Igor B. Morozov

Heterogeneity of the uppermost mantle beneath Russian Eurasia from the ultra-long-range profile QUARTZ

The 3850-km long Deep Seismic Sounding profile QUARTZ crosses six major geologic provinces in Eurasia and is sourced by 3 nuclear and 48 chemical explosions. We present the first interpretation of the entire data set, using two dimensional (2-D) ray tracing and inversion, resolution analysis, and 1-D amplitude modeling. Our interpretation shows a 42-km-thick, high-velocity crust under the Baltic Shield, a 29-km-thick crust and high-velocity upper mantle under the Mezenskaya depression, 52-km-thick crust with high-velocity lower crust and uppermost mantle under the Urals, and 40-km-thick crust under the West Siberian basin deepening to 45 km under the Altay-Sayan fold belt. High-velocity (8.4 km/s) uppermost mantle is found under the Mezenskaya depression and under the east flank of the Urals. One almost continuous upper mantle boundary occurs at 65- to 80-km depth, and another with an approximately 40-km-thick LVZ occurs at 120- to 140-km depth. The shallow upper mantle blocks and the two extensive interfaces indicate strong upper mantle heterogeneity. Resolution analysis based on direct multivariate model perturbations, artificial neural network and principal component analysis, indicate the depth uncertainty of the 410-km discontinuity within ±6 km, and also its trade-off with dip and velocities above and below the discontinuity. Decreased near-critical amplitudes of reflections from the 410-km and 660-km discontinuities indicate that these boundaries are most likely represented by gradient zones about 15-20 km thick. Lithosphere thins, asthenospheric velocity decreases, and the 410-km discontinuity dips to the SE approaching the Himalayan orogenic belt.

Wide-angle seismic imaging across accreted terranes, southeastern Alaska and western British Columbia

Abstract This study addresses the question of crustal, Moho, and uppermost mantle structure across an accreted terrane, continental arc, and fold and thrust belt in southeastern Alaska and western British Columbia. The 186-km-long Portland Canal line of the ACCRETE wide-angle seismic dataset across the Coast Mountains is analyzed using a combination of travel-time analysis techniques, including delay-time tomography, turning ray tomography, reciprocal time analysis, intercept-time inversion, and forward ray tracing. Additional constraints on the model are obtained from a coincident vertical-incidence seismic section and from correlation with the mapping of geologic structures plunging into the seismic section. The study reveals moderately high crustal velocities, low-velocity gradient in the middle crust, and decreasing average crustal velocity and a north-deepening Moho. Termination of crustal reflectivity across a vertical zone (the Coast Shear Zone, CSZ) indicates that the CSZ is most likely a strike-slip fault associated with a transpressive tectonic regime. A mid-Cretaceous thrust system mapped on the surface to the west of the CSZ is imaged by several groups of mid- to lower crustal reflectors extending close to the Moho indicating it was a thick-skinned thrust system. NE-dipping fabric imaged within the Coast Mountains batholith (CMB) is associated with a ductile deformation during Early Eocene crustal extension. The crustal section under the CMB, which has an average velocity of 6.55 km/s and shallower than average crustal thickness of 31 km, can be considered as corresponding to the lower two thirds of an average crustal section which has been inflated by intrusions of high-velocity tonalite to gabbro sills.

Generation of new continental crust and terrane accretion in Southeastern Alaska and Western British Columbia: constraints from P- and S-wave wide-angle seismic data (ACCRETE)

Abstract The ACCRETE study addresses the question of continental assemblage in southeastern Alaska and western British Columbia through accretion of exotic terranes and generation of new crust by magmatic addition in a former continental arc. We present results of wide-angle P- and S-wave seismic interpretation of a 300-km long marine-land seismic line across the contacts between accreted terranes and Coast Mountains. Additional constraints on the model are obtained from correlation with geologic mapping. Our results indicate that the Coast Shear Zone (CSZ) is a nearly vertical fault zone probably related to a transpressive regime. West of the CSZ, the mid-Cretaceous (90 Ma) thrust belt is rooted in the deep crust and is truncated by the CSZ. From the interpretation of the imaged sub-vertical reflecting zones, we infer the positions of the Alexander–Wrangellia terrane boundary (AWB) and of Tertiary extensional grabens within Dixon Entrance near its intersection with the profile. The observed values of Vp and Vp/Vs in the lower crust of the Alexander terrane are similar to those of oceanic crust and distinctly different from the Coast Mountains Batholith (CMB) to the northeast. The crust under the CMB (32 km) is thinner than the average continental crust, and the Moho is sharp (∼200 m) and highly reflective. The low-velocity mantle (7.9 km/s) suggests high temperature consistent with the stability of garnet in mafic rocks in the lower crust. The lower crustal velocity of 6.9 km/s supports a lower crust composed of interlayered garnet pyroxene granulite and quartzofeldspathic-restite related to batholith generation. The crustal section under the CMB is seismically identical to the lower two thirds of normal crust, heated and inflated by intrusions of tonalite, and gabbro interlayered with restites from batholith generation and uplifted during exhumation.

Instantaneous polarization attributes and directional filtering

We introduce a systematic definition of instantaneous attributes for an arbitrary multicomponent seismic signal. The definition is a natural generalization of known complex trace attributes of a one‐component signal. Instantaneous amplitude and all polarization parameters are defined as invariants of “instantaneous phase rotation.” The principal feature of our approach is the unique definition of the instantaneous phase for a signal with any number of components. Plots of subtle polarization parameters of multicomponent seismic data are easily obtained using conventional seismic plotting routines. We illustrate our approach on a synthetic example and apply it to real 3‐component, wide‐angle crustal data. Plots of polarization attributes provide evidence for shear‐wave splitting in an Sg arrival. Having determined the instantaneous polarization vector, we design a new type of time‐domain spatial directional filter. The filter enhances linearly polarized events with specified instantaneous polarization. The...

Coda of Long-Range Arrivals from Nuclear Explosions

Short-period, three-component recordings from peaceful nuclear explosions (PNE) of the profile QUARTZ, located in Russia, are used to constrain the nature of the coda of PNE arrivals. In particular, we examine the unusually strong and extensive coda of the long-range P n (interpreted as a whispering-gallery, WG) phase propagating to beyond 3000 km. Energy-balance considerations in three dimensions show that such an extensive coda is inherent not only to WG but to all other P -wave phases and can be explained by crustal scattering. The long coda is a result of excitation of short-period scattered waves ( Pg, Sg, Lg, Rg ) within the crust by the waves incident from the mantle, or, conversely, by generation of mantle phases from crustal guided waves within the source region. The resulting estimates of coda Q range between Q = 380 near 2 Hz and Q = 430 near 5 Hz and can be associated with crustal attenuation including the sediments. Our coda model also explains quantitatively the observed build-up of the arrival amplitude with time and the apparent lack of a pronounced coda of the body-wave arrivals from the mantle transition zone. These effects result from adding up the energy of the later arrivals arriving during the codas of earlier arrivals. We “deconvolve” the overlapping coda patterns and show that the true relative energies of the arrivals are significantly lower than the apparent energies measured from the raw records. A whispering-gallery interpretation of the long-range P n and crustal scattering accounts for the entire range of observations of kinematic, spectral, and amplitude pattern of the PNE wavefield and allows the derivation of constraints on attenuation.

Seismic studies around the Kola Superdeep Borehole, Russia

Abstract The Kola Superdeep Borehole (SG-3) provided an ideal opportunity to test hypotheses on the origins of crustal reflections and on the presence and seismic expression of fluids in the upper crust. The alternative sources of crustal reflections include compositional changes, shear zones, fluids, and metamorphic facies changes, all of which are represented at the well. Both the 38-km-long CDP section and the borehole VSPs in the range 2.2–6.0 km demonstrate the presence of reflections from dipping compositional layering, shear zones, and fluid-filled zones. Subhorizontal reflectivity zones are interpreted as horizontal fluid-filled fracture-type reservoir rocks. Results suggest the presence of fluids down to a depth of at least 12 km in the upper crust; the presence of these fluids lowering seismic velocity causes estimates of upper crustal composition to be too felsic.

A new system for multicomponent SEISMIC processing

Abstract A new seismic data-processing program package was developed at the Program for Crustal Studies at the University of Wyoming. The new system has a well-developed job-description language similar to that used by some commercial analogs. Unlike commercial systems, our seismic monitor supports a structured multicomponent seismic data stream, which facilitates programming of complex processing algorithms. The monitor employs a unique backpropagation processing strategy, giving additional flexibility to the processing flow. The organization of the system allows parallelizing of the computations in a natural way. Over 60 seismic processing modules have already been developed for our system. Special interfaces allow inclusion of older modules developed for other processing systems. The system was written using C programming language, with some modules using FORTRAN. It operates under the UNIX environment on SUN Sparc and Silicon Graphics Inc. (SGI) Indigo 2 workstations, and on a four-processor SGI Power Challenge L.

Integrated software framework for processing of geophysical data

We present an integrated software framework for geophysical data processing, based on an updated seismic data processing program package originally developed at the Program for Crustal Studies at the University of Wyoming. Unlike other systems, this processing monitor supports structured multi-component seismic data streams, multi-dimensional data traces, and employs a unique backpropagation execution logic. This results in an unusual flexibility of processing, allowing the system to handle nearly any geophysical data. A modern and feature-rich graphical user interface (GUI) was developed for the system, allowing editing and submission of processing flows and interaction with running jobs. Multiple jobs can be executed in a distributed multi-processor networks and controlled from the same GUI. Jobs, in their turn, can also be parallelized to take advantage of parallel processing environments, such as local area networks and Beowulf clusters.

AVO modeling of pressure-saturation effects in Weyburn CO2 sequestration

The Weyburn Field in southeastern Saskatchewan, Canada (Figure 1) was discovered in 1954 and has been water-flooded since the 1960s. In October 2000, injection of CO2 for enhanced oil recovery was started by EnCana, concurrently with a multidisciplinary International Energy Agency Greenhouse Gas (IEA GHG) CO2 Monitoring and Storage Project (CO2MSP). In order to monitor CO2 injection, storage, and oil recovery, several vintages of 3D and 3D/3-C data were acquired, starting with a baseline survey in December 1999. These time-lapse data sets were used to evaluate the quality and safety of CO2 sequestration, and monitor the reservoir pressure front, water flooding, and bypassed oil (White et al., 2004).

3D First-Arrival Regional Calibration Model of Northern Eurasia

Seismological monitoring of the Comprehensive Test Ban Treaty (CTBT) requires detailed knowledge of travel-time characteristics of seismic phases across large areas. We use first-arrival travel times from several of Russian Deep Seismic Sounding (dss) profiles, primarily those from Peaceful Nuclear Explosions (pnes), to construct a 3D regional travel-time model of northern Eurasia. The method used, which can be viewed as a generalization of the existing regionalization techniques, is based on apparent-velocity-based spatial interpolation of the travel times picked from dss records. The resulting travel-time field is described in terms of an apparent velocity model in the 3D space of geographic coordinates and ray parameter. In the travel-time calibration, this model could be used to construct approximate source-specific station correction surfaces for any location within the region and also as a region-specific reference model to be used as a background for further calibration effort. This approach allows incremental refinement of the model as additional travel-time data become available. By converting the travel-time model into depth, a 3D regional velocity model is obtained, providing a description of the general features of the upper mantle in northern Eurasia.