Seismic multiples are a serious hindrance to hydrocarbon exploration in Australia. In particular, water bottom multiples can be very difficult to attenuate. This is because there often exists a strongly reflective sea floor which gives multiples large amplitudes when compared with the primary events they overlay, and secondly, because of a widely occurring velocity inversion, which seriously reduces the effectiveness of a very important class of multiple attenuation techniques.Multiple attenuation techniques can be classified according to the characteristic of the data which is used to discriminate against the multiples in conjunction with the operation behind the demultiple process. Common multiple attenuation processes include FK demultiple, Radon Demultiple, predictive deconvolution, wave equation based demultiple procedures and the family of techniques which come under the umbrella of Surface Multiple Attenuation (SMA). All of these techniques, given the right conditions, can be very effective. They also vary in price from very cheap (FK demultiple) through to expensive (wave equation based demultiple procedures).However, despite these procedures, and fifty odd years of research, there is no effective general solution to multiple problems off the coast of Western Australia and indeed in many regions around the world.Two new wavefield transformations, Multiple MoveOut (MMO) and IsoStretch Radial Trace (ISR), have been developed in this research to precondition data prior to the removal of surface related multiples by existing techniques. These form the basis of a new multiple attenuating procedure.MMO shifts the data so that the water bottom primary event is flattened and the simple water bottom multiples are also flat and periodic. Water bottom peg leg multiples are made approximately periodic.To solve the stretch problem introduced by the MMO transform, ISR ++ / interpolates oblique traces of constant stretch, which also map constant shot emergence angles. The water bottom primary and multiple events form a stationary time series after MMO and ISR. They are then amenable to removal by autoconvolution and predictive deconvolution.The results of the new procedure are demonstrated on two case studies from offshore Western Australia. It is shown to be more effective at removing both simple and peg leg water bottom multiples than traditional techniques. Finally, it is an inexpensive procedure, which does not require velocity analysis prior to its application.

Vertical cable seismic methods are becoming more relevant as we require high quality and high resolution seismic data in both land and marine environments. Our goal in this thesis is to demonstrate the impacts of vertical cable surveying in these areas. Vertical cable methods have been applied to the marine environment with encouraging results. Data quality is similar to that of traditional towed-streamer data, without the long, cumbersome towed-streamers which are difficult to maneuver in congested areas. The current marine vertical cable processing schemes tend to use primaries and receiver ghosts of primaries for imaging. Therefore, we demonstrate the ability of the current multiple attenuation algorithms developed by Ikelle (2001) to preserve either primaries or the receiver ghosts of primaries. As we focus on land acquisition, we discover that vertical cable surveying can overcome many of the traditional problems of land seismics. In fact, our investigations lead us to believe that problems such as ground roll, guided waves and statics can be avoided almost entirely using vertical cable acquisition methods. Furthermore, land vertical surveying is naturally suited for multi-component acquisition and time-lapse surveying. To fully analyze the applicability of vertical cable surveys in marine and land environments, we also investigate the problem of cable spacing and sampling within each cable. We compare the resolution of vertical cable data and horizontal data by calculating the maximum angular coverage of each acquisition geometry and measuring the occurrence of each angle within this coverage, such that more occurrences means better resolution. From our investigations, we find that by using vertical cables of no more than 500 m in length at 500 m intervals, we can acquire higher resolution seismic data relative to horizontal surface methods for an image point, horizontal reflector or a dipping reflector. The key tool used in these investigations is fully elastic finite-difference modeling. We chose this technique based on its ability to properly and accurately model the full wavefield through complex models, all the while preserving amplitudes and the phase of reflected, diffracted and converted wavefields.

Modern seismic imaging tools for oil and gas exploration and production (E&P) assume that seismic data contain responses only of waves that bounce (e.g., reflect, diffract) only once at each interface in the subsurface. This type of response is called a primary. Unfortunately, actual seismic data also contain responses of waves that bounce at several interfaces in the subsurface. This type of response is called a multiple. In general, multiples in seismic data fall into two categories: (1) events that bounce at least once at the free surface in addition to any other bounce in the sub- surface and (2) events that do not bounce at the free surface but instead inside the subsurface, at two or more interfaces. The first category has the greater amount of energy; therefore most of the research and development efforts in E&P have so far focused on attenuating this category of multiples accurately. At present, more knowledge of the subsurface is expected from seismic imaging. To avoid any misinterpretation of these details, there is a growing need in the E&P industry to also attenuate the second category of multiples, known as internal multiples. In this work I describe a new method of attenuation of internal multiples. The method consists of predicting the internal multiples and then subtracting them from the data. The prediction of internal multiples from seismic data is made possible by the discovery of a new type of seismic scattering event known as a virtual event. Seismic virtual events constitute a calculational device, which is becoming an important part of seismic data processing. Virtual events combine forward and back- ward wave propagation in such a way that their convolution with real events allows us to predict internal multiples. In addition to showing how virtual events can be constructed from real seismic events, I also show that virtual events obey physical laws, despite their counterintuitive wavepath. I have illustrated the findings in this thesis with synthetic examples. In particu- lar, I have shown the effectiveness of my internal-multiple-attenuation method for a 1D data set, which includes several primaries and internal multiple interferences.

The three key steps of modern seismic imaging are (1) multiple attenuation, (2) velocity estimation, and (3) migration. The multiple-attenuation step is essentially designed to remove the energy that has bounces at the free surface (also known as "multiples"), since velocity estimation and migration assume that data contain only primaries (i.e., seismic events that have reflected or diffracted only once in the subsurface and have no free-surface reflection). The second step consists of estimating the velocity model such that the migration step can be solved as a linear inverse problem. This thesis concerns the multiple attenuation of towed-streamer data. We have proposed a new method for attenuating multiples and discussed how this method affects velocity estimation and migration. The multiple-attenuation approach used today in the E&P industry is based on the scattering theory. It is carried out in two steps: (1) the prediction of multiples using data only, and (2) the subtraction of multiples contained in the data using predicted multiples. One of the interesting features of these multiple-attenuation methods is that they do not require any knowledge of the subsurface. However there are still two drawbacks that limit the usage of these methods. They are (1) the requirement of acquiring very large 3D datasets which are beyond the capability of current seismic acquisition technology, and (2) the requirement of acquiring near-offset (including zero-offset) data. The method developed in this thesis can potentially overcome these two problems. The novelty of our approach here is to image receiver ghosts of primaries--events which have one bounce in the subsurface and one bounce at the free-surface that is also the last bounce--instead of primaries themselves. We propose to predict two wavefields instead of a single wavefield, as is presently done. One wavefield contains all free-surface reflections, including receiver ghosts of primaries, ghosts of multiples, and multiples. The other wavefield does not contain receiver ghosts of primaries. We pose the problem of reconstructing receiver ghosts of primaries as solving a system of two equations with three unknowns. The two wavefields are used to construct the two equations. The three unknowns are (1) the receiver ghosts of primaries, (2) the multiples contained in the wavefield containing the receiver ghosts of primaries, and (3) the multiples contained in the other wavefield. We solve this underdetermined system by taking advantage of the fact that seismic data are sparse. We have validated our approach using data generated by finite-difference modeling (FDM), which is by far the most accurate modeling tool for seismic data. Starting with a simple 1D model, we verified the effectiveness of predicting data containing multiples and receiver ghosts of primaries. Then we used the sparsity of seismic data to turn the system of two equations with three unknowns into a system of two equations with two unknowns on a datapoint basis. We have also validated our method for complex geological models. The results show that this method is effective, irrespective of the geology. These examples also confirm that our method is not affected by missing near-offset data and does not require special seismic 3D acquisition.

Made available in DSpace on 2015-03-13T17:08:35Z (GMT). No. of bitstreams: 1 MarcosALL_DISSERT.pdf: 5173840 bytes, checksum: 26fa2fd6ccf8445fa5e27cbfeebf642c (MD5) Previous issue date: 2012-12-18 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / The seismic reflection is used on a large scale in oil exploration. In case of marine acquisition the presence of high impedance contrast at the interfaces water/air generates multiple reflections events. Such multiple events can mask primary events; thus from the interpretational viewpoint it is necessary to mitigate the multiples. In this manuscript we compare two methods of multiple attenuation: the predictive multichannel deconvolution (DPM) and the F-K filtering (FKF). DPM is based in the periodicity of the multiples while FKF is based in multiples and primaries splitting in F-K domain. DPM and FKF were applied in common-offset and CDP gathers, respectively. DPM is quite sensible to the correct identification of the period and size of the filter while FKF is quite sensible to an adequate choice of the velocity in order to split multiples and primaries events in the F-K domain. DPM is a method that is designed to act over a specific event. So, when the parameters are well selected, DPM is very efficient in removing the specified multiple. Then DPM can be optimized by applying it several times, each time with a different parameterization. A deficiency of DPM occurs when a multiple is superposed to a primary event: in this situation, DPM can attenuate also the primary event. On the other hand, FKF presents almost the same performance to all multiples that are localized in the same sector of the F-K domain. The two methods can be combined in order to take advantage of their associated potentials. In this situation, DPM is firstly applied, with a focus in the sea bed multiples. Then FKF is applied in order to attenuate the remaining multiples / A s?smica de reflex?o ? utilizada em grande escala na explora??o de petr?leo. No caso de aquisi??o marinha, devido ao alto contraste de imped?ncia nas interfaces ?gua/ar, podem ocorrer eventos de reflex?o m?ltipla. Tais m?ltiplas podem mascarar eventos prim?rios, sendo necess?rio atenu?-las para facilitar o processo de interpreta??o. Neste trabalho faremos a compara??o usando dados sint?ticos e reais de duas t?cnicas de atenua??o de m?ltiplas: a deconvolu??o preditiva multicanal do tipo Wiener-Levinson (DPM) e a filtragem F-K. A primeira t?cnica ? baseada na periodicidade das m?ltiplas enquanto a segunda ? baseada nas diferen?as de mergulho dos eventos. A DPM foi aplicada em fam?lias de afastamento comum e a filtragem F-K em fam?lias CDP. Constatamos que a efici?ncia da t?cnica DPM ? bastante sens?vel ? identifica??o correta do per?odo e do tamanho do filtro. Por sua vez, a filtragem F-K ? bastante sens?vel ? escolha da velocidade adequada para separar as m?ltiplas dos eventos prim?rios. A DPM ? uma t?cnica que ? focada num dado evento; quando bem parametrizada, ela ? bastante eficiente para remover a m?ltipla especificada, podendo ter atua??o menos eficiente em outras m?ltiplas. A DPM pode ser ent?o otimizada aplicando-se a t?cnica v?rias vezes, em cada vez com uma diferente parametriza??o. Uma defici?ncia da t?cnica DPM ? quando h? sobreposi??o de m?ltiplas com eventos prim?rios, em que a DPM pode remover tamb?m uma parcela do evento prim?rio. Por sua vez, a filtragem F-K tem aproximadamente o mesmo desempenho em todas as m?ltiplas que estejam localizadas em um mesmo setor do espectro F-K. As duas t?cnicas podem ser combinadas de modo a tomar partido do potencial de cada uma delas, aplicando-se primeiro a t?cnica DPM, focada na m?ltipla do fundo do mar, seguida da filtragem F-K para a atenua??o das demais m?ltiplas