2.3 Seismic reflection and refraction methods

Seismic reflection and refraction methods are crucial tools for peering into Earth's subsurface. These techniques use sound waves to map underground structures, helping geologists understand what lies beneath our feet.

Both methods have unique strengths. Reflection is great for detailed imaging of complex structures, while refraction excels at determining large-scale velocity patterns. Together, they paint a comprehensive picture of Earth's hidden layers.

Seismic Reflection vs Refraction

Principles and Methods

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• Seismic reflection and refraction are two primary methods used in exploration geophysics to image the subsurface and determine Earth's internal structure
• Seismic reflection methods involve measuring the two-way travel time of seismic waves that are reflected off subsurface interfaces (sedimentary layers, faults)
• Reflection methods are based on the principle of acoustic impedance contrast across interfaces
  • Acoustic impedance is the product of seismic velocity and density
  • Reflections occur when there is a significant change in acoustic impedance between layers
• Seismic refraction methods measure the travel times of critically refracted waves along subsurface interfaces
• Refraction methods rely on the principle of Snell's law and critical refraction
  • Snell's law describes the relationship between the angles of incidence and refraction at an interface
  • Critical refraction occurs when the angle of incidence reaches a critical value, causing the refracted wave to travel along the interface at the velocity of the lower layer

Applications and Survey Design

• Seismic reflection is typically used for high-resolution imaging of complex geologic structures (sedimentary basins, hydrocarbon reservoirs)
  • Reflection surveys require a dense spatial sampling and higher frequency content compared to refraction surveys
  • Reflection data processing involves various steps, such as deconvolution, stacking, and migration
• Seismic refraction is more suitable for determining large-scale velocity structures and crustal layers
  • Refraction surveys can have larger receiver spacing and lower frequency content
  • Refraction data processing mainly focuses on travel time analysis and tomographic inversion
• The choice between reflection and refraction methods depends on the target depth, resolution requirements, and available resources

Seismic Survey Design

Survey Planning and Parameter Selection

• Designing a seismic survey requires careful consideration of the geologic target, desired resolution, and available resources
• Key factors in survey design include the choice of seismic source (explosives, vibroseis, air gun), receiver type and spacing, survey geometry, and recording parameters
  • Explosives and vibroseis are commonly used in land surveys, while air guns are used in marine surveys
  • Receiver types include geophones (land) and hydrophones (marine)
• The target depth, size, and geologic complexity dictate the required source-receiver offset ranges, fold coverage, and frequency content of the seismic data
  • Deeper targets require longer offsets and lower frequencies
  • Higher resolution requires denser spatial sampling and higher frequencies
• Seismic data acquisition parameters, such as sampling rate, record length, and filters, should be optimized based on the target and desired signal-to-noise ratio

Survey Logistics and Quality Control

• In land surveys, factors such as terrain, accessibility, and environmental constraints influence the survey layout and logistics
  • Rugged terrain may require specialized equipment (helicopters, vibroseis trucks) and survey designs (sparse 3D, crooked line 2D)
  • Environmental regulations may limit the use of certain sources or access to sensitive areas
• Marine seismic surveys involve towed streamers with hydrophone arrays and air gun sources, requiring specialized vessels and navigation systems
  • Streamer length and spacing determine the subsurface coverage and resolution
  • Accurate positioning of sources and receivers is critical for data quality
• Quality control measures, such as monitoring source and receiver performance, are crucial during data acquisition to ensure data integrity
  • Source signatures and receiver responses should be consistent and within specifications
  • Noise levels and data gaps should be minimized through proper survey planning and execution

Seismic Data Interpretation

Reflection Data Processing and Imaging

• Seismic data processing aims to enhance the signal-to-noise ratio, remove artifacts, and generate accurate subsurface images
• Reflection data processing steps include trace editing, amplitude recovery, deconvolution, velocity analysis, normal moveout correction, stacking, and migration
  • Trace editing removes noisy or dead traces
  • Amplitude recovery compensates for geometric spreading and attenuation losses
  • Deconvolution removes the effect of the source wavelet and improves temporal resolution
  • Velocity analysis estimates the seismic velocities for normal moveout correction and stacking
  • Normal moveout correction aligns reflections from different offsets
  • Stacking enhances signal-to-noise ratio by summing traces with common midpoints
  • Migration repositions reflectors to their true subsurface locations and improves spatial resolution
• Advanced imaging techniques, such as pre-stack depth migration and full-waveform inversion, can improve the accuracy and resolution of subsurface models in complex geologic settings (salt bodies, thrust belts)

Refraction Data Processing and Velocity Modeling

• Refraction data processing involves picking first arrival times, identifying refracted phases, and performing travel time inversion or tomography
• Travel time curves and intercept time analysis provide initial estimates of layer velocities and thicknesses
  • The slope of the travel time curve represents the reciprocal of the layer velocity
  • The intercept time is related to the depth and velocity of the overlying layers
• Tomographic inversion techniques, such as ray tracing or wavefront methods, are used to obtain detailed velocity models
  • Ray tracing simulates the propagation of seismic waves through a velocity model
  • Wavefront methods compute travel times by solving the eikonal equation
• Interpretation of processed seismic data requires integration with well logs, geologic information, and other geophysical data (gravity, magnetics)
• Seismic attributes, such as amplitude, phase, and frequency, can be extracted to characterize reservoir properties and stratigraphic features (porosity, lithology, fluid content)

Seismic Method Limitations

Resolution and Accuracy Constraints

• Seismic resolution refers to the ability to distinguish individual features in the subsurface and is limited by the wavelength of the seismic waves
• Vertical resolution is determined by the dominant frequency and velocity of the seismic waves, with higher frequencies providing better resolution
  • The vertical resolution is approximately 1/4 of the dominant wavelength
  • Thin layers below the vertical resolution limit may not be detectable
• Horizontal resolution depends on the Fresnel zone size and is improved by migration processing
  • The Fresnel zone is the area on a reflector that contributes to a single reflection point
  • Migration collapses the Fresnel zone, improving horizontal resolution
• Seismic velocity models obtained from reflection and refraction methods are affected by the accuracy of travel time picks, the assumptions in the inversion algorithms, and the spatial coverage of the data
• Seismic anisotropy, caused by oriented fractures or layering, can lead to errors in velocity estimation and depth conversion if not accounted for

Geologic Complexity and Interpretation Challenges

• The presence of complex geologic structures, such as salt bodies, volcanic intrusions, or steeply dipping layers, can cause seismic imaging challenges and reduce the accuracy of subsurface interpretations
  • Salt bodies have high velocities and irregular shapes, causing distortions in seismic images
  • Volcanic intrusions can scatter seismic energy and create velocity anomalies
  • Steeply dipping layers may not be adequately imaged due to limited illumination and aperture
• Seismic methods have limited sensitivity to fluid content and pore pressure, which may require integration with other geophysical methods or borehole data for reservoir characterization
  • Amplitude variations with offset (AVO) analysis can provide insights into fluid and lithology changes
  • Well logs and core data are essential for calibrating seismic interpretations
• Non-uniqueness in seismic interpretation arises from the inherent ambiguity in relating seismic reflections to geologic interfaces, particularly in areas with limited well control or complex stratigraphic relationships
  • Multiple geologic scenarios may explain the same seismic response
  • Interpreters must rely on geologic knowledge, regional context, and other data to constrain interpretations
• The cost and environmental impact of seismic surveys can limit their applicability in certain areas, such as environmentally sensitive regions or urban settings
  • Seismic surveys may be restricted or prohibited in protected habitats or near populated areas
  • Mitigation measures, such as using low-impact sources or avoiding sensitive seasons, may be required to minimize environmental disturbance