Seismic surveys are vital in oil and gas exploration, enabling energy companies to locate and assess potential hydrocarbon reservoirs beneath the Earth’s surface. Whether conducted onshore or offshore, the process involves a combination of advanced technologies, data acquisition methods, and interpretation techniques.
In this blog post, we will break down the step-by-step process of seismic surveys for both onshore and offshore exploration, highlighting the similarities, differences, and unique challenges in each environment.
What is a Seismic Survey?
A seismic survey is a geophysical method used to map subsurface geological formations by generating and analyzing sound waves. These sound waves travel through the Earth and reflect back to the surface when they encounter different geological layers.
The process generally involves:
1. Generating sound waves.
2. Recording the reflected waves using sensors.
3. Processing and interpreting the data to create images of the subsurface.
Step-by-Step Process of Seismic Surveys
1. Planning and Site Preparation
• Onshore:
• Conduct geological studies to identify target areas.
• Obtain permits and assess environmental impact.
• Clear paths for survey equipment and vehicles, minimizing disruption to the environment.
• Offshore:
• Perform bathymetric (seafloor mapping) surveys to plan safe operations.
• Secure regulatory approvals and assess potential impacts on marine life.
2. Generating Seismic Waves
• Onshore:
• Method: Vibrations are generated using seismic vibrators mounted on trucks or through controlled explosions (small charges placed in shallow holes).
• Purpose: Produce sound waves that penetrate subsurface layers.
• Offshore:
• Method: Air guns are used to release compressed air, creating sound waves that travel through water and the underlying seabed.
• Purpose: Generate consistent and powerful sound waves suitable for deepwater exploration.
3. Recording Seismic Waves
• Onshore:
• Sensors Used: Geophones are placed on the ground to detect reflected waves.
• Setup: Geophones are arranged in linear arrays (2D surveys) or grid patterns (3D surveys) for optimal coverage.
• Offshore:
• Sensors Used: Hydrophones embedded in streamer cables towed behind vessels or placed on the seafloor in Ocean-Bottom Seismic (OBS) surveys.
• Setup: Streamers can extend for kilometers, with multiple hydrophones capturing reflected sound waves.
4. Data Acquisition
• Process:
• Sound waves travel through the Earth, reflecting back to the surface when they encounter geological boundaries.
• Reflected waves are recorded by geophones or hydrophones.
• The strength and travel time of the reflections provide clues about the subsurface structure.
• Differences:
• Onshore data acquisition can be influenced by terrain and environmental noise.
• Offshore data acquisition is affected by water depth, wave noise, and seafloor conditions.
5. Data Processing
• Objective: Convert raw seismic data into interpretable images of the subsurface.
• Steps:
1. Noise Removal: Eliminate interference and enhance signal quality.
2. Migration: Adjust the recorded data to account for wave travel paths, creating accurate subsurface images.
3. Stacking: Combine multiple data sets to improve image clarity.
• Technologies Used:
• Advanced algorithms.
• High-performance computing for large datasets.
6. Interpretation
• Process:
• Geophysicists analyze seismic images to identify geological structures like traps, faults, and reservoirs.
• Amplitude variations and wave patterns help determine rock types, porosity, and fluid content.
• Onshore Focus: Evaluate sedimentary basins and shallow reservoirs.
• Offshore Focus: Analyze deepwater formations, including complex structures like salt domes.
7. Validation and Further Exploration
• Onshore:
• Drill exploratory wells to validate seismic findings.
• Use Vertical Seismic Profiling (VSP) to refine subsurface images near the well.
• Offshore:
• Drill test wells in deepwater locations.
• Perform additional high-resolution site surveys to assess hazards like shallow gas pockets.
Advancements in Seismic Survey Techniques
1. Full Waveform Inversion (FWI): Delivers higher-resolution subsurface images.
2. Autonomous Vehicles: Enable efficient data collection in deepwater environments.
3. Machine Learning: Speeds up data processing and interpretation.
4. Eco-Friendly Technologies: Reduce environmental impact, especially offshore.
Conclusion
Seismic surveys are indispensable in oil and gas exploration, providing critical insights into subsurface geology. Whether onshore or offshore, the process requires careful planning, advanced technology, and skilled interpretation. By understanding the step-by-step process, energy professionals can appreciate the complexity and importance of seismic surveys in unlocking the Earth’s hydrocarbon potential.
