The data

The synthetic example is based on a field data set from the Gulf of Mexico. A well was drilled on location which hit gas, and was logged for sonic velocity and density. The reservoir level is at about 2.960 km depth, and the reservoir sand dips 5$^{\circ}$-10$^{\circ}$. The target zone chosen to perform the Kirchhoff elastic prestack depth migration/inversion is a 2x2 km image area, which is 1.75-3.75 km in depth (1.5-3.0 seconds on the stacked time section).

A synthetic shot gather was calculated based on a 1-D estimate of the true earth model at this site, as shown in Figure . The synthetics were generated by the paraxial raytracing algorithm PRX87 (Beydoun, 1985, Beydoun and Keho, 1987). The shot gather contains $\grave{P}\!\acute{P}$ primaries only (no multiples) and includes the amplitude effects of 3-D geometrical divergence, elastic reflection/transmission loss, and high frequency anelastic Q attenuation (but not dispersion). We used a 25 Hz peak frequency zerophase Ricker wavelet to simulate the source waveform.

 

synth1
Figure 1 Synthetic shot gather.

The 1-D elastic model properties are given in Table 1, and plotted in Figure .

 

$$\rho$$

630. 1860. 930. 2.04 65

830. 1990. 995. 2.07 75

1330. 2271. 1135. 2.14 97

1960. 2604. 1302. 2.21 128

1970. 2697. 1686. 1.85 137

2610. 2697. 1350. 2.23 137

2960. shale 2949. 1475. 2.35 164

3110. gas ss 2770. 1731. 2.00 25

3130. brine ss 3321. 1660. 2.20 208

3330. shale 2949. 1475. 2.41 164

4000. 3250. 1625. 2.50 199

The eight reflections correspond to prominent events in the raw shot gathers. The shale/gas sand contact at the top of the reservoir is at about 2.6 seconds in the synthetic gather. The later events correspond to the reservoir base and a deeper shale boundary. The recording geometry is identical to the marine field survey: the near and far offsets are 318 m and 3293 m respectively, and the group and shot interval are both 25 m. The traces are 6.0 seconds in total length (of which we used 3.5 seconds for the m/i), sampled at 4 msec intervals.

 

emodel
Figure 2 Elastic model for marine synthetics and migration.