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The module refraction traveltime
analysis allows to analyse and interpret picked first
arrivals. The module consists of two different parts. The first part (traveltime
processing) contains the possibility to put together the picked
traveltimes from several shots and to assign the picks to special layers.
The second part (traveltime interpretation) contains the interpretation
tools like wavefront-inversion, forward raytracing and interactive
1-dimensional adaptation. The module is useful for the following
applications:
refraction seismics
Traveltime processing
| Within this menu it is possible to analyse and interpret picked
first arrivals (refraction seismics). Precondition is that all
chosen data are located along one line within one acquisition plane.
You may put together the traveltimes from several shots and assign
the traveltimes to special layers. Those traveltimes stemming from
several shots and belonging to one layer may be combined together to
one forward and reverse traveltime curve. These combined traveltimes
are the basis for a subsequent 2D wavefront-inversion which allows
to invert both the structure of the layer and the smoothed refractor
velocity. |
The traveltime processing part offers comprehensive tools for
- sorting of the traveltime data
- interactive editing (moving a set of traveltimes, cancelling
of single traveltimes, combining different branches, ...)
- comparison of traveltime data (e.g. real and synthetic data);
calculation of the discrepancies
- CMP-sorting (e.g. for 1-D-modelling)
- interactive assignment of arrival times to layers
- manual or automatic phantoming
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Traveltime interpretation
The first arrivals may be interpreted in the following ways:
- 2-dimensional wavefront inversion of the complete forward and
reverse traveltime curves
- 2-dimensional forward raytracing and comparison of the real and
the calculated traveltime data
- interactive 1-dimensional traveltime adaptation
wavefront inversion
The wavefront inversion allows to migrate the combined forward and
reverse traveltimes into depth using a Finite Difference approximation
of the eikonal equation. The following traveltime processing steps must
have been performed before:
- put the different traveltime curves together
- assignment to the actual layer
- combination to one single forward and reverse traveltime curve
The method allows:
 interactive back propagation of the
wavefronts using finite differences approximation of the eikonal
equation; the backpropagation is exact, even for very complicated
overburdens.
no parameter adjustments are necessary
inversion of layer interfaces and layer
velocities
the topography can directly be included in
the inversion process (no static correction is necessary)
The complete forward and reverse wavefronts are continued downward
based on the given overburden model. The new refractor is
automatically constructed at those points where the sum of the
downward traveltimes is equal to the reciprocal traveltime. The
refractor velocity is determined from the mean of the slopes of the
forward and reverse wavefronts at the new calculated refractor
points.
The method is iterative. This means that each layer must be inverted
separately and that the overburden must be existent. It may contain
any 2-dimensional structure.
The results (interfaces of the layers and layer velocities) can
easily be manipulated (e.g. smoothed). A priori information can
easily be incorporated to the overburden prior to the inversion of
the next interface. This guarantees that all available information
contributes to the inversion result. |
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forward raytracing
A fast and reliable traveltime calculation for arbitrarily
complicated 2D-models is possible. The method is based on a finite
difference approximation of the eikonal equation for calculating
first arrivals. It takes into the account the existence of different
propagation waves like transmitted, diffracted or head waves.
Therefore no practical limitation concerning the complexity of the
medium is given. The method is very suitable for near surface
investigations, because there is no need for approximations
concerning the complexity of the models. The wavefronts and
therefore the raypaths can be stored and displayed.
The information about the geometry (shot and receiver positions) can
automatically be adopted from the shot records or from the
traveltime files. Editing, if necessary, is easily possible. The
number of shots (e.g. a complete refraction seismic line) is not
limited. |
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1-dimensional traveltime adaption
The 1-dimension traveltime adaptation allows the interactive
calculation of a one-dimensional velocity-depth-distribution from
refraction shot or CMP-data.
The intercepttime option allows to calculate a first starting model
which may be refined interactively (depth and velocities). The
resultant diving waves, reflections (incl. overcritical) and surface
multiples are displayed in real time. A comparison can be done with
either real traveltime data or the complete data set (in this case
picking is not necessary). |
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