C-wave Imaging Research Articles

Converted-wave diodic moveout and application in the land gas cloud area

PS converted-waves (C-waves) have been commonly used to image through gas clouds but the C-wave imaging may also be degraded by the diodic effect introduced by the gas cloud. It may be compensated for using a velocity perturbation method which decouples the diodic moveout into two parts: the base velocity and the velocity perturbation. Gas clouds are widely distributed in the Sanhu area in the Qaidam basin of northwest China which is rich in natural gas. A land 2D3C seismic dataset is analyzed from the Sanhu area and significant diodic effects are observed in the data which harm the C-wave imaging. The diodic correction is applied to this data and the resultant C-wave imaging and the details of the reservoir structure are significantly improved. The diodic moveout plays an important role in working out the residual shear wave statics and the association of diodic correction and shear wave residual statics computation is a key step of C-wave high fidelity imaging in the gas cloud area. Finally, the new process workflow with diodic moveout is given.

Imaging Lomond Field using C-wave anisotropic PSTM

In this paper we present the results from an anisotropic converted-wave (C-wave) prestack time migration (PSTM) of a 2D multicomponent (4-C) line over Lomond Field. This is a gas-condensate field 233 km east of Aberdeen, in the Central North Sea, Block 23/21, close to the border between the UK and the Norwegian sectors, on the east flank of the Central Graben. The geologic setting is excellent for testing C-wave imaging tools; the quite complex structure is dominated by a salt-induced anticline (fractured on top) and a large gas cloud (leaked from the fractured reservoir). In addition, a thick overburden of laminated shales is likely to produce polar anisotropic (VTI) effects. The large gas cloud was the main motivation for C-wave acquisition. In fact, the imaging results obtained using P-waves (Figure 1) include a large zone of amplitude dimming, circled in black, and the push-down effects on the above events. This cloud partially obscures the area of the reservoir, which is composed of Paleocene sandstones of the Forties Formation, 200 m in thickness, and sealed by mudstones from the Rogaland Group. The presence of polar (VTI) anisotropy was confirmed by observations of angular-dependent velocity in deviated sonic logs.