## 4 Material and Methods
### Data Processing  
The obtained seismic refraction data were processed and interpreted using the SeisImager/2D\({}^{\circledR}\) software (Plotrefa v.3.0.0.6, 2014, and Pickwin v.5.1.1.2, 2014) offered by Geometrics. Shot records provided the geometry, including the shots and the geophone positions and altitudes, for all the acquired seismic data [34]. After visual verification with Pickwin, the first arrival times were manually selected for both the P- and S-wave shot records. Figure 4 illustrates the first-break picking for the P- and S-waves on Pickwin's line no. 1. Plotrefa software was used to produce the travel time-distance curves by plotting the first arrival times for all the shots along every surveyed line versus the source-to-geophone distances. In Plotrefa, these curves were displayed, examined, and some of them were modified for more precise data explanation. The research site suffered lateral velocity variation because of the lateral lithologic, fluid content, and geotechnical variations in the subsurface. Therefore, the interpretation of this work employed seismic refraction tomographic inversion. A refraction seismic survey's ultimate goal is to derive seismic velocity-depth sections that detail the depths and thicknesses of each layer and their respective velocities. The initial model was created using data from prior studies, including previous geological and geophysical data, field outcrops and exposures, accessible borehole data, and modeling outcomes from previous works of the same authors of electric resistivity tomography and borehole data in the researched area [35]. The inversion for both P- and S-wave data was carried out using the same software (Plotrefa), which employs nonlinear travel time tomography [36; 37], consisting of ray tracing for forward modeling and a simultaneous iterative reconstruction technique (SIRT) for inversion. The default inversion parameters for this software are 10 iterations, 3 nodes, and a smoothing weight of 0.5. There are no limitations on the number of layers or their thicknesses in the Plotrefa package's inversion method. As a result, these parameters are not constant in the models. The initial model was created mostly using the ERT section from the prior investigation of the same study region [35]. The minimum and maximum velocities used in the initial models were calculated using another software package that inverted the data based on the homogeneous function method [38]. The authors utilized an initial model with 15 layers (the program's default setting). The used velocities were those that gave the lowest RMSE. Figure 3: The configuration of the seismic refraction lines for P- and S-waves in the current investigation. Figure 4: _Cont._  
A velocity model without small-scale aberrations was produced by iteratively modifying the resulting model to decrease the RMS error between the estimated and actual travel times. The raytracing application from Plottrega was used to evaluate the models. An important consideration when assessing a seismic refraction interpretation is the model-data compatibility. This is done by comparing the differences between the synthetic data produced by models and the first arrival data in the field [39]. Using this raytracing, the theoretical travel times were computed and presented along with the observed data and the RMS error. The RMS error between the calculated and observed values was less than 4 ms, indicating the best fit between the observed and estimated travel times, as shown in Figure 5.  
Figure 4: Examples of the obtained picked refraction seismic data for profile 1, which involved two segments.