oa Estimating porosity and thickness from combined seismic amplitude and isochron analysis

We present a deterministic method and a statistical method for analyzing amplitude anomalies. Both methods use, as a constraint, the isochron measured from a shallower event. In our example, the amplitude anomaly is modeled as due to a target consisting of three thin layers. Thickness changes in any of the layers results in both amplitude and isochron variations. The deterministic approach consisted of creating one 3-D cube of synthetic zero-offset traces by varying the porosity and thickness of one layer. Each inline of the cube corresponds to a thickness value, and each x-line corresponds to a porosity value. Additional cubes were created by varying the properties of other layers. These data were compared to determine which porosity-thickness pair resulted in the<br>best match with the observed amplitude and isochron. In the statistical approach, we varied two layers simultaneously. The parameters that were varied were P-wave velocity, density, and thickness. We generated 50,000 synthetics by randomly sampling uniform distributions for the six parameters. Before proceeding, we used the isochron as a constraint. If the peak amplitude in a synthetic trace was delayed by more than observed, that trace was rejected. The conclusion from the statistical approach<br>was consistent with that from the deterministic approach; namely that the anomaly is most likely due to decreased impedance in layer 1. In addition, it illustrates that the properties of layer 1 dominate to such an extent that variations due to changes in the porosity of layer 2 would be difficult to observe. Thus, two different, yet reasonable, approaches to the modeling gave similar results, increasing confidence in our conclusions.