about the target local normal vector. These pairs are consistent with the reflection condition, and are equivalent to a ray propagating from a shot on the surface down to the target, reflecting from the target and propagating back up to the surface. The pairs of rays are also binned according to the offset that they represent, and the azimuth. The counts are then plotted according to offset and azimuth. The figure shows that the east-west and north-south azimuths would not produce any ray pairs. However, the northwest-southeast direction produces ray pairs at long offsets.

Hit map for the new survey Introduction Hit map for the new survey Repeating the streamer survey using a northwest southeast sailing direction does indeed fill the illumination hole. Repeating the streamer survey using a northwest southeast sailing direction does indeed fill the illumination hole. N125 Complete illumination

Introduction Overview This short presentation gives some applications of NORSAR-3D ray tracing. Survey aperture Fold and Amplitude, Planned v Modelled Survey offset and azimuth Migration aperture Effect of Overburden S.R.M.E. Aperture This presentation is intended for seismic acquisition geophysicists who may not have considered the application of ray tracing to survey design questions.

Max. CMP-CRP, 10km Streamer Introduction Max. CMP-CRP, 10km Streamer Again using the SEG/EAGE salt model, the CMP-CRP distance attribute is plotted in the reflection point domain. Away from the salt the CMP-CRP distance is effectively zero, indicating that small apertures are required for imaging reflections. However, significant apertures are needed under the salt. This is answered by plotting the horizontal Common Mid-Point to Common Reflection Point distance. Again using the SEG/EAGE salt model, the CMP-CRP distance attribute is plotted in the reflection point domain. Away from the salt the CMP-CRP distance is effectively zero, indicating that small apertures are required for imaging reflections. However, significant apertures are needed under the salt.

Introduction Overview This short presentation gives some applications of NORSAR-3D ray tracing. Survey aperture Fold and Amplitude, Planned v Modelled Survey offset and azimuth Migration aperture Effect of Overburden S.R.M.E. Aperture This presentation is intended for seismic acquisition geophysicists who may not have considered the application of ray tracing to survey design questions.

Total Illumination Amplitude Introduction Total Illumination Amplitude The illumination map here was made using the SEG/EAGE salt model and a streamer survey. All rays that reflected from the target were used, no matter what part of the model they had traveled through. The illumination map here was made using the SEG/EAGE salt model and a streamer survey. All rays that reflected from the target were used, no matter what part of the model they had traveled through.

Illumination Amplitude, No Salt Introduction Illumination Amplitude, No Salt This illumination map was made in the same way as the previous one, except that any ray that had passed through the salt was rejected. It shows the image that requires only sedimentary ray paths, and can be called the “high confidence” image, because it is not subject to uncertainties in the salt model. This illumination map was made in the same way as the previous one, except that any ray that had passed through the salt was rejected. It shows the image that requires only sedimentary ray paths, and can be called the “high confidence” image, because it is not subject to uncertainties in the salt model.

Introduction Overview This short presentation gives some applications of NORSAR-3D ray tracing. Survey aperture Fold and Amplitude, Planned v Modelled Survey offset and azimuth Migration aperture Effect of Overburden S.R.M.E. Aperture This presentation is intended for seismic acquisition geophysicists who may not have considered the application of ray tracing to survey design questions.

Surface Bounce Points Bounce Bounce Introduction Surface Bounce Points Bounce Bounce The SRME method estimates the multiple by using the surface bounce. The 2D SRME method can be used when the surface bounce occurs within the streamer array. Ray tracing can be used to see if this is a valid assumption. Surface Bounce Points Streamer Array The SRME method estimates the multiple by using the surface bounce. The 2D SRME method can be used when the surface bounce occurs within the streamer array. Ray tracing can be used to see if this is a valid assumption.

Salt Top Multiple Introduction If the surfaces generating the multiple are approximately horizontal, then the bounce points will occur within the streamer array. This extreme example illustrates ray paths for the top salt multiple from a single shot. If the surfaces generating the multiple are approximately horizontal, then the bounce points will occur within the streamer array. This extreme example illustrates ray paths for the top salt multiple from a single shot.

Salt Multiple, Surface Bounce Points Introduction Salt Multiple, Surface Bounce Points The surface bounce points for the top salt multiple from the single shot are depicted in red. Clearly the bounce points are not contained within the streamer array, indicating that a 3D SRME technique would be necessary. The surface bounce points for the top salt multiple from the single shot are depicted in red. Clearly the bounce points are not contained within the streamer array, indicating that a 3D SRME technique would be necessary.

NORSAR-3D Work Flow Introduction Surfaces Properties The next few slides are an overview of the NORSAR-3D workflow. The first step is the import of horizon surfaces and property fields for building the depth model.

Model Components Generated internally Imported surfaces Introduction Model Components Generated internally Imported surfaces Depth/time grids GoCAD trimeshes GoCAD model Imported properties SEGY property field These can be generated internally as a simple set of analytic functions, or more commonly, imported.

NORSAR-3D Work Flow Introduction Surfaces Survey Properties Build Model Depth Model The surfaces and property fields can be used to build a depth model. A seismic survey must then be defined for ray tracing.

Surveys Simple Surveys Internally Complex surveys import Introduction Surveys Simple Surveys Internally Marine streamer Ocean Bottom Sensor Complex surveys import P1/90 SPS ASCII (for VSP) Simple regular surveys, particularly marine surveys can be generated internally. Complex or irregular surveys must be created externally and then imported in P1/90 format, or SPS format, or a general ASCII table. The latter is particularly suited for VSP survey definition.

NORSAR-3D Work Flow Introduction Surfaces Survey Properties Ray Code Ray Trace Build Model Ray Code Depth Model A ray code is a description of how the rays will propagate in the model. For example, the ray needs to propagate through the horizons until it reaches the target horizon, where it reflects and propagates back through the horizons until it reaches the surface.

3D Wavefront Construction Introduction 3D Wavefront Construction Direct wavefront Shot point and streamer array. Salt NORSAR-3D uses the Wavefront Construction technique for ray tracing. A number of rays are propagated radially time step by time step from each shot point. The wavefront is represented by the triangulated surface in the slide. An adaptive algorithm keeps the ray density high enough to ensure an accurate result. Reflected wavefront returning to surface.

3D Wavefront Construction Introduction 3D Wavefront Construction This shows the result of the ray traced shot represented as the more conventional ray diagram. This shows the result of the ray traced shot represented as the more conventional ray diagram.

NORSAR-3D Work Flow Introduction Surfaces Properties Survey Map Illumination Map Ray Trace Build Model Ray Code Make Seismogram Seismogram Depth Model The output from the ray tracer is a file containing a long list of captured events with their associated attributes. NORSAR-3D contains tools for analyzing these results. Illumination Mapper is for generating and plotting illumination maps. The Seismogram Generator is for making synthetic seismograms in SEGY format. The Event Extractor is for making tables of the event attributes in ASCII format for easy access by other software. The Programmer’s Toolkit is for the development of custom application for the fast extraction of event attributes into the user’s own format. Events Export Attributes ASCII Table Toolkit Processing User Format

Summary A variety of import formats for NORSAR-3D models and surveys. Introduction Summary A variety of import formats for NORSAR-3D models and surveys. Wavefront Construction is fast and robust Variety of survey analysis tools in NORSAR-3D

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