Seismic attribute inversion for velocity and attenuation structure using data from the GLIMPCE Lake Superior experiment

A simultaneous inversion for velocity and attenuation structure using multiple seismic attributes has been applied to refraction data from the 1986 GLIMPCE Lake Superior experiment. The seismic attributes considered include envelope amplitude, instantaneous frequency, and travel time of first arrival data. Instantaneous frequency is converted to t* using a matching procedure which approximately removes the effects of the source spectra. The derived seismic attributes are then used in an iterative inversion procedure referred to as AFT inversion for amplitude, (instantaneous) frequency, and time. Uncertainties and resolution of the velocity and attenuation models are estimated using covariance calculations and checkerboard resolution maps. A simultaneous inversion of seismic attributes from the GLIMPCE data results in a velocity model similar to that of previous studies across Lake Superior. A central rift basin and a northern basin are the most prominent features with an increase in velocity near the Isle Royale fault. Although there is an indication of the central and northern basins in the attenuation model for depths greater than 4 km, the separation is not evident for shallower depths. This may result from microstructures masking compositional variations in the attenuation model for shallower depths. Attenuation Q values range from approximately 60 near the surface to ear 500 at 10 km depth. A relationship between inverse Q and velocity of Q¯¹=0.0210-0.0028*v was found between Q¯¹ and velocity beneath Lake Superior which supports previous laboratory results. The invereted velocity and attenuation models provide important constraints on the lithology and physical properties of the Midcontinent rift beneath Lake Superior.Copyrighted by American Geophysical Union

Seismic imaging for crustal velocity and attenuation structure

In this study, seismic attributes are used to determine seismic attenuation and velocity structure of the subsurface. The seismic attributes considered include instantaneous frequencies, amplitudes and travel-times of selected phases. Complex trace analysis is used to estimate the instantaneous frequencies. Instantaneous frequency matching is then used to obtain the differential t* values between a reference pulse and the observed pulses. The differential t* values computed using instantaneous frequency matching, along with travel-time and amplitude information, are then utilized in a simultaneous inversion of seismic attributes for velocity and attenuation structure. Uncertainties in the model are estimated using covariance calculations and checkerboard resolution maps. The method is then applied to seismic refraction data along line A of the GLIMPCE Lake Superior experiment to determine velocity and attenuation structure. The inverted velocity structure was found to be similar to that found in previous studies. The prominent features include a central rift basin and a smaller northern basin along with an increase in velocity near the Isle Royale fault. The inverted attenuation model indicates that the basin structures are more attenuative than the surrounding areas. In the shallower portions of the model, microfracturing may have masked any compositional variations between the basins and the rocks along the Isle Royale fault. The instantaneous frequency matching procedure is then applied to seismic reflection data from line A of the GLIMPCE Lake Superior experiment. The computed differential t* values are converted to apparent {\cal Q}\sp{-1} layers by a fitting procedure that simultaneously solves for all the interval ${\cal Q}$ values using non-negative least squares. The bootstrap method is used to estimate the uncertainties in the computed models. Comparison of the {\cal Q}\sp{-1} profiles obtained from the seismic reflection data shows a close correlation with the results obtained from the seismic refraction data. This correspondence suggests that the effects of wave propagation and scattering on the apparent attenuation are similar for the two data sets. The attenuation model from the seismic reflection data is then related to the interpreted reflectivity structure. The highest interval {\cal Q}\sp{-1} values ($\u3e$0.01) were found near the surface, corresponding to the upper Keweenawan sedimentary rock sequence. Low {\cal Q}\sp{-1} values ($\u3c$0.0006) were obtained beneath the central rift basin. The surrounding crystalline rocks had {\cal Q}\sp{-1} values similar to those found for the basaltic flows which included some interflow sedimentary rocks