Helmut Jakubowicz

Wave Equation Prediction And Removal of Interbed Multiples

The success of multiple suppression for seismic data often hinges on the accuracy with which the multiple behaviour can be predicted. This is especially true for interbed multiples where small details can be essential for distinguishing the multiples from surrounding primaries. In this paper we describe a new, wave-equation method of predicting interbed multiples from primary events. This new scheme enables the multiples to be estimated directly from measured data without the use of an explicit earth model. Furthermore, it includes surface-related multiples as a special case and can be readily exploited for multiple suppression within the framework of existing demultiple techniques. The application of the method to multiple removal is illustrated on a synthetic dataset using an extension of the surface-related demultiple technique of Berkhout and Verschuur (1997).

Enhanced refractor imaging by supervirtual interferometry

Refraction surveys are a well-established method of imaging subsurface velocities, both in terms of the deep crustal structure at global scales and in the shallow near surface. These surveys generally involve deploying an array of receivers on the surface (or water bottom) and recording arrivals from a seismic source initiated at or near the surface.

Least‐squares reverse‐time migration in a matrix‐based formulation

This paper describes least-squares reverse-time migration. The method provides the exact adjoint operator pair for solving the linear inverse problem, thereby enhancing the convergence of gradient-based iterative linear inversion methods. In this formulation, modified source wavelets are used to correct the source signature imprint in the predicted data. Moreover, a roughness constraint is applied to stabilise the inversion and reduce high-wavenumber artefacts. It is also shown that least-squares migration implicitly applies a deconvolution imaging condition. Three numerical experiments illustrate that this method is able to produce seismic reflectivity images with higher resolution, more accurate amplitudes, and fewer artefacts than conventional reverse-time migration. The methodology is currently feasible in 2-D and can naturally be extended to 3-D when computational resources become more powerful.

Electron-photon angular correlations in electron-molecule scattering

The density-matrix formulation of the theory of electron-photon coincidence experiments is applied to the case of electron scattering from molecules obeying the Hunds case (b) coupling scheme. The results are given in a form in which the measured quantities are related directly to the scattering amplitudes. The general results are then specialised to the case in which the scattered electrons are not observed, and are compared to those obtained by other authors. Threshold polarisation is discussed briefly and discrepancies between previously obtained results considered.

Marine source signature measurement using a reference seismic source

The quality and reliability of seismic data is controlled by a number of factors, but is ultimately constrained by the nature of the seismic source. As a result, it is desirable to determine the source output (the “signature”) and correct for its effect using deconvolution (see, for example, Ziolkowski, 1982, and Ziolkowski, 1991). Indeed, signature deconvolution is not only a necessary prerequisite for maximising the temporal, vertical and lateral resolution of seismic data, but is also important for reconciling data in time-lapse studies, the accurate determination of amplitude variations with offset, and wave-equation multiple removal and inversion schemes.

Issues regarding the use of time‐lapse seismic surveys to monitor CO2sequestration

A rock physics model was built to provide a link between geological modeling, reservoir engineering and the timelapse seismic response to CO2 injection. This model was used to examine the influence of factors such as reservoir depth and fluid distribution on the seismic attributes. Depth was shown to be an important control on the magnitude of the response, with the largest change found at shallow depths. Therefore a tradeoff is required between the need to sequester CO2 deeply for safety and the ability of seismic surveys to detect CO2.

Processing Low-frequency Data From the Faroe-Shetland Basin For Sub-basalt Imaging

Figure 1: Basalt covered areas of the North-East Atlantic Margin and associated igneous centres. Penetrating boreholes and hydrocarbon discoveries near the Shetlands are also shown. Following the findings that one has to use low frequency data to image beneath a multi-layered basalt sequence (Mack, 1997), Veritas DGC and Texaco UK re-shot a profile in August 2001 using a solid streamer and an additional low-frequency setting (Ziolkowski et al., 2001). Comparing the new low-frequency data with the old data shows not only a superior data quality due to the solid streamer used but also improved low-frequency response from beneath the basalt.

Fracture-related Amplitude Variations With Offset And Azimuth In Marine Seismic Data

Fractures are of great interest for hydrocarbon production. In particular, they can provide conduits for fluid flow, hence knowledge of their distribution and orientation can be critical, especially for horizontal wells. Unfortunately, most fractures are relatively small, and well beyond the imaging range of conventional seismic data. On the other hand, it is increasingly recognized that fractures do have a strong influence on shearwave energy (S waves). Indeed, amplitude variations in stacked (qS2) shear-wave data have successfully been used to identify productive fracture clusters in the Austin Chalk formation in Texas (Mueller 1991). Although seismic information on fracturing is most readily obtained from shear waves, direct measurements of shear-wave energy require both specialized sources and multicomponent receivers, and are not feasible in marine environments, except perhaps at the sea floor. Of importance is the fact that fractures have an impact on the entire seismic wavefield, with the imprint of the fracture symmetries leading to a general azimuthal behaviour. As a result, although compressional (P) waves are relatively insensitive to fracturing at near offsets, they can become linked to the larger effects experienced by the shearwaves through mode conversion at oblique incidence, and also through their own specific sensitivity. Thus, it is feasible that anisotropic variations in P-wave behaviour can provide a way of obtaining information on fracture properties at subseismic scales (MacBeth 1995). This then offers a potential means of studying fracture distributions and orientations in marine areas. In this paper we present evidence to show that information on both the presence and nature of fractures can be obtained from conventional marine (P-wave) seismic data using AVO (amplitude variation with offset) analysis. In particular, our results demonstrate that fracture-related AVO has a distinct