Attenuation

An obvious approach to improving resolution of seismic data is to use higher frequencies, thus generating waves of shorter wavelength. However, in practice it can be difficult to transmit high frequencies through sediments, since intrinsic attenuation (i.e., attenuation due to intergranular friction) is proportional to the number of wave oscillations and thus increases with frequency. A second form of attenuation is scattering due to small heterogeneities (smaller than a seismic wavelength) or thin layers, which can also be frequency-dependent. Attenuation thus causes an inherent trade-off between resolution and penetration of seismic waves: greater penetration generally requires lower frequencies, which have lower resolution. Seismologists often describe attenuation by referring to its inverse, the quality factor (Qp or Qs).

Research into the characteristics of P- and S-wave attenuation in hydrate/gas deposits is in its infancy. Little quantitative information is available about the effects of hydrate on Qp and Qs, though hydrate should increase Qp and Qs (decrease attenuation) if it acts as an intragranular cement. Gas-charged sediments attenuate P-waves, which often precludes imaging beneath gas zones. A recent study of P-wave attenuation in the Blake Ridge hydrate province showed a strong decrease in Qp from the hydrate zone to the gas zone (Wood et al„ 2000).

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