Pre-deconvolution step includes the first iteration of noise attenuation, such a linear, coherent and random noise, ground-roll, ambient noise, seismic interference, corrupted samples. Surface-consistent deconvolution step requires that ground roll and linear waves are attenuated before the application.

In order to attenuate ground roll and spikes we are going to use FDNA and Despike modules and 2D Radial trace denoise for linear waves (noise). Notice this is the first iteration of a noise attenuation step and it should be done in soft a mode in order to make low frequencies wider after deconvolution, because low frequencies on the current data mostly consist of linear waves (noise) rather than signal. Accordingly, low frequency noise would contaminate deconvolution operator which lead to negative effect on the resulting seismic traces. Group of these modules gives acceptable attenuation of lower frequent and high amplitude noise (ground roll) and spikes with no any signal damaging. Traces sorting is source gather order.

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There are many different modules for noise attenuation such a LNA, FK filter, Adaptive ground roll attenuation and others, so you can try to use them and make some extra tests in this step.

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Create a new workflow 0050_Noise_attenuation_before_deconvolution and add all the following modules:

1. Read seismic traces

2. Sort traces

3. Seismic loop

4. Despike

5. FDNA (+Flow - Mute by velocity)

6. 2D Radial trace denoise

7. Save seismic by gather

1) Read seismic traces. Load seismic data set 0040_SCAC1.

2) Sort traces. Here we need to sort seismic traces for Seismic loop. Add Sort traces module and set SOURCE_SP header for sorting as it is shown below:

3) Seismic loop  (prepare seismic data for amplitude calculation) . Connect trace headers vector (Input sorted headers) from the Sort traces module output and seismic (Input SEG-Y data handle) from Read seismic traces.

4) Despike module allows the user to remove amplitude spikes (high amplitude values that are many times larger than adjacent values) from seismic traces.  Spikes are not normal seismic signal, they are usually created by the recording equipment and hence should be removed. Spike identification and removal is based on the average (depending on the chosen parameter) amplitude calculated in a time window and spatial window for each sample of the seismic trace in accordance with the chosen threshold coefficient.

Parameters:

Horizontal sliding window - number of traces to include in the amplitude calculations.

Number of samples that will be moved - number of samples of the seismic trace feasible for suppression;

Sample window - number of samples to include in the amplitude calculations;

Time Table - amplitude calculation type to use in the requested time/threshold parameters;

Threshold criterion - selected the threshold type from the drop down menu. By default, it's Median;

Apply threshold - by default, it is FALSE. If checked this option, it will apply the threshold;

Threshold value - this is will be activated if the user selects the Apply threshold option;

Min frequency - provide the minimum frequency value;

Max frequency - provide the maximum frequency value;

Time window start - define the starting time window to do the despiking;

Time window end - define the end time window;

Taper window - define the taper window.

Define parameters and change visual setting for gathers:

Time Table:

    Execute Despike and check the result:

Measure a amplitude frequency spectrum:

5) FDNA (+Flow - Mute by velocity) (Frequency Dependent Noise Attenuation) module uses frequency-dependent and time-variant algorithm where an amplitude threshold values are defined in a trace neighbor area (T-X) for detecting and noise attenuation according to different frequencies and different time windows. The attenuation process consists of two steps – First prepare the etalon (model) and noise attenuation data sets. The etalon should contain only the signal zones, with the noise zone muted. The module estimates the median value of the amplitude spectrum in the sliding windows of the etalon data set, and for each window computes an operator using the threshold value. The procedure attenuates amplitudes whose values exceed the specified threshold.

Input data – Two seismic data sets are input to the FDNA module.

The Input gather are 2D/3D seismic gathers in source or receiver sort order. It is better if they are NMO corrected and static corrections have been applied, but for this step we can apply denoise sequence in a soft mode without NMO correction.

The Model gather is the etalon of 2D/3D seismic gathers in source or receiver sort order. The same idea: it is better if they are NMO corrected and static corrections have been applied, but for this step we can apply denoise sequence in a soft mode without NMO correction.

Seismic loop makes automatic connections between all modules in a sequence under that seismic loop and the user cannot disconnect them. In this case, we need to use an additional Flow module inside of the Seismic loop for etalon preparation. Put the mute modules into a Flow inserted into the Seismic loop and then connect the output muted data to the Model gather field of the Input data tab of the FDNA module. The Input gather on the Input data tab will be automatically connected to the previous module in the Seismic loop processing flow.

Firstly, we need to create an etalon for FDNA by using Mute by velocity module:

Next, we need to connect input Model gather (etalon) with output from the Mute by velocity module:

Input data is taken from Despike:

FDNA parameters:

Collection frequency time windows:

Parameter definition:

Trace window type: one window or multi-window;

Trace window[-1 - all traces]

Number of traces to use for the sliding window. This window is used for local attenuation.

Default: -1 (all traces)

Range: from 1 to max traces in the input gather

Time window

Time in milliseconds for sliding window. This window used for local attenuation. The taper zone between sliding windows is ¼ of time window length default 200ms.

Sliding time window - Amount to advance the sliding time window each shift

Iteration number - Define the number of iterations to perform. The more number of iterations, more run time.

Collection frequency time windows

Three parameters of basic control noise attenuation:

• Frequency– frequency for attenuation;

• Time– time value for threshold;

• Threshold– the limiting parameter for calculating threshold values to attenuate excessive amplitudes.

Carefully prepare the etalon (model) data, try to mute the high amplitude zones and include the signal zones.

Control the noise attenuation by using the threshold parameters and the ability to specify the thresholds in a time variant manner and frequency.

This procedure is more effective in the first steps of the seismic data processing flow when noise zones still have high amplitudes. The input seismic data should not be amplitude normalized.

Execute FDNA and check the results, source gathers before - after - difference:

6) 2D Radial trace denoise is a simple transform to map gathers in x-t domain to into an apparent velocity and travel time domain. The apparent frequency of the ground roll will also be transformed in the RT domain. Ground roll can be extracted via the use of filters in the RT domain, to generate a noise model. This noise model can then be adaptively subtracted from your original input data. Results show that this method produces better results than an F-K filter, with less reduction in the amplitude spectrum.

Parameters definition:

Max velocity - the maximum of velocity for transformation;

Velocity step - The step increment of velocities for transformation. This determines the resolution of the gather in R-T domain.

Using a Band-pass filter we can attenuation (low-frequency) linear noise in defined frequency range:

Execute 2D Radial trace denoise and check the result:

7) Save seismic by gather. Define a name for data set 0050_Denoise1. Turn off all difference calculations and launch  denoise workflow for the entire seismic data. 

Create a stack before and after denoise, as was described in the Spherical divergence chapter:

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Pay attention that in g-Platform system we are able to execute workflows by using many threads and nodes in parallel mode. For example, Distributed Seismic loop module may be used in case of large 3D data set.

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Next step >>> Sufrace consistent deconvolution.

If you have any questions, please send an e-mail to: support@geomage.com    

Despike - Geomage g-Platform - YouTube

Frequency Dependent noise attenuation (FDNA) - Geomage g-Platform - YouTube

Linear Noise attenuation (LNA) - Geomage g-Platform - YouTube