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Scattering Versus Intrinsic Attenuation in the Near Surface: Measurements from Permanent Down-hole Geophones



Scattering Versus Intrinsic Attenuation in the Near Surface:

Measurements from Permanent Down-hole Geophones


Maria-Daphne Mangriotis

Doctor of Philosophy in Civil and Environmental Engineering

University of California, Berkeley

Professor James Rector III, Chair

The study of attenuation, equivalently of the quality (Q) factor, in the near-surface has three main applications. Firstly, low Q values, which are fairly common in near-surface materials, aside from decreasing seismic energy, also distort the waveforms; treatment of this disturbance effect with inverse-Q filters requires reliable Q estimates. Secondly, attenuation is a seismic parameter which improves interpretation of seismograms, as it is correlated with lithological properties. Thirdly, establishing near-surface Q is important in assessing site effects on strong ground motion events in applications of earthquake modeling and seismic engineering design. In view of these applications, theoretical treatments of attenuation, as well as laboratory and field tests, aim at estimating Q as a function of frequency and strain level. To determine the applicability of using different types of Q measurements, laboratory vs. in-situ measurements, to predict Q behavior across the different frequency bands and strain-levels of interest, it is necessary to model and separate the attenuation mechanisms into scattering (heterogeneity of elastic properties causing energy to be redistributed in space) and intrinsic (energy absorption due to conversion to heat) components. The objective of the presented study was to separate scattering versus intrinsic attenuation in the near-surface from a shallow VSP experiment conducted in the Lawrence Livermore National Laboratory (LLNL) facility using permanent down-hole geophones and a vertical impact source. Given that the VSP array was above the watertable, the Q characterization lies within the vadose zone.

The first arrival of the vertically-incident transmitted P-wave was used to estimate the P-wave attenuation in the field data. Scattering attenuation estimates were established for a selected range of elastic models, which addressed both the effect of the variance of the elastic properties (density and velocity), as well as the effect of the structure of the variation, i.e. 1D versus 3D heterogeneity, on scattering. The elastic profiles were constructed from a superposition of interval values determined from log information (for the density profile) and first-break arrivals (for the velocity profile) and a high-frequency random component with variance range typical of sedimentary basins. The results for the scattering Q estimates related to one-way transmission and multiple reflections are in the order of 20 to 100, as obtained from 1D analytical and elastic finite-difference models. Given the short propagation pathlengths in the experiment, the results show that attenuation due to lateral heterogeneity is non-significant. In addition, given the experimental geometry of shallow VSP studies, it is shown that the scattering estimates are affected from the presence of the near-field, local impedance, and interference effects, which are termed `pseudo-Q' factors. The pseudo-Q factors result in a biased estimate for scattering Q derived from both time-domain and frequency-domain methods. Hence, to accurately model the scattering vs. intrinsic components of attenuation, the bias due to the pseudo-Q factors was accounted for.

The intrinsic attenuation was deduced from comparison of the field data Q estimates, which contain scattering attenuation, intrinsic attenuation and effects from pseudo-Q factors with the elastic synthetic Q estimates. Results yield very low intrinsic Q values, in the order of 4 to 15, for the low and high scattering attenuation estimates respectively. The intrinsic attenuation is attributed to the interaction of the free gas present in the vadose zone with the compressional wave, which is the only known mechanism that can lead to absorption at seismic frequencies (White, 1975; Dutta and Seriff, 1979). Visco-elastic modeling shows that aside from amplitude decay, an intrinsic attenuation mechanism is required to produce the pulse broadening observed in the field data. For a typical set of conditions in the vadose zone, analytical modeling shows that it is possible for the effect of the free gas to account for the intrinsic Q values estimated for the LLNL profile. It is anticipated that attenuation will be very high in vadose environments similar to the LLNL profile, with intrinsic attenuation being the primary loss mechanism. Further research is required to establish the Q filter characteristics due to the free gas effect in the vadose zone and verify if an approximation with standard visco-elastic models, such as the SLS, is appropriate.

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