Deep‐water peg legs and multiples: Emulation and suppression

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.

Download Full-text

Ray‐tracing‐based prediction and subtraction of water‐layer multiples

Geophysics ◽

10.1190/1.1442853 ◽

1990 ◽

Vol 55 (4) ◽

pp. 443-451 ◽

Cited By ~ 7

Author(s):

Andrew J. Calvert

Keyword(s):

Ray Tracing ◽

Seismic Data ◽

Computational Cost ◽

Water Layer ◽

Computational Effort ◽

Lateral Variation ◽

Multiple Reflections ◽

Seafloor Topography ◽

Arrival Times ◽

Data Matching

Many methods of multiple suppression break down when the structure that produces the reverberation possesses significant lateral variation; a common example of this situation occurs in marine data with the multiple reflections that are generated by seafloor topography. Such multiples may be suppressed by techniques based upon wave‐equation extrapolation; the recorded seismic data are mathematically propagated through a simulated water layer to generate a set of multiple arrivals which may, after data matching, be subtracted. However, the computational effort required to propagate prestack data to a laterally varying datum is very large. In this paper, a method of suppressing selected multiples with arbitrary moveout is presented. In order to reduce the computational cost, prediction of the multiple arrival times is performed by ray tracing through a model of the laterally varying water layer and, possibly, the subsurface. An estimate of the multiple waveform on each trace is obtained by stacking a window of data about the calculated arrival times. The multiple arrival can then be attenuated by subtracting this wavelet from each trace in the prestack gather from which the estimate is derived. In practice, calculations of the variation in multiple amplitude and of any errors in the moveout correction require the multiple reflections to be of comparable, or higher, amplitude than contemporary primary events, a situation that is often the case where multiple contamination is a problem.

Download Full-text

WATER REVERBERATIONS—THEIR NATURE AND ELIMINATION

Geophysics ◽

10.1190/1.1438579 ◽

1959 ◽

Vol 24 (2) ◽

pp. 233-261 ◽

Cited By ~ 74

Author(s):

Milo M. Backus

Keyword(s):

Seismic Data ◽

Structural Information ◽

Water Layer ◽

Original Data ◽

Present Form ◽

Linear Filtering ◽

Inverse Filtering ◽

Multiple Reflections ◽

Filtering Techniques

In offshore shooting the validity of previously recorded seismic data has been severely limited by multiple reflections within the water layer. The magnitude of this problem is dependent on the thickness and the nature of the boundaries of the water layer. The effect of the water layer is treated as a linear filtering mechanism, and it is suggested that most apparent water reverberation records probably contain some approximate subsurface structural information, even in their present form. The use of inverse filtering techniques for the removal or attenuation of the water reverberation effect is discussed. Examples show the application of the technique to conventional magnetically recorded offshore data. It has been found that the effectiveness of the method is strongly dependent on the instrumental parameters used in the recording of the original data.

Download Full-text

Wave Equation Datuming Applied to Seismic Data in Shallow Water Environment and Post-Critical Water Bottom Reflection

Energies ◽

10.3390/en10091414 ◽

2017 ◽

Vol 10 (9) ◽

pp. 1414 ◽

Cited By ~ 1

Author(s):

Umberta Tinivella ◽

Michela Giustiniani ◽

Ivan Vargas-Cordero

Keyword(s):

Wave Equation ◽

Shallow Water ◽

Seismic Data ◽

Water Environment ◽

Shallow Water Environment ◽

Water Bottom

Download Full-text

SPECTRA OF WATER REVERBERATIONS FOR PRIMARY AND MULTIPLE REFLECTIONS

Geophysics ◽

10.1190/1.1440300 ◽

1972 ◽

Vol 37 (5) ◽

pp. 788-796 ◽

Cited By ~ 2

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.

Download Full-text

Approximating subsurface structure in areas of severe subsea erosion—A nonprocessing technique

Geophysics ◽

10.1190/1.1442603 ◽

1989 ◽

Vol 54 (11) ◽

pp. 1397-1409

Author(s):

Fred W. Lishman ◽

Michael N. Christos

Keyword(s):

Seismic Data ◽

Seismic Reflection ◽

Velocity Model ◽

Subsurface Structure ◽

Submarine Canyons ◽

Arrival Times ◽

Sea Floor ◽

Static Corrections ◽

Map Migration ◽

Made In

Severe subsea erosion distorts seismic reflection times and velocity analyses and makes determining subsurface structure difficult. Although data reprocessing is the logical solution for removing these distortions, reprocessing can be expensive. We present a case history describing a nonprocessing depth‐conversion technique using a geologic erosional model. A grid of common‐midpoint seismic data located in and around several submarine canyons was used for this study. Establishing a geologic erosional model requires an accurate representation of the sea floor, which we obtain by map migration of the sea‐floor reflection. A velocity model was developed using only those analyses not adversely affected by sea‐floor erosion. To remove the effects of erosion from the arrival times of a mapped horizon, static corrections (velocity replacement and compaction) were developed. We replaced the water velocity in the eroded section with depth‐equivalent rock velocities from the velocity model. The compaction correction, which was derived empirically, is based on the assumption that porosity restoration occurred in the sediments beneath the canyons when erosion reduced the overlying pressure. Compaction correction in conjunction with velocity replacement produced structure maps (time and depth) that exhibit only minor effects of erosion. These results were further improved by applying dynamic corrections obtained by ray tracing a subsurface model to determine the traveltime through the water for the reflection from the mapped horizon. Our final structure maps demonstrate that a geologically reasonable structural interpretation in depth can be made in areas of severe subsea erosion without reprocessing the data.

Download Full-text

SOME CHARACTERISTICS OF MARINE SPARKER SEISMIC DATA

Geophysics ◽

10.1190/1.1440272 ◽

1972 ◽

Vol 37 (3) ◽

pp. 462-470 ◽

Cited By ~ 4

Author(s):

F. T. Allen

Keyword(s):

Seismic Data ◽

Water Layer ◽

Seismic Survey ◽

Multiple Reflections ◽

Time Profiles ◽

Survey Technique ◽

Recording Conditions ◽

Single Detector ◽

Marine Seismic ◽

The Relationship

The marine seismic survey technique of frequent recordings with a single detector group can provide intricate details valuable to relatively shallow investigations. Velocities may be computed from time anomalies, under certain circumstances. The extent of multiple energy response is an indication that the 100,000-joule source is strong enough for the purpose. Recognizable minor details in primary reflections are important clues in identifying related multiple reflections. Bounces off the underside of the water layer are rarely found. The pattern and character of reflections are influenced by recording conditions; thus, the relationship between recorded events and sedimentary beds is not simple. Seismic time profiles frequently give wrong impressions of structural attitudes because of the horizontal‐to‐vertical exaggeration, time anomalies, and multiple reflections, as well as the usual effects of velocity differences. The interpreted cross‐section gives a reasonably correct (even if velocities are assumed) impression of structure; the profile often does not.

Download Full-text

Submarine canyons: Velocity replacement by wave‐equation datuming before stack

Geophysics ◽

10.1190/1.1442207 ◽

1986 ◽

Vol 51 (8) ◽

pp. 1572-1579 ◽

Cited By ~ 26

Author(s):

John R. Berryhill

Keyword(s):

Wave Equation ◽

Sea Water ◽

Original Data ◽

Acoustic Velocity ◽

Computational Method ◽

Common Source ◽

Submarine Canyons ◽

Sea Floor ◽

Exposed Rock ◽

Corrected Data

Submarine canyons incised into the continental slope interfere with the quality of common‐midpoint (CMP) stacked seismic data obtainable from reflectors beneath the sea floor. The interference problem is caused by rough topography in conjunction with the contrast between the acoustic velocity of sea water and the velocity of the exposed rock layers. Geophysicists have long recognized that part of the solution is to replace the traveltimes of raypaths through the water by their traveltimes through an identical thickness of rock. However, use of wave‐equation datuming to effect velocity replacement yields an additional correction for the change in raypath direction that occurs in crossing from rock to sea water; the wave‐equation datuming implementation of velocity replacement is more comprehensive and complete. The wave‐equation datuming method requires an accurate sea‐floor profile as part of the input, along with values of replacement velocity; it does not require knowledge of geology or velocities at depths much greater than the sea floor. Unstacked common‐source and common‐receiver records are processed to appear as if sources and receivers were moved to the water bottom; the velocity of water is replaced; and the sources and receivers are moved back to the sea surface through the replacement medium. The computational method is well‐suited to the irregular surfaces and laterally variable velocities inherent in the problem of submarine canyons. The advantage of this method is that the corrected seismic records accurately emulate the data that would actually be observed if the acoustic velocity of water could be changed physically. The normal‐moveout (NMO) velocity for optimum CMP stacking becomes the root mean square of the layer velocities, including the velocity substituted for that of water. The spurious lateral variation of stacking velocity in the original data is eliminated. Processing of the corrected data through velocity analysis, stacking, migration, and conversion to depth is therefore more reliable.

Download Full-text

THE G-LOG* SEISMIC INVERSION PROCESS

The APPEA Journal ◽

10.1071/aj80018 ◽

1981 ◽

Vol 21 (1) ◽

pp. 155

Author(s):

D. B. Hays ◽

J. Wardell

Keyword(s):

Wave Equation ◽

Seismic Data ◽

Acoustic Impedance ◽

Mean Squared Error ◽

Seismic Inversion ◽

Synthetic Seismogram ◽

Inversion Method ◽

Multiple Reflections ◽

Seismic Section ◽

Impedance Model

The G-LOG process is a method of seismic inversion which provides direct estimates of subsurface acoustic impedance from wavelet process stacked or migrated data. The fundamentals and characteristics of the inversion method will be discussed and examples of its use on Australian seismic data will be presented.G-LOG functions are derived by an iterative subsurface modelling technique based on a rigorous inversion of one- dimensional wave equation. This process finds the acoustic impedance model, or log, whose resulting wave-equation- consistent synthetic seismogram best matches the input seismic data in a least mean squared error sense. Multiple reflections are included in the synthetic seismogram, so that they become useful information in the determination of the log.Interval velocity logs are derived from the acoustic impedance logs. The results can be displayed in various forms, including detailed velocity logs, and colour-coded log 'sections' to match with the seismic section. Several examples of such results are presented.The G-LOG process is a revolutionary technique of subsurface modelling, and the logs it provides are strong indicators of subsurface lithology and will be an important tool in the evaluation and re-evaluation of potential hydrocarbon-bearing prospects.*Trademark of G.S.I.

Download Full-text

PENERAPAN METODE F-K DEMULTIPLE DALAM KASUS ATENUASI WATER-BOTTOM MULTIPLE

JURNAL GEOLOGI KELAUTAN ◽

10.32693/jgk.11.1.2013.229 ◽

2016 ◽

Vol 11 (1) ◽

pp. 33

Author(s):

Subarsyah Subarsyah ◽

Sahudin Sahudin

Keyword(s):

Seismic Data ◽

Final Section ◽

Signal To Noise Ratio ◽

Multiple Reflections ◽

Seismic Section ◽

Multiple Attenuation ◽

Seismic Acquisition ◽

Marine Seismic ◽

Second Category ◽

Water Bottom

Keberadaan water-bottom multiple merupakan hal yang tidak bisa dihindari dalam akuisisi data seismik laut, tentu saja hal ini akan menurunkan tingkat perbandingan sinyal dan noise. Beberapa metode atenuasi telah dikembangkan dalam menekan noise ini. Metode atenuasi multiple diklasifikasikan dalam tiga kelompok meliputi metode dekonvolusi yang mengidentifikasi multiple berdasarkan periodisitasnya, metode filtering yang memisahkan refleksi primer dan multiple dalam domain tertentu (F-K,Tau-P dan Radon domain) serta metode prediksi medan gelombang. Penerapan metode F-K demultiple yang masuk kategori kedua akan diterapkan terhadap data seismik PPPGL tahun 2010 di perairan Teluk Tomini. Atenuasi terhadap water-bottom multiple berhasil dilakukan akan tetapi pada beberapa bagian multiple masih terlihat dengan amplitude relatif lebih kecil. F-K demultiple tidak efektif dalam mereduksi multiple pada offset yang pendek dan multiple pada zona ini yang memberikan kontribusi terhadap keberadaan multiple pada penampang akhir. Kata kunci : F-K demultiple, multiple, atenuasi The presence of water-bottom multiple is unavoidable in marine seismic acquisition, of course, this will reduce signal to noise ratio. Several attenuation methods have been developed to suppress this noise. Multiple attenuation methods are classified into three groups first deconvolution method based on periodicity, second filtering method that separates the primary and multiple reflections in certain domains (FK, Tau-P and the Radon domain) ang the third method based on wavefield prediction. Application of F-K demultiple incoming second category will be applied to the seismic data in 2010 PPPGL at Tomini Gulf waters. Attenuation of the water-bottom multiple successful in reduce multiple but in some parts of seismic section multiple still visible with relatively smaller amplitude. FK demultiple not effective in reducing multiple at near offset and multiple in this zone contribute to the existence of multiple in final section. Key words : F-K demultiple, multiple, attenuation

Download Full-text

Wave‐equation velocity replacement of the low‐velocity layer for overthrust‐belt data

Geophysics ◽

10.1190/1.1443795 ◽

1995 ◽

Vol 60 (2) ◽

pp. 573-579 ◽

Cited By ~ 36

Author(s):

William A. Schneider ◽

Lindy D. Phillip ◽

Ernest F. Paal

Keyword(s):

Wave Equation ◽

Seismic Data ◽

Water Layer ◽

Estimation Procedure ◽

Low Velocity ◽

Near Surface ◽

Replacement Procedure ◽

Low Velocity Layer ◽

Overthrust Belt ◽

Velocity Layer

Seismic land data are commonly plagued by nonhyperbolic distortions induced by a variable near‐surface, low‐velocity layer (LVL). First‐arrival refraction analysis is conventionally employed to estimate the LVL geometry and velocities. Then vertical static time shifts are used to replace the LVL velocities with the more uniform, faster velocities that characterize the underlying refracting layer. This methodology has earned a good reputation as a geophysical data processing tool; however, velocity replacement with static shifts assumes that no ray bending occurred at the LVL base and that waves propagated vertically through the LVL (even though conventional refraction analysis methods, which are used to derive LVL models from seismic data, are less restrictive). These assumptions often are inadequate in thick, complex LVL situations, where resulting errors may considerably hamper a statics‐based velocity replacement procedure. Wave‐equation datuming may be used to perform LVL velocity replacement when statics are inadequate. This method extrapolates the seismic data from the surface to the LVL base with the LVL velocities. Then it extrapolates the data from the LVL base to an arbitrary datum, with the replacement velocity field. The marine analog of such a procedure has been well documented in the geophysical literature, where the object is to remove distortions caused by an irregular water layer. Application of wave‐equation datuming to land data is more difficult because of certain common characteristics of land data (irregular shooting, large data gaps, and crooked line geometry, combined with lower signal/noise) and because the LVL estimation procedure is considerably more difficult. We demonstrate wave‐equation velocity replacement on land data from a western U.S. overthrust belt. The LVL in this region was particularly thick and complicated and ideal for a wave‐theoretical velocity‐replacement procedure. Standard refraction analysis techniques were employed to estimate the LVL, then wave‐equation datuming was used to perform the velocity replacement. Our derived LVL model was not perfect, so some imaging errors were expected because wave‐equation datuming is highly dependent upon the LVL model. Nevertheless, our results show that wave‐equation datuming generally allowed better shallow reflector imaging than could be achieved with conventional statics processing.

Download Full-text