For inversion modeling, we use ZONDST2D, a two-dimensional processing and interpretation software of seismic prospecting data.
With inversion modeling we determine the velocity structure at a known time path of the refracted waves from source to receiver. Where the calculations are based on special ray tracing algorithm (Shortest path’s method) of graph theory. Shortest path methods aims to find the minimal traveltime of the refracted wave from one point to all other points in a network of nodes and edges.
The main advantages of the shortest path method are its simplicity, capacity for simultaneous calculation of the first arrival times and the associated ray paths of each mode without missing any receiver in a complex geological structure.
The model shows the actual seismic velocity distribution obtained with seismic tomography inversion. Isolines are plotted every 0.5 km/s.
The ray path coverage
The background color is the rays density per model cell. As a standard, we use the following grid: horizontal spacing 1 meter, vertical spacing 0.25 meter at the surface and increasing by 10% with depth. Based on the actual ray paths coverage the areas of the model are blanked, where there is no seismic coverage. Important note: as mathematically seismic tomography requires crossing seismic rays to work properly – the areas on the edges of the model are poorly constrained and should not be used for interpretation.
Semi-automatic simplified structural interpretation based on the tomography recovered seismic velocities. Loose sediments including soil, clay, sand, morene, and “ur” infill usually do not have seismic velocities exceeding 2.0-2.5 km/s.
The isolines for these values are used as a proxy to bedrock depth. The possible extent of the top bedrock is shown as a gridded zone bounded by blacklines – this is our in-person interpretation for possible upper and lower limits of the bedrock depth. This interpretation takes into account uncertainties of the seismic modeling, in-field observations, and general geological background for the project area.
Quality ensurance model
The geophysical data interpretation is carried out in two waves in order in order to ensure quality and reliability in our results presented.
The manual interpretation is based on the ABC method that is fed by the observed geophysical and geological data. This method gives a layered model of the geophysical features of the Earth's sub-surface and present the layer boundaries and bedrock curve along the seismic profiles.
Parallel to the manual interpretation, we also carry out a software based interpretation of the geophysical data that is presented in an inversion model. The inversion model gives a good visualisation of the geological structures that is calculated by using the Shortest Path method based on Graph theory.
Concluding the interpretation process, we create a combined model by aligning and comparing the two methods against each other, therefore, ensuring quality and reliability in our results.
Traditional layered model
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