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Interval Attenuation Estimationby Submitted to the Department of Earth, Atmospheric, and Planetary Sciences on May 8, 1998 in partial fulfillment of the requirements for the degree of Master of Science ABSTRACT
The seismic quality
factor (Q) is estimated from synthetic and ultrasonic laboratory shotgathers
using the spectral ratio method. Synthetic seismograms are used to estimate
constant and frequency dependent Q functions of a target layer. The overburden
anelasticity causes target Q estimates to increase with offset. Error caused
by the attenuation of the overburden is negligible, relative to other sources
of error, for highly attenuating targets (Q<30). The accuracy of estimating
frequency dependent Q functions degrades as the reflectivity induced amplitude
losses increase. As a result, inversion for frequency dependent Q from far
offset traces failed to retrieve the target Q function accurately.
In the laboratory, Q values of Lucite, rubber and Berea sandstone slabs are estimated using ultrasonic
top and bottom reflections of each slab. Each target was submerged in a water
tank and a shotgather is acquired using a source and center frequency of 250
kHz and receiver hydrophones with offsets ranging from 2 cm to 16 cm at 2
cm trace interval. Both the source and the receiver are located at the water
surface. The Lucite and Berea sandstone Q estimates are 25% and 15% off the
actual value, respectively. The rubber Q estimate contains about 40% error
which is due to inherent limitation in the spectral ratio method. The width
of the error bounds is found to be inversely proportional to the S/N ratio. Return to Theses Return to ERL Home Updated: April, 1999
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