Creating a Vibroseis sweep with user defined parameters
What Is a Vibroseis Sweep?
A Vibroseis sweep is a controlled vibration generated by a vibratory seismic source (vibrator truck). Instead of producing an explosive impulse, the vibrator emits a smoothly varying tone that starts at a low frequency and gradually moves to a higher frequency over a specific time interval.
This controlled signal travels into the ground, reflects off geological layers, and is recorded by geophones. The reflected sweep is later correlated with the original sweep to produce a clean seismic trace.
Why Use a Vibroseis Sweep?
•Allows safe, repeatable, controllable energy
•Produces broad frequency content
•Better penetration and better signal-to-noise ratio
•Adaptable to surface conditions (through custom sweep design)
•Works well in environmentally restricted areas
Compared to impulsive sources, Vibroseis gives much better control over the transmitted energy.
Key Characteristics of a Sweep
A Vibroseis sweep is defined by:
a. Frequency Range
•Start frequency: where the sweep begins
•End frequency: where the sweep finishes
For example, a sweep may start at 6 Hz and end at 80 Hz.
b. Duration
•Total time over which the sweep runs
•Includes gentle fade-in at the start and fade-out at the end
Modern sweeps often last 8–16 seconds.
c. Frequency Progression (Sweep Type)
This defines how the frequency changes with time:
1.Linear Sweep
Frequency increases at a constant rate.
2.dB/Hz Sweep
Energy is balanced so each unit of frequency contains equal energy.
3.dB/oct Sweep
Energy is distributed evenly per octave (doubling frequency).
4.T-power Sweep (Nonlinear)
Frequency progression is curved, allowing faster or slower movement through certain frequency bands.
Each type is chosen based on ground conditions, noise levels, and processing needs.
d. Amplitude Shaping (Tapers)
A Vibroseis sweep does not start or stop abruptly. It uses:
•Start taper: gradually increases amplitude from zero
•End taper: gradually reduces amplitude to zero
This prevents mechanical shocks to the vibrator and reduces unwanted noise.
How a Vibroseis Sweep is created?
Below is a clear, step-by-step description of how sweeps are generated in processing systems.
Step 1 — Define the Sweep time window
Engineers choose four key time points:
•T1 – Sweep start time
•T2 – End of starting taper
•T3 – Beginning of ending taper
•T4 – Sweep end time
Between T1 and T2, the sweep fades in.
Between T2 and T3, the sweep runs at full strength.
Between T3 and T4, the sweep fades out.
Step 2 — Choose the Sweep type
The sweep type defines how the frequency increases.
•Linear sweep - steady, uniform frequency increase
•dB/Hz or dB/oct sweeps - control energy distribution
•T-power sweeps - flexible control over speed of frequency movement
This decision is influenced by ground conditions (soft, hard, noisy, etc.) and processing goals (better penetration, more high-frequency content, etc.).
Step 3 — Specify the Frequency range
The sweep designer sets:
•Start frequency (low end)
•End frequency (high end)
This determines the bandwidth of the transmitted energy.
Step 4 — Create the Frequency progression
Software calculates a smooth change in frequency over time according to the chosen sweep type.
For example:
•A linear sweep increases frequency evenly
•A T-power sweep moves slowly at the beginning and faster at the end
In all cases, the progression is smooth, with no jumps.
Step 5 — Build the Amplitude Envelope
The sweep’s amplitude envelope is built using:
•Cos² taper (smooth, rounded) or
•Linear taper (straight line)
This ensures the sweep:
•starts gracefully
•maintains steady energy in the middle
•ends gracefully
The amplitude envelope is applied to shape the final sweep signal.
Step 6 — Generate the Sweep signal
Once the frequency progression and amplitude envelope are created, the software combines them to generate the final sweep.
This produces a continuous vibration signal with:
•smoothly changing pitch
•controlled amplitude
•defined start/end conditions
•consistent mechanical load for the vibrator truck
There are input data requirements for this module. Everything should mention in the Parameters tab.
Number of harmonics - define the number of harmonics. If more than 1 harmonic is added then the sweep energy will be more complex. Each additional harmonics add a sinusoidal component and increases the fundamental frequency.
Start Frequency Fr1 - specify the starting frequency of the sweep. By default, 1.
End Frequency Fr2 - specify the ending frequency of the sweep. By default, 5.
Sweep type { Linear, DB/Hz, DB/oct, T-power } - select what kind of sweep to create. By default, Linear.
Sweep type - Linear - most common sweep type. In this linear type, frequency increases linearly from Fr1 to Fr2.
Sweep type - DB/Hz - this type of sweep is used for ground roll attenuation. Higher frequencies receive smaller amplitudes to remain the flat spectrum.
Decibels per hertz - specify decibels per Hertz. By default, 0.
Sweep type - DB/oct - energy is distributed evenly for per octave, which means doubling the frequency.
Decibels per octave - specify the decibels per octave. By default, 0.
Sweep type - T-power - frequency progression is curved in this type of sweep allowing faster or slower movement through certain frequency bands.
Power of time - used to control ground roll or match geophone response. If power of time value is greater than 1, sweep accelerates faster at the end. If the power of time is less than 1, sweep moves faster in the beginning. By default, 0.
Taper Type { Cos2, Linear } - these tapers are helpful in smoothing of the sweep at the start and end by avoiding any jumps. By default, Cos2.
Cos2 - It is the most common taper type. It is smooth and rounded. This taper reduces the leakage.
Linear - It is a simpler taper where amplitudes are increasing or decreasing in a liner fashion.
Sweep dt - this is the sampling interval of the sweep. Specify the sampling interval in ms. By default, 2 ms.
Sweep Start time T1 - this is the starting time of the sweep. At this time, amplitude of the sweep is zero. By default, 0
Taper end time T2 - this is the initial taper end time. The amplitude between T1 & T2 gradually increases. By default, 500
Taper Begin time T3 - this is the ending taper time. The amplitude between T3 & T4 gradually decreases. By default, 20000.
Sweep End time T4 - this is the ending time of the sweep. At this time, amplitude of sweep is zero. By default, 20500.
Skip - By default, FALSE(Unchecked). This option helps to bypass the module from the workflow.
Sweep with harmonics - generates and outputs sweep with harmonics. If the number of harmonics are three then there will be 4 traces (1 sweep + 3 harmonics)
Modeled sweep signal - this is the final generated sweep.
There is no information available for this module.
In this example workflow, we generate a sweep with the default parameters.
Adjust the parameters with difference number of harmonics, taper types and sweep types.
There are no action items available for this module so the user can ignore it.
YouTube video lesson, click here to open [VIDEO IN PROCESS...]
Yilmaz. O., 1987, Seismic data processing: Society of Exploration Geophysicist
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