3 Types of Seismic Reflection Surveys in geophysics
Seismic data acquisition is the systematic process at which the modified seismic waves from seismic reflectors are recorded by various seismic receivers such as geophones or hydrophones. That is simple to define, however to know the way through which this process occurs is very important as this will give you an idea when dealing with seismic interpretation tasks.
And this is why I have written this piece of article to help you to have an idea on how seismic surveys can be carried out.
There are two common techniques at which field seismic data acquisition can be conducted, these are 2-D and 3-D techniques. Due to advancing in technology there is another term known as 4D - seismic technique especially in oil and gas industry,
However deciding which one you can deploy will depend on various factors such as geophysical seismic objectives, budget, size of survey area, nature of geological terrain being surveyed.
Let's start to describe these terms in more convenient manner as shown here below,
Types of seismic reflection surveys
(1) 2D - seismic survey
Here the seismic array is straight line, meaning that your seismic source and receivers are arranged in one line throughout the survey area.
If it is a ground based seismic reflection survey make sure seismic lines have been cleared out even if they are not totally straight let them be at least rough straight.
Example, let's say 5 receivers are placed in a straight line at which seismic source (shot) is moving into a different position along that line as the survey is proceeding. See the figure 1, there below so as to gain a better insight regarding this. The key for the figure 1 is that S - seismic source while R - seismic receivers.
This means that your end seismic section will have only two dimensions such as X and Y only. I have used X and Y as just a representation for any parameters that you are interested in. However most seismic sections have time - depth parameters.
It means 2D - seismic represent 1D of X parameter plus 1D of Y parameter.
We can take an example for simplicity of the term 2D section as X - Y coordinates plotted in X - Y plane.
By this example it can be said thatthe 2D - seismic section is profiling section.
Figure 1: A simple 2D- seismic survey representation.
2D - seismic survey is generally simple to carry out even for ground seismic survey, however one of the challenges you will face on this technique is that your section will miss some important geology plane structures such as faults or any other linear structures. So make sure you have the plan on how you will extract these structures if your survey objective will have interest with them.
(2) 3D - seismic survey
As the number 3 in this term is telling us, that the parameters should be three such as X, Y and Z. These are just representations however in seismic section X and Y would stand for sections such as inline and crossline respectively. The Z would stand for depth parameter.
See figure 2 as the blue line (inline) is where the seismic survey ship is moving. The crossline (blackline) must be at 90° with inline as illustrated in figure 2.
It means 3D - seismic represent 2D plus 1D of Z parameter.
Figure 2: A simple representation of 3D- seismic section.
So carrying out a 3D seismic reflection survey requires multiple arrangements of seismic receivers in different seismic lines at which a seismic shot deployed in such a way that its seismic energy would spread throughout the receivers, so as to increase Signal to Noise ratio (S:N).
Of course this is complex technique, and more expensive to deploy than 2D seismic reflection survey because very serious planning is required on how your shot should give the best results. Let us limit ourselves as this talk is not on how to design your seismic reflection survey.
You may look at figure 3, here below to gain more insight.
Figure 3: A simple representation of 3D - seismic survey.
As explained above the 3D - seismic reflection survey is the best because those plane geology structures such as faults would not be missed because we can rotate this section whatever we want, at any angle so there will be no need for another plan of extracting these structures. And the best news is if you carry out 3D seismic it means you also carried 2D seismic reflection survey indirectly.
(3) 4D - SEISMIC SURVEY
4D seismic survey stands for 4 dimension seismic reflection survey, which technically is the 3D seismic technique that repeated over a period of time to create a set of time lapse (3D) images of the earth subsurface. It is also known as 3D-lapse seismic reflection survey, So it apply the same basic principle as a regular 3D seismic survey as described above.
The 4D seismic survey is mostly utilized in oil production, as this survey is used to monitor the production fluid parameters such as temperature, pressure over a specified period throughout the well production life cycle.
Let say before production a baseline survey is carried out, then after a specified period say 5 years another survey will be carried out to monitor how the pre production conditions varies over the well production cycle. Any difference recognized in 3D Lapse images can be regarded as 4D signatures. So the fourth parameter in 4D - seismic survey is "Time".
All in all, any of these three (3) techniques can give good results in your seismic reflection survey, however if you want to have abundant seismic reflection data with useful hidden information 3D - seismic is the way you can go, however you have to know that this survey would cost you more so plan it carefully.
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