Seismic Reflection Interpretations: 1-5 Acquisition Basics Part 2

Summary

This video script delves into seismic data acquisition methods for both land and marine environments. It compares modern vibrator technology with older dynamite methods, highlighting the improved signal-to-noise ratio of vibrators. The importance of survey geometry, fold coverage, and acquisition artifacts in interpreting seismic data is emphasized. In marine settings, the script discusses the challenges of ocean currents on data quality and the advantages of ocean bottom cables and nodes. The script also touches on how acquisition geometry affects the visibility of geological structures, stressing the need for careful interpretation in seismic surveys.

Takeaways

• 😀 Seismic data acquisition can be done through two main methods: vibrator (modern) and dynamite (older). Vibrator provides better signal-to-noise ratios and clearer reflections.
• 😀 Data from vibrator shots often exhibit more coherent reflections, making them easier to interpret compared to dynamite-acquired data, which can be noisy and uncertain.
• 😀 In seismic surveys, the 'sweet spot' refers to the ideal area to acquire data, usually larger than the reservoir to allow for proper noise reduction and coverage.
• 😀 Fold coverage in a seismic survey refers to how many data points are collected over a specific area. Higher fold coverage results in better signal-to-noise ratios and more accurate interpretations.
• 😀 Perimeter shooting and cross spread methods help manage obstacles (e.g., roads, buildings) by placing sources and receivers around these barriers, increasing survey flexibility.
• 😀 Marine seismic acquisition involves towing air gun arrays and hydrophone streamers from a source vessel, often following racetrack patterns to ensure complete coverage.
• 😀 Ocean currents are a significant challenge in marine acquisition, as they can cause streamer feathering, affecting the accuracy of subsurface imaging.
• 😀 Ocean Bottom Cables (OBCs) and Ocean Bottom Nodes (OBNs) provide better data quality than streamers, especially for complex subsurface structures, but are more expensive to deploy and retrieve.
• 😀 Ocean Bottom Cables offer better low-frequency response and can record both pressure and shear waves, unlike streamers, which can only record pressure waves.
• 😀 Survey geometry—how seismic data is acquired relative to geological features (like faults)—greatly impacts how those features are imaged and interpreted in the data.
• 😀 Acquisition footprint artifacts from survey geometry can sometimes mimic geological structures in seismic data, requiring careful attention during interpretation to avoid misidentification.

Q & A

What are the two common ways to acquire land seismic data?

The two common methods for acquiring land seismic data are using vibrators, which are more common today, and dynamite, which is an older technology.

How does the vibrator method compare to dynamite in terms of signal quality?

The vibrator method generally produces a better signal-to-noise ratio, making the reflections clearer and easier to interpret compared to the dynamite method.

What is the concept of 'sweet spot' in seismic surveys?

'Sweet spot' refers to the optimal area in a seismic survey where the target is located. It’s typically surrounded by a larger survey area to ensure adequate coverage, larger offset angles, and noise reduction.

Why is it important to have fold coverage in seismic data?

Fold coverage is important because it helps in stacking the data, which reduces noise and improves the signal-to-noise ratio, making the interpretation of seismic data clearer.

What are acquisition artifacts in seismic data?

Acquisition artifacts are unwanted patterns, such as striping, that can appear in seismic data due to the survey geometry. These artifacts are often mistaken for geologic features and must be accounted for during interpretation.

How does survey geometry impact seismic data interpretation?

Survey geometry directly affects how subsurface features are illuminated in seismic data. For example, shooting seismic surveys parallel or perpendicular to faults can result in different appearances of those faults in the data.

What is the difference between cross spread shooting and perimeter shooting?

Cross spread shooting and perimeter shooting are two different methods of land seismic acquisition. Cross spread shooting uses a regular geometry for placing sources and receivers, while perimeter shooting places sources around the perimeter of the target area, which is useful for avoiding obstacles.

What challenges are associated with marine seismic acquisition?

Marine seismic acquisition faces challenges such as ocean currents causing streamer feathering, which affects the accuracy of subsurface coverage. Additionally, dealing with infrastructure like platforms can require complex shooting plans.

What is the advantage of ocean bottom cables in marine seismic acquisition?

Ocean bottom cables provide better low-frequency response and allow for the recording of both pressure and shear waves, which are not possible with traditional streamers. However, their deployment is more complex and costly.

How do ocean bottom nodes compare to traditional streamer acquisition methods?

Ocean bottom nodes offer higher resolution and better imaging of complex subsurface structures compared to traditional streamer methods. They are more expensive and complex to deploy but can be essential for accurate subsalt imaging.