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.
Despite this, similarity these two seismic techniques operate differently and objected to produce different results. And the fact that if you are not an expert in this field of geophysics it would be very challenging to make a clear distinction between these two seismic techniques.
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This post is intended to make clear to you the differences between seismic reflection and seismic refraction. So after reading this article I believe you can realize specifically what made this difference.
What is seismic reflection?
Seismic reflection is the bouncing (throwing) back of the seismic waves after hit à n 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?
The following are the differences between seismic reflection and seismic refraction methods
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. This is known as Laws of reflection, which simply states that, " the angle of incidence is equal to the angle of reflection". You can read more regarding the basic physics of seismic reflection method while considering the figure
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. The seismic refraction can occurs in single to multiple layers as shown in the figure 1 below, with letters i and r stands for angles of incidence and refraction respectively. While number stand to represent the corresponding layer. You can read more regarding the basic physics of seismic refraction.
Figure 1: Seismic ray refraction in multiple layers.
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. The reflected waves become modified and returned to the surface where they are recorded by seismic receivers such as geophones. See figure 2 below
Figure 2: Simple basic Principle of seismic reflection
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. Shock waves propagate within the subsurface until the ray approaches the interface between two types of rocks at critical angle with the layer above it having low velocity compared to the layer below it. See figure 3 below
Figure 3: Simple principle of seismic refraction
The ray refracted along the upper boundary. After refraction the ray travels along this interface with velocity V2. According to the theory of elasticity the material at the boundary are subjected to stress from below it while generates the new disturbance along the boundary that travel upwards (head waves), see figure 4 below, through the low velocity layers and then reach the surface where it get recorded by seismic receivers known as geophones.
Figure 4: Seismic waves propagation along interface and head waves generation.
3. Targets
Seismic reflection methods work best for relatively deep structural discontinuities as the acoustic impedances of shallow ( < 0.1 km) reflectors are generally weak.
However the depth of illumination is limited by the attenuation of seismic energy which in turn depends on the size of the source and initially generated frequency ranges. Example typical depths of interest are less than 10 km, while the typical frequencies are in the order of 10 -100 Hz. This means seismic Reflection can which can resolve structures of 30 - 300 m in size. In order, however, to resolve both small and large scale structures, a wide frequency band source is required.
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 (Greenhouse et al. 1998).
While the primary objective of seismic refraction is to determine the internal subsurface properties such as depth of refractor, velocity of subsurface layers from time - distance plots that involves plotting arrival times against geophone spacing.
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. Example processing reflection data using the Common Depth Point (CDP) method generally requires software to perform the proper trace sorting and corrections.
Another example is Lower amplitude of the recorded reflections which require noise-reducing techniques such as Stacking.
Stacking can be defined as the process of assembling source/receiver combinations from different short records that reflect from the same subsurface point (common depth point). These combinations arise from different source/receiver locations; thus they have different travel path lengths.
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.
However this statement is not generally true in all real practical cases . Because the seismic velocity is always governed by the elastic modulus of the material medium which in turn is related with density of the material medium through which they propagate.
It is generally true that the denser the material medium, then the higher velocity than the less denser material medium. So there is a rise in the Masking Problem. For example a dense rock layer near the surface will mask weathered rock (low velocity) underneath. Another example is that Low velocity sands cannot be detected if they lie beneath denser high velocity soil materials, so this masked known as Hidden (Blind) Layer
Advantages of seismic reflection over seismic refraction.
The main advantage of the seismic reflection method is that it provides more detail and high resolution in the interpreted results compared to the seismic refraction method. However in many cases this is not necessary as a result of the acquisition method itself, but it is due to the powerful use of sophisticated processing and correction techniques.
It produce subsurface Image, hence the seismic reflection technique is more preferred in Oil and gas industry and other industry for subsurface imaging purposes as geophysical Imaging technique
Advantages of seismic refraction over seismic reflection
Velocity and Depth estimation
If we consider a single layer case with a higher velocity layer (V2) at depth Z, from the surface. Then the depth to the higher velocity layer, can be obtained from time - distance plots, by using equation below,
Z = 0.5x( V2 - V1/V2 + V1)
Where, Z = depth, V1 = Low velocity layer, V2 = higher velocity layer, x = critical distance
However also it can be applied to the case of multiple layers see the post related to multiple layer
Since velocity of shock waves are related to elastic modulus such as Young's modulus (E), and other elastic properties such as poisson ratio (v). This in turns allows accessing of the engineering performance of the ground by using the following equation.
E = p(Vp)2 [(1 + v)(1 - 2v)/(1 - v)] and E = 2Vs2p (1 + v)
Where, E = Young's modulus, p = density of the medium, Vp = Velocity of primary waves, Vs = Velocity of secondary (shear) waves, v = poisson Ratio
Reasonable results, simplicity and less expensive
Seismic Refraction methods generally produce reasonable results in areas of thick alluvial or glacial fill and where large velocity contrasts exist, such as buried bedrock valleys.
Another reason is that Personnel and equipment requirements for seismic refraction are generally simpler and less expensive compared to seismic reflection surveys.
Also processing and interpretation of Seismic refraction data is less ambiguous compared to seismic reflection methods.
Seismic reflection and Seismic refraction are both important geophysical methods of subsurface studies , so each method is the best on its own and each has some limitations. However to choose which method to apply will still remain to the geophysical objective of the study itself, nature of geophysical problem, tools, resources and equipment availability, budget and the geology of the study site.
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