Anisotropy Evaluation Using an Array Walk-Away VSP

Abstract VSP surveys provide the best way to measure anisotropic parameters for formations with transverse isotropy. Two types of anisotropic parameters can be determined from a walkaway VSP survey: effective parameters for overburden formations above and below VSP receivers, and local parameters for formations within the depth range of a receiver array. Each type has advantages and disadvantages. We use a case study to demonstrate the determination of these two types of anisotropic parameters and use them to build anisotropic depth profiles. Walkaway VSP data were acquired offshore East Coast Canada. The receiver array was centered at a depth of 2051 m and covered a vertical depth range of 143 m. The survey well was deviated 46 °from vertical at the array and rotated 61 ° from the plane of the shot lines. The maximum sourcereceiver offset was 4000 m. Effective anisotropic parameters were calculated from direct and reflected P-wave traveltimes using traveltime inversion. Local anisotropic parameters were computed from the direct P-wave data using a local phase slowness analysis technique. The estimated effective anisotropic parameters Õand ?were 3.7% and 16.7% for the overburden. The estimated local parameters Õ and e were 5.6% and 20.7% over the receiver depth range, confirmed by the interval parameters computed using traveltime inversion for the same depth range. The effective parameters Õand?were -1.9% and 23.8% for the formations just below the array. Combinations of these parameters, in conjunction with well log and seismic data, were used to build anisotropic profiles. This case study shows that the joint use of local slowness and traveltime inversion methods reliably allows theextraction and maximization of anisotropy information from walkaway VSP data. It provides a means to build anisotropic profiles that best approximate formation anisotropy. Introduction Transverse Isotropy with a Vertical symmetry axis (TIV or VTI) is a relatively common type of anisotropy observed in seismic data. A walkaway VSP (WVSP) has a wide angular aperture and well-determined direct arrivals. By placing receivers within the suspected anisotropic zone, a walkaway VSP offers the best dataset available for in-situ seismic anisotropy measurements. Three parameters define weak P-wave anisotropy in a general VTI medium; the P-wave velocity in the direction of the symmetry axis, V0, and the Thomsen parameters Õand ? 1. For horizontally layered VTI formations, the vertical interval velocity function Vv obtained in a VSP velocity survey is a good approximation for V0. The anisotropic parameters Õand ?can be determined from WVSP traveltime and polarization data using a variety of techniques 2, 3, 4. Three sets of anisotropic parameters can be determined from WVSP; effective parameters for overburden formationsabove VSP receivers, local parameters for formations within the depth range of a receiver array, and effective parameters for formations below the array. In practice, receivers are usually placed in a small depth range in the borehole because of acquisition constraints and economic conditions. In such cases, local anisotropic parameters can be derived for a limited depth range.

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