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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
 


 


 

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. 
 

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.
 

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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