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

Geophysics ◽  
1999 ◽  
Vol 64 (3) ◽  
pp. 838-851 ◽  
Author(s):  
Are Osen ◽  
Lasse Amundsen ◽  
Arne Reitan
Keyword(s):  
Vertical Velocity ◽  
Water Layer ◽  
Real Data ◽  
Physical Principle ◽  
Sea Floor ◽  
Numerical Tests ◽  
Sea Bottom ◽  
Marine Source ◽  
Suppression Technique ◽  
Fundamental Physical

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.

Decomposition of multicomponent sea‐floor data into upgoing and downgoing P‐ and S‐waves

Geophysics ◽  
1995 ◽  
Vol 60 (2) ◽  
pp. 563-572 ◽  
Author(s):  
Lasse Amundsen ◽  
Arne Reitan
Keyword(s):  
Particle Velocity ◽  
Layered Medium ◽  
Synthetic Data ◽  
Water Layer ◽  
S Wave ◽  
Sea Floor ◽  
S Waves ◽  
Sea Bottom ◽  
Marine Source ◽  
Horizontal Particle

A method for decomposing multicomponent sea‐floor measurements into upgoing and downgoing P‐ and S‐waves is presented. We assume that a marine survey employing a marine source in the water layer is conducted over a plane‐layered medium. From recordings of the pressure just above the sea floor and the particle velocity vector just below the sea floor, decomposition filters can be determined by plane‐wave analysis. The decomposition filter coefficients depend on the P‐ and S‐wave velocities and the density at the sea bottom. We show how to decompose the multicomponent measurements into upgoing and downgoing P‐ and S‐vertical traction components, vertical‐particle velocity components, and horizontal particle velocity components. The decomposition filters are applied with good results to synthetic data modeled in a plane‐layered medium.

scholarly journals The Formation and Composition of the Gas Content of Sea Ice

1979 ◽  
Vol 22 (86) ◽  
pp. 67-81 ◽  
Author(s):  
V. L. Tsurikov
Keyword(s):  
Sea Ice ◽  
Sea Water ◽  
Major Effect ◽  
Relative Volume ◽  
Gas Content ◽  
Dominant Factor ◽  
Sea Floor ◽  
Sea Bottom ◽  
Initial Freezing ◽  
Air Pockets

Abstract The different factors contributing to the formation of the gas porosity of sea ice are: (Ia) gases captured during the formation of the initial ice cover, (Ib) gases released from solution during the initial freezing of sea-water, (Ic) the inclusion of gases rising from the sea bottom, (2a) the substitution of gas for brine drained from the ice during times of melting, (2b) the release of gas from the brine within the ice during the course of partial freezing, and (2c) the formation of voids filled with water vapour during the course of internal melting. An analysis is made of each of these processes and it is concluded that processes Ib, 2a, and 2C are important. Process Ic may also be a major effect but it is difficult to evaluate until the rate of gas release from the sea floor is better known. The migration of air pockets into the ice from the overlying snow is shown to be a possible but not a significant effect. Available data on the composition of gas in sea ice are reviewed and it is shown to be significantly different from air. Possible causes for these differences are discussed. The porosity of sea ice, i.e. the total relative volume of its gas plus its brine inclusions, is one of the factors strongly affecting its strength, as has been shown by Tsurikov (1947) and by Weeks and Assur (1968). In seas with high salinities the effect of the presence of brine within the ice will usually be the dominant factor. However on water bodies with low salinities the effect of the gas included within the ice may be greater than the effect of the brine. Despite its significance there have not been any attempts at a quantitative analysis of the entrapment of gas in sea ice. This paper is an attempt at such a study.

Impact—Response Behavior of Offshore Pipelines

1982 ◽  
Vol 104 (4) ◽  
pp. 325-329 ◽  
Author(s):  
P. G. Bergan ◽  
E. Mollestad
Keyword(s):  
Plastic Material ◽  
Material Behavior ◽  
Hydrodynamic Drag ◽  
Offshore Pipelines ◽  
Sea Floor ◽  
Drag Forces ◽  
Sea Bottom ◽  
Nonlinear Dynamic Equations ◽  
Numerical Formulation ◽  
Inertia Forces

A method for analyzing the dynamic behavior of marine pipelines subjected to impact loads or sudden forced movements is outlined. Inertia forces (also from hydrodynamic mass), hydrodynamic drag forces as well as friction and lift effects for a pipe at the sea bottom are accounted for. An extensive nonlinear formulation is used for the pipe itself; it includes large displacements and elasto-plastic material behavior. Aspects of the numerical formulation of the problem and the solution of the nonlinear dynamic equations are discussed. The examples show computed dynamic response for pipelines lying on the sea floor and for a pipe section freely submerged in water when subjected to various force and displacement histories.

SPECTRA OF WATER REVERBERATIONS FOR PRIMARY AND MULTIPLE REFLECTIONS

Geophysics ◽  
1972 ◽  
Vol 37 (5) ◽  
pp. 788-796 ◽  
Author(s):  
John Pflueger
Keyword(s):  
Reflection Coefficient ◽  
Seismic Event ◽  
Theoretical Study ◽  
Water Layer ◽  
Multiple Reflection ◽  
Multiple Reflections ◽  
Quick Method ◽  
Sea Floor ◽  
Marine Data ◽  
Amplitude Characteristics

A theoretical study shows that passage of a seismic event through the water‐layer filter imposes amplitude characteristics on the resultant reverberating event which are independent of whether the event is a primary reflection or a multiple reflection. The phase characteristics of each order of event are, however, different. It is also shown that the reverberating sequence from a multiple reflection can be “whitened” by deconvolution but will still exhibit ringing. This phenomenon explains why some marine data, containing dominantly multiple reflections, are not amenable to deringing using standard deconvolution approaches. In addition, a quick method of obtaining the approximate reflection coefficient of the sea floor is derived.

Data‐driven adaptive decomposition of multicomponent seabed recordings

Geophysics ◽  
2004 ◽  
Vol 69 (5) ◽  
pp. 1329-1337 ◽  
Author(s):  
Remco Muijs ◽  
Johan O. A. Robertsson ◽  
Klaus Holliger
Keyword(s):  
Water Layer ◽  
P Wave ◽  
Data Driven ◽  
Minimal Amount ◽  
S Wave ◽  
Sea Floor ◽  
Decomposition Scheme ◽  
Wave Potential ◽  
Dual Sensor ◽  
Adaptive Decomposition

Dual‐sensor (hydrophone and three‐component geophone) data recorded on the sea floor allow the elastic wavefield to be decomposed into its upgoing and downgoing P‐ and S‐wave components. Most decomposition algorithms require accurate knowledge of the elastic properties of the sea floor in the vicinity of the receivers and properly calibrated sensors, in order for the data to be a faithful vector representation of the ground motion. We present a multistep adaptive decomposition scheme that provides the necessary information directly from the data by imposing constraints on intermediate decomposition results. The proposed scheme requires no a priori information and only a minimal amount of user‐defined input, thus allowing multicomponent data to be decomposed in an automated data‐driven fashion. The performance of the technique is illustrated using seabed data acquired in the North Sea with prototype single sensors (multicomponent geophones individually sampled). Realistic sea floor properties and sensor calibration operators are obtained, and elastic decomposition of the calibrated data generally yields good results. Dominant water‐layer reverberations are successfully attenuated and primary reflections are substantially enhanced in the computed upgoing P‐wave potential just below the sea floor. In contrast, the result for the upgoing S‐wave potential is somewhat less convincing; although the energy of water‐layer multiples is substantially reduced, notable amounts of undesired multiple energy remain in this section after decomposition, particularly at high offsets. These imperfections may point to inaccuracies in the parametrization of the sea floor or remaining inaccuracies in the vector fidelity of the horizontal geophone recordings. Nevertheless, the results obtained with the extended data‐driven decomposition scheme are at least comparable to previously published results.

Model‐based removal of water‐layer multiple reflections

Geophysics ◽  
1999 ◽  
Vol 64 (6) ◽  
pp. 1816-1827 ◽  
Author(s):  
Guochun Lu ◽  
Bjørn Ursin ◽  
Jan Lutro
Keyword(s):  
Water Layer ◽  
Computationally Efficient ◽  
Multiple Reflections ◽  
Model Based ◽  
The North ◽  
Sea Bottom ◽  
Receiver Array ◽  
Sea Bed ◽  
Reflectivity Function ◽  
Calculated Amplitude

We have developed a procedure to attenuate water‐layer multiple reflections. We estimate the sea‐bottom reflectivity function and use it plus calculated amplitude functions to model all order water‐layer multiple reflections, taking into account both amplitude and waveform shape. We model the primary and multiple reflections from the sea bottom in the frequency‐slowness domain. The amplitude function in the data modeling includes the source directivity function, source ghost response, receiver array directivity function, receiver ghost response, and offset‐dependent geometrical spreading. For small offsets we can assume that the seabed reflectivity depends only on frequency, and it is estimated using a least‐squares algorithm. An unknown scaling constant in the data is estimated using the amplitude of the primary and first multiple reflection from the sea bed. The composite sea‐bottom reflectivity is estimated as a function of frequency for each common midpoint (CMP) position. We apply the algorithm to high‐resolution seismic data from the North Sea. The modelled data match the recorded data well, and the estimated primary reflectivity is more geologically meaningful than the stacked trace. By comparison with Radon transform multiple removal applied to the same data, the model‐based method was more computationally efficient and left less residual multiple energy.

Traveltime and relative geometric spreading approximation in elastic orthorhombic medium

Geophysics ◽  
2020 ◽  
Vol 85 (5) ◽  
pp. C153-C162 ◽  
Author(s):  
Shibo Xu ◽  
Alexey Stovas ◽  
Hitoshi Mikada
Keyword(s):  
Seismic Data ◽  
Real Data ◽  
P Wave ◽  
Form Expression ◽  
Amplitude Variation ◽  
Rational Form ◽  
Amplitude Variation With Offset ◽  
Practical Applications ◽  
Numerical Tests ◽  
Geometric Spreading

Wavefield properties such as traveltime and relative geometric spreading (traveltime derivatives) are highly essential in seismic data processing and can be used in stacking, time-domain migration, and amplitude variation with offset analysis. Due to the complexity of an elastic orthorhombic (ORT) medium, analysis of these properties becomes reasonably difficult, where accurate explicit-form approximations are highly recommended. We have defined the shifted hyperbola form, Taylor series (TS), and the rational form (RF) approximations for P-wave traveltime and relative geometric spreading in an elastic ORT model. Because the parametric form expression for the P-wave vertical slowness in the derivation is too complicated, TS (expansion in offset) is applied to facilitate the derivation of approximate coefficients. The same approximation forms computed in the acoustic ORT model also are derived for comparison. In the numerical tests, three ORT models with parameters obtained from real data are used to test the accuracy of each approximation. The numerical examples yield results in which, apart from the error along the y-axis in ORT model 2 for the relative geometric spreading, the RF approximations all are very accurate for all of the tested models in practical applications.

Inversion of Pressure Data on a Vertical Array for Sea Floor Geoacoustic Properties

1998 ◽  
Vol 06 (01n02) ◽  
pp. 269-289 ◽  
Author(s):  
Purnima Ratilal ◽  
Peter Gerstoft ◽  
Joo Thiam Goh ◽  
Keng Pong Yeo
Keyword(s):  
Ambient Noise ◽  
Historical Data ◽  
Low Frequency ◽  
Real Data ◽  
Vertical Array ◽  
Trial Site ◽  
Matched Field Processing ◽  
High Frequencies ◽  
Sea Floor ◽  
Line Array

Estimation of the integral geoacoustic properties of the sea floor based on real data drawn from a shallow water site is presented. Two independent inversion schemes are used to deduce these properties. The first is matched-field processing of the pressure field on a vertical line array due to a projected source. The second approach is the inversion of ambient noise on a vertical array. Matched-field processing has shown to be successful in the inversion of high quality field data. Here, we show that it is also feasible with a more practical and less expensive data collection scheme. It will also be shown that low frequency inversion is more robust to variation and fluctuation in the propagating medium, whereas high frequencies are more sensitive to mismatches in a varying medium. A comparison is made of the estimates obtained from the two techniques and also with available historical data of the trial site.

Deep‐water peg legs and multiples: Emulation and suppression

Geophysics ◽  
1986 ◽  
Vol 51 (12) ◽  
pp. 2177-2184 ◽  
Author(s):  
J. R. Berryhill ◽  
Y. C. Kim
Keyword(s):  
Wave Equation ◽  
Seismic Data ◽  
Water Layer ◽  
Original Data ◽  
Equation Method ◽  
Multiple Reflections ◽  
Step Method ◽  
Arrival Times ◽  
Sea Floor ◽  
Water Bottom

This paper discusses a two‐step method for predicting and attenuating multiple and peg‐leg reflections in unstacked seismic data. In the first step, an (observed) seismic record is extrapolated through a round‐trip traversal of the water layer, thus creating an accurate prediction of all possible multiples. In the second step, the record containing the predicted multiples is compared with and subtracted from the original. The wave‐equation method employed to predict the multiples takes accurate account of sea‐floor topography and so requires a precise water‐bottom profile as part of the input. Information about the subsurface below the sea floor is not required. The arrival times of multiple reflections are reproduced precisely, although the amplitudes are not accurate, and the sea floor is treated as a perfect reflector. The comparison step detects the similarities between the computed multiples and the original data, and estimates a transfer function to equalize the amplitudes and account for any change in waveform caused by the sea‐floor reflector. This two‐step wave‐equation method is effective even for dipping sea floors and dipping subsurface reflectors. It does not depend upon any assumed periodicity in the data or upon any difference in stacking velocity between primaries and multiples. Thus it is complementary to the less specialized methods of multiple suppression.

Generation of metal deposits on the sea floor

1976 ◽  
Vol 13 (1) ◽  
pp. 126-135 ◽  
Author(s):  
B. J. Fryer ◽  
R. W. Hutchinson
Keyword(s):  
Sea Water ◽  
Metal Sulfide ◽  
Base Metals ◽  
Base Metal Sulfide ◽  
Gold Ores ◽  
Sulfide Deposits ◽  
Sea Floor ◽  
Metalliferous Sediments ◽  
Sea Bottom ◽  
Metal Abundances

Recent studies of volcanogenic base metal sulfide deposits and of metalliferous sediments in the Red Sea indicate precipitation of iron and base metals under conditions varying from reducing to oxidizing, at or near sites of fumarolic brine emission onto the sea floor. Differing lithofacies of iron-rich sediments were apparently deposited penecontemporaneously, mainly in response to changing chemical, biological, and sedimentary lithofacies conditions.Iron-rich sediments associated with the cupriferous pyrite bodies of Cyprus have been studied to determine the behavior of Fe, Mn, Cu, Zn, Pb, Ni, Co, Cr, Sn, Mo, Ag, and Au, when these fumarolic brines enter the sea bottom environment. Variations in metal abundances and ratios indicate that rapidly changing Eh is a major factor controlling metal deposition on the sea floor. The Fe/Mn ratio in these sediments is a useful indicator of the amount of interaction of these fumarolic brines and normal oxygenated sea water. Results suggest that zinc, copper, and gold are concentrated in the high Fe/Mn ratio proximal sediments; nickel is concentrated in the low Fe/Mn ratio distal sediments; and lead, silver, tin, and molybdenum are relatively unaffected by oxidation of the fumarolic brine solution by normal sea water.These concepts of sea floor deposition controlling the distribution of metals may also be applicable to other types of stratabound metalliferous deposits, like certain skarn, greisen, and gold ores, heretofore considered to be of epigenetic origin.

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