Residual multiple attenuation (Radon)

The first iteration of multiple attenuation should be not very harsh, because the second iteration will be after migration step due to the fact that multiples are focused after PSTM/PSDM process. g-Platform provides several radon modules and we are going to use Radon multiple attenuation by velocity (high resolution Radon).

Keep in mind that you have several radon modules with a bit different functionality and results:

•Radon multiple attenuation: vista views: input, output, difference gathers, user-defined time curves on gather;

•Radon - TauP - High resolution: vista views: input, output, difference gathers, Tau-P domain, interactive curve on gather;

•Radon - TauP: vista views:  input, output, difference gathers, Tau-P domain, interactive curve on gather;

•Radon multiple attenuation by velocity: vista views: input, output, difference, NMO-corrected gathers, interactive velocity spectrum for constrain-polygon picking;

•Radon - by picking: vista views: input, output, difference gathers, interactive Tau-P domain for constrain-polygon picking.

If you are not happy with one result, try another module or mix them and create a complex workflow.

Create a new workflow 0140_Multiple_attenuation_(iteration_1):

Add all necessary modules to the workflow:

Above we've two work flows to choose. In the previous tutorials, we've covered on Radon multiple attenuation module and it's functionalities. So in this chapter, we look into Radon multiple attenuation by velocity module. Though both are same in working principle, the setup is bit different. Inside the Radon multiple attenuation by velocity module, the user should input the velocity information. So we can avoid the NMO (forward & reverse) modules inside the previous workflow. Similarly Velocity analysis (before & after radon) semblance QC display is also not required since Radon multiple attenuation by velocity itself has the vista items to QC.

1. Read seismic traces - load traces after noise attenuation after deconvolution

2. Header manipulation - change offsets (make it positive only)

3. Create velocity model - read the internal/external velocity file to create the velocity model

4. Sort traces - sort traces by CMP

5. Seismic loop - process every sorted gather in a loop (one by one)

6. Radon multiple attenuation by velocity - remove multiples with the velocity function

7. Save seismic by gather - save seismic traces

1) Read seismic traces. Load date set from the previous step on multiple attenuation.

2) Headers manipulation. We can modify trace headers. Headers Manipulation module can be useful in doing all sorts of mathematical operations by means of mathematical expressions for changing/manipulating trace headers.

We can use any of the following mathematical operations to create our own equation/expression.

g-Platform uses following mathematical expressions in designing your equation.

•  Mathematical operators (+, -, *, /, %, ^)

•  Functions (min, max, avg, sum, abs, ceil, floor, round, roundn, exp, log, log10, logn, root, sqrt, clamp, inrange)

•  Trigonometry (sin, cos, tan, acos, asin, atan, atan2, cosh, cot, csc, sec, sinh, tanh, d2r, r2d, d2g, g2d, hyp)

•  Equalities, Inequalities (=, ==, <>, !=, <, <=, >, >=)

•  Assignment (:=, +=, -=, *=, /=)

•  Boolean logic (and, nand, nor, not, or, xor, xnor, mand, mor)

•  Control Structures (if-then-else, ternary conditional, switch case)

•  Loop Structures (while loop, for loop, repeat until loop, break, continue)

Modify OFFSET header -> covert negative values to positive:

3) Create velocity model - Create the velocity model by selecting the Input velocity type as "Picks file". Next click on icon and browse through the folder to provide the input velocity file.

4) Sort traces. sorting by CDP - OFFSET:

Execute those modules.

5) Seismic loop. Put two modules inside: Radon multiple attenuation and Save seismic by gather.

6) Radon multiple attenuation by velocity. This module transforms seismic traces into Tau/P (intercept time and slowness dt/dx = p) domain where we can easily separate multiples and primaries. Such separation allows to attenuate energy of multiple waves before reverse transformation into time domain (T-X). Basic idea is separation primaries and multiples by their velocity (moveout). Input traces are decomposed so hyperbolic events map to elliptical curves in Tau-P domain.

Input seismic gather must be sorted by Common Middle Point (CMP) - Offsets and the primaries should be flattened by applying Normal Move Out (stack velocity) correction before Radon transform. In this case the primary energy will be near P=0, which produces difference in moveout makes it possible to flatten the primary reflections while leaving the multiples under-corrected with a moveout approximately parabolic.

                                                                  Slowness = 1 / Velocity

Model of CMP gather: 1) Time domain (T-X): Primaries+Multiples;

                                       2) Tap-P domain: Primaries+Multiples:

                                       3) Time domain (T-X): Primaries.

The module performs a model of primary and multiple events. This computation is based on data decomposition into user-defined parabolas and calculated by high-resolution algorithm, de-aliased least-squares method in the frequency space domain for every frequency of the pass-band which is defined by frequency min (Hz) and frequency max (Hz) parameters. Events corresponding to parabolas with a bigger curvature are considered as multiples. Events corresponding to parabolas smaller than this constrain are primary events. The area limits between primaries and multiples is user defined parametrization.

Parameters:

Time-velocity factor

                 This is where the user has to define the time-velocity factors. At what time how much percentage of input velocity should be kept.

In the above table, at time 0ms we are keeping 100% of the original velocity. At 2000ms, we consider 90% of the input velocity which means the central velocity trend is shifted (lowered) 10% from the input velocity. Any velocity semblance bellow the user defined velocity percentage will be muted.

In the above images, we have original velocity curve(Orange) displayed on the semblance. On the right hand side, we have defined velocity curve with time - velocity factor (Red) displayed on the semblance. Anything below the RED curve will be considered as multiples and attenuated accordingly.

P min

Minimum p-value for transform data from t-x into tau-p domain.

P mid

Middle p-value for transform data from t-x into tau-p domain.

This parameter is start for modeling multiple waves.

P max

Maximum p-value for transform data from t-x into tau-p domain.

(Choose this parameter with care because run time for this process increases with the square of the  number of P-values).

Delta P

Amount of modeling waves.

Should be approximately equal signal response time interval.

Taper P

Taper zone between P min/mid/max-values.

Taper Tau

Taper zone between start time - filtered - end time.

TimeOffset

Time varying reference offset for multiple modeling.

Time– time.

Reference offset– reference offset.

Frequency min

Minimum frequency for filtering

Frequency max

Maximum frequency for filtering

Pre-Whitening Factor

Pre-whitening factor for stabilizing tau-p to t-x transformation.

Use AGC

Apply automatic gain control pre radon filtering and remove after radon filtering.

AGC Window

Window for automatic gain control.

Execute the module and open all vista windows:

7) Save seismic by gather. Define an output file 0200_DEMULTIPLE1 name and execute calculations for the entire data set.

Next step >>> Residual diffraction multiples.

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