John Brittan

Size and morphology of the Chicxulub impact crater

The Chicxulub impact in Mexico has been linked to the mass extinction of species at the end of the Cretaceous period. From seismic data collected across the offshore portion of the impact crater, the diameter of the transient cavity is determined to be about 100 km. This parameter is critical for constraining impact-related effects on the Cretaceous environment, with previous estimates of the cavity diameter spanning an order of magnitude in impact energy. The offshore seismic data indicate that the Chicxulub crater has a multi-ring basin morphology, similar to large impact structures observed on other planets, such as Venus.

Anisotropic parameters of layered media in terms of composite elastic properties

In elastic earth models, at a wide variety of scales, the subsurface is a layered sequence of different constituent media. It is therefore important to understand the elastic properties of such a sequence, in particular to determine the response of the layering to an investigating seismic wave. It can be shown that if the individual layer thicknesses are much less than the wavelength of a seismic wave passing through the stack, the wave will propagate as though it were traversing a homogenous, anisotropic medium (Postma, 1955). This property has been subjected to rigorous testing both experimentally (Melia and Carlson, 1984) and numerically (Carcione et al., 1991). The elastic properties of this “equivalent medium” can be derived algebraically from the elastic properties of the materials that compose the layers (Backus, 1962). The homogenous equivalent medium will be transversely isotropic (hereafter referred to as TI), the axis of symmetry lying perpendicular to the layering.

Seismic velocity, heterogeneity, and the composition of the lower crust

Abstract Many wide-angle seismic experiments attempt to constrain lower crustal seismic velocity and thus composition. Deep near-normal-incidence seismic data recorded during the past decade suggest a widely varying and highly complex pattern of lower crustal heterogeneity. Synthetic seismic modelling indicates that velocities measured from wide-angle experiments may depend significantly upon the geometrical arrangement and scale of the heterogeneities. Heterogeneities with a correlation length considerably large than the seismic wavelength will tend to produce observed velocities biased towards high values. Heterogeneities much smaller than the seismic wavelength can lead to a velocity bias of either sense depending upon their arrangement. Seismic velocities greater than 7.3 km/s measured near the base of the crust in underplated and intruded passive continental margins suggest compositions that are markedly more mafic than mantle melting calculations predict. Our seismic modelling suggests that the mismatch between modelled and observed velocities at passive margins may be attributable to large-scale (correlation length greater than seismic wavelength) heterogeneities.

Target-oriented adaptive subtraction in data-driven multiple removal

Data-driven multiple removal methods have proven a valuable addition to the demultiple toolbox for three main reasons. Firstly, such methods do not make use of any a priori information about the subsurface geology and, therefore, such information cannot bias the solutions. Secondly, data-driven methods can be applied in either 1D, 2D, or 3D mode and can therefore account for the full, multidimensional complexity of the earth. Finally, because no a priori information is used, the required user interaction is minimized. These methods are now applied in production-style processing environments with comparable speed and turn-around as conventional techniques.

Applications of Adaptive Noise Attenuation to Dual Sensor Seismic Data

When ocean bottom cables (OBC) are used for seismic surveying it has become standard practice to remove water layer reverberations by combining the geophone and hydrophone data. One of the most important processing concerns in such dual-sensor summation has been high levels of geophone noise. While many techniques have been tried to address strong geophone noise (such as match filters and coherent noise rejection) in many cases the only workable solution is not to use the majority of the geophone data for a given receiver in the dual sensor summation (often by allocating it very small scalar multipliers). This paper describes a new technique, Dual Wavefield Noise Attenuation (DWNA), which attempts to attenuate high magnitude geophone noise on dual sensor data. DWNA uses dual sensor theory to identify signal and noise and separate the two elements of the seismic recording in an adaptive manner. One of the main advantages of DWNA is that it is not limited by spatial aliasing issues that constrain the effectiveness of coherent noise rejection schemes such as f-k filtering or -p deconvolution. In addition, and in contrast to many other methods of coherent noise rejection, DWNA is not affected by the fact that much of the noise on the geophone is nonlinear and has significant move-out.

Using virtual reality for quality control and interpretation of 4-D reservoir characterization and AVO analysis

Development of stereographic visualization has opened the third and fourth dimensions in seismic analysis and interpretation. The ability to look, simultaneously, at several data volumes and attributes in a true 3-D sense opens the door to visualization systems applied to regional AVO analysis and volume-based repeatability analysis, as part of the 4-D processing flow.

Dense multi‐offset reflection tomography

In seismic reflection tomography, the velocity model of the subsurface is updated by back-projecting travel-time residuals along ray-paths. The travel-time residuals are picked from the seismic data itself and the methodology used to gather these picks is a fundamental part of any velocity inversion workflow. In particular, the density at which the residuals are picked in the four-dimensional data space (inline, crossline, offset and depth/time) appears to have a significant effect on the precision of the velocity updates that are output from the tomographic inversion. Dense, single-offset picking samples the residuals in the data space very finely but does not necessarily represent their true values with great accuracy. Dense, multi-offset picking offers a similarly fine sampling but with greater adherence to the true residual value. These two methodologies are compared and contrasted on a complex synthetic dataset.

Attenuation of multiple diffractions using a cascaded noise removal sequence

Multiple diffractions are a significant noise problem on seismic data from many of the world’s most important hydrocarbon-rich provinces. These noises are most prevalent in the case of deep water and a complex near-surface reflection sequence. In such regimes the multiples of diffracted energy by the near surface geology are often coincident in time with primary reflections within the subsurface. The energy composing the primary reflections will have a considerably reduced amplitude and frequency content due to the long travel-paths through the Earth; in contrast the multiples from the near-surface reflectors have travelled most of their propagation path through the very weakly absorbing sealayer. Thus the deeper parts of the seismic section are dominated, particularly at high frequencies, by the incoherent, high amplitude diffracted multiple arrivals. Due to their aliased, non-hyperbolic nature these arrivals are difficult to suppress using standard demultiple methods e.g. 2-D SRME, parabolic Radon demultiple. It has been shown that the use of high-fold, multiazimuth data acquisition can considerably increase the effectiveness of CMP stacking in removing diffracted multiple energy (Widmaier et al., 2002); however, for standard marine acquisition, a rigorous noise attenuation methodology must be adopted. Indeed, while techniques such as 3-D SRME hold great promise for multiple diffraction attenuation (van Borselen et al., 2004), in this paper we discuss how for typical field data a cascaded sequence of (up to) four different

Application of 3-D SRME And Multiple Diffraction Removal In the Makassar Straits, Indonesia.

It is recognized that 3-D related noise problems and multiple attenuation in particular can be benefit from currently non-standard acquisition techniques, like, for instance, multi-azimuth (Widmaier et al. 2002) and streamer overlap (van Borselen et al. 2005). However processing solutions are still required for historical and new standard marine acquisition datasets. Especially for the application of 3-D Surface Related Multiple Elimination (SRME), the issues of limited crossline sampling and aperture have to be addressed. A variety of methods for dealing with these issues have been proposed in the literature, which can be broadly categorized into a) a full wavefield interpolation or reconstruction, followed by a large number of 3-D inline convolutions to create a dense grid of multiple contributions (MCs) and finally summation in the crossline direction of those MCs (see, for instance, Baumstein and Hadidi, 2004), or b) calculation of 3-D inline convolutions to create MCs wherever possible using the existing data followed by sparse inversion in the crossline direction (van Dedem and Verschuur, 2002; Hokstad and Sollie, 2003). The hybrid approach described in van Borselen et al. (2005) consists of a more limited sailline regularisation and reconstruction in the crossline direction and sparse inversion of the subsequently increased number of crossline MCs. Major advantages are the reduced number of 3-D inline convolutions that need to be calculated compared to full reconstruction approaches, while with more MCs the sparse inversion becomes easier to parameterize and provides better results. This 3-D prediction approach is used in the current case study.

Case studies in quality control for 4-D seismic processing

Summary Quality control in 4D seismic projects is vital due to the small magnitudes of the production-related change s that are being sought. Comprehensive quantitative comparisons between base and monitor data are frequently required, as differences in amplitude, frequency, and phase are indicators of non-repeatability. The non-repeatability that is a function of acquis ition and processing effects has to be removed during the processing sequence to increase the resolution of the production related changes. One very efficient 4D quality control strategy is to calculate relevant QC attribute volumes as part of the processing flow and then to utilize the capabilities of a visualization system for interactive and simultaneous analysis of the 4D seismic and QC attribute volumes. We present a number of case studies showing the importance and advantages of such a methodology.