A prestack time migration is presented that is simple, efficient, and provides detailed velocity information. It is based on Kirchhoff prestack time migration and can be applied to both 2-D and 3-D data. The method is divided into two steps: the first is a gathering process that forms common scatterpoint (CSP) gathers; the second is a focusing process that applies a simplified Kirchhoff migration on the CSP gathers, and consists of scaling, filtering, normal moveout (NMO) correction, and stacking. A key concept of the method is a reformulation of the double square‐root equation (of source‐scatterpoint‐receiver traveltimes) into a single square root. The single square root uses an equivalent offset that is the surface distance from the scatterpoint to a colocated source and receiver. Input samples are mapped into offset bins of a CSP gather, without time shifting, to an offset defined by the equivalent offset. The single square‐root reformulation gathers scattered energy to hyperbolic paths on the appropriate CSP gathers. A CSP gather is similar to a common midpoint (CMP) gather as both are focused by NMO and stacking. However, the CSP stack is a complete Kirchhoff prestack migrated section, whereas the CMP stack still requires poststack migration. In addition, the CSP gather has higher fold in the offset bins and a much larger offset range due to the gathering of all input traces within the migration aperture. The new method gains computational efficiency by delaying the Kirchhoff computations until after the CSP gather has been formed. The high fold and large offsets of the CSP gather enables precise focusing of the velocity semblance and accurate velocity analysis. Our algorithm is formulated in the space‐time domain, which enables prestack migration velocity analysis to be performed at selected locations and permits prestack migration of a 3-D volume into an arbitrarily located 2-D line.
Magnetovariation fields were recorded by the land part of the EMSLAB array, August‐September 1985. Data from two complete days and two shorter sequences, chosen for moderate magnetic activity, are here used to map induced currents, which flow preferentially in the more conductive rocks, in Washington and Oregon and adjoining areas. Fourier transform anomaly maps, induction vectors, and hypothetical event anomaly maps are used to delineate conductive structures. These include the ocean and suboceanic asthenosphere and extended regions of attenuated vertical component caused by highly conductive layers under the Basin and Range Province and the Canadian Cordillera, both considered associated with partial melting and hydrothermal water. The Cascade Range of volcanoes has a north‐south strip of high conductivity some 100 km wide beneath it, which is named the Deep Cascades conductor. Between 46° and 47°N the Deep Cascades currents appear to flow up through a narrow upper crustal conductor, well known from prior work by others, into Puget Sound. The Deep Cascades conductor correlates with high heat flow and probably delineates the partial melting beneath the volcanic arc. At its southern end it appears to be continuous with the Basin and Range regional conductor. Northward it terminates between 47.5° and 48°N, south of Mounts Baker and Garibaldi. The Blue Mountains of NE Oregon and SE Washington coincide approximately with a kidney‐shaped block of resistive lithosphere, which lies between the Klamath‐Blue Mountain Lineament and the conductive mantle of the Basin and Range. The Columbia Embayment and plateau basalts occupy a triangular block of lithosphere more resistive than the Canadian Cordilleran Regional Conductor, which abuts against it, but less resistive than the Blue Mountains. Boundaries between these tectonic units are mapped from the associated magnetovariation anomalies, that is, in terms of electrical conductivity.
Application of bandpass and directional filtering to potential-field data in northwestern Canada allows separation of anomalies due to northwest-oriented upper crustal sources that are associated with Cordilleran structures from anomalies due to northeast-oriented lower crustal sources that are primarily associated with Precambrian Shield rocks. In northeastern British Columbia, northeast-trending lower crustal structures of the Canadian Shield as represented in the gravity patterns appear to project west of the Tintina fault -Northern Rocky Mountain Trench at about 56°N. About 400-500 km farther north, the Tintina fault may penetrate into the lithospheric mantle. It is, thus, likely that the depth extent of the Tintina fault rises southward into the crust, as dextral strike-slip motion is transformed into contractional structures of the southern Cordillera.Résumé : L'application de filtres directionnels à bandes passantes sur des données de champ potentiel dans le Nord-ouest canadien permet de séparer les anomalies dues à des sources de la croûte supérieure orientées vers le nord-ouest (qui sont associées aux structures de la Cordillère) des anomalies causées par des sources de la croûte inférieure orientées vers le nord-est (qui sont surtout associées à des roches du Bouclier précambrien). Dans le Nord-ouest de la Colombie-Britannique, les structures à direction nord-est de la croûte inférieure du Bouclier, telles que représentées dans les patrons de gravité, semblent se projeter à l'ouest de la faille Tintina -sillon des Rocheuses septentrional à environ 56ºN. Environ 400-500 km plus au nord, la faille Tintina pourrait pénétrer dans le manteau lithosphérique. Il est donc probable que la profondeur de la faille Tintina s'élève vers le sud à l'intérieur de la croûte alors que le décrochement dextre est transformé en structures de contraction de la Cordillère méridionale.[Traduit par la Rédaction] 961 Geiger and CookCan. J.Earth Sci. 38: 953-961 (2001)
In areas of rugged topography, the application of datum statics may lead to non-hyperbolic moveout in common scatter point (CSP) gathers. Equivalent offset migration (EOM), originally derived for a flat datum, can be extended to handle source, receiver and output surface scatter point locations on different datums. CSP gathers are created for a constant velocity model data set with a single scatter point in the subsurface. Correct hyperbolic moveout curves are obtained for both positive and negative datum shifts, although large shifts may move data of a single input trace over a wide range of offsets. Processing of an Alberta Foothills data set using rugged topography EOM produces a good image of the subsurface.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.