Analytical solutions of seismic wave propagation are available in restricted cases such as homogenous or layered homogenous models, or targets with regular shape or smooth property variations. As seismologists try to quantify the Earth with high resolution, these models are oversimplified and only valid for particular purposes. Heterogeneities commonly exist in the Earth. Their scale length ranges from the grain size to lithological units with extensions of many kilometers. In seismic exploration, complex geological settings are frequently encountered. Conventional approaches like ray tracing are not applicable in those situations. Parallel 3-D viscoelastic Finite Difference (FD) modeling codes numerically solve the seismic wave equation in 3-D and output seismograms and snapshots with high accuracy depending on the computing resources. The investigation of seismic wave behaviours in strong contrast situations demonstrates that the modeling codes are important to study seismic attributes from complex structures and to optimize seismic acquisition geometries.