The difference between Seismic Reflection and Refraction Explained

Seismic Reflection and Seismic Refraction are the two geophysical techniques of active seismology. These seismic techniques measure the travel time of the acoustic seismic waves after they either reflect or refract from different material mediums. Also the fact that they utilize the same data acquisition instruments and tools ranging from seismic sources like sledge hammer, dynamite, to seismograph receivers like geophone and hydrophones still continue to pose questions regarding their differences.

What is seismic reflection?

Seismic reflection is the bouncing (throwing) back of the seismic waves after hit àn interface known as a reflector. Seismic reflection method relied on recording the reflected seismic waves from the geological interface. Seismic reflection applies the same Principle of wave ray theory as that of reflection of light waves.

What is seismic refraction?

Seismic refraction is the bending of seismic waves after passing through two different media of different densities. The bending of Seismic waves is due to either increase or decrease of its velocity when passing from one medium to another.

Seismic refraction method relied on recording the refracted head waves after passing through an interface between two velocity layers. The seismic refraction applies the same Principle as bending of light waves.

What is the difference between Seismic reflection and seismic refraction?

1. Basic physics

Basic principle of seismic reflection is based on the fact that when seismic waves reach the reflector at a certain angle of incidence, then they may bounce back with a certain angle of reflection which is always equal to that of incidence assuming that the nature of the reflector is smooth.

While Basic principle of seismic refraction is based on Snell's Law, the ratio of sine of angle of incidence to the sine of angle of refraction gives the constant term known as Refractive index.

2.Fundamental Principle

The fundamental principle of seismic Reflection method is that when seismic waves from seismic energy sources are sent down into the subsurface, they become reflected on subsurface interfaces (reflector) such as discontinuities where there is acoustic impedance contrast.

Fundamental principle of seismic refraction is that when the ground is disturbed by either mechanical pounding such as sledge hammer, vibrator or detonation by explosive, the surface generates shock waves (P and or S - waves) that radiates out in a hemispherical wave front from point of release.

3.Targets

The  seismic refraction method is useful when the target is shallow and low-dipping and consists of materials with different elastic properties. However the type of target will often depend on the field at which it applied. Example in hydrogeological investigation, seismic Refraction surveying is often used to detect the water table and to determine its depth from the surface. Also in shallow engineering investigation seismic refraction can be used to determine the depth of a shallow bedrock interface.

4. Primary objective

The primary objective of seismic Reflection is to determine the internal elastic properties where there is  impedance contrast (velocity × density) and finally produce the subsurface image .The seismic reflection method is useful for mapping variable subsurface topography and structure and stratigraphy in the overburden.

5. Limitations

Data processing capability

Limitation of Most seismic reflection is on Data processing capability, since seismic reflection surveys  involve complex and  large amounts of data sets which in turn require integrated processing resources including computer softwares.

Velocity increase with depth

Limitation of seismic refraction is that the velocity must increase with depth, meaning that the velocity of the certain layer should be lower compared to another layer underneath (below) it. Such that the layer buried at a depth of 10 feets should have (transmit) low seismic velocity than the layer buried at depth of 30 feets. This means that the seismic refraction method will detect only horizons which increase in velocity with depth.