Large lakes have always represented a problem for regional gravity databases; the difficulty of access means gaps or coarse spacing in the sampling. Satellite, airborne, and shipborne gravity techniques are options, but the resolution and/or cost of these systems make them impractical or inaccurate for exploration or environmental studies, where the required resolution is [Formula: see text]. In this study, the feasibility of a ground gravity survey over a frozen lake where ice moves because of windy conditions is assessed. Lake Wanapitei, widely accepted as resulting from the impact of a meteorite 37 million years ago, is one of these cases in which the necessity of expanding coverage over poorly sampled regions arose from a significant gap between surface and airborne geophysical maps. Two gravity surveys were completed on the ice of Lake Wanapitei in the winters of 2003 and 2004. To study the structure, longtime series of gravity field measurements were recorded for 98 stations, allowing for improved control over the noise sources in the data. Final processing and integration with an existing regional data set in the area and the application of terrain corrections reduced the amplitude of the circular anomaly from 15 to [Formula: see text] and its diameter from 11 to [Formula: see text]. The feasibility of gravity surveys on ice was assessed, and we determined that for large-scale studies such as this one, the quality of the data, even under noisy conditions, was acceptable. However, for more detailed mapping, calm wind conditions and long time series are required.
Imaging and characterizing time-lapse changes in reservoir properties rely on accurate estimation of the corresponding time shifts and amplitude changes. We evaluated the accuracy of competing time-shift extraction algorithms — cross correlation, nonlinear inversion, dynamic warping, the correlated-leakage method, and the optical-flow-warping method — for estimating 4D time shifts, particularly from subsalt reservoirs. A synthetic model of a Gulf of Mexico field was used for testing these methods. The model contains both subsalt and extra-salt reservoirs. We imposed 4D velocity changes within both reservoirs and in the overburden of the subsalt reservoir. Each of the time-shift extraction methods was evaluated, and each was found to have various strengths and weaknesses. The correlated-leakage and optical-flow-warping methods reproduced the model time shifts most accurately and with the best resolution.
Lake Wanapitei, located within the Southern Province of Ontario, Canada, provides the setting for a unique study of an impact crater situated within a shield environment. Evidence for the 7.5-km-diameter Wanapitei impact includes a circular Bouguer gravity low centered over the central area of the lake and features of shock metamorphism in samples of glacial drift found on the southern shores. Geophysical studies of craters in hard-rock environments are often limited by the lack of markers used for exploration; this may be overcome with the use of the large igneous dike swarms that characterize Archean terrains. The 1.2 Ga Sudbury dike swarm predates the impact that is suggested to have generated Lake Wanapitei and provides the setting for a study to constrain the size and location of the impact crater. The swarm is clearly visible on aeromagnetic maps as high amplitude, linear features, suggesting they could be used as vertical markers indicative of structural changes having an effect on target rock susceptibilities.To fully establish the size of the crater, a total fi eld magnetic map was produced to trace the Sudbury dikes through the proposed crater center. A gap in their signature, expressed as a 100 nT low, 2-3 km in width, constrains the size of the crater to <5 km. Numerical modeling suggests that a crater of this size will demagnetize target rocks, producing a low in the total magnetic fi eld, up to a maximum diameter of 3 km. Dikes L'Heureux, E., Ugalde, H., Milkereit, B., Boyce, J., Morris, W., Eyles, N., and Artemieva, N., 2005, Using vertical dikes as a new approach to constraining the size of buried craters: An example
An accurate solution of the wave equation at a fluid-solid interface requires a correct implementation of the boundary condition. Boundary conditions at fluid-solid interface require continuity of the normal component of particle velocity and traction, whereas the tangential components vanish. A main challenge is to model interface waves, namely, the Scholte and leaky Rayleigh waves. This study uses a nodal discontinuous Galerkin (dG) finite-element method with the medium discretized using an unstructured uniform triangular meshes. The natural boundary conditions in the dG method are implemented by (1) using an explicit upwind numerical flux and (2) by using an implicit penalty flux and setting the modulus of rigidity of the acoustic medium to zero. The accuracy of these methods is evaluated by comparing the numerical solutions with analytical ones, with source and receiver at and away from the interface. The study shows that the solutions obtained from the explicit and implicit boundary conditions provide the correct results. The stability of the dG scheme is determined by the numerical flux, which also implements the boundary conditions by unifying the numerical solution at shared edges of the elements in an energy stable manner.
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