In this chapter we will pick first breaks, create velocity model via tomography algorithm and calculate (refraction) static corrections.

Notice, that tomography procedure requires high quality picks, so it is better to have accurate manual picking on the 2D line (manual picking if you have enough time). The seismic is vibro data and it is another obstacle of using auto picking method as well as trying to use tomography procedure, the result may be far from being sufficiently addressed in comparison with the conventional reciprocal method static solution.

Let’s look at the workflow with all necessary modules for static calculations:

Read seismic traces. For the first break picking step we should use seismic without any processing in order to have original first response from guide-waves.

Refraction FB picking - azimuthal solver guide / phase / aperture. This module computes the static correction based on the first breaks of the data. It allows the user to interactively pick or correct the automatically picked first breaks. Subsequently, the first breaks can be used for refraction static solution, generating static corrections for source and receiver positions. The module calculates only the refraction correction, that is, replacing the near surface low velocity layers from the surface to the bottom of the calculated near surface. The module does not include an elevation correction. Elevation static is a shift which moves the data from topography to final datum. This correction to final datum is performed with a separate module Shift Data. Since most processes in g-Platform that would require the statics applied (for example NMO) can be run from topography, being able to easily separate the refraction correction from the final datum correction is very useful. How to use floating and constant datum planes will be discussed in Datum planes chapter.

Refraction FB picking uses first breaks and  parameters for the solver to calculate the source and receiver static correction at each surface location. The solver includes options to calculate a smoothed solution, as well as to solve into multiple azimuths. Additionally, a residual correction can be calculated and applied at each source and receiver giving a short-wave correction. The replacement velocity is calculated internally by the module. Upcoming changes will allow specify a weathering velocity as well as a replacement velocity. With these parameters the module will build a near surface weathering model and then the user defined values will be used for static corrections calculation.

Fill the Refraction FB picking – azimuthal solver guide / phase / aperture parameters before performing calculation and displaying the data set. For the training seismic data set we will use the following example, but you should do some extra tests by changing parameters for better understanding:

Input data:

Parameters:

Further step is setting up user work area, it means making configuration of all necessary Vista Windows for QC and interactive engagement. Click RMB on the module and open all Vista Groups:

On the first Vista Window, there is a Linear Move Out (LMO) picking on source gather. The LMO is one of the main parameters for FB picking, in view of the fact that it is used as a guide function for auto FB picking. Consequently, we should pick LMO guide function on a few source gathers for different locations through the acquisition area. Picking may be located exactly on the first arrivals or a little bit above them, it depends on what option (algorithm) and parameters you use for auto picking. For the training data it is enough to make LMO picking only for one source due to the fact the there is a flat relief.

All the visual setting were configured and we can propagate and save them as default:  

Next, open the source gather FB picking window and execute one source FB picking by pressing Auto picking current SRC<Alt+,S> button:

Then we are going to launch auto picking for the entire data set by clicking on Auto picking action button:

Check the result, it is obvious that picks became more accurate because the algorithm uses neighbor gathers for statistics:

Eventually we have all FB picks and need to check it, open the Map sources and receiver location and activate Selected source option:

Now we can choose any source from the location map and perform FB picks QC and editing. Other windows are connected to each other automatically, so it allows to perform interactive QC on fly:

Or choose another option for picks removing:

There are other QC windows available from the Refraction FB picking - azimuthal solver guide / phase / aperture module.

The final step is export FB picking to binary file that we will use for tomography process. Find Export picking action and choose Save in Binary, because tomo statics module requires binary format:

Define a name of the output binary file FBpick, we will use this file in the next step - tomography.

Now we have all FB picks and there two options for static calculation, by using the following modules:

• Refraction FB picking – azimuthal solver guide / phase / aperture;

•Tomo statics 2D.

Execute Refraction FB picking – azimuthal solver guide / phase / aperture, now we have refraction static correction and it is easy to do a QC. Go to vista window Current source gather and do a quick statics QC:

It was a fast QC of static corrections calculated by Refraction FB picking – azimuthal solver guide / phase / aperture. Of course we are going to calculate stacks, but before we do it there is another option for static calculation by using tomography - module Tomo statics 2D.

Tomo statics 2D. Tomography plays a key role in seismic imaging. Be it is a building depth velocity model or calculating the refraction statics by generating the near surface model. Refraction Tomography statics are used in the event of regular refraction statics are not giving optimum results or to improve the refraction statics solution. In this method, the weathering zone (WZ) is parametrized as a number of cells that used for filling up a depth velocity model. Travel times are goes through the cell-model, and residuals times are converted into velocity variations in 2D/3D cells. Here we are able to calculate vertical velocity gradient due to the fact that tomography is a nonlinear move out modeling of the first arrivals. 

The tomographic is sensitive to the initial model due to the fact that the nonlinear solution performs by iterating the ray trace calculation and velocity is updated by local linearization, so the solution of tomography is depending on quality of first breaks picking. Therefore, if we have 2D data set it is more reasonable to make FB picking more accurate by doing it manually.

Linear (left) and nonlinear (right) solutions comparison:

The tomo-statics modules in the g-Platform system are based on the Fresnel seismic travel time tomography. A Fresnel volume approach is applied to represent wave propagation for seismic travel time tomography instead of rays. A Fresnel volume is defined as a set of many waves delayed after the shortest travel time by less than half a period. It is derived by calculating travel times both from a source and from a receiver. Tracing rays from sources to receivers is completely avoided.This considerably reduces computational time. We solved the eikonal equation by using a finite-difference method to calculate travel times. The advantage of this approach is as follows: first, the frequency of wave can be introduced into analysis. Therefore, we can evaluate the resolution of seismic tomography. Next, the smoothing feature can be naturally introduced. Finally, Fresnel volumes with finite bandwidth considerably reduces the sparseness of ray distribution.

The more physically-realistic representation of wave propagation is to treat a ray path as a beam with finite width. Using a Fresnel volume is a natural and an effective approach. A Fresnel volume is a set of many rays delayed after the shortest travel time by less than half the period of wave. The rays in a Fresnel volume are added constructively to form the first-arrival of wave. There have been several studies on the application of Fresnel volumes to seismic tomography since Harlan(1990). In this study, first, we discussed the characteristics of Fresnel volumes. Next, we formulated the inversion procedure. Then, we investigated the resolution of tomography with respect to the frequency (Toshiki W. Seismic travel time tomography using Fresnel volume approach).

Travel times scheme:

A module Tomo statics 2D is used for tomography and it requires the first break picks. In this module, it start with an initial model and performs updating the model during each iteration until it converges to the best solution.

Scheme of tomography steps:

Demo data also consists of the FB picking file on your disk in the same folder with the input seismic data set, but you just need to convert ASCII FB pick file into Binary one via Refraction FB picking - azimuthal solver guide / phase / aperture module (import->export). So, we can use the module Tomo statics 2D which requires first breaks as mandatory input data set.

Tomography procedure requires high quality picks, consequently preferable to have manual picking on the 2D line due to relatively small amount of source gathers. The seismic is vibro data and it is another obstacle of using auto picking method as well as trying to use tomography procedure, the result may be far from being sufficiently addressed in comparison with the conventional reciprocal method static solution.

For tomography procedure we will use Tomo statics 2D module, add it to the workflow RMB->Modules Next->Find module->Tomo statics 2D and set all the parameters.

Connect all the input data vectors from the previous module and import the FBpick binary file:

Usually, testing parameters starts with wider cells size of the model because initially the main goal is to find the most appropriate values for other parameters. When we have optimal result, it is reasonable to reduce cell size and increase the number of iterations. Eventually, a dense gradient depth velocity model is calculated as well as static corrections (ms) for sources and receivers. A depth velocity model may be used for the depth migration step as the upper part of the PSDM model.

Tomography parameters:

By the way, for 3D type tomography Tomo statics 2D/3D there is an option for define the low velocity zone by using Window for calculation V0 parameter which determines an average velocity in the interval (datum - window).

Now we have defined all the parametrization and the next step is launching the tomography calculation by double LMB click on the module or pressing Solve action. Use GPU and distribution mode for time consuming procedures or big acquisition. Those functions must be configures on a cluster by your system administrator:

To visualize tomography velocity model and convergence, user should generate vista items. The module generates many QC windows: the location map, convergence refraction tomo, tomography velocity view and Current source/receiver/bin gather.

For tomography velocity view, the user can define how many number of iteration required in the parameters tab. Once the solution is computed we can do a QC of the statics result by going to the any one of the current source/receiver/bin gather and enable/disable the Current source gather and Current source gather applied options from the View properties of the Current source/receiver/bin gather. Interactive fast QC by applying statics correction on gathers with different sorting: source, receiver and bin.

Check the source gather before and after statics applying. Choose by LMB clicking on any source on the location map:

And look at the source gather ViewVista window and check gather before and after statics applying:

Source gather before (left) and after (right) tomography statics applying:

Source gather and different types of picks for QC: User picks (purple), tomography (green) and residual (red):

Another important QC tool is convergence graphs where you can understand how many tomography iterations should be enough. Every iteration must reduce an error/mistie between observed and modeled times. If the error is too high or the tomo convergence is not matching with the residual convergence then the user can adjust the solver parameters and/or other parameters and click on the Solve option from the action item menu. It will recalculate the solution and update the displays.

Graphs of the main-tomography iteration (purple) and residual (red) solution:
 

Depth velocity model after tomography (try to change default color palette):

Static QC. The next step is checking statics corrections by using Statics QC module.

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For static QC we can use two modules:

• Statics QC if input statics correction from Tomo statics 2D

• Statics azimuthal QC if input statics correction from Refraction FB picking - azimuthal solver guide / phase / aperture

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Make input data item connection:

Use default parameters:

Open all vista groups. There are many QC tools like source, receiver statics graphs and maps, source and receiver gathers before and after static applying, tables with static corrections etc. Statics QC vista windows: source statics graph (left top), source statics map (bottom), source gathers overlaying before and after static correction applying (right):

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An internal data base of the g-Platform includes seismic data sets and all types of libraries.

Library is all other types of data except the seismic, i.e. velocity, statics, muting, etc.

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Before we continue, let's save library of tomo statics into data base. Add new module to the workflow: Save item, it saves any libraries into DB. Define a name of statics library - Refraction_statics and execute the module. After saving a library we can remove Save item module or save it in the workflow:

Check that your static library was saved into DB. Open g-Platform main window and go to Project data tab and write a mask for filtering static library:

Now we can do a quick QC on a stack section. Apply statics corrections to seismic traces and build a stack section via Stack imaging.

Stack section before (left) and after (right) tomography statics applying: