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Compensating for the effects of gas clouds on C-wave imaging

A case study from Valhall

British Geological Survey, Edinburgh, Scotland, U.K.

Michael C. Mueller

BP, Houston, Texas, U.S.

Olav I. Barkved

BP Norge AS, Jorpeland, Norway

Corresponding author: Xiang-Yang Li, xyl@bgs.ac.uk

The first 20% of the full text of this article appears below.

P-to-S converted waves (or C-waves) have been successfully used to image beneath gas clouds in many areas. However, C-waves often suffer from severe diodic effects due to the gas clouds; that is, the C-wave amplitude and traveltime may be different in the forward and reverse shooting directions, giving rise to different C-wave stacking velocities (diodic V_C) and velocity ratios (diodic _eff). In some cases, whether a horizon (a geologic target) beneath a gas cloud can be undershot also depends on the shooting directions (diodic illumination). These effects, compounded with the asymmetric raypath of the C-wave and the uncertainties in the P-wave data due to the gas clouds, will further increase the difficulties and costs of processing C-wave data. In this article, using the 2-D Valhall data as an example, we examine these effects and discuss ways to compensate for them during processing for improving the C-wave imaging.

	Effects of gas clouds

 
The 2-D Valhall data were acquired in 1996. In the survey area, the overburden layers are highly charged with gas. These gas clouds are not thought to be present in economic quantities, but they attenuate the P-wave energy substantially and reduce the P-wave velocity, inducing dimming and push-down effects on the P-wave data (Figure 1a).

View larger version (115K): [in this window] [in a new window]   Figure 1. (a) The final prestack-migrated P-wave imaging from the vertical geophone of the Valhall data. Note the dimming and push-down effects due to gas clouds. The vertical axis is two-way P-wave time. (b) The initial poststack-migrated image of the C-wave from the in-line horizontal geophone of the Valhall data. The processing involves asymptotic common-conversion-point (ACCP) binning, NMO, DMO, stack, and poststack migration. The vertical axis is two-way C-wave time. The picks shown are examples of P. . . [Full Text of this Article]

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