The complexity of the crust from refraction/wide-angle reflection data

ThePmP wide-angle reflected signal from the Moho shows a wide variation in its characteristics when cornpared from region to region. If the earth's crust is simple and homogeneous and the Moho is a sharp discontinuity, supercriticalPmP wavelets are large and isolated in the 100 km to 300 km distance range when compared to the preceding signals on the seismic traces. If the crust itself has numerous short reflectors and the Moho transition zone is thick or badly disrupted from past tectonic disturbances, thenPmP is often poorly defined and difficult to identify in the coda of earlier arriving signals. These signals are poorly correlated because the reflectors themselves are in general not only discontinuous but tend to be distributed at various depths within the crust. The effect of the vertical velocity gradient in the crust is to make the reflective field for downgoing waves much greater than for upgoing waves. The large variations in reflection coefficients wiht angle of incidence has the effect of making the coda generated by a reflective machanism distance dependent. The reflected signals are also contaminated with scattered signals from smaller scale heterogeneities which may be distributed more uniformly within the crust.In conventional seismic inversion methods applied to crustal refraction experiments, emphasis is usually placed on obtaining a velocity model of the subsurface structure. These models are relatively simple and are limited in the amount of complexity which can be uniquely inferred from the data. The main problem arises because of the difficulties in the identification of the origin of the signals which are themselves often incoherent from trace to trace.In this paper it is shown how the conventional record section may be complemented with a normal moveout corrected intensity section which emphasizes areas of large signal complexities. Data from this intensity section is then used as input to obtain a quantitative measurement of a complexity parameter. These measurements may be used to infer or compare differences in crustal heterogeneity from one region to another. The discussion is illustrated with both numerical modeling data as well as data from recent crustal experiments which were conducted over the Canadian Shield.     

Changes in the early part of the seismic coda due to localized scatterers: The estimation ofQ in a stope environment

A single scattering model was used to analyse the temporary changes in the mean density of scattered waves in a discrete random medium. The model of the mean energy density, originally proposed bySato (1977) for spherical radiation and isotropic scattering, has been modified and applied to a medium in which the scatterers are confined to a specified volume. The time variation of the early part of the mean energy density function for the different source durations was investigated. The dominant effect on the theoretical mean energy density is caused by the specified volume containing scatterers. The duration of the source pulse influences the early part of the coda fort/t ₀<1.2, wheret is the lapse time measured from the source origin time, andt ₀is arrival time of the body wave.The analysis of the coda signal of micro-events occurring immediately in front of the face enables us to estimate the size of the fracture zone induced by the stope. The model of the mean energy density of coda for a medium containing scatterers close to the seismic source was used to analyse a large number of events recorded close to an advancing mine face in a deep level gold mine in South Africa. The coda decay rate has two trends: the first, with a steep decay of coda, is produced by a larger deviation of rock parameters and/or larger size of the scatterers; the second trend, which decays more slowly, has the corresponding mean-free path ranging from 20 m to 200 m. The analysis indicates that the rock mass about 15–20 m from the stope contains a large proportion of fractured and blocked rock, which is the source of scattering. The scattering of theS-wave was much stronger and more stable, with the mean-free path varying from 11 m to 45 m. This is due to the shorter wavelength of theS wave in comparison with theP wave. The quality factor for theP coda wave varies from 30 to 100 in the fracture zone of stope and outside this zone it has a value of 300. The quality factor of theS wave varies from 20 to 78 in the equivalent volume. For rock surrounding the stope the ratioQ _sp ^–1 /Q _ss ^–1 varied from 0.31 to 0.69. This suggests that the radii of scatterers are smaller than 3.5 m.