Seismic FAQ
What is Seismic Trace?
Seismic trace is a graphical representation of seismic wave recorded by the geophone. The seismic trace contained the information like – Amplitude, Time, Wave types, Reflection and Refraction Patterns, Frequency Contents, and Noise Patterns.
Why Frequency Analysis of Seismic Signal is Carried Out at -12dB?
The signals are affected by background noise and attenuation as they propagate through the Earth.
A seismic signal at -12 dB retains about 25% of the maximum amplitude, that means the frequency components are still strong enough to be relevant without being overwhelmed by noise.
It ensures that the frequencies still contribute meaningfully without being drowned out by signal losses.
The signal analysis at -12 dB, can filter out both very high and low frequencies of noise, ensuring the signal carries genuine seismic information.
Higher thresholds (e.g., -6 dB) might exclude important low frequency components, while much lower thresholds such as (e.g., -20dB), would include the frequency of noise.
Therefore -12 dB is commonly used as a practical benchmark for understanding how much the signal has decayed from its peak value.
The frequency analysis at -12 dB helps balance between retaining useful signal components and eliminating noise, making it an optimal reference for seismic data analysis.
What is Reason for Frequency Shifts in Depth Domain?
When seismic data is converted from time to depth, the relationship between time and depth is non-linear due to variable subsurface velocities.
High velocities for a given depth will correspond to a shorter time interval. Conversely, in low-velocity zones, the time interval will be stretched. This vertical stretching or wavelets stretching affects the apparent frequency content in the depth domain.
The accuracy and complexity of the velocity model used for time-to-depth conversion also play a major role. If the velocity model is not accurate, the depth domain data may experience frequency shifts.
The uncertainties in the velocity model may result in smearing or blurring of seismic events, which may reduces higher frequencies in depth domain.
The seismic conversion from time to depth involves data resampling of data, which can also suppress higher frequency components in the depth domain.(e.g. time domain sampling is 2ms and depth sampling is 3m).
What is Active and Passive Seismic?
In active seismic, a control energy source is used to generate seismic wave. Data is recording is completed within few second to a minutes.
In passive seismic, we normally used an uncontrolled energy sources such as earthquakes, ocean waves and tsunami are used to generate the seismic waves. Data recording is generally done from 24 hours to several days continuously.
What is Time and Depth Domain Processing?
The time domain seismic data processing is also known as signal processing, it is carried out to filter or remove noise, gain balancing and geometry corrections (data regularisation) and event migration to actual time locations.
Whereas the depth processing is carried out to update velocity model for anisotropic variations in the subsurface, and providing an accurate image at correct depth. Velocity modelling, Seismic tomography and Migration are the fundamental steps in depth imaging.
Learn More About Seismic
Click 👉 Seismic Data Acquisition
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Click 👉 Seismic Data Interpretations
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