What is Seismic Refraction 

Seismic refraction is a geophysical method used to investigate the subsurface properties of the Earth by analysing how seismic waves travel through it. It relies on the principle that seismic waves change speed when they pass through different layers of material with varying densities and elastic properties.

In this method, seismic waves (often generated by a small explosion or a hammer strike) travel through the ground and are refracted (bent) when they encounter boundaries between layers with different seismic velocities. The refracted waves are detected by geophones placed on the surface.

By analysing the arrival times and the angles of the refracted waves, geophysicists can estimate the depth and composition of subsurface layers, such as soil and rock. This technique is particularly useful for mapping bedrock. These surveys are widely employed for geotechnical engineering applications. 

How Does Seismic Refraction Work

Seismic refraction surveys generate compressional waves (P-waves) that travel through the earth, refracting at boundaries between layers with different seismic velocities. The seismic source is usually either a sledgehammer, weight drop or explosives on land or various types of airguns when used over water. The seismic waves are recorded using specialised sensors—geophones for land-based surveys and hydrophones for marine seismic surveys. 

The velocities of the seismic waves depend on the density and elastic properties of subsurface materials through which they travel. The velocities of seismic layers are measured by recording the arrival times of direct and refracted P-waves with geophones positioned along the survey line. 

The method assumes that the seismic velocity of layers increases with increasing depth.  The simplest case for seismic refraction consists of two seismic layers (e.g., soil underlain by bedrock). In this example, direct waves which travel along the ground surface are measured by geophones as they travel away from the seismic source. Their arrival times at different distances from the source allow us to calculate the velocity of the upper layer.  The waves also travel deeper into the ground and refract at the soil/bedrock interface.  The waves then travel through the bedrock and also refract back up to the surface.  Because the waves travel faster through the rock, at a certain distance from the seismic source, the refracted arrivals with begin to arrive before the direct arrivals.  Their arrival times at different distances from the source allows us to calculate the velocity of the deeper bedrock layer.   Using the velocities calculated for the two layers, Snell’s law can be used to calculate the refraction angles between layers of differing velocity allowing us to determine the theoretical refracted ray paths through the subsurface and the corresponding seismic velocities and depths to the different layers.

 

image source:  https://en.wikipedia.org/wiki/Seismic_refraction#/media/File:Refraction_2layers.png

 

Common Applications of Seismic Refraction

Seismic refraction surveys play a crucial role in geotechnical site investigations, as they help determine soil and rock layer depths, bedrock stability, and groundwater presence. Typical applications include rippability assessments and depth to bedrock investigations for roads, pipelines, housing developments and major infrastructure projects.

Data Processing

In seismic refraction surveys, data processing focuses on analysing wave travel times to calculate subsurface layer depths and velocities, which is crucial for engineering projects.  A number of approaches to interpretation are possible which can generally be divided into two main categories:  

1. Calculation of discrete layer models (using the Reciprocal Method and/or other methods); and
2. Calculation of smooth models using Tomographic Inversion.

Discrete layer models may be preferred for applications such as rippability investigations where absolute depths and velocities of layers are required. This approach works best where the subsurface materials are best represented as discrete layers.  On the other hand, a tomographic approach to interpretation may be preferable where the velocity increases with depth more gradually or where velocity decreases with depth (known as a velocity inversion, which violates one of the main assumptions of the method).

 

Example of a seismic refraction profile produced using the reciprocal method supplemented by the intercept time method (Refract 2006 software).

Example of a seismic refraction profile produced using a tomographic approach (2D Wavepath Eikonal Traveltime inversion, Rayfract software).

Advantages of Seismic Refraction

The main advantages of seismic refraction include:

 

1. Effective for Shallow Layers: It works well for mapping shallow subsurface layers, such as the depth to bedrock or groundwater levels, which is useful in construction, environmental studies.

 

1. High lateral and vertical resolution: depending on survey geometry.

 

1. High Accuracy for Velocity Profiling: It provides accurate information about the velocities of seismic waves, which can be used to estimate the material properties of subsurface layers (e.g., soil, rock, or water).

 

1. Non-destructive: The method does not require drilling or disturbing the subsurface, making it non-invasive and suitable for sensitive environments or areas where drilling might be impractical.

 

Challenges and Limitations of ERT

While seismic refraction has several advantages, it also has its limitations. Some of the key limitations include:

 

1. Seismic Noise: Seismic refraction relies on a source of energy (such as a hammer strike or explosive charge) to generate seismic waves which have a significantly stronger signal compared to background seismic noise. Ambient seismic noise can be a significant problem for seismic refraction in urban environments, in adverse weather or in areas where plant or machinery are operating nearby.  

 

1. Limited to Shallow Depths: Seismic refraction is primarily effective for shallow subsurface investigations. For deeper investigations explosive sources are generally required which are more expensive and may require permits etc.

 

1. Velocity Reversals/Inversions: Seismic refraction relies on seismic velocity increasing with depth. In some environments this may not be the case.  While some software can image velocity inversions, the accuracy of the calculated thickness and velocity of inversion layers is relatively poor.

 

1. Hidden Layers: relatively thin intermediate layers may remain undetected if first arrivals from deeper faster layers arrive first. This can lead to large inaccuracies of calculated depths to the deeper layer.

 

1. Low Resolution for Small-Scale Features: Seismic refraction may not effectively detect small-scale subsurface features, such as narrow faults or thin layers.

 

1. Difficulty in Identifying Layer Boundaries: In some cases, especially where layers have similar seismic velocities, seismic refraction may have difficulty distinguishing between different layers or determining the exact boundaries between them.

Poor Performance in Unconsolidated Materials: In areas with loose or unconsolidated soils (e.g., loose sand), seismic refraction can be less effective, as the seismic waves are attenuated more rapidly.