Modeling effect

Illustrating the effect of the modeling in the amplitudes, Figure 6 shows the picked amplitudes (without any calibration) corresponding to a 2-layer modeling case (no overburden). Comparing this figure with Figure 5, we can see a very good qualitative agreement between the relative amplitudes of the different interfaces (shale/brine, shale/oil, and shale/tuff) and the corresponding reflection coefficients. We can see some difference for incident angles > 25 degrees; for this reason, and because Shuey's approximation is valid for incident angles < 30 degrees (see Figure 7), we will restrict the data to be used for the AVO inversion up to 25 degrees of incident angle.

We can also notice that Zoeppritz equation predicts a polarity change for the shale/oil case. We observed the same polarity change in the data (see Figure 8); however, this is not present in the picked amplitudes because we are using an automated picking program that picks the maximum absolute value in a moving window in time. We consider the automatic picking still valid in this modeling case because the polarity change is evident in the data after offset > 2 km, which corresponds to angles > 27 degrees, and we used angles up to 25 degrees for the AVO inversion. We did not use the automatic picking on migrated data, where the events are supposed to be flattened by the migration; rather, in this case, we followed the constant time corresponding to the event.

 

shuey Figure 7 Shuey's approximation of P-wave reflection coefficient

 

2lay_all
Figure 8 Modeling of shot gather for a shale/brine, shale/oil, and shale/tuff interfaces (1-D, 2-layer model). Notice the change in polarity for offset > 2 km in the shale/oil case