Python Project 1

Python Project 1

TU 12-Advanced seismic interpretation

ACUNA Ezequiel - PGS/RGE 2025 Mathilde Adelinet

Abstract

This presentation was developed as part of TU12: Advanced Seismic Interpretation for the PGS/RGE 2025 program. The project involves programming tools in Python to address the first part, which focuses on calculating the fold in land and marine seismic acquisition, as well as generating useful plots. Additionally, a mock seismic acquisition proposal for a potential client in the Wight Monocline area was included. The second part of the project consists of solving a seismic imaging exercise, covering concepts such as NMO, prestack, DMO, and poststack migration in a basic case of a dipping horizon. The study includes relevant plots along with explanations to support the analysis.

Content

Introduction: Seismic acquisition

Part 1: Fold Calculation for land seismic acquisition

Part 1: Fold Calculation for marine seismic acquisition

Part 1: Wight Monocline case

Part 2: Seismic Imaging excercise

Conclusions and considerations

About Seismic Acquisition...

Seismic acquisition requires the use of a seismic source at specified locations for a seismic survey, and the energy that travels within the subsurface as seismic waves generated by the source gets recorded at specified locations on the surface by what is known as receivers (geophones or hydrophones).A source, such as a vibrator unit, dynamite shot, or an air gun, generates acoustic or elastic vibrations that travel into the Earth, pass through strata with different seismic responses and filtering effects, and return to the surface to be recorded as seismic data. The acquisition is planned using a grid of source and receiver points to ensure uniform coverage and optimal subsurface imaging.

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Python Project: Part 1

Fold calculation for land and marine seismic acquisition

Part 1: Fold Calculation for land seismic acquisition

In this first exercise can be observed a simple seismic acquisition setting on land which shows the reflection ray-path considering one seismic shot-point. The Common Depth Point (CDP) spacing is half the receiver spacing, and the subsurface coverage per shot is about half the spread length. By advancing the spread by half its length per shot, each CDP is sampled once, producing a single-fold section.

Number of shot-points: 1 Number of geophones: 9 Geophones spacing: 10 m. Horizon depth: 1500 m.

Part 1: Fold Calculation for land seismic acquisition

In order to achieve enough fold-coverage for imaging, each reflecting point must be recorded more than once. This is why, in this other case can be observed that more shot-points were included (10). Fold reaches the number of 9 at a distance of 42.5 m from the source.

Number of shot-points: 10 Number of geophones: 9 Shot-points spacing: 10 m. Geophones spacing: 10 m. Horizon depth: 1500 m.

Part 1: Fold Calculation for marine seismic acquisition

In 2D marine seismic acquisition, data collection occurs along a line of receivers. In this case, we are considering a marine acquisition setup with 1 source and 1 streamer, and 80 receivers (RC), each spaced 12.5 meters apart. The spacing between the source and the first receiver is 60 meters, while the SP (source point) spacing is double the RC spacing, resulting in 25 meters. The formula for calculating the 2D fold is as follows:

Thus, the calculated fold is 20, meaning there are 20 unique sampling points or traces at the CDP located at 160 - 420 meters from the source, as shown on the figure.

Wight Monocline Case

Brief commercial proposal for Seismic acquisition

Part 1: Wight Monocline case

The Wight Monocline, is a geological feature formed during the Tertiary period, of significant interest for hydrocarbon exploration. It consists of alternating layers of chalk, marl, and sandstone. While structurally simple overall, small-scale faults and fractures in chalk units can create heterogeneities that complicate reservoir modeling and fluid flow. Regarding seismic acquisition, handling chalk can be challenging, as it can be difficult to obtain high-quality seismic data. Additionally, the highly fractured zones in the region, particularly around faults, can cause seismic signal scattering, making it harder to accurately interpret subsurface structures. In this brief presentation, three different approaches to perform seismic acquisition will be presented to our client.

Part 1: Wight Monocline case

Seismic acquisition proposal

2D Seismic acquisition

Detailed 2D Seismic cquisition

3D Seismic cquisition

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Part 1: Wight Monocline case | Recommendation

Based on the balance between cost and resolution, the detailed 2D seismic survey offers sufficient coverage for the Wight Monocline, providing a high level of detail for most project objectives without the high costs of the 3D option. Additionally, the 2D survey can be effectively complemented with any prior seismic data, as stated by Evans et al. (2011), allowing for a more comprehensive understanding of the subsurface without the need for the high cost and extended timeline of a 3D survey. Finally, the detailed 2D survey offers a reasonable timeline of 1 - 1.5 months, balancing speed and quality of data collection.

Python Project: Part 2

Seismic Imaging Exercise

Part 2: Seismic Imaging Exercise

Objective

Apply the concepts of NMO, prestack time migration, DMO and poststack migration.

Assumptions

It is assumed a single dipping horizon within a constant velocity earth. Assume basic units of time or depth without mentioning if is in m, km, sec or msec. Since the velocity V is constant, depth (d) and two-way seismic time (t) are simply related by the simple equation: d = Vt/ 2 The used system is an orthonormal one with x-axis as horizontal axis and z-axis the vertical one, corresponding to depth.

Exercise Workflow

3.

2.

1.

4.

5.

6.

7.

Post-stack

Pre-stack

NMO assumption

Reflection

Geometry

Post-stack migration after DMO

DMO

Seismic reflection raypath

Post-stack migration

Pre-stack time ellipse

Dipping horizon, source, receiver and mid-point.

Partial pre-stack time migration

Apparent travelpath

Little summary of the workflow and results.Each step will be explained.

Part 2: Seismic Imaging Exercise

1. Geometry

In this step, a simple dipping reflector is drawn in a constant velocity medium. The horizontal surface represents the Earth's surface (black), and the reflector is drawn with a dip angle where tan() = 1/2 (red). The source (S) (10, 0) and receiver (R) (30, 0) are placed on the surface, with their midpoint (M) (20, 0), which is key for later calculations.

Part 2: Seismic Imaging Exercise

2. Seismic Reflection Raypath

The image point technique is applied to construct the raypath from the source (S) to the reflector at the reflection point (P) and then up to the receiver (R). Total distance covered by the reflexion raypath: 23.45 Total distance P-N: 6.71

Part 2: Seismic Imaging Exercise

3. NMO Assumption

Without knowing the reflector’s dip, is applied the Normal Moveout (NMO) equation:

Assuming the subsurface geometry is unkown and knowing that the reflection traveltime between S and R is 8√10, a zero-dip reflector is considered, meaning the reflecting point P* is below M. The NMO raypath is drawn in purple, showing and incorrect assumption of a horizontal reflector.

Part 2: Seismic Imaging Exercise

4. The Prestack Time Ellipse

P''

P''

At this stage, it was recognized that there is not just one possible reflector location, but an entire range of possible points forming an ellipse:

This corresponds to the prestack time migration ellipse which connects all possible reflectors with dips between 0° and 90°.Following the correct processing workflow: NMO correction Spread over prestack elipse Stack

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Part 2: Seismic Imaging Exercise

5. The Poststack Migration

The poststack migration assumes zero-offset imaging, leading to a circle of d radius, where L = d:

Once the post-stack migration ellipse is plotted, can be observed that is centered at M. True reflector is not reached by the circle, so can be stablished that NMO, followed by stack and poststack migration is not applicable for dipping reflectors.

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Part 2: Seismic Imaging Exercise

6. Partial Prestack Time Migration (DMO Correction)

To correct for the dip effect, it is introduced the Dip Moveout (DMO), which modifies the prestack ellipse equation:

Can be observed that DMO ellipse passes through P*, S and R points. Then, point Q is plotted as the depth Z corresponding to the x-coordinate of N point using the DMO ellipse equation. If it is used the ellipse equation, it can be found that the depth for Q is 6.71, same as the lenght of the normal raypath (PN).

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Part 2: Seismic Imaging Exercise

7. Poststack Migration after DMO

The final step is to apply poststack migration again, but now centered at N instead of M:

It can be noticed that the circle intersects the dipping reflector in a correct position, proving that a correct processing sequence is given by: NMO + DMO + stack + migration

Part 2: Seismic Imaging Exercise

Final plot

Part 2: Seismic Imaging Exercise

Conclusion

By following the imaging workflow outlined in the Python project, it was demonstrated how NMO alone is insufficient and incorrect for dipping reflectors. Instead, the proper sequence involves:

• NMO correction
• DMO correction (prestack time migration)
• Stacking
• Final migration

This approach ensures the reflector is properly positioned in the seismic image.

References

A. Chaouch, J. L. Mari, 3-D Land Seismic Surveys: Definition of Geophysical Parameter, Oil & Gas Science and Technology. 2010. Revue d'IFPEN, Volume 61, Issue 5, Pages 611-630, ISSN 1294-4475, https://doi.org/10.2516/ogst:2006002. D.J. Evans, G.A. Kirby, A.G. Hulbert. 2011. New insights into the structure and evolution of the Isle of Wight Monocline, Proceedings of the Geologists' Association, Volume 122, Issue 5, Pages 764-780, ISSN 0016-7878.

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Knowing the base equation for an ellipse:

And knowing that L is the lenght of each raypath, L= 8√10/2 = 4√10, and knowing P*, we can define the equation for the pre-stack ellipse as:

Knowing the base equation for the ellipse:

And knowing that the circle is centered at M, d= -7.75, and h=10, the equation of the post-stack ellipse is:

2D Seismic Acquisition

• Number of Lines: 5

• Line Spacing: 2000 m

• Line Orientation: East-West

• Number of Sources: 350 sources (300 m spacing.

• Number of Receivers: 1050 receivers (100 m spacing)
• Estimated fold:
• Estimated acquisition time: ~1/2 weeks.

This option offers a reliable approach for seismic data acquisition with 5 seismic lines and 2km spacing between each. This option provides general coveage of the area and uses a low number of sources and receivers, making it ideal for projects with limited budgets. This survey will allow for the collection of seismic data, though with lower resolution and quality than the other options.

Pros

Cons

• Limited resolution
• Less accurate coverage

• Reduced costs.
• Quick implementation.
• Lower environmental impact

Estimated Cost: ~$525,000.00

Detailed 2D Seismic Acquisition

• Number of Lines: 20

• Line Spacing: 500 m

• Line Orientation: East-West

• Number of Sources: 1400 sources (300 m spacing)

• Number of Receivers: 4200 receivers (100 m spacing)
• Estimated fold:

• Estimated acquisition time: ~4 weeks.

The Detailed 2D Seismic Survey provides a higher density of data compared to the first option. With 20 lines spaced 500 meters apart, this survey improves both the resolution and accuracy of the seismic data. The denser configuration of sources and receivers allows for better coverage of the area, making it an ideal choice for projects that require more detailed seismic information while still maintaining a balance between cost and resolution.

Pros

Cons

• Higher cost
• Longer acquisition time

• Higher Resolution
• Good Cost-to-Benefit Ratio
• Improved Coverage

Estimated Cost: ~$2,200,000.00

Seismic acquisition on land

Marine seismic acquisition

• Uses vibratory sources (vibroseis) or explosives to generate seismic waves.
• Geophones are placed on the ground to record reflected waves.
• Conducted in various terrains like deserts, jungles, and mountains, requiring specific logistics and accessibility.

• Uses air guns as the seismic source.
• Hydrophones, arranged in floating streamers, capture reflected signals from the seabed.
• Can be 2D, 3D, or 4D, depending on data density and reservoir monitoring needs.

Knowing the base equation for a circle:

And knowing that the circle is centered at M, and that d= -7.75, the equation of the post-stack circle is:

Detailed 2D Seismic Acquisition

• Number of Lines (East-West): 33

• Receiver and Source Line Spacing: 500 m

• Line Orientation: East-West and North-South

• Number of Sources: 5412 sources (50 m spacing)

• Number of Receivers: 3960 receivers (50 m spacing)
• Estimated fold: ~ 40 (explained in additional window)

• Area Covered: 336 km²

• Estimated acquisition time: ~2/3 months.

The 3D Seismic Survey provides the most comprehensive and detailed data for subsurface analysis. This approach uses a grid system of sources and receivers, allowing for full three-dimensional imaging of the subsurface. It is the best choice for obtaining high-resolution data. This option is ideal for high-precision projects where a complete and accurate subsurface model is necessary.

Pros

Cons

• High cost
• Long acquisition time

• High Spatial Resolution
• Detailed Visualization
• Complete Coverage

Estimated Cost: ~$7,623,000

Fold Calculation - 3D Seismic acquisition

Chaouch and Mari (2010)

1. Total number of shots

5. Fold calculation

2. Total number of receivers

3. Bin size

• Number of Lines (East-West): 33

• Receiver and Source Line Spacing: 500 m

• Line Orientation: East-West and North-South

• Number of Sources: 5412 sources (50 m spacing)

• Number of Receivers: 3960 receivers (50 m spacing)
• Estimated fold: ~ 40 (explained in additional window)

• Area Covered: 336 km²

4. Number of bins