Comparing Near-Surface Investigations: Full-Waveform Inversion (FWI) vs. Multichannel Analysis of Surface Waves (MASW) in Seismic Earth Science

Near Surface Inspection: FWI vs. MASW

When it comes to near-surface investigations in seismic and earth sciences, two widely used and effective methods are Full-Waveform Inversion (FWI) and Multichannel Analysis of Surface Waves (MASW). Both techniques provide valuable insight into subsurface properties, but they use different approaches and have distinct advantages and limitations. Understanding the characteristics and applications of FWI and MASW is critical for researchers and practitioners in fields such as geotechnical engineering, environmental studies, and hydrocarbon exploration.

In this article, we will review the principles, methods, and comparative analysis of FWI and MASW, shedding light on their respective strengths and weaknesses. By exploring these two techniques, we aim to provide a comprehensive understanding of their applications and assist practitioners in selecting the most appropriate approach for their near-surface investigations.

Full Waveform Inversion (FWI)

Full-Waveform Inversion (FWI) is an advanced imaging technique that uses the complete seismic waveform recorded at the surface to estimate the subsurface velocity model. FWI involves iteratively solving the wave equation for the best-fit model that matches the recorded data. By minimizing the misfit between the modeled and observed waveforms, FWI provides high-resolution images of the subsurface, allowing precise characterization of geological structures and material properties.

One of the key advantages of FWI is its ability to handle complex subsurface structures and heterogeneous media. It is particularly effective in imaging near-surface features such as faults, fractures and small-scale velocity anomalies. FWI can provide detailed velocity models that allow accurate interpretation of subsurface lithology and fluid content. In addition, FWI can be applied to both active and passive seismic data, making it a versatile tool for a variety of near-surface investigations.

Multichannel Analysis of Surface Waves (MASW)

Multichannel Analysis of Surface Waves (MASW) is a geophysical method that relies on surface wave analysis to determine the near-surface shear wave velocity profile. MASW measures the dispersion characteristics of Rayleigh waves propagating along the surface to estimate the shear wave velocities at different depths. By analyzing the dispersion curves, which represent the relationship between wave velocity and frequency, MASW provides valuable information about subsurface stratigraphy and mechanical properties.

MASW offers several advantages that make it a popular choice for near-surface investigations. First, MASW is a non-invasive and cost-effective technique, requiring only a single seismic source and a linear array of receivers. This makes it suitable for large-scale surveys and rapid data acquisition. Second, MASW provides a continuous shear wave velocity profile, which is critical for assessing site conditions and evaluating the potential for ground hazards such as liquefaction or landslides. MASW is also effective in mapping shallow bedrock and delineating depth to the water table, making it valuable for geotechnical and environmental assessments.

Comparative Analysis

While both FWI and MASW are valuable tools for near-surface investigations, they have different characteristics that make them suitable for different scenarios. FWI excels in complex geological environments where accurate imaging of subsurface structures and high-resolution velocity models are required. Its ability to handle heterogeneities and image small-scale features makes it a preferred choice for hydrocarbon exploration, geothermal studies and environmental applications.

MASW, on the other hand, is well suited for rapid and cost-effective site characterization. Its ability to provide continuous shear wave velocity profiles makes it indispensable in geotechnical engineering, infrastructure development, and seismic hazard assessment. MASW can quickly identify potential problem areas and assist in the design of safe and efficient construction projects. In addition, MASW is relatively less computationally intensive than FWI, making it a practical choice for large-scale surveys.
In some cases, a combination of FWI and MASW can be used to leverage the strengths of both techniques. This integrated approach provides a more complete understanding of the subsurface by combining the high-resolution imaging capabilities of FWI with the efficient and cost-effective site characterization of MASW.

Conclusion

Near-surface investigations play a critical role in a variety of seismic and geoscience disciplines. Full waveform inversion (FWI) and multichannel analysis of surface waves (MASW) are two powerful techniques that provide valuable insight into subsurface properties. FWI provides high-resolution imaging and accurate velocity models, making it ideal for complex geological environments. MASW, on the other hand, provides cost-effective and rapid site characterization, making it suitable for geotechnical engineering and infrastructure development.

By understanding the principles and applications of FWI and MASW, practitioners can make informed decisions about which technique to use based on their specific project requirements. In addition, the integration of both techniques can provide a more comprehensive understanding of the subsurface, enabling a holistic approach to near-surface investigations.
It is important to note that advances in technology and ongoing research in seismic and geoscience continue to enhance the capabilities of FWI and MASW. As these techniques evolve, they will continue to contribute to our understanding of the near surface and facilitate more accurate and efficient exploration, engineering and environmental assessments.

FAQs

Near-Surface Investigations: FWI vs. MASW

Near-surface investigations play a crucial role in various geophysical applications. Two commonly used methods for near-surface investigations are Full-Waveform Inversion (FWI) and Multichannel Analysis of Surface Waves (MASW). Let’s explore some questions and answers to understand these methods better.

Question 1: What is FWI?

FWI, or Full-Waveform Inversion, is a seismic imaging technique used to estimate subsurface properties by iteratively minimizing the difference between observed and modeled seismic waveforms. It provides high-resolution images of the subsurface by inverting the full waveform data, including amplitude and phase information.

Question 2: What is MASW?

MASW, or Multichannel Analysis of Surface Waves, is a geophysical method used to determine the shear-wave velocity profile of near-surface materials. It analyzes the dispersion characteristics of surface waves recorded by an array of geophones or seismometers to estimate the shear-wave velocity as a function of depth.

Question 3: How does FWI differ from MASW?

FWI and MASW differ in terms of the data they utilize and the information they provide. FWI uses the complete waveform data, including amplitude and phase, to estimate subsurface properties with high resolution. On the other hand, MASW analyzes the dispersion characteristics of surface waves to determine the shear-wave velocity profile of near-surface materials.

Question 4: What are the advantages of FWI?

FWI offers several advantages in near-surface investigations. It provides high-resolution images of subsurface structures and properties, including velocity and density, which can be valuable for various applications such as geotechnical studies, hydrocarbon exploration, and environmental assessments. FWI can also handle complex subsurface structures and has the potential for imaging beyond the classical resolution limits of traditional methods.

Question 5: What are the advantages of MASW?

MASW has its own advantages in near-surface investigations. It is a relatively quick and cost-effective method that can provide valuable information about the shear-wave velocity profile of near-surface materials. This information is crucial for assessing soil liquefaction potential, determining site response for seismic design, and characterizing shallow geological layers. MASW can be applied in various environments, including urban areas, where access for traditional borehole methods may be limited.