Use of Seismic Frequencies in Hydrocarbon Exploration

Satyesh Bhandari

In the last century, when seismic data were first available for use in oil exploration, the seismic data were used only for subsurface geologic mapping. Later, it was realized that use of characteristics of seismic data, especially the seismic amplitude can be effectively used for more meaningful and confident interpretation.

The use of seismic frequencies in hydrocarbon exploration is also crucial for analyzing subsurface geological features and hydrocarbon reservoirs.

Here’s how frequencies play a role in exploration:

Data Processing and Interpretation:

• Seismic data are processed to filter and analyse different frequency components. Techniques such as spectral analysis help to separate and interpret high and low-frequency signals, aiding in the identification of specific geological features.
• Frequency filtering can also help in noise reduction. For instance, high-frequency noise might be filtered out to enhance the clarity of the data related to the desired subsurface features.

Resolution and Detail:

• Vertical Resolution: Higher frequencies improve vertical resolution, allowing better distinction between closely spaced reflectors. This is essential for identifying thin beds and subtle stratigraphic features.
• High Frequencies: High-frequency seismic waves have shorter wavelengths, which provide higher resolution images of the subsurface. They can reveal finer details of geological structures and shallow features. However, high frequencies are more likely to be absorbed or scattered by the earth, which can limit their depth penetration.
• Horizontal Resolution: Influenced by the dominant frequency and velocity of the seismic waves. Higher frequencies can also enhance horizontal resolution but are more susceptible to attenuation.
• Low Frequencies: Low-frequency waves have longer wavelengths, allowing them to penetrate deeper into the earth. They are useful for identifying larger geological structures and deeper resources, but they provide less detailed resolution of shallow features.

Depth of Investigation:

• Seismic surveys often need to balance between resolution and depth. For deeper targets, low-frequency data are preferred because they penetrate further into the earth’s crust. For shallower targets, high-frequency data can provide more detailed images.

Tuning Effects

• Seismic Tuning: Occurs when the thickness of a geological layer is approximately a quarter of the wavelength of the seismic wave. This can enhance or diminish the amplitude of reflections, impacting the interpretation of bed thickness and lithology.

Attenuation and Absorption

• Frequency-dependent Attenuation: Higher frequencies are absorbed more quickly than lower frequencies as they travel through the Earth. This can indicate changes in lithology, fluid content, and porosity.
• Q-factor (Quality Factor): Describes the attenuation of seismic waves. Lower Q-values indicate higher attenuation and can be used to infer fluid presence, such as gas in a reservoir.

Spectral Decomposition

• Frequency Analysis: Decomposing seismic data into its frequency components helps identify subtle features not visible in conventional time or depth domain data.
• Spectral Attributes: Can highlight features like channels, faults, and fractures by analyzing the frequency content of the seismic signal.

Seismic Inversion and Impedance

• Inversion Techniques: Convert seismic reflection data into a quantitative rock property, such as acoustic impedance. This process often relies on understanding the frequency content to improve the resolution and accuracy of the inversion results.
• High-frequency vs. Low-frequency: High-frequency components provide detailed layer information, while low-frequency components give the overall trend and are crucial for inversion stability.

Frequency Filtering

• Band-pass Filtering: Used to enhance specific frequency bands of interest while suppressing noise and irrelevant frequencies.
• Frequency Filtering Applications: Helps in identifying and enhancing particular seismic events, improving signal-to-noise ratio, and focusing on specific subsurface features.

Interpretation of Geophysical & Geological Features

• Lithological Variations: Different rock types have distinct frequency responses. For instance, shales might attenuate high frequencies more than sands.
• Fluid Content: The presence of hydrocarbons can be inferred from frequency anomalies, such as bright spots (high amplitude reflections) that are frequency dependent.
• Geological and Physical Property Differentiation: Different geological materials and structures have varying impacts on seismic wave propagation. Frequencies can help differentiate between types of rocks, fluid reservoirs, and fault lines. For example, low frequencies might be used to differentiate between rock types or detect fluid reservoirs, while high frequencies can help identify small-scale features like fractures.

Advanced Techniques

• Time-Frequency Analysis: Techniques like Continuous Wavelet Transform (CWT) provide a detailed time-frequency representation, allowing for more precise interpretation of seismic events.
• Seismic Attributes: Attributes like instantaneous frequency and spectral bandwidth are used to highlight and interpret geological features and reservoir characteristics.
• Enhanced Imaging Techniques: Advanced techniques like full-waveform inversion and multi-component seismic analysis use frequency-dependent information to create more accurate models of subsurface structures. These methods exploit various frequencies to build detailed 3D models of the subsurface.

Understanding and utilizing frequency in seismic interpretation enables geophysicists to extract more detailed and accurate information about the subsurface, improving the identification of potential hydrocarbon reservoirs and other geological features.

Overall, the effective use of frequencies in seismic data allows geophysicists to tailor their exploration techniques to specific targets and improve the accuracy of their geological models, which is critical for resource exploration and management.