ISSN:
1365-246X
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Geosciences
Notes:
The width of a seismic pulse increases monotonically with distance and with Q-1. Estimates of Q from pulse width measurements are often not robust for oscillatory arrivals or for impulsive arrivals in the presence of noise. We present a method to estimate Q from two arrivals using measurements of any signal attribute, ɛ, that is sensitive to propagation loss. The propagation loss is defined as the change in ɛ divided by the difference in traveltime between the arrivals. The first data arrival is used as the reference wavelet. The Q-gram method is based on propagating the reference wavelet with a plane-wave Q-propagator for various values of Q-1. The Q-propagator includes a dispersion relation and the measured difference in traveltime between the data arrivals. The plot of synthetic propagation loss between the reference and propagated wavelets, versus Q-1, is called a Q-gram. The Q-gram, together with the measured propagation loss of the data, gives the Q of the data.The averaged instantaneous frequency f̄ and the averaged instantaneous pulse width f̄ make good signal attributes. Tests on synthetic seismograms show that the Q-gram method, using either f̄ or τ for ɛ, is applicable to both impulsive and oscillatory arrivals and is relatively robust with regard to noise, phase changes and signal clipping.We apply the Q-gram method to horizontal-component airgun ocean-bottom seismometer (OBS) data using the basement-converted shear-wave reflection, PS, as the first arrival and PSSS as the second arrival. We estimate Qβ, the effective sediment shear-wave Q, with an f̄-type Q-gram and a τ-type Q-gram for the PS and PSSS sediment shear-wave reflections. The data indicate that Qβ∼ 75 ± 15, in agreement with results from the application of the spectral-ratio method using windows that exclude interfering arrivals identified by means of the instantaneous frequency.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1365-246X.1995.tb05911.x
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