Subsurface Imaging with Reverse Vertical Seismic Pro les

Abstract

A novel imaging process, referred to as vector image isochron (VII) migration, is specifically designed to reduce artifacts caused by arrays with limited apertures. By examining the assumptions behind generalized Radon transform (GRT) migration, a new approach is found which identities and suppresses array artifacts, based on the array geometry and the migration earth model. The new method works in four steps: 1) The conventional image is broken down according to the orientation of imaged planes within the image space, forming a vector image of the earth; 2) the earth model and the geometry of the arrays are used to derive vector image isochrons, which define the shape of reflection events in the vector image space; 3) the vector image is transformed by summing along the isochrons so that it depends on subsurface location and reflector orientation, rather than imaged plane orientation. This process is referred to as vector image isochron (VII) transformation; and 4) the transformed vector image is collapsed to a scalar image by summing over reflector orientations. The VII imaging method is derived in both 2D and 3D with the assumption that at least one of the arrays, source or receiver, is oriented horizontally. The surface array can have any distribution along the surface. The other array can have any orientation, although in this paper it will be assumed to be either another surface array or a vertically oriented downhole array. Downhole surveys in deviated wells, or in multiple wells, can be imaged with VII migration, at the likely cost of more computation time. The VII imaging method is tested on field data acquired in 1998 by MIT and several industry partners. The dataset is a 3D reverse vertical seismic profile (RVSP) over a hydrocarbon-bearing pinnacle reef in the northern Michigan reef trend. The survey exhibited two features of note: 1) A new, strong, downhole vertical vibrator, and 2) a random distribution of surface receiver locations. Due to adverse conditions, a large portion of the surface spread had to be abandoned. The reduced spatial coverage presents a challenge to the new migration method, but also limits the extent of the migrated image, precluding an evaluation of the reflectiveness of the random receiver spread. The limited nature of the receiver array also causes artifacts in the image which resemble migration "smiles". These are partially suppressed by limiting the dip aperture of the migration, but this also limits the reflector dips that can be imaged. The new VII imaging scheme, on the other hand, removes the artifacts without diminishing dipping reflectors. The VII images show more continuity along reflectors than images made with the conventional method.