The CO 2 storage operation at Sleipner in the Norwegian North Sea provides an excellent demonstration of the application of time-lapse surface seismic methods to CO 2 plume monitoring under favourable conditions. Injection commenced at Sleipner in 1996 with CO 2 separated from natural gas being injected into the Utsira Sand, a major saline aquifer of late Cenozoic age. CO 2 injection is via a near-horizontal well, at a depth of about 1012 m bsl, some 200 m below the reservoir top, at a rate approaching 1 million tonnes (Mt) per year, with more than 11 Mt currently stored.A comprehensive time-lapse surface seismic programme has been carried out, with 3D surveys in 1994, 1999, 2001, 2002, 2004, 2006 and 2008. Key aims of the seismic monitoring are to track plume migration, demonstrate containment within the storage reservoir and provide quantitative information as a means to better understand detailed flow processes controlling development of the plume in the reservoir.The CO 2 plume is imaged as a number of bright sub-horizontal reflections within the reservoir, growing with time ( Figure 1). The reflections mostly comprise tuned wavelets arising from thin (mostly < 8 m thick) layers of CO 2 trapped beneath very thin intra-reservoir mudstones and the reservoir caprock. The plume is roughly 200 m high and elliptical in plan, with a major axis increasing to over 3000 m by 2008. As well as its prominent reflectivity, the plume also produces a large velocity pushdown caused by the seismic waves travelling more slowly through CO 2 -saturated rock than through the virgin aquifer. This paper summarises some of the quantitative methods that have been applied to the Sleipner seismic datasets.
Abstract. Reciprocity theorems have proven their usefulness in the study of forward and inverse scattering problems. Most reciprocity theorems in the literature apply to the total wave field and are thus not compatible with one-way wave theory, which is often applied in situations in which there is a clear preferred direction of propagation, like in electromagnetic or acoustic wave guides and in seismic exploration. In this paper we review the theory for one-way wave fields (or bidirectional beams), and we extensively discuss the symmetry properties of the square root operator appearing in the one-way wave equation. Using these symmetry properties, it appears to be possible to derive reciprocity theorems of the convolution type and of the correlation type for electromagnetic or acoustic one-way wave fields in dissipative inhomogeneous media along the same lines as the usual derivation of the reciprocity theorems for the total wave field. The one-way reciprocity theorem of the convolution type provides a basis for representations of scattered one-way wave fields in terms of generalized Bremmer series expansions or generalized primaries. The one-way reciprocity theorem of the correlation type finds its application in reflection imaging based on inverse one-way wave field propagators. IntroductionReciprocity In many wave propagation problems one can define a "preferred direction of propagation." Electromagnetic and acoustic waveguides are obvious examples, but also in laterally unbounded media it is often advantageous to define a preferred propagation direction, such as in seismic exploration. In all those situations it is useful to decompose the wave equation into a system of coupled equations for oppositely propagating waves. In the literature on electromagnetic wave theory these oppositely propagating waves are known as "bidirectional beams" [Hoekstra, 1997; van Stralen, 1997]; in the acoustic literature one usually speaks of "one-way wave wave fields" [Claerbout, 1971; Fishman et al., 1987]. In this paper we adopt the latter terminology.The reciprocity theorems discussed above apply to the total electromagnetic or acoustic wave field. Obviously, these reciprocity theorems are not compatible with one-way wave theory. In a recent paper we derived reciprocity theorems for acoustic one-way wave fields in lossless inhomogeneous fluids [Wapenaar and Grimbergen, 1996]. With some minor modifications these theorems are also applicable to transverse electric or transverse magnetic one-way wave fields in lossless inhomogeneous media. The aim of the current paper is to extend those one-way reciprocity theorems to electromagnetic or acoustic wave 851
Ultrasonic experiments on a dry Colton sandstone placed in a triaxial pressure machine show that effective stress changes lead to distinct anisotropic velocity changes in compressional waves and shear waves. The stress imprint can be recognized from the associated velocity pattern by relating the velocities to the three normal stress directions. The ultrasonic velocities indicate that the sensitivity of the different waves to stress predominantly depends on stresses applied in the polarization and propagation directions of the particular wave mode. Also, stress‐induced changes in shear‐wave splitting are observed.
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