Richard Bale

Plane Wave Deghosting of Hydrophone And Geophone OBC Data

Summary The combination of hydrophone and geophone data recorded from ocean bottom surveys can be used to cancel down-going energy and so remove the receiver ghost. The combination weights are determined by an overall scaling factor and the water bottom reflectivity. These are often estimated using near offset or stacked data without regard to angle dependence. The estimation of the scalar is particularly sensitive to offset, since it is requires knowledge of the lag time due to travel through the water layer, which varies with angle of incidence. Use of a plane wave decomposition, or tau-p transform, makes it possible to extend the method to larger offsets, both for the estimation of the scaling factor and for the summation itself. This results in a more effective cancellation of the receiver ghost.

The impact of attenuation on the resolution of multicomponent seismic data

Summary An important practical question for multicomponent seismic surveys is how absorption impacts shear or converted wave resolution compared with that of P-waves. In this paper we undertake a comparative analysis of the expected effect of constant Q absorption on different modes, illustrating these effects by modeling absorption for homogeneous and layered models. We find that when S and P-wave attenuation filters are compared in depth, they are exactly equal for the same Q value, in the homogeneous case. Higher wavenumbers for given frequencies in the source wavelet gives an initial advantage to S-wave resolution in depth, which may be lost to attenuation if Swave Q is less than P-wave Q, and/or if there are very low shear velocities in the near surface. Shear wave resolution is most likely to benefit from additional low frequencies in the source spectrum. Finally, dispersion, which inevitably accompanies attenuation, will differ for P and S modes with different Q values, resulting in event correlation errors.

Robust estimation of fracture directions from 3-D converted-waves

Summary We present a new method for estimating fracture directions from 3-D converted wave data. The method is designed to alleviate some of the usual requirements placed upon the acquisition geometry – in particular regarding the distribution of azimuths within an analysis gather. We present a synthetic example, comparing the new method with a standard approach based on polarity changes, to illustrate the superior accuracy of the new method. We also present initial results on a 3-C field dataset, which indicate the new method performs well compared to an existing technique.

Prestack depth migration of converted wave data in anisotropic media

Depth imaging of converted wave (P-SV) data is complicated by the need to develop a consistent depth model for both P and SV velocities. Given such a model prestack depth migration may then be done by the use of separate travel-time tables for the source to image point and the image point to receiver rays. We show that construction of a consistent model cannot be done without regard to anisotropy, and conclude that it is desirable to include anisotropic corrections in the migration travel-time tables.

TTI wave‐equation migration

We describe a phase-shift plus interpolation (PSPI) method for wave-equation migration in TTI media. To apply the PSPI methodology for anisotropy, we generate reference operators based upon phase error criteria with respect to the symmetry axis direction, and exploit correlations between parameters. The method is demonstrated on an elastic synthetic dataset generated over a thrust-belt setting, as found in the Canadian Foothills.

Offset-to-angle Transformations For PP And PS AVO Analysis

The quest for quantitative interpretations of seismic amplitudes requires that higher-order approximations supplant those considered as “industry standard”. A layered earth with VTI anisotropy requires a non-hyperbolic moveout equation to reproduce long-offset arrival times. If use is to be made of long-offset amplitude information, we must honor this theory in each step. We describe an offsetto-angle transformation that is applicable for both PP and PS reflections, based on the non-hyperbolic equation. The effects are illustrated on a model and on field data.

Up‐down deconvolution and subsurface structure: theory, limitations and examples

In this paper we investigate the use of up-down deconvolution to simultaneously eliminate both source-side and receiver-side multiples in ocean-bottom data processing. In theory, this method is strictly valid for a horizontally layered earth only, but we demonstrate analytically that it is robust even in the presence of structure, provided that at least the sea floor is flat or gently dipping. We successfully apply the method on synthetic and real data examples.

TTI Wave Equation Migration by Phase-Shift Plus Interpolation

Summary We describe a phase-shift plus interpolation (PSPI) method for wave-equation migration in TTI media. To apply the PSPI methodology for anisotropy, we generate reference operators based upon phase error criteria with respect to the symmetry axis direction, and exploit correlations between parameters. The method is demonstrated on an elastic synthetic dataset generated over a thrust-belt setting, as found in the Canadian Foothills.

A new approach to remove the water column effect from 4‐D ocean bottom data

Repeated ocean bottom seismic (OBS) surveys are an effective tool for time-lapse reservoir monitoring. It is desirable to acquire high-fidelity and repeatable 4-D OBS data in order to reveal small reservoir changes. However, compensating for water-column variations occurring between repeated surveys is a challenging task. In this paper, we propose using the up-down deconvolution (U/D) method to remove changes in water velocity and water depth from 4-D ocean bottom seismic data. We successfully apply the U/D method to time-lapse synthetic examples.

Near-surface Issues in TTI Wave Equation Migration from Topography

Summary Wave-equation migration, which uses more accurate wavefield extrapolation techniques than ray-based Kirchhoff migration, produces high-quality images beneath complex overburdens. However, near-surface images resulting from wave equation methods are sometimes less satisfactory than those from Kirchhoff migration, even though more computing effort is required to obtain them. Two filters are shown to be useful in improving the near surface image: a k-filter with an elliptical cutoff, and a subsurface p-filter. In addition, it is useful to introduce a zero-slowness layer in connection with topography.