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Master Dissertation

Multiples Attenuation in Western Offshore BasinA comparative study of available techniques of multiple attenuation by processing a long offset deep marine seismic data.

Pankaj K Mishra M.Sc. Geophysics IIT Kharagpur

UNDER THE GUIDENCE OF

Prof. S.K.Nath Former Head of Department Department of Geology and Geophysics IIT Kharagpur, India

Mr. S. Basu Senior Geophysicist, SPIC Oil and Natural Gas Corporation Ltd India

INDIAN INSTITUTE OF TECHNOLOGY

ACKNOWLEDGEMENT It is a great pleasure to pay my gratitude to my academic supervisor Prof. S.K Nath (former Head, Department of Geology and Geophysics, IIT Kharagpur) for providing me enough theoretical background as well as such a opportunity to do my master dissertation in industry and also for his guidance throughout my master degree. I am grateful to Mr. S.Basu (SPIC, ONGC, Mumbai) for his true guidance throughout this project .Their full and continual support helped me a lot. I am thankful to Mr. D. Chatterjee (GGM, SPIC ONGC Mumbai) for providing all the facilities necessary for this project. I am also thankful to Prof. Biswajit Mishra, (Head of the Department of geology and geophysics, IIT Kharagpur) for providing me all possible facilities throughout the program. I would also like to express my heartfelt thanks to Mr. T.K. Bharti (SPIC, ONGC), who has contributed his time in helpful discussion and created friendly atmosphere which led to the successful completion of the work and preparation of my thesis. Finally, I acknowledge Indian Institute of Technology, Kharagpur for providing me such a great platform.

-Pankaj K Mishra

Chapter1: Introduction1.1 The Problem of Multiple ReflectionsSubsurface images provided by the seismic reflection method are the single most important tool used in oil and gas exploration. Almost exclusively, our conceptual model of the seismic reflection method, and consequently our seismic data processing algorithms, treat primary reflections, those waves that are scattered back towards the surface only once, as the signal. The travel times of the primary reflections are used to map the structure of lithology contrasts while their amplitudes provide information about the magnitude of the lithology contrasts as well as other information such as presence or absence of fluids in the pore spaces of the rock. In seismic exploration the problem of multiple reflections contaminating seismograms and thus disguising important information about subsurface reflectors is well-known. Today, the majority of all oil and gas resources are discovered in offshore continental shelf areas both in shallow and deep water. Before oil-producing wells can be drilled, geophysicists have to provide an image of the physical properties in the subsurface that shows where reservoirs can be expected. In a marine exploration we encounter the problem that the water layer often behaves as a wave trap (Backus, 1959), where seismic waves are multiply reflected between sea surface and sea bottom. Waves that are transmitted through the sea bottom can also reverberate between deeper reflectors. The energy of these interbed multiples and water layer reverberations can become so strong that the primary reflection arrivals of deeper target reflectors become completely invisible. As a result, marine seismograms often show a ringy character with strong multiples superposed on most of the primary arrivals from deeper reflectors. For correctly locating a target reflector that might indicate an oil reservoir, these interfering multiple reflections have to be eliminated, or since this is only rarely possible, they have to be at least attenuated. The efficient elimination of multiples from marine seismic data is one of the outstanding problems in geophysics. The efficient elimination of multiples requires large amounts of computer time. The marine seismic industry is a multi-million dollar market, and improvement of the accuracy and efficiency of the removal of multiples will lead to cost reduction and shorter turnaround times in this industry.

1.2 Classification of Multiples:Multiples can be either short period or long period. In recorded marine seismogram most multiple reflections arise from an interface with a strong impedance contrast such as free surface and water bottom. Figur1.1 shows ray path diagrams for: (a).water bottom multiples of first and second order (b).free surface multiples of first and second order (c).peg-leg multiples of first and second order (d).intrabed multiples of first and second order (e).interbed multiples of first and second order

These are few of the numerous configurations of ray paths associated with multiple reflections encountered in marine data. Regardless of the type of multiples, they all have two common properties that can be exploit to attenuate then varying degree of successperiodicity and moveout that is different from primaries. The shot records over the deep water contain long period water bottom multiples and peg-leg multiples associated with reflectors just below the water bottom. Whereas the shot records over the shallow water

contain short period multiples and reverberations. The guided waves in the shallow water records also contain multiples which have ray paths within the water layer. Same kind of multiples can be shown in the stack section also as demostrated in the figure below.

1.3 Attenuation of Multiples: The standard approach in seismic data processing isto attenuate the multiples before imaging, that is, in data space. Most algorithms for the attenuation of multiples in data space are based on three main characteristics of the multiples: (1).their periodicity in arrival time (predictive deconvolution), (2).their difference in moveout with respect to the primaries in CMPs (f-k and radon) (3). their predictability as the auto-convolution of the primaries (Surface Related Multiple Elimination (SRME)). Predictability has always been important in multiple removal. In the early days of seismic processing (the 1960s), single trace statistical prediction was very successful (Robinson, 1957). In the early 1980s prediction-error filtering has been given a wave theoretical base, providing a unified theory for surface-related and internal multiples (Berkhout, 1982). It has increased the effectiveness of multi-channel, prediction-error filtering significantly (Verschuur, 1991). Nowadays, multiple removal algorithms are to a large extend presented by wave theory based, multi-channel, prediction-error filters. Each of these approaches has distinctive advantages and disadvantages.

In this thesis, I refer to data space as the un-migrated space. This means data as a Function of time. I consider two main sets of data: source gathers and CMP gathers.The first are function of the source co-ordinates, offsets and time while the second are function of the CMP co-ordinates, half-offsets and time.

Chapter 2: Geometry Merging and Raw Data AnalysisFor processing we are given a raw seismic data and its geometry. In land acquisition this geometry is called self processing sequence SPS and contains information about shot points, receivers, static corrections etc. In marine acquisition this is called UKOOA and similarly it contains information about source location, receiver location etc. Apart from these we have an observer report which gives some additional information about the geometry. As we start processing the seismic data our first job is to merge the corresponding geometry with the data. However it doesnt change the data anyway but puts header values in it and by doing this we can excess the data in desired format, either FFID or CDP shorted etc. Once geometry is merged the data is ready to be processed. Our next job is to analyze the given data I will start from here with the real data provided. The raw data display is shown in figure (2.1). Seeing the data we can analyze that.. This data has gone through some very initial processing because since this is a deep marine data, this data must be full of swell and cable noises but here we dont see anything like that. Hence this data must have gone a low cut filtering that eliminated these noises. Some straight lines at the upper left corner are the direct arrivals. This is not desired in our output seismogram. Elimination of these direct arrivals is very easy task we simply mute the part over water bottom applying a top mute. Since this dissertation is mainly concerned about multiple elimination I will not be describing this procedure and I will apply this mute somewhere in between the processing. Water Bottom starts at 2 second approximately. And since it is a long offset data primary reflections associated with water bottom multiple go straight as a linear event at longer offset because there occurs refraction. The refraction ending in between 8 and 9 second (the upper dark one) is to be removed .We will attenuate this by F-K filtering. We can see first order and second order multiples of the primary events at 2 second, near 4 second and 6 second. These are also continuing up to long offset. We will try to eliminate this in two parts. The near offset part by 2D SRME and the far offset part by muting in parabolic Radon transform domain. Apart from these we observe a series of multiples in between primary and first order multiple. These multiples are short period multiples and can be treated by predictive deconvolution. However predictive deconvolution is not enough effective to eliminate all these. But it can work well after 2D SRME. We do not see significant linear noise as it is a deep marine data.

Figure 2.1: the raw seismic record however with a low cut filter.

Chapter 3: F-K Filtering3.1: Principle- Data in T-X domain is transferred to Frequency Wave number domainusing FFT. Noises and multiples are separated because of their different dip