Seismic Methods for Petroleum Subsurface Imaging
Introduction to Seismic Exploration
Seismic exploration is the most widely used geophysical method in petroleum engineering for understanding subsurface rock formations. It works on the principle of sending artificial seismic waves into the Earth and recording the waves that return after interacting with subsurface layers.
Because seismic waves travel at different velocities through different lithologies, fluids, and densities, they provide highly reliable information about subsurface structures, stratigraphy, and potential hydrocarbon traps. The entire method is based on elastic wave propagation, where generated waves travel through the Earth, reflect or refract at boundaries of different densities (acoustic impedance contrasts), and are recorded at the surface by receivers called geophones or hydrophones. This technique helps in identifying the depth, thickness, geometry, and continuity of subsurface layers.
Types of Seismic Waves
Seismic waves used in exploration are mainly categorized into two types:
Body Waves
These travel through the interior of the Earth:
- P-waves (Primary or Compressional waves): These are the fastest waves; particles move in the direction of wave propagation. They are the first to arrive at geophones.
- S-waves (Secondary or Shear waves): These are slower; particles move perpendicular to the direction of propagation. They cannot travel through liquids.
Surface Waves
These travel along the Earth’s surface and are generally slower and noisier:
- Rayleigh waves: Characterized by elliptical motion.
- Love waves: Characterized by horizontal shearing motion.
Body waves, especially P-waves, are primarily used in petroleum exploration because they offer superior imaging capabilities.
Seismic Exploration Methods
There are two major seismic methods used in the industry:
Seismic Reflection Method
This is the primary method used in oil and gas exploration because it provides high-resolution images of deep subsurface structures.
Principle: When a seismic wave hits a boundary between two rock layers with different acoustic impedances (Velocity × Density), part of the wave is reflected back to the surface. These reflected waves are recorded, and their Two-Way Travel Time (TWT) is used to calculate depth.
Key Features:
- Produces detailed seismic sections.
- Shows layer boundaries, faults, folds, and traps.
- Helps identify hydrocarbon-bearing zones.
Uses:
- Structural interpretation (anticlines, faults, and domes).
- Stratigraphic interpretation (channels and reefs).
- Reservoir characterization and depth mapping.
Seismic Refraction Method
Used to determine near-surface velocity variations and identify shallow geological layers.
Principle: When seismic waves strike a deeper, high-velocity layer at the critical angle, they refract along the boundary and return to the surface.
Uses:
- Mapping weathered layer thickness.
- Groundwater studies and engineering projects.
- Correcting near-surface effects in reflection surveys (static correction).
Limitations: It cannot image deep structures and has poor resolution compared to reflection.
Advantages of Seismic Methods
Advantages of Seismic Reflection
- High Resolution: Can detect thin beds and small geological features.
- Deep Penetration: Can image up to 10–15 km in depth.
- Hydrocarbon Indication: Bright spots, flat spots, and AVO anomalies can predict gas or oil.
- 3D Imaging: Produces very accurate subsurface maps.
- Widely Applicable: Works on land, offshore, in deserts, and in jungles.
Advantages of Seismic Refraction
- Simple and Fast: Easy field setup and quick data acquisition.
- Low Cost: Much cheaper than reflection surveys.
- Shallow Investigation: Ideal for engineering sites, groundwater, and weathered zones.
- Static Corrections: Improves the quality of reflection data.
Limitations of Seismic Methods
Limitations of Seismic Reflection
- Very Expensive: Requires heavy equipment, explosives, or vibroseis trucks.
- Complex Processing: Needs advanced computer processing and interpretation.
- Noise Sensitive: Weather, traffic, and cultural noise affect data quality.
- Risk of Misinterpretation: Poor velocity models lead to depth errors.
Limitations of Seismic Refraction
- Velocity Inversion: Cannot detect low-velocity layers beneath high-velocity layers.
- Poor Structural Detail: Only gives layer boundaries; cannot show faults or traps.
- Limited Depth: Useful only for shallow to medium-depth surveys.
- Complex Geology: Not suitable for dipping beds or irregular structures.
Interpretation of Seismic Data
Interpretation is the most critical step in seismic exploration. It involves examining seismic sections to understand:
- Subsurface layering (stratigraphy).
- Structural geometry, faults, and fractures.
- Thickness and continuity of layers.
- Indications of hydrocarbons.
Common Interpretation Tools:
- Horizon Mapping: Tracking reflectors across seismic lines.
- Velocity Analysis: Converting time to depth.
- Seismic Attributes: Amplitude, phase, and frequency for reservoir identification.
- Seismic Facies Analysis: Recognizing depositional environments.
Identifying Geological Structures
- Anticlines: Upward-arched structures with strong continuous reflectors; ideal structural traps for hydrocarbons.
- Synclines: Downward-curved layers showing concave-down reflectors; usually not hydrocarbon traps unless stratigraphic components exist.
- Faults: Displacements of reflectors appearing as breaks, offsets, or discontinuities; indicate migration pathways or trapping mechanisms.
- Fault Blocks (Horst and Graben): Alternating uplifted and down-dropped blocks; important for structural traps in rift basins.
- Salt Domes: Chaotic or noisy seismic signatures where surrounding strata appear pushed upward; major traps in the Gulf of Mexico and the Middle East.
- Unconformities: Obvious truncation of reflectors indicating missing geological time; important for stratigraphic traps.
- Channels and Reefs: High-amplitude anomalies with characteristic geometries suggestive of reservoir bodies.