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.
|
