9.0 Seismic Response Features

Strata has the ability to predict the seismic response of the modeled basin. Strata can determine the velocity and density of the basin sediments as long as the porosity is everywhere defined (if compaction was used -- if compaction was turned off in setbasin, seismic response will not be available in plotbasin) and composition of the sediments is known. In our case we determine the composition by either the diffusion or the water depth (see help entry in setbasin). Velocities are determined from the Wylie equation:

where, f is the percentage of sand in the sediment; vsand, vshale, and vwater are the velocities of sound through those materials; vb is the bulk velocity of the sediment. Typical acoustic values of sand, shale, and water are (from Schlumberger, 1987):

Bulk density is determined from the following equation:

9.1 Impedance Display

We illustrate the use of the seismic tool with the file seis.dat. The first exercise is to view the simulation in two-way travel time by clicking on the button in plotbasin and selecting . The user may wish to alter twtmax and/or twtmin (in the parameters window) to expand/contract the view. The user can now view any of a range of properties in log, fill, or contour format, including velocity, porosity, density, diffusion, and impedance. We show one example of the log image of the impedance in Figure 16.a. This display was created by first selecting to display the curve and then by selecting in the menu. The log node sampling, controlled in the parameters window, was set to five. The amplitude scale is changed by placing the cursor arrow over the scale value and using button 1 on the mouse to increase the value, or button 3 to decrease the value. Click on to redraw the image.

Figure 16.a: Example of impedance display. This image was displayed by changing the amplitude of the curve to 5 and changing skip log nodes in parameters to 5.

9.2 Reflection Coefficients

The next step is to calculate the reflection coefficients. Change back to empty and toggle the reflection coefficients entry on (i.e. so that it shows `reflec coeffs visible'). An example of the reflection coefficients display is shown in Figure 16.b. The reflection coefficients are defined by:

Figure 16.b: Display of reflection coefficients (seis.dat).

9.3 Convolved Seismic Traces

The final step is to convolve the input signal with the reflection coefficient plot. First click on and select `signal'. This step will take a few moments as the model does the convolution. Toggle off the reflection coefficient entry if you wish to clear some clutter from the display. Reduce the scale to two, go back into parameters and change skip log nodes to 2 and the resulting image is shown in Figure 16.c. This can also be shown as a color `field' by selecting . (Signal cannot be contoured.)

Figure 16.c: Convolved traces resulting from reflections (seis.dat). For this display, the scale was changed to 2 and skip log nodes in parameters was changed to 2.

Please note that it is not necessary to go through all of the above exercises to view the convolution; the model will do whatever calculations are necessary to display the data selected.

Last Modified: 01:48pm EST, February 22, 1996 - Steven E. Nelson