Are Osen

Removal of water-layer multiples from multicomponent sea-bottom data

A method for suppressing water-layer multiples in multicomponent sea-floor measurements is presented. The multiple suppression technique utilizes the concept of wavefield separation into upgoing and downgoing modes just below the sea floor for eliminating the sea-floor ghost, the sea-surface ghost, and the accompanying water-layer reverberations. The theory applies to each of the recorded components: pressure, vertical velocity, and horizontal velocities. The fundamental physical principle for the multiple suppression technique rests on identifying these multiples as downgoing waves just below the sea floor, while the primaries of interest arriving from the subsurface are upgoing waves. White presented this realization for the pressure component three decades ago; hence, the theory for the velocity field is an extension of the theory. In this paper, the theory is derived for an experiment with a marine source in the water layer above a locally flat, elastic sea floor with known elastic parameters. The method is otherwise multidimensional and operates on a shot-to-shot basis; hence, it is computationally fast. Aside from this, we show that this demultiple method removes the strongest multiples in sea-floor data without knowledge of the source wavelet. Synthetic and real data examples are provided to illustrate the application of the algorithms to the pressure, in-line velocity, and vertical velocity components. The numerical tests show that strong multiples have been attenuated on the pressure and the velocity recordings, producing promising results.

Wavelet estimation from marine pressure measurements

A new and alternative procedure for the deterministic estimation of the seismic source time function (wavelet) is proposed. This paper follows a series of reports on source signature estimation, requiring neither statistical assumptions on the signature nor any knowledge about the earth below the receivers. The proposed estimation method, which in principle is exact, uses conventional recordings of the pressure on a surface below the source and recordings of the pressure at one or more locations above the receiver surface. The derivation of the method is based on the Kirchhoff-Helmholtz integral equation. The formulation ensures that the scattered energy is filtered from the wavelet estimation, enabling the wavelet to be detected from the direct pressure field. Along with the derivation, we present a numerical example.

Toward optimal spatial filters for demultiple and wavefield splitting of ocean‐bottom seismic data

Suppressing the receiver‐side sea‐surface ghost with its accompanying water‐layer reverberations in multicomponent sea‐floor data can be performed in both the frequency–wavenumber and the frequency–space domains. Similarly, separation of ocean‐bottom seismic (OBS) data into upgoing P‐ and S‐mode responses can be formulated in both domains. The frequency–space domain algorithms can be attractive when the sea‐floor medium has lateral velocity variations or the data are irregularly sampled along the sea floor, which requires that the data be processed with local, compact decomposition filters having optimal angular (wavenumber) response.By deriving the frequency–space domain formulas, we find an exact representation for the demultipled pressure response. This formula involves filter convolutions over the data, with filters having a long spatial impulse response. The filtering can be approximated in the wavenumber domain by, for example, power series, Chebyshev polynonomial fitting, Pade approximants, or cont...

A New Local Method for Seismic Deghosting of Towed Streamer Data

The recent interest in broadband seismic technology has driven us into searching for new and improved deghosting methods. The ghost effects in recorded towed streamer data are caused by the reflection of data back down from the sea surface. The interference between the up-going primary wavefields and the down-going ghosts creates notches in the recorded spectrum which reduce the useful bandwidth of the spectrum especially at increasing streamer depth. The purpose of this abstract is to propose a new approximative deghosting method for towed streamer data that works well for streamer depths down to around 15m. Compare to other proposals for deghosting, our method is local, which means the recorded pressure can be deghosted using only itself and its horizontal spatial derivatives at that receiver location together with the receiver’s depth. Numerical example we are showing in this abstract indicates a promising result in deghosting data between the first and second notch frequency.

Fast Deterministic Designature/Demultiple Of 2D And 3D 4C-OBS Data

Four-component ocean bottom seismic (4C-OBS) surveying, in contrast to conventional towed streamer acquisition, records the complete seismic wavefield. When developing seismic processing tools for 4C-OBS data one therefore can liberate one’s mind from the line of thought used in the development of seismic processing methodology for streamer seismic. In particular, using the complete wavefield source signature deconvolution and the troublesome problem of attenuation of sea-surface related multiples can be specially designed for 4C-OBS data. The attenuation of these events is an essential prerequisite for an accurate seismic imaging. In 2001, Amundsen et al. published a fully data-driven method that without any information transforms, during processing in a computer, the recorded 4C-OBS data with the sea surface present into those data that would be recorded in a hypothetical 4C-OBS experiment with the sea surface absent. Removing the sea surface is equivalent to removing all seasurface related multiples. Further, this method automatically designatures the 4C-OBS data. Under the model assumption of a layered earth, the designature/demultiple method reduces to deterministic deconvolution where the deconvolution operator is designed from the inverse of the downgoing acoustic field. This assumption, however, does not severly limit the method’s practical use in attenuating free-surface related multiples even in geologically quite complex areas. Further, this designature/demultiple scheme is the only method in the class of free-surface demultiple methods that straightforwardly is implemented in 3D for removing free-surface multiples in today’s geometries of 4C-OBS surveys. The method, which is numerically fast, is implemented by several seismic contractors. During the presentation we show results of designature/demultiple processing on half a dozen 4C-OBS deep-water and medium-water depth exploration lines. Introduction Soon after the introduction of four-component ocean bottom seismic (4C-OBS) technology (Berg et al., 1994a,b; Ikelle and Amundsen, 2004) efforts to develop 2D and 3D wave-equation prestack depth imaging intensified (Amundsen et al., 2000; Arntsen and Røsten, 2002). To obtain accurate and detailed imaging, however, attenuation of multiple energy while preserving the character of primaries is required. Through the 1990’s till today one has seen many excellent developments of wave-equation based free-surface demultiple algorithms for 2D and 3D streamer seismic and land seismic data (see, e.g., Fokkema and van den Berg, 1990; Verschuur et al., 1992; Matson and Weglein, 1996; Matson, 1997; Weglein et al., 1997; Ziolkowski et al., 1999; Lokshtanov, 1999; Ikelle, 1999a,b; Ikelle et al., 2003; Kleemeyer et al., 2003; Hokstad and Sollie, 2003). The attractiveness of these methods is that they do not require any information about the subsurface. A possible disadvantage is that these methods require information of the source signature. Further, the streamerseismic demultiple methods can not straightforwardly be adapted to 4C-OBS data, in particular not for 3D ocean bottom seismic surveys. Amundsen (2001), Amundsen et al. (2001), and Holvik (2003) published an alternative way of removing free-surface multiples in the case that the complete wavefield is recorded in the seismic experiment. For streamer seismic, the complete wavefield on the streamer is the pressure field and the vertical component of the particle velocity (or vertical pressure gradient). For 4C-OBS the complete wavefield is the pressure field and the three components of particle velocity (or acceleration). From the recording of the complete wavefield Amundsen and coworkers posed a solution to the free-surface demultiple problem that does not require any knowledge of the source signature. The essence of the method is to design a demultiple operator from the inverse of the downgoing part of the acoustic wavefield (downgoing pressure or downgoing component of the particle velocity). See Figure 1. The method has the following additional characteristics: it preserves primary amplitudes; it requires no knowledge of the subsurface; it removes without any information all variations in the water layer; it accommodates source arrays; and no information of the physical source array, its volume, and its radiation characteristics (wavelet) is required. The method thus has a potential for use in time-lapse seismic as any water velocity changes and tidal changes are eliminated. Source designature is an implicit part of the demultiple process; hence, the method is capable of transforming recorded reflection data excited by any source array below the sea OTC 16943 Fast Deterministic Designature/Demultiple Of 2D And 3D 4C-OBS Data Lasse Amundsen, Børge Arntsen, Hans Aronsen, Are Osen, Geir Richardsen, and Mark Thompson/STATOIL ASA

Multidimensional Multiple Attenuation of OBS Data

Amundsen (2001) has presented a general wave-equation method for multidimensional signature deconvolution (“designature”) and elimination of free-surface related multiples (“demultiple”) from four-component ocean bottom seismic (OBS) data. Contrary to other free-surface multiple attenuation schemes, the method requires no information about the source signature. Here, we consider implementation and testing of the multidimensional multiple attenuation and designature scheme for general inhomogeneous media.

Source signature estimation in the seismic experiment

The source signature is a necessary input to many algorithms in seismic data processing for extracting reliable information of the earth`s structure. In the last few years a number of papers have addressed the problem of deterministic source signature estimation using the acoustic wave equation. In particular, many algorithms utilizing two marine field measurements have been proposed. The most general source signature identification method for marine seismic (acoustic) data has been given by Weglein and Secrest (1990), who showed that the source signature could be estimated by solving a Kirchhoff-like integral over the receiver surface. The algorithm requires no knowledge of the scattering medium below the receivers. When the data are measured along horizontal surfaces, the authors show that the source signature problem easily can be solved in the frequency-horizontal wavenumber domain by a linear least-squares inversion technique. Furthermore, if the sources in the array are located at the same depth, the source signature estimation algorithm corresponds to frequency-wavenumber algorithms published previously.

Efficient reverse time migration with amplitude encoding

Reverse time migration (RTM) is an accurate seismic imaging method for imaging the complex subsurface structure. Traditional common shot RTM suffers from low efficiency due to the large number of single shot gathers, especially for marine seismic data. Phase encoding is commonly used to reduce the computational cost of RTM. Phase encoding in the frequency domain is usually related to time shift in the time domain. Therefore, phase-encoding-based RTM needs time padding to avoid information loss which degrades the efficiency of the time-domain wavefield extrapolator. In this paper, an efficient time-domain RTM scheme based on the amplitude encoding is proposed. This scheme uses the orthogonal cosine basis as the encoding function, which has similar physical meaning to plane wave encoding (i.e. plane-wave components with different surface shooting angles). The proposed scheme can generate a qualified imaging result as well as common shot RTM but with less computational cost. Since this scheme does not need time padding, it is more efficient than the phase encoding schemes and can be conveniently implemented in the time domain. Numerical examples on the Sigsbee2a synthetic dataset demonstrate the feasibility of the proposed method.