Investigating the low frequency content of the Hussar data with impedance inversion

Heather J. E. Lloyd and Gary F. Margrave

ABSTRACT

Acoustic impedance inversion can easily be computed by the BLIMP (BandLimited IMPedance) algorithm. This algorithm uses well logs to fill in the low-frequency information that is missing in bandlimited seismic data. The transition from well log low-frequency information to the seismic data spectrum is marked by a cut-off frequency, fc. The choice of fc depends on the low-frequency content of the seismic data, and it is generally desirable to push fc as low as possible to make the inversion less dependent on well control. For the Hussar data, with 10 Hz geophones and dynamite sources, an fc as low as 2 Hz gives good results. This is relatively low compared to the 5 to 10 Hz that is commonly chosen for most seismic data. Three wells intersect the Hussar line and all were used to calculate inversions as well as a well log that was prepared by averaging the three impedance logs. The average log was found to produce the best inversions with a mean impedance error of 8.5% from .2 to 1.05 seconds, where wells 12-27, 14-27 and 14-35 produced errors of 11%, 10% and 10% respectively over the same interval. Other cut-off frequencies were also examined and the best choice appears to be non-stationary, as frequencies down to 1.5 Hz can be trusted in the shallower section. This study has shown that the Hussar data set has trusted frequencies down to 1.5 - 2 Hz.

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