Key Differences Between Seismic Reflection and Seismic Refraction

Seismic reflection and seismic refraction are two primary seismic methods used to explore the Earth’s subsurface, especially in the petroleum industry. Both methods rely on the behaviour of seismic waves as they travel through different layers of the Earth, but they operate differently.

What is Seismic Reflection?

Seismic reflection is a geophysical method used to map the subsurface of the Earth by analyzing the reflection of seismic waves off different geological layers. When seismic waves, generated by an energy source, travel through the Earth, they encounter boundaries between different rock types.

At these boundaries, part of the seismic energy is reflected back to the surface, where it is detected by sensors (geophones on land or hydrophones in water). This method is particularly valued in the petroleum industry for its ability to create detailed images of subsurface structures, such as faults and folds. Seismic reflection offers high accuracy, resolution, and penetration depth, making it the preferred method for deep crustal studies.

What is Seismic Refraction?

Seismic refraction is another geophysical technique that involves measuring the refraction of seismic waves as they pass through different subsurface layers. Unlike reflection, where the waves bounce back to the surface, refraction focuses on the waves that are bent and continue to travel through the subsurface layers.

The primary path of these waves is along the interface between layers, making this method effective for determining the depths of subsurface interfaces and the velocities of seismic waves in each layer. Seismic refraction is often used in engineering and environmental studies, where it is less common but highly useful for delineating subsurface layers.

Difference between refraction and reflection seismic?

Refraction and reflection seismics differ mainly in their focus on seismic wave motion and the type of results they produce. Refraction seismics concentrates on seismic waves traveling laterally along subsurface layer interfaces, producing a layered velocity model as its primary output. Reflection seismics, on the other hand, emphasizes vertically propagating waves that reflect off subsurface boundaries, creating a detailed image of the subsurface stratigraphy and structure.

While both methods respond to changes in the density and elastic properties of subsurface materials, their distinct focuses and outputs make them suitable for different geological and industrial applications. Refraction is commonly used for building velocity models, whereas reflection is preferred for detailed structural imaging.

Advantages and Disadvantages of Seismic Methods

While seismic techniques are generally more expensive to undertake than other geophysical methods, they can produce remarkably detailed images of the subsurface. However, this high level of detail comes with a significant economic cost.

Therefore, when selecting the appropriate geophysical survey method, it is crucial to assess whether the increased resolution and detail provided by seismic methods are justified in terms of the costs associated with conducting and interpreting the survey.

How to Choose the Right Method for Subsurface Imaging？

Refraction Surveys are commonly used for tasks such as predicting depth to bedrock, assessing rock strength, determining layer thickness, identifying fault and fracture zones, evaluating soil stability, and investigating landfills.

They provide quantitative results through p-wave (pressure wave) velocities in soils and bedrock, with a relatively straightforward field procedure and processing workflow. Refraction surveys only require an accurate record of the first seismic wave arrival at each geophone, making them less susceptible to noise in urban areas.

However, they produce a predictive model rather than a true image of the subsurface and rely on the assumption that ground velocity increases with depth. This method may not be suitable in conditions where a high-velocity layer lies above a low-velocity layer, such as in cold climates where the top layer is frozen.

Reflection Surveys are ideal for mapping layer thicknesses, bedrock levels, horizon profiling, and detecting voids, sinkholes, faults, and horizontal fracture zones. They are particularly valuable in complex geological environments where high detail is necessary.

Reflection surveys theoretically provide a “true” image of subsurface stratigraphy and are effective even when a high-velocity layer overlies a low-velocity layer. However, they require more shots and receiver locations than refraction surveys, involve more complex data processing, and can be challenging to execute in noisy environments, where achieving good results may be difficult.

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References:

Seismic Reflection and Refraction Methods A. K. Chaubey National Institute of Oceanography, Dona Paula, Goa-403 004. chaubey@nio.org