SeisOpt™ @2D - Refraction
Comprehensive Subsurface Solution
Seismic refraction is a method used to identify rock properties, geological structures, and much more. Once the data is collected and analyzed, it produces a 2D image displaying velocity contrast, i.e. geological boundaries.
Setting up a refraction survey involves laying out a linear array of sensors (geophones) which record ground movement over a set amount of time. Ground shots are created using a sledgehammer or accelerated wait drop to create seismic energy which travels down to the top of a distinct rock layer (or other density contrast) and is refracted along the top of the rock then returns to the surface as a head wave. The refracted seismic wave is observed as a first-arrival signal at the geophones.
Shots are created off each end of the seismic line and along the line as well. When collecting refraction data, it is important to have multiple different shot locations because it will increase raypath coverage of the area below the seismic array. The data is then processed using SeisOpt™ Picker and SeisOpt™ @ 2D.
SeisOpt Picker is a first arrival picking module that correlates raw shots and performs automatic and manual picking of first breaks. The picks and the survey geometry information can then be easily exported into files that can be used directly by Optim’s refraction interpretation software, SeisOpt @2D.
The module allows the user to interactively input and edit survey geometry, manipulate various display options, perform basic processing like Automatic Gain Control (AGC) and filtering, display and edit trace header values, compute frequency spectra, and do a linear velocity analysis. It also has a printing module that enables the user to produce report-quality output.
SeisOpt @2D is an automatic refraction interpretation package that contains modules for performing velocity optimization and visualization, virtual survey design, and output report quality images. SeisOpt @2D is the Graphical User Interface to the suite of modules for performing the velocity model optimization and visualization, interactive seismic survey design, and outputting postscript images of the results for printing.
SeisOpt @2D uses only the first-arrival travel times and the survey geometry to derive subsurface velocity information. For this reason, accurate picks are important. It uses a nonlinear optimization technique called adaptive simulated annealing and involves forward modeling. Test velocity models are created, through which travel times are calculated. These calculated travel times are compared with the observed data. Testing every possible velocity model would take far too long, so SeisOpt @2D uses Optim’s proprietary algorithm to search through only a small percentage of the many possible models, yet still finds the best model.
It is called an optimization because the discrepancy, or error, between the calculated and observed travel times is optimized. In this case the optimal solution is the velocity model with the minimum travel-time error. Running the optimization on the same data with different resolutions and with different depth ranges at the same resolution of the velocity model can increase confidence in the results. Features that are present in models with different resolutions can be assumed to be realistic with much greater confidence.
For each velocity model optimization, SeisOpt @2D produces a file showing the number of times each cell in the model was sampled. This determines which parts of the velocity model are controlled by the data. Any gaps in the sampling at the end of the model correspond to unconstrained areas of the velocity model. Therefore, this sampling information should be considered before interpreting the optimized velocity model.
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