🌋Seismology Unit 15 – Seismology in Exploration Geophysics

Key Concepts and Terminology

• Seismology studies the propagation of seismic waves through the Earth's interior to understand its structure and composition
• Exploration geophysics applies seismology principles to investigate subsurface geology for resource exploration (oil, gas, minerals)
• Seismic waves are elastic energy that travels through the Earth, generated by earthquakes, explosions, or other sources
  • Body waves travel through the Earth's interior (P-waves and S-waves)
  • Surface waves travel along the Earth's surface (Rayleigh waves and Love waves)
• Seismic velocity is the speed at which seismic waves propagate through a medium, dependent on its elastic properties and density
• Seismic reflection occurs when a seismic wave encounters a boundary between two materials with different acoustic impedances, causing part of the energy to be reflected back to the surface
• Seismic refraction involves the bending of seismic waves as they pass through layers with varying velocities, following Snell's law
• Seismic anisotropy refers to the directional dependence of seismic velocity in a medium due to factors such as layering, fractures, or stress

Seismic Wave Basics

• Seismic waves are characterized by their velocity, amplitude, frequency, and wavelength
• P-waves (primary or compressional waves) are the fastest seismic waves, traveling through both solids and fluids by compressing and expanding the medium parallel to the direction of wave propagation
• S-waves (secondary or shear waves) are slower than P-waves and can only travel through solids, causing particles to oscillate perpendicular to the direction of wave propagation
• Surface waves (Rayleigh and Love waves) have lower frequencies and travel along the Earth's surface, resulting in ground roll in seismic data
• Seismic wave attenuation is the loss of energy as waves propagate through a medium due to factors such as absorption, scattering, and geometrical spreading
• Seismic wave dispersion occurs when different frequencies of a seismic wave travel at different velocities, leading to a change in the shape of the waveform over time
• Seismic wave interference can result in constructive or destructive interference patterns, affecting the amplitude and character of the recorded signals

Seismic Data Acquisition Methods

• Seismic surveys involve the controlled generation and recording of seismic waves to create images of the subsurface
• Land seismic surveys use vibroseis trucks or explosives as seismic sources and deploy geophones to record ground motion
  • Vibroseis trucks generate a sweep signal that is cross-correlated with the recorded data to produce a seismic trace
  • Explosives (dynamite) provide a high-energy, impulsive source but require drilling shot holes and have environmental concerns
• Marine seismic surveys use air guns as seismic sources and hydrophones to record pressure changes in the water column
  • Air guns release compressed air to create a bubble pulse that generates seismic waves
  • Streamers containing hydrophones are towed behind the survey vessel to record the reflected seismic energy
• Ocean-bottom seismic (OBS) surveys place seismic receivers directly on the seafloor, allowing for better imaging in complex geologic settings and the recording of shear waves
• Vertical seismic profiling (VSP) involves placing seismic receivers in a borehole to record both downgoing and upgoing seismic waves, providing high-resolution velocity information and improved tie to well data
• Microseismic monitoring uses sensitive seismic arrays to detect and locate small-scale seismic events associated with hydraulic fracturing or reservoir depletion

Seismic Data Processing Techniques

• Seismic data processing aims to enhance the signal-to-noise ratio, remove artifacts, and create an accurate image of the subsurface
• Deconvolution is a process that removes the effect of the seismic source wavelet and attenuates reverberations, improving temporal resolution
• Velocity analysis involves estimating seismic velocities from the data to enable accurate time-to-depth conversion and seismic migration
  • Normal moveout (NMO) correction removes the effect of offset on reflection arrival times, allowing for velocity estimation
  • Dix equation converts RMS velocities to interval velocities, which represent the velocity of each layer
• Seismic migration repositions reflectors to their true subsurface locations, collapsing diffractions and improving spatial resolution
  • Pre-stack time migration (PSTM) is applied to common-offset gathers before stacking, providing better imaging in complex geologic settings
  • Pre-stack depth migration (PSDM) uses a velocity model to directly migrate the data in depth, accurately handling lateral velocity variations
• Seismic inversion estimates rock properties (e.g., acoustic impedance) from seismic data by minimizing the difference between the observed and modeled data
• Seismic attributes are derived quantities that highlight specific features or properties of the seismic data (e.g., amplitude, phase, frequency, coherence)
• Seismic data interpolation and regularization techniques aim to reconstruct missing or irregularly sampled data, improving the quality of the final image

Seismic Interpretation Strategies

• Seismic interpretation involves the analysis of processed seismic data to understand the subsurface geology and identify potential hydrocarbon accumulations
• Structural interpretation focuses on mapping faults, folds, and other geologic structures that control hydrocarbon trapping and migration
  • Fault interpretation relies on identifying discontinuities and offset reflectors in the seismic data
  • Horizon picking involves tracing continuous seismic reflectors that represent geologic layers or boundaries
• Stratigraphic interpretation aims to understand the depositional history and sedimentary architecture of the basin
  • Seismic facies analysis characterizes the seismic response of different depositional environments based on their amplitude, frequency, and geometry
  • Sequence stratigraphy uses seismic data to identify and interpret key surfaces (e.g., unconformities, flooding surfaces) and depositional sequences
• Direct hydrocarbon indicators (DHIs) are seismic anomalies that suggest the presence of hydrocarbons, such as bright spots, flat spots, and gas chimneys
• Seismic attribute analysis enhances the interpretation process by highlighting specific features or properties of the data (e.g., coherence for fault detection, RMS amplitude for reservoir characterization)
• Seismic geomorphology uses 3D seismic data to visualize and interpret ancient depositional systems and landforms, aiding in the prediction of reservoir distribution and quality

Applications in Exploration Geophysics

• Seismic exploration is widely used in the oil and gas industry to identify and characterize potential hydrocarbon reservoirs
  • Seismic data helps to delineate structural and stratigraphic traps, estimate reservoir properties, and guide well placement
  • 4D seismic (time-lapse) surveys monitor reservoir changes over time, such as fluid movement or pressure depletion, to optimize production strategies
• Seismic methods are applied in geothermal exploration to map subsurface heat sources, fault systems, and fluid pathways
• In mineral exploration, seismic surveys can image subsurface structures, alteration zones, and ore bodies, guiding drilling programs and resource estimation
• Seismic investigations support geologic hazard assessment and risk mitigation, such as identifying active faults, evaluating earthquake potential, and characterizing landslide-prone areas
• Seismic data is used in carbon capture and storage (CCS) projects to characterize storage reservoirs, monitor CO2 injection, and ensure containment integrity
• Seismic methods contribute to geotechnical engineering applications, such as site characterization for infrastructure projects, foundation design, and monitoring of underground excavations

Advanced Topics and Current Research

• Full-waveform inversion (FWI) is an advanced seismic imaging technique that iteratively updates a velocity model by minimizing the difference between observed and simulated seismic waveforms
• Least-squares migration (LSM) aims to improve the amplitude fidelity and resolution of seismic images by minimizing the difference between the observed and modeled data in the image domain
• Seismic anisotropy analysis investigates the directional dependence of seismic velocity to characterize fracture networks, stress fields, and reservoir properties
• Machine learning and artificial intelligence techniques are being increasingly applied in seismic data processing and interpretation, such as for noise attenuation, fault detection, and facies classification
• Distributed acoustic sensing (DAS) uses fiber-optic cables as seismic sensors, enabling continuous, high-resolution monitoring of subsurface processes and infrastructure
• Ambient noise seismology utilizes naturally occurring seismic noise to image the subsurface, providing a cost-effective alternative to active-source surveys in certain applications
• Integration of seismic data with other geophysical methods (e.g., gravity, magnetics, electromagnetics) and geologic data (e.g., well logs, core samples) enhances the understanding of the subsurface and reduces interpretation uncertainty

Practical Skills and Tools

• Proficiency in seismic data processing software (e.g., Schlumberger Omega, CGG GeoSoftware, SeisSpace) is essential for handling and analyzing large seismic datasets
• Seismic interpretation workstations (e.g., Petrel, Kingdom, DecisionSpace) provide integrated environments for visualizing and interpreting seismic data alongside other geologic and geophysical information
• Programming skills (e.g., Python, MATLAB) are valuable for automating workflows, implementing custom algorithms, and performing data analysis tasks
• Understanding of geologic concepts and principles is crucial for accurate seismic interpretation and integration with other subsurface data
• Effective communication and collaboration skills are important for working in multidisciplinary teams and conveying technical findings to stakeholders
• Familiarity with geophysical data formats (e.g., SEG-Y, SEG-D) and metadata standards facilitates data exchange and interoperability between different software platforms
• Knowledge of geospatial data management and GIS tools (e.g., ArcGIS, QGIS) is useful for integrating seismic data with other spatial datasets and creating maps and visualizations