Ray and finite-difference modelling of CDP seismic sections for shallow lignite deposits

Abstract

A numerical experiment carried out to investigate the structural model of the Domenico lignite site is discussed. The model is a 2D structure containing several lignite layers at different depths, and a low-velocity layer at the top of the model. The experiment consists in simulating a measured CDP section by two independent techniques, based on completely different concepts: the finite-difference method and the ray method. Due to the incompleteness of the ray synthetic wave field, as well as to numerical problems of the finite differences at higher frequencies, the agreement between the synthetic seismogram sections for the individual shot points is poor. However, the CDP stacked sections modelled by the ray and finite-difference methods agree rather well. This is because the main differences between the wave fields computed by the two methods are due to the presence of the low-velocity layer (ground roll, head waves, etc.), and just these parts of the wave field can be suppressed by routine data processing such as f–k filtration. Synthetic ray and finite-difference CDP stacks agree relatively well with the observed data. They confirm three lignite seams and a fault in the shallower one. The synthetic data also indicate that many apparent horizons of the measured section may be due to the multiple reflections within the subsurface low-velocity layer.