3C Seismic and VSP: Converted Waves and Vector Wavefield Applications
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Abstract
3C seismic applications provide enhanced rock property characterization of the reservoir that can complement P-wave methods. The continued interest in converted P- to S-waves (PS-waves) and vertical seismic profiles (VSPs) has resulted in the steady development of advanced vector wavefield techniques. Shear waves are coupled with P-waves, and although they do not respond to fluid properties of the medium they are nevertheless very sensitive to anisotropy and provide direct estimates of shear moduli (rigidities). When the full elastic response is recorded in a VSP survey, vertical components of the wavefield are obtained to calibrate surface 3C seismic data in depth. PS-wave images along with VSP data can be used to help P-wave interpretation of structure in gas obscured zones, of S-wave impedance and density characterization in unconventional reservoirs for lithology and elastic property discrimination, and of fracture characterization and stress monitoring from S-wave birefringence analysis. The course will give an overview of 3C seismic theory and practical application: from fundamentals of PS-waves and VSPs, through to acquisition and processing including interpretation techniques. The emphasis will be on unique aspects of vector wavefields, anisotropy, and the important relationships that unify S-waves and P-waves. Various applications and case studies will demonstrate image benefits from PS-waves, elastic properties from joint inversion of amplitude variations with offset/angle (AVO/A), and VSP seismic methods for improved reservoir characterization.
Course Objectives
Students will obtain an understanding of theoretical and practical aspects of 3C seismic and VSP, including how to use PS-wave and vector wavefield data to improve rock property applications, as well as:
- Basics of PS-wave registration, velocities and birefringence (S-wave splitting).
- Elastodynamic processes that generate converted waves and how they relate to elastic rock properties
- Issues of PS-wave asymmetry and illumination, and how 3C surface and VSP wavefields are related
- Unique characteristics of PS-wave processing: time registration with P-waves, S-wave splitting,VP/VS analyses, velocities, and conversion-point gathering.
- Identifying and accounting for potential vector infidelity effects
- Interpretation of converted-wave and VSP wavefields
- Applications of 3C seismic and VSP data for migration and elastic impedance inversion, imaging through gas, fracture/stress characterization, and time-lapse.
Who Should Attend
The course is intended for geophysicists, geologists and engineers. The emphasis is on practical understanding and application of vector wavefields, thus a basic prerequisite knowledge of P-waves is assumed. The course would be most relevant to those currently involved with, or considering the use of AVO/A inversion, fracture/stress characterization analyses, or interpretation in gas-obscured reservoirs.
Summary
The following topics will be addressed in the course:
Introduction:
Definitions and wavefield properties of 3C seismic and VSP data are covered, including anisotropy, coordinate systems, vector wavefields, and S-wave applications. Challenges our industry has faced in the development of S-wave technology are reviewed to obtain a perspective of the current PS-wave emphasis.
S-waves and VSP in the 20th century:
An overview of the history and development of S-wave and VSP technology in the 20th century is discussed, including S-wave source development, the influence from P-wave AVO, and the emphasis on vertical transverse isotropy (VTI) and azimuthal anisotropy. Also, the early development of PS-wave and VSP technology is reviewed.
Fundamentals:
A tutorial of the elastodynamic theory of PS-wave generation is described, along with reflection and transmission coefficients, coordinate systems, and polarity standards. Conversion-point illumination, modeling and interpretation of 3C seismic and VSP, NMO velocity in anisotropic media, and the resolution of PS-waves are also reviewed.
Acquisition:
Basic source radiation patterns, free surface and seabed responses to P- and S-wave arrivals are described as well as source, receiver, and VSP systems. Various 3C acquisition configurations are examined in terms of PS-wave illumination, minimal datasets, and common-offset vector (COV) gathers, including VSP geometries.
Processing and Analysis:
Unique 3C processing steps such as rotation, S-wave statics and splitting analyses are emphasized in addition to noise attenuation, vector infidelity corrections, elastic-wavefield decomposition, common conversion-point gathering, and VP/VS analyses. Essentials of VSP wavefield separation, anisotropic velocity analyses, and conventional processing are described along with interferometry application.
Imaging and Inversion Applications:
Applications of PS-wave seismic demonstrating anisotropic imaging, velocity model building, and tomography are presented in addition to case studies imaging through gas, and imaging with VSP. Also, various inversion applications are presented: layer stripping for fracture/stress properties and joint AVO/A for rock properties, including unconventional reservoir, microseismic imaging, and time-lapse applications. Current research directions of 3C seismic and VSP include investigations using reverse-time migration, AVAz and full-waveform inversion, near surface velocity model building, distributed acoustic sensing, and rotational sensors. Business model considerations are discussed along with improving the economic viability of 3C seismic and VSP to increase productivity, and to reduce processing costs and turnaround times.
