Produção Científica



Artigo em Revista
09/10/2010

Source parameters and rupture velocity of small? 2.1 reservoir induced earthquakes.
We calculate stress drop and rupture speed for ML ‚ȧ 2.1 shallow reservoir induced earthquakes and Ô¨Ānd them to be similar to those of large, natural earthquakes. Previous studies have suggested that hydrofractures, mining and reservoir-induced earthquakes have lower average stress drop than natural tectonic earthquakes. This difference might result from the different tectonic setting or the shallower hypocentral depths of induced earthquakes. Alternatively,
difÔ¨Āculties in correcting for attenuation and site effects in earlier studies may lead to underestimation of stress drop. In addition, most studies assume the rupture velocity of small
reservoir induced earthquakes to be the same as for the large earthquakes. We analyse a set of 101 ML ‚ȧ 2.1 earthquakes induced by changing water level in the Ac¬łu Reservoir, NE Brazil. The earthquakes are shallow, (depth <5 km) and the region has negligible natural seismicity. We use three different approaches to calculate the source parameters of the six largest (1.9 ‚ȧ ML ‚ȧ 2.1) earthquakes. We model the individual spectra to Ô¨Ānd corner frequency, frequency-independent Q, and long period amplitude. We use collocated small earthquakes as empirical Green‚Äôs functions to calculate the spectral ratios, and determine the relative source
time functions. Estimates of the source duration and corner frequency imply stress drops in the range of 26‚Äď179 MPa. These are similar to, or higher than tectonic earthquakes suggesting
that the shallow hypocentral depth and the presence of water do not affect stress drop. We observe clear directivity for one of the earthquakes, and use the azimuthal variation in pulse
width to estimate a rupture velocity of ‚Č•0.6ő≤.
Artigo em Revista
09/10/2010

SS-traveltime parameters from PP and PS reflections.
The SS-wave traveltimes can be derived from PP- and PS-wave data with the previously derived method. We have extended this method as follows. (1) The previous requirement that sources and receivers be located on a common acquisition surface is removed, which makes the method directly applicable to PS-waves recorded on the ocean bottom and PP-waves recorded at the ocean surface. (2) By using the concept and properties of surface-to-surface propagator matrices, the second-order traveltime derivatives of the SS-waves are obtained. In the same way as for the original method, the proposed extension is valid for arbitrary anisotropic media. The propagator matrix and geometric spreading of an SS-wave reflected at a given point on a target reflector are obtained explicitly from the propagators of the PP- and PS-waves reflected at the same point. These additional parameters provided by the extended method can be used for a partial reconstruction of the SS-wave amplitude as well as for tomographic estimation of the elastic velocity model. A full simulation of the SS-wave, which includes reflection and transmission coefficients, cannot be obtained directly from recorded PP- and PS-wave amplitudes.
Artigo em Revista
09/10/2010

A new stabilized least-squares imaging condition.
The classical deconvolution imaging condition consists of dividing the upgoing wave field by the downgoing wave field and summing over all frequencies and sources. The least-squares imaging condition consists of summing the cross-correlation of the upgoing and downgoing wave fields over all frequencies and sources, and dividing the result by the total energy of the downgoing wave field. This procedure is more stable than using the classical imaging condition, but it still requires stabilization in zones where the energy of the downgoing wave field is small. To stabilize the least-squares imaging condition, the energy of the downgoing wave field is replaced by its average value computed in a horizontal plane in poorly illuminated regions. Applications to the Marmousi and Sigsbee2A data sets show that the stabilized least-squares imaging condition produces better images than the least-squares and cross-correlation imaging conditions.
Artigo em Revista
09/10/2010

Fundamentals of coaxial and coplanar coil arrays in induction tools.
Por meio s√©culo, dede sua cria√ß√£o, todas as sondas comerciais por indu√ß√£o eletromagn√©tica (EM) utilizavam o tradicional arranjo coaxial de bobinas. Somente a partir da virada do s√©culo XX que estas sondas passaram a incorporar o arranjo coplanar, devido a necessidade de investigar reservat√≥rios finamente laminados ou anomalias sem simetrias de rota√ß√£o (fraturas ou cavidades). Com o objetivo de melhor compreender a aplica√ß√£o do arranjo coplanar nas sondas de indu√ß√£o, elaboramos um estudo comparativo de suas respostas com as do tradicional arranjo coaxial, atrav√©s de modelagem unidimensional, em alguns ambientes comuns √† geof√≠sica de po√ßo: 1) meios homog√™neos, isotr√≥picos e ilimitados; 2) camadas espessas com invas√£o de filtrado de lama e forma√ß√£o de annulus, 3) sequ√™ncias de multicamadas horizontais e inclinadas; 4) transi√ß√£o gradacional entre duas camadas espessas; e 5) forma√ß√Ķes finamente laminadas. Este estudo comparativo entre os arranjos coaxial e coplanar permite concluir que: 1) o efeito pelicular √© mais acentuado nas respostas do arranjo coplanar; 2) as respostas do arranjo coplanar s√£o mais sens√≠veis a movimenta√ß√£o de fluidos na forma√ß√£o, principalmente nas zonas de annulus, 3) os perfis do arranjo coplanar apresentam picos devidos √† polariza√ß√£o nas interfaces, que podem ser bons indicadores das fronteiras entre camadas; 4) o arranjo coplanar √© mais sens√≠vel para detectar e delinear reservat√≥rios finamente laminados.
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