Since seismic data are mostly important tools in hydrocarbon exploration and production monitoring. Apart from that there are other applications based on target scale such as in groundwater, engineering and environmental studies (10's m) and studies on crustal structures (10' km). There are two (2) common transient seismic methods such as seismic reflection and seismic refraction. These techniques are based on wave ray theory in their operation.
However, today through this post, I would like to talk to you about seismic reflection. Without wasting time let's go to see how this technique is operating.
What is Seismic Reflection?
Seismic Reflection is the geophysical technique that uses seismic acoustic waves to determine the subsurface condition such as elastic properties from reflected seismic waves.
Seismic Reflection is also known as Reflection seismology. However In oil exploration industry it simply abbreviated as " Seismic"
The reflection methods work best for relatively deep structural discontinuities as the acoustic impedances of shallow ( < 0.1 km) reflectors are generally weak.
The depth of illumination is limited by the attenuation of seismic energy which depends on the size of the source and initially generated frequency ranges.
In this technique, typical depths of interest are less than 10 km, while the typical frequencies are in the order of 10 -100 Hz, which can resolve structures of 30 - 300 m in size. In order to resolve both small and large scale structures, a wide frequency band source is required.
The basic working Principle of seismic reflection
It relies on the wave ray theory that utilizes acoustic waves measured in millisecond time scales. Also it applies the basic Laws of reflection the same as those used in optics geometry such that the angle of incidence is equal to the angle of reflection on the plane surface.
When seismic waves from seismic energy sources are sent down into the subsurface, they become reflected on subsurface interfaces (reflector) such as Discontinuities. The reflected waves become modified and returned to the surface where they are recorded by seismic receivers such as geophones. See figure 1.
The receivers records the travel times, since the velocity of sent waves is known then you can estimate the depth of an interface by using the formula below
V = D/t
Where V - is the velocity of the waves in that layer, D - depth of the interface and t - is one way travel time.
N.B The interface here is assumed to be single layer which is horizontal as shown in the figure 1 below.
When we consider many layers with number n, their depth can be estimated as
Va = Dn/Tn
Where Va - Average Velocity of waves in n layers, Dn - Is the total depth of the n layers, and Tn - Is the total one way travel time in n layers.
You can process reflected Signals into Depth - time sections at which 2D/3D image about the subsurface can be produced. This is why seismic reflection can be considered as a Geophysical Imaging technique.
It is this reflected wave from an elastic interface that is interested in this technique, since the amplitude of the reflected wave from an interface is related to the contrast in the acoustic impedance.
Figure 1: Simple Model representation of the seismic reflection.
Figure 2: Simple representation of a shipborne seismic reflection.
Amplitudes of Reflected and Transmitted waves.
Amplitudes of these rays are given by ZOEPPRITZ EQUATION.
It describes rays as a function of incident angle
Acoustic impedance (Z)
It can be defined as the product of the velocity of the ray in the medium and the density of that medium.
Z = V x d
where, d is the density of that medium. and
V is the velocity of the ray in that medium.
Reflection Coefficient (Fs)
Is the ratio of amplitudes of reflected rays to that of incident rays.
Fs = Ar/Ai
Fs = Z2 - Z1/Z2 + Z1
Where,
Ar : Amplitude of reflected ray
Ai : Amplitude of incident ray
Z1 and Z2 : Acoustic impedances of first medium and second medium respectively
N.B, Fs is usually small since only about 1% of energy is reflected.
Transmission Coefficient (T)
Is the ratio of amplitudes of transmitted ray to that of incident ray.
T = At /Ai
T = 2Z1/Z2 + Z1
Since, T = 1 - Fs
T = 1- (Z2 - Z1 /Z2 + Z1)
Then,
T = 2Z1 / Z2+Z1
However Seismic reflection has some limits in Near-surface applications particularly in detection of cavities, Since high frequency tends to attenuate more hence resulting in weak reflected and modified signals with low resolution that make difficult to detect these structures effectively.
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