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About XTomo-LM  2.0

Introduction

 

The product continues the Firstomo-XTomo product line and represents the result of evolvement of the XTomo 1.0 seismic tomography system into a more powerful instrument of the arrival times processing. Like XTomo, it is a 2D tool. 

Already XTomo is considerably richer in capabilities than the classic first arrival tomography. In particular, it implements tomography for reflections and refractions, if position and shape of a reflector or refractor is known. With a powerful forward problem solver in stock, XTomo was applicable to some problems of the layered model interpretation.  



The ability of XTomo to work easily with curved seismic boundaries is rooted in ray-tracing algorithm that uses a curvilinear grid of special kind. If a horizon is identified with one of grid lines, it is capable of tracing reflected and refracted rays. That makes XTomo potentially an adequate instrument for the layered model study. However, version 1.0 was adjusted, in the first place, for tomography problems. As regards the layered model interpretation, it was  “well underdone”.

Under the pressure of needs of particular seismic investigations, in which the developers have been involved in recent years, they made it a point to fully evolve XTomo features and turn it into a multi-purpose instrumental system for wave arrival times processing, or for kinematic interpretation, for those who use such term. XTomo-LM 2.0 is the first result of these efforts. “LM” in the title reminds of “layered model” as the major target.





The main XTomo-LM features are implemented in the following software units (modules):

Model Editor enables the user to modify the model, i.e. velocity distribution and grid geometry. Changing mesh geometry is called remeshing. One of the most important methods of model remeshing is implanting an arbitrary curve that may represent topography, or bottom line, or a seismic horizon. 

Forward Problem Solver provides ray tracing on a specific mesh for arbitrary source-receiver configuration and for a given set of waves – diving, reflected or/and refracted (head).

Forward Problem Viewer allows viewing samples of rays and corresponding travel-time curves. In inversion problems, it allows viewing both observed and computed travel times and examining time error (residual) statistics and distributions.

Tomography Inverse Problem Solver improves initial velocity distribution of the whole model or a layer to fit best the observed traveltimes.

Horizon Builder solves inverse problem for a set of reflection/refraction traveltime curves and known velocity distribution. The solution represents a reflector or refractor. Inversion of this kind is akin to migration in seismic record processing.

 

Remeshing is a crucial point of interpretation. The user can start with a normal (that is, orthogonal) grid; then, with a horizon curve given, he or she can implant it in the grid and so turn it into a grid line, which makes ray-tracing and tomography inversion as simple as for the normal mesh.  Dynamic remeshing is what makes XTomo-LM an effective instrument of fitting models to the observed time-distance curves. That was practically impossible to do with XTomo 1.0.



If the observed data are rich enough, the layered model can be studied within the layer-by-layer interpretation scheme, in which layer velocity is computed by tomography inversion, while seismic horizons are are introduced as a priori information or determined by downward continuation of reflection or refraction time-distance curves with Horizon Builder. Even studying a layer can be an iterative process with using tomography, first on first arrivals and then on reflection or refraction arrival times.




     

XTomo-LM 2.0 Release 3. September 2007

Release 3 includes add-ons that had been planned still before Release 1 was issued in the beginning of 2007. On the other hand, it is a response to the users' reports that has been received since then. Below is the list of the most important adds-ons and amendments.

Creating Primary Model

The primary model or the data for its construction are required at XTomo-LM project creation time. Now the list of types of such data includes all practically significant cases:

·        Orthogonal grid with velocity defined in the corner cells.

·        Creating model on orthogonal grid using a set of velocity columns Vk(z) stored in an ASCII file of the VC format.

·        Creating curvilinear grid using a "wire-frame" of base h-lines (typically, seismic horizons), stored in an ASCII file of the MG format.

·        Importing model with arbitrary grid from ASCII-file of the legacy VFT format, used in Firstomo and  XTomo 1.0.

Utilities for SRT Files Preprocessing

The XTomo-LM first release included two utilities for work with SRT files. These files contain observation data for inversion projects. The most important application is SRT Data Viewer (UDV) for viewing observed time-distance curves in graphic form. As a side result, it verifies input data for correctness. Two new utilities are used for SRT data preprocessing before import. They work with verified and sorted data rather than initial SRT-file, so UDV is now appended with the command of saving SRT data to the temporary internal storage.

UTX

The purpose of the UTX utility is to prepare refraction time-distance curves for inversion. The inversion module requires, together with SRT data, a list of reciprocal TX-curves with their reciprocal times. The list must be stored in an ASCII file of the TXS format that is imported together with the SRT file. Making up such file manually may prove too complicated a job. The utility compose such file interactively, allowing, in some cases, to compute lacking reciprocal times using TX-curve extrapolation.

UMB

The UMB utility is targeted at fast building of the primary model using the diving wave time-distance curves recorded at inline observations.  The utility does not fit a model to observed times and so does not use any optimization procedures; it works with "raw" noisy data. Using a regularized algorithm, UMB "copes" with such data in majority of cases, producing, in the end, an acceptable initial approximation for tomography inverse problem.  The utility let the user to compose a sample of time-distance curves from those containing in an input data; perform TX-curves inversion resulting in a set of velocity columns; view each velocity column as a table and graph; select some for storing in a VC file, which is later imported by Project Manager to create the primary model.


 



 


 
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