1985 Theses Doctoral

Sonic attenuation in consolidated and unconsolidated sediments

Goldberg, David S.

Measurements of attenuation are extracted from sonic waveforms recorded in a consolidated limestone/shale interval in the Anadarko Basin, Oklahoma and in unconsolidated marine chalk in the Baltimore Canyon Trough. In the consolidated sediments, amplitude and coupling changes were effective lithology and fracture discriminators, even if other measurements appeared to be insensitive. The extraction of compressional-wave Q using the spectral ratio technique is performed at each source depth by a least-squares statistical fitting of the spectra. In a study on synthetic waveforms, Q measurements were sensitive to windowing, bandwidth, fluid properties, and geometrical effects on the waveforms. Frequency-independent coupling was calculated from the mean of the spectra and was correlated with high resolution to changes in the physical state of the borehole. Although, scattering at bed boundaries was significant in the spectral ratio measurements, mean Q values were estimated to be 45 in the limestone and 25 in the shale. These field measurements are typically lower than published seismic or ultrasonic Q values and support the existence of a peak in attenuation near sonic frequencies.

Sites 612 and 613 were drilled in a transect across the Atlantic margin during Leg 95 of the Deep Sea Drilling Project. The impedance contrast across a diagenetic boundary in the middle-Eocene sediments is sharp on the slope and gradational on the rise, generating differences in amplitude and phase of modeled seismic reflections. At Site 613, compressional attenuation was extracted from sonic waveforms by the spectral ratio technique and at the peak frequency. As the sediment frame stiffens in the process of lithification, both estimates of attenuation increase, and spectral ratio mean Q values decrease from 69 to 28 across the diagenetic boundary. A decrease in pore size and aspect ratio due to diagenetic effects is observed in the recovered sediment core and can explain the increase in attenuation by stress relaxation due to local fluid flow in thin pores. These results suggest that changes in elastic and anelastic effects across this boundary may generate seismic reflections which cannot be simply related to lithologic changes.