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MEASUREMENTS OF EARTH ATTENUATION FROM DOWNHOLE AND SURFACE SEISMIC RECORDINGS *
Author(s) -
RAIKES S. A.,
WHITE R. E.
Publication year - 1984
Publication title -
geophysical prospecting
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.735
H-Index - 79
eISSN - 1365-2478
pISSN - 0016-8025
DOI - 10.1111/j.1365-2478.1984.tb00745.x
Subject(s) - attenuation , anelastic attenuation factor , geology , amplitude , interference (communication) , broadband , noise (video) , seismic wave , acoustics , seismology , optics , computer science , physics , telecommunications , channel (broadcasting) , artificial intelligence , image (mathematics)
A bstract Frequency‐dependent attenuation of compressional waves within the earth has been estimated in the vicinity of wells from1 spectral power ratios of the coherent events in separate time gates on the seismic section 2 matching a broadband synthetic trace with seismic data at the well, and 3 determining the operator that transforms one down(up) going pulse recorded in the well into another recorded at a deeper (shallower) level.The accuracy of estimation of all three methods was insufficient to estimate attenuation over small depth intervals, and it was not possible to distinguish between the contribution due to internal multiples and that of genuine absorption with much confidence. Spectral ratios from (1) showed a smoother variation with frequency—and one more consistent with other estimates—when they were compensated for the spectra of the reflectivities over the time gates employed, but they did not provide more than a broad indication of attenuation over a substantial depth interval. Approach (2) was hampered by the restricted durations over which synthetic trace and seismic data can be reliably matched; approach (3) gave the best results. Here matching is a much more powerful tool than the spectral‐ratio techniques that are commonly applied since it can yield the form of the attenuation operator, i.e., both its amplitude and phase response, together with properly defined measures of its accuracy, while at the same time it minimizes the influence of noise and local interference effects at each recording level. For seismic target depths where internal multiple activity was low the logarithms of the amplitude responses of the estimated attenuation operators decreased approximately linearly with frequency and the phase responses showed no significant dispersion. Application of approach (3) to downgoing and upgoing waves estimated from a vertical seismic profile revealed the importance of changes in frequency‐dependent geophone coupling and their effect on values of Q determined from downgoing pulses only.