It is obviously known that when you talk about seismic refraction we simply mean that it is bending phenomena with the same bending concepts as that of light waves. This bending is directly related to the changes of velocity of seismic waves as it passes from one medium to another of different elastic properties (density).
Here down I have described to you a brief on this matter. Let's be together till the end of this description.
What is seismic refraction?
Seismic Refraction Is the geophysical technique that applies seismic acoustic waves to determine the internal elastic properties of the subsurface by using refracted seismic waves.
Basic Principles of Seismic Refraction
Before going directly to describe the working principle of this technique, Let's first become familiarized with the basic Laws of refraction which can be explained by Snell's Law.
Snell's Law described that the ratio of the sine of angle of incidence (i°) to the sine of angle of refraction (r°) is the constant term known as "Refractive Index".
Different medium (materials) have their refractive index such as water has 1.33, while air has 1, It may be denoted by the letter n.
Such as n = sin i° / sin r°
Figure 1: Simple representation of Snell's Law
Also Refractive Index of material can be obtained as the ratio of the Velocity of wave in medium 1 to that of medium 2. Consider the figure 1 above
Such that n = V1 / V2 provided that V2 > V1.
Then n1 sin i° = n2 sin r°
When you rearrange this, you will have the equation below,
n2 / n1 = sin i°/ sin r° = V1 / V2
By considering the figure 2 below, When the seismic waves from the seismic source (at A) are sent down into the subsurface (AB) they will propagate with the same velocity until they reach the material where there is density contrast (at B). At the transition between the overlying and underlying layers these waves tend to bend as the result of changing in Velocity.
At critical refraction they will propagate horizontally (BC) at that boundary until they reach a point where the angle of incidence to the normal is the same as that of the incident wave.
At that point they emerge as Head waves (CD) which then propagate to the point on the surface where they will be recorded by receivers (geophones at D).
The travel times of received waves is recorded, and the depth (Z) of the boundary to the surface can be estimated.
The diagram below shows how seismic waves propagate from source via subsurface to seismic receiver.
Where , A - Source and D - Receiver
AB - incident (sent) wave
BC - Refracted wave
CD - Head wave
AD - Direct wave
X - Offset distance (distance between source and receiver)
Z - Depth to the surface from the boundary.
Head waves can be defined as refracted waves that leave the high velocity medium ( V2) at the critical refraction.
Critical refraction occurs when waves bend away from normal until the angle of refraction is 90°, hence the angle of incidence becomes known as Critical Angle.
From Snell's Law
Recall, n = sin i° / sin r°
When r° = 90°, then i° = ic (Critical Angle).
Then, n = sin ic
Seismic refraction plays an important role in providing assistance to civil Engineering works in characterizing subsurface elastic Velocities and estimates depth of bedrocks (layers), rippability and hence the stability of the ground can be revealed.
Seismic refraction brings the best results when seismic velocity in layers increases with depth. However the great challenge came during the signal processing and interpretation of acquired data from non - uniform subsurface geology, because this situation may cause a hidden layer problem as a result of seismic velocity inversion (velocity decreases with depth).
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Do you want to carryout Electrical resistivity and Seismic refraction Surveying? If Yes, here below is the simple list of equipment and tools that may help you.
BASIC RESISTIVITY EQUIPMENTS and TOOLS
- Electrical resistivity meter
- Geological Hammer (The friend of Geologists)
BASIC SEISMIC TOOLS
- 3D Components - Geophones, detector, vibration sensors
- GD - One Geophone and Exploration equipment
- GDH - 4.5 Seismograph and Vibration sensors
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