Earth models

Four different Earth models were constructed (Figure 4):

1.

A simple horizontal plane layer

2.

A dipping plane layer

3.

Two plane layers with opposite dips

4.

A faulted block model

 

models
Figure 4 Models for finite-difference experiments: 1. Simple plane layer, 2. Single dipping layer, 3. Two dipping layers, and 4. Fault model. All axes show km's. Velocities vary from 1500 m/s in the uppermost layer to 2000 m/s in the half-space below.

The velocities in each model varied from 1500 m/s in the uppermost layer to 2000 m/s in the half-space below. The density values were engineered artificially to give large impedance contrasts at the boundaries and hence strong reflections. In all cases the top boundary was a free surface.

The simulations were done using a versatile finite-difference modeling program written by Martin Karrenbach 1995. A cell size of 5 m by 5 m were used, with time-steps of 2 ms. The input grids were surrounded by an exponential damping boundary that was 20 cells wide, and a one-way boundary, 4 cells wide. To reduce artifacts due to the discrete nature of the mesh, the models were slightly smoothed. This can be seen in Figure 4.