## 3 Methods and Materials
### Methodology and Parametrization of Kirchoff PreSDM  
Prestack depth migration was performed using the Kirchhoff summation algorithm which performs a migration by applying a Green's function to each CDP location using a traveltime map. Kirchhoff Migration is commonly implemented following the technique outlined by [PERSON] [43] for the integral solution of the scalar wave equation. The migrated image is constructed by summing weighted amplitudes along diffraction curves. These curves are determined by traveltimes from surface to subsurface scatters. Kirchhoff depth migration estimates traveltimes from a velocity model (supplied as vertically and laterally variant interval velocity field in-depth domain), using ray-tracing or solving an expression such as the Eikonal equation. The Kirchhoff PreSDM procedure was performed in two steps. In the first step, the travel times were calculated from an imaged underground point to shot and receiver points using the given model of interval velocities. In the second stage, the weighted summation of trace amplitudes was computed along the supposed trajectory of the diffraction surface [39; 40]. The calculation method of arrival times plays a key role in the results of migration because it determines the correctness of the shape of the diffraction surface. The calculation algorithm for traveltimes was based on the solution of the eikonal equation, which is a high-frequency approximation of the wave equation [44]. It comprises the two methods: the seismic ray tracing [44; 45] and the previously developed finite difference [46]. For the purposes of our task, the input data consisted of fully preprocessed CMP gathers, located on floating datum, after offset regularization and sorted into the specified group bin size. Migration parameters were determined based on tests, especially migration aperture, maximum dips, maximum frequency, traveltime computation method, offset groups and output depth step. Finally, the following main parameters were used: offset group: 120 m, aperture size: 7000 m, maximum dip up to 75 deg, maximum offset: 10,080 m, ray tracing method: Eikonal solver, frequency range: 8-90 Hz, output sampling rate: 5 m.  
Additional processing was performed on the migrated gathers. First, the angle mute up to 45 deg was conducted, and then residual moveout correction RMO based on interpreted horizons was computed and applied. Maximum moveout for nominal offset was put as \(+/-800\) ms. Trim statics calculation was the last step performed on CRP gathers before stack. Poststack processing consisted of signal-to-noise enhancement realized using coherency dip filters in the F-X domain. The imperfection of Kirchhoff PreSDM methodology in the areas of complicated tectonic structure results from the fact that under such conditions, the values of Green's function cannot be unambiguously determined due to the presence of multiplied wavefronts generated by a large number of discordant reflectors [40; 44]. Additionally, in the geological structures strongly refracting the acoustic waves, the true reflectors are accompanied by false reflection horizons resulting from the migration procedure itself and unrelated to the errors contained in a given seismic velocity model. If such false horizons are accepted as true ones they can be erroneously included and analyzed in the estimation procedures of seismic velocities [40; 47; 48]. If seismic datasets originated from the Carpathians and the Carpathian Foreland are considered the main obstacles in their usage are heterogeneities in data recording procedures. It is revealed by a large number of successive omitted shotpoints, which additionally hinder the correct calculation of velocity-depth models then applied to the computation of static corrections. Their erroneous values may cause unreal changes in the course of hodographs, which, in turn, may deteriorate the imaging of analyzed rock formations with the PreSDM procedure using the floating datum migration variant.