Estimation Of Seismic Attenuation From Vsp Data In Two Cooper/Eromanga Basin Wells Using The Spectral Ratio Method.
Degree of Master of Science, 1994
University of Adelaide
Anelastic or intrinsic attenuation is the loss of energy through frictional processes from a propagating seismic wave, resulting in both a decrease in amplitude and a change in frequency content. Laboratory and in-situ measurements indicate that, within the bandwidth of conventional seismic data, attenuation obeys a negative exponential law, the exponent of which is a linear function of frequency and may be parameterised by a dimensionless quantity, Q, a constant for any particular rock.
The spectral ratio method is commonly used to estimate attenuation from vertical seismic profile data. The technique involves comparing the amplitude spectrum of a reference signal with successive downhole VSP first arrival spectra to determine cumulative attenuation. Q is then calculated from the rate at which cumulative attenuation increases with depth. The attenuation measured in this way is a combination of both the intrinsic attenuation, due to rock properties, and the apparent attenuation resulting from short-period multiple reflections.
Attempts to estimate attenuation using the spectral ratio method were made on VSP data recorded in two wells penetrating representative Cooper and Eromanga Basin sediments in South Australia and Queensland. Measured attenuation from one of these wells, Watson #1, implies Q values ranging from 26 to over 400. Synthetic VSP?s created from log data, both with and without multiples provide the method to determine the apparent attenuation, which appears to be responsible for between 4% and 82% of the measured attenuation. In particular, cyclic coal sequences encountered in the Permian give rise to very high apparent attenuation rates.
Spectral ratios for the second well, Tirrawarra #22, could not be interpreted for Q, due to severe notching in the spectra of the downhole data.
The accuracy of the intrinsic attenuation estimates over small depth intervals is insufficient to make correlations with lithology, particularly in reservoirs of limited vertical extent. A Q log constructed from intrinsic attenuation and interval velocity is compared with known geological information.
Attention to data acquisition procedures, the minimizing of noise and "ghosting" effects, are necessary in recording data of a quality suitable for analysis. The use of a far field signature to compute the spectral ratio for each VSP level is shown to give a better result than using a shallow VSP trace as the reference signal.
The attenuation measurements presented here should prove useful in quantifying the known problem in collecting wide bandwidth seismic data in the area, and in the design of inverse-Q filters.