Lecture 3c - Reflection Seismics and Joint inversion

Reflection Seismics and Joint Inversion

Reflection Seismics: Basics

• Reflection seismics and joint inversion covered.
• Joint inversion: Combines data from different geophysical methods, enhancing interpretation.
• Reflection seismics: Data collection and complex processing, applications of the method, subsurface features investigation.
• Similarities between reflection seismics and ground-penetrating radar.

Joint Inversion

• Combines seismic data with data from other geophysical methods to improve interpretations.
• Models from geophysical methods are subject to uncertainty, which can be reduced by integrating different types of independent observations.
• Joint inversion works to combine measurements from velocity models with electric measurements.

Seismic Reflections

• Seismic reflections occur when seismic waves are reflected by interfaces between layers that have different seismic velocities.
• The angle of reflection from an interface is equal to the angle of incidence.
• Reflection survey setup:
  • Surface shock points (A to F).
  • Array of geophones (1 to 13).
  • Imaging a subsurface interface/reflector.
  • Multiple shot points and detectors are characteristic of reflection surveys.

Data Acquisition and Signal Processing

• All points on the subsurface are imaged by multiple ray paths.
• Example: Point D on the reflector imaged by rays from shot points A, B, and C, measured by geophones 7, 8, and 9.
• Multiple rays imaging enhance the signal-to-noise ratio.
• Large surveys with 2D geophone arrays can lead to computationally intense data processing.
• Forward modeling is relatively straightforward, while reflection data processing is more complex and uses numerical data inversion techniques.
• Processing by inversion requires greater data volumes than forward modeling but is more automated and can lead to more detailed models.

Shot Records

• Shot records represent a collection of primary seismic data.
• Site-scale survey: ~10 geophones, ~1000 samples per shot.
• Large 3D marine survey: Several thousand detectors, thousands of samples per shot.
• The figure illustrates a collection of shot records arranged for visualizing raw data.
• Travel time is on the vertical axis (increases downward), also called two-way travel time.
• The horizontal axis represents the individual separation between the geophones and their traces.
• Data presented in a shot gather format.
• Travel time increases towards the bottom of the plot.
• Lower signals represent deeper reflectors, but it's not a direct one-to-one relationship.
• Data only represent reflections in travel time; processing is required to convert them to a configuration more directly representative of the subsurface.

Seismic Noise

• Seismic noise can reduce the quality of final processed results.
• In land-based surveys, noise can result from wind, geophone deployment issues, or anthropogenic sources (traffic, railways).
• Repeating measurements and averaging can reduce the impact of random noise.
• Averaging data from multiple identical measurements can improve the quality of results by enhancing the signal-to-noise ratio.
• The concept can be extended in seismic surveys through a process called stacking, which allows data to be combined that image the same subsurface point, but it allows us to do this for data collected along different ray paths.

Stacking

• Data is combined that images the same subsurface point from different ray paths.
• CDP: Common depth point.
• Rays from shot points 1 to 4 are detected by detectors 1 to 4 after reflecting off the CDP.
• Rays have traveled different distances, so they can't immediately be combined by simple averaging.
• Reflection time increases with increasing distance from the source.
• Travel time from a reflector increases with the increasing distance between the source and the detector, known as move out.
• To combine signals, estimate and subtract the extra time by estimating seismic velocity.
• $v = \sqrt{\frac{displacement}{time}}$
• The dashed lines represent the theoretical position of the signal if different seismic velocity estimates are used.
• The optimal seismic velocity can be estimated as part of the stacking process.
• Stacking enhances the signal-to-noise ratio.
• Understanding move out helps interpret shot gathers.

Shot Gather Interpretation

• Detectors closest to the source are in the center.
• Shot occurred in the center, data from detectors symmetrically arranged.
• This is known as a common midpoint gather.
• First arrivals are where each shot trace first deviates from the background levels.
• Hyperbolic patterns in the data represent subsurface reflectors.
• The central region of large amplitude signals are surface waves.

Data Processing

• Data processing converts raw shot records into processed seismic sections.
• Distance is on the horizontal axis, and two-way travel time is on the vertical axis.
• Reflections are aligned to represent subsurface lithologies.
• Artifacts, such as hyperbolas, are processed out.
• Parallel reflectors suggest parallel variations in lithology.
• Disrupted regions may indicate faults.
• Reflectors and interfaces are indicative of lithological changes.
• Do not interpret individual dark regions on a processed seismic section as representing an actual layer or bed; they are representative of seismic reflectors.
• The vertical access is time. Seismic velocities must have been used to convert the original time axis into distance in order to show seismic sections presented with depth in the vertical axis.

Applications

• Reflection seismic is advanced through commercial hydrocarbon exploration, mainly marine.
• Large data volumes enable 3D models.
• Green shaded surface highlights a suspected salt dome.
• Deformations of source and reservoir rocks sealed against salt form natural traps for oil and gas.
• Seismic reflection is used to infer geological details, assess structural details for infrastructure (faults near dams).
• Facies relationships and details, such as paleo sediment source directions and sedimentation rates for deltas give information on crystal processes and erosion.
• Faulting and tilted crystal blocks illuminate tectonic processes associated with failed rifting.
• Detailed subsurface data at depths of a few meters to multiple kilometers can be explored.

Joint Imaging

• Joint imaging combines seismic data with other geophysical methods.
• Synthetic example: Vertical cross-section showing seismic velocity and electrical conductivity.
• Seismic survey results in a seismic model, and electrical conductivity survey provides a model.
• Models have uncertainty and limitations.
• Joint imaging inverts different datasets together to fit seismic and electrical data consistently.
• Combines the best of both methods in terms of detectability.
• Consistent models describe seismic and electrical variations equally well.

Joint Inversion Process

• Requires a concept to constrain the modeling by linking seismic and electrical properties.
• Changes in one property (conductivity or seismic velocity) occur in the same direction as changes in the other property.
• Does not require constraints on absolute values but assumes that where one property is changing, the other property is also likely to change.
• Real example: Joint inversion of seismic and electromagnetic data.
• The electromagnetic data are modeled in terms of subsurface electrical resistivity.
• Generated models are self-consistent.
• Compare resistivity and velocity data directly together.

Interpretive Units

• Each point on the 2D section, the resistivity value has been plotted against the velocity value.
• The graph shows that the different regions follow different trends.
• Model results collect themselves into different regions that can be identified.
• Interpretive unit 9: Small change in velocity, wide range in resistivity.
• Transfer the classification back into the 2D cross-sectional view.
• Colors show regions with particular trends in seismic and electrical properties but don't necessarily represent specific mythologies.
• Link variations to changes in physical properties (rock type, fracture density, water content).

Summary

• Reflection seismic surveying converts raw shot data into representative subsurface models.
• Reflectors can be identified and interpreted in terms of geological structure.
• Joint inversion combines seismic methods with other geophysical methods for consistent subsurface models.
• Joint inversion illustrates the use of independent methods to reduce uncertainty in geophysical models.