TIflE,rnl, FREDUENCYl hZ 1 FIG. 2. Pulse estimated from the reflectivity function and the stacked trace at CDP 278.bound to depend upon bandwidth, time gate, pulse length, signal-to-noise ratio, and a priori information. We have not been able to establish a theoretical resolution limit, and we leave this as an open question.Real data example Figure 1 shows a part of a stacked, unprocessed seismic section. The sampling interval is 4 ms. A reflection coeffi-FIG. 3. Stacked traces and their estimates. Reflectivity function obtained from the well at CDP 278 is included for comparison.Cient series obtained from well data at CDP 278 is also displayed. The sampling interval of the reflection coefficient series iS 2.0 ms. We focus our attention on the area between 2082 and 2222 ms two-way traveltime. Reflection coefficients in this interval together with the stacked trace for CDP 278 are used to estimate the pulse displayed in Figure 2. AS can be seen from its "tail," the estimated pulse is distorted by multiple reflections and/or edge effects. These effects cannot be expected to be spatially invariant. Hence the estimated pulse will only be suited for inversion of traces close to the well.The results displayed in Figure 3 are obtained using the estimated pulse and with zero starting values for r and c. For LS estimation, the trace had to be resampled to 8.0 ms sampling interval. The event at approximately 2100 ms twoway traveltime is resolved, but the rest of the trace is corrupted by spurious events with alternating polarity. Maximum-likelihood estimation was performed with 4.0 ms sampling interval and NC equal to five. The result of ML estimation appears to be a smoothed version of the reflection coefficient series obtained from the well log. The event at approximately 2100 ms two-way time is nicely resolved, but the negative peak at approximately 2150 ms is not properly recovered. The polarity changes from 2150 to 2220 ms vary consistently along the traces.
Conclusions
Synthetic and real data examples demonstrated that the maximum-likelihood method gives better resolution andimproved numerical stability when compared to the leastsquares method. When least-squares estimation is applied, the sampling rate must be chosen approximately equal to the Nyquist rate of the trace. Maximum-likelihood estimation yields resolution that is superior to the resolution provided by least-squares estimation. The ML method also appears to be numerically more stable. In real data applications, inversion is distorted by multiple reflections and also by edge effects when only a limited part of the seismic trace is considered.
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