| Summary: | Previous workers have demonstrated that the anelastic energy loss of a seismic wave is related to the petrophysical and mineralogical properties of the rock through which it is propagating. The objectives of the work described in this thesis are the design and development of stable computer techniques for measuring anelastic energy loss (attenuation) from full-waveform sonic data and surface seismic surveys of hydrocarbon reservoirs, to explore these relationships in-situ. Software for anelastic attenuation analysis of borehole and surface seismic data was developed within the ProMAX processing system environment. New and established attenuation estimation techniques were implemented using borehole and surface seismic data from a gas reservoir, and seismic data from shallow boreholes in fractured rocks. The Logarithm Spectral Ratio (LSR) and the Instantaneous Frequency (IF) methods were developed for full-waveform sonic data from a long spacing multi-receiver tool. The two methods provide independent estimates of attenuation, as the LSR method uses the whole spectrum of the seismic pulse whilst the IF method detects shifts in the centre frequency of the pulse spectra. Attenuation was successfully estimated with both of these methods from sonic data acquired within a gas-bearing reservoir, and the IF method was found to be superior to the LSR method. An indirect measure of attenuation from sonic data, the Peak Instantaneous Frequency attribute (PIF), gave very stable measurements in low attenuation regions where the conventional techniques were unstable. - The resolution and stability of the attenuation logs was increased by combining attenuation estimates from multiple receiver pairs of the sonic tool, using a least-squares inversion approach. Both P- and S-wave attenuation were estimated from waveform sonic data acquired in a gas-bearing reservoir. Although the P-wave attenuation estimates had smaller measurement errors compared to the S-wave estimates, they were more strongly affected by borehole fluid invasion. The relationships between attenuation and other petrophysical measurements from the same well were examined. Difficulties were encountered in the interpretation of the P-wave logs because of the possibility of invasion whilst neural network analysis of the S-wave attenuation measurements ii suggested potential correlation between attenuation and permeability. Manual and automated techniques were implemented for detecting attenuation anomalies from surface reflection data in the same gas reservoir. The Weighted Peak Instantaneous Frequency attribute (WPIF) which provides an indirect measure of the level of attenuation gave maximum stability and resolution, and indicated high energy absorption in the reservoir region. Comparison of the WPIF estimates from log and surface seismic data, using forward WPIF modelling, showed similar frequency variation within the gas reservoir. Attenuation estimates obtained from shallow borehole seismic measurements 111 fractured rocks, using similar techniques, suggested potential correlation between high attenuation regions and fracturing and gave indications of the high sensitivity of attenuation to fracturing.
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