Amplitude scaling in seismic inversion

Introduction to amplitude scaling in seismic inversion

Seismic inversion is a powerful technique used in geophysics to estimate the physical properties of subsurface formations from seismic data. A crucial aspect of seismic inversion is the proper scaling of seismic amplitudes, as this can significantly affect the accuracy and reliability of the results. Amplitude scaling is the process of adjusting the relative magnitudes of seismic reflections to account for various factors that can affect the amplitude of the recorded signal. In this article we will explore the importance of amplitude scaling in seismic inversion and discuss the various methods and considerations involved.

Seismic amplitudes are influenced by a number of factors, including the properties of the subsurface formations, the characteristics of the seismic source, and the acquisition and processing of the seismic data. Understanding and accounting for these factors is critical to seismic inversion, as the inverted properties, such as density, porosity and fluid content, are directly related to the recorded amplitudes. Incorrect amplitude scaling can lead to erroneous interpretations and poor estimates of subsurface properties.

Factors affecting seismic amplitudes

Seismic amplitudes are influenced by a number of factors, both geological and geophysical. Geological factors include the lithology, porosity and fluid content of the subsurface formations, as well as the presence of fractures, faults and other structural features. Geophysical factors include the characteristics of the seismic source, the attenuation of seismic energy as it propagates through the subsurface, and the effects of the acquisition and processing techniques used.

Understanding the effects of these factors is crucial in seismic inversion, as the inverted properties are directly related to the recorded amplitudes. For example, the presence of hydrocarbons in a formation can have a significant effect on the seismic amplitudes, resulting in a ‘bright spot’ or ‘dim spot’ in the seismic data. Accurately accounting for these amplitude variations is essential for reliable estimates of subsurface properties.

Amplitude scaling techniques

A number of amplitude scaling techniques have been developed to address the challenges posed by the various factors affecting seismic amplitudes. These techniques can be broadly classified into two main categories: data-driven methods and model-based methods.
Data-driven methods rely on analysis of the seismic data itself to determine the appropriate scaling factors. This can involve techniques such as automatic gain control (AGC), which adjusts amplitudes based on the overall energy of the seismic signal, or more advanced methods that use statistical analysis or machine learning to identify and correct for amplitude variations.

Model-based methods, on the other hand, use a priori information about the subsurface, such as well log data or geological models, to estimate the appropriate scaling factors. These methods often involve the use of forward modelling, where the seismic response of the subsurface is simulated and compared with observed data to determine the appropriate scaling factors.

The choice of amplitude scaling technique depends on the specific requirements of the seismic inversion project, the available data and the complexity of the subsurface geology. In many cases, a combination of data-driven and model-based methods may be used to achieve the best results.

Amplitude scaling challenges and considerations

Although amplitude scaling is a critical component of seismic inversion, it is not without its challenges. One of the main challenges is the inherent uncertainty and ambiguity in the seismic data, which can make it difficult to accurately determine the appropriate scaling factors. In addition, the presence of complex geological features such as fractures, faults and heterogeneous formations can introduce additional complexities that need to be addressed.

Another consideration in amplitude scaling is the impact of the seismic processing workflow. The various processing steps such as deconvolution, filtering and stacking can have a significant impact on the seismic amplitudes and it is important to ensure that these effects are properly accounted for in the amplitude scaling process.

Furthermore, the choice of amplitude scaling technique can have a significant impact on the final results of the seismic inversion. It is essential to carefully evaluate the assumptions and limitations of each method and to validate the results against independent data sources, such as well log data or other geophysical measurements.
In conclusion, amplitude scaling is a critical component of seismic inversion and its proper application can significantly affect the accuracy and reliability of the inverted subsurface properties. By understanding the factors affecting seismic amplitudes and the various amplitude scaling techniques available, geophysicists can improve the quality and reliability of their seismic inversion results, ultimately leading to better decision making in exploration and development activities.

FAQs

Here are 5 questions and answers about “Amplitude scaling in seismic inversion”:

What is amplitude scaling in seismic inversion?

Amplitude scaling in seismic inversion refers to the process of adjusting the amplitudes of seismic data to match the expected amplitudes of the subsurface reflections. This is an important step in seismic inversion, as the amplitudes of the seismic data can be influenced by factors such as source strength, receiver sensitivity, and absorption in the earth, which need to be accounted for in order to accurately estimate the reflectivity of the subsurface.

Why is amplitude scaling important in seismic inversion?

Amplitude scaling is important in seismic inversion because the amplitudes of the seismic data are directly related to the acoustic impedance contrasts in the subsurface. If the amplitudes are not properly scaled, the resulting inversion may not accurately represent the true subsurface properties, leading to incorrect interpretations and potentially erroneous decisions in exploration and development activities.

What are the main methods for amplitude scaling in seismic inversion?

The main methods for amplitude scaling in seismic inversion include:

Scaling to well log data: The seismic data is scaled to match the amplitudes of the corresponding reflections in the well log data.

Scaling to a reference horizon: The seismic data is scaled to match the amplitudes of a known, high-quality reflector in the seismic data.

Automatic gain control (AGC): The seismic data is scaled using an automatic gain control algorithm to adjust the amplitudes based on the overall statistical properties of the data.

How does amplitude scaling affect the results of seismic inversion?

Proper amplitude scaling is critical for obtaining accurate results from seismic inversion. If the amplitudes are not correctly scaled, the resulting inversion may not accurately represent the true subsurface properties, such as acoustic impedance, density, and porosity. This can lead to incorrect interpretations of the subsurface geology and potentially erroneous decisions in exploration and development activities.

What are some common challenges in amplitude scaling for seismic inversion?

Some common challenges in amplitude scaling for seismic inversion include:

Identifying the appropriate reference horizons or well log data for scaling.

Accounting for complex absorption and attenuation effects in the subsurface.

Dealing with missing or unreliable well log data.

Ensuring consistency in the scaling process across multiple seismic surveys or time-lapse data.

Validating the accuracy of the amplitude scaling through independent means, such as seismic-to-well tie or other geological constraints.