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Papers by Chaitanya Mehta

Research paper thumbnail of Wave Equation Based Deghosting of OBC Data

Towed streamer operations in congested areas lead to gaps in 3D coverage making the OBC (Ocean Bo... more Towed streamer operations in congested areas lead to gaps in 3D coverage making the OBC (Ocean Bottom Cable) method employing detectors on the ocean bottom a necessity. In order to ensure that high quality seismic data is obtained using this method one has to address the problem of water column reverberation at the acquisition stage itself. Every primary arrival at a detector location is followed by secondary arrivals due to reverberation of the seismic energy between the water surface and water bottom. If the water depth is small (typically below 10m), the time gap between the primary and secondary arrivals is small and the undesired secondaries can be removed by deconvolution algorithms.

Research paper thumbnail of Anisotropic PSDM: A case study from Western Offshore, India

SEG Technical Program Expanded Abstracts 2004, 2004

Summary Isotropic pre-stack depth migration (PSDM) in the presence of anisotropy in the subsurfac... more Summary Isotropic pre-stack depth migration (PSDM) in the presence of anisotropy in the subsurface, suffers from positioning errors; vertical as well as lateral. It is well known that layering in shaly formations produces anisotropy which can be modeled as transverse Isotropy (TI). In the present paper we present a case study from shallow offshore India. The subsurface in the area is characterized by massive shale (800-1000 m) underlain by thick limestone ‐ shale alternations. The isotropic PSDM showed a 50 m mismatch at this level with hockey-stick effect visible at far offsets on PSDM gathers. We have estimated the heterogeneity and anisotropy parameters i.e. velocity gradient (G), and Thomson parameters (e and δ) from seismic and well data. These parameters, estimated in a number of wells spread over 1000 sq. km., were found to be practically constant. These parameters were incorporated in depth velocity modeling and PSDM along 2D profiles. The study of PSDM depth image gathers has shown that in this area non-hyperbolic effect due to heterogeneity dominates beyond offsets of 1140 m (offsetdepth ratio of 1.4). The similar effect due to anisotropy is visible only after offsets of 1440 m (offset depth ratio of 1.8). However after Anisotropic PSDM (APSDM) the gathers are flattened up-to 1940 m (offsets depth ratio 2.5). APSDM images match well with well-tops within seismic resolution with accompanying improvements in overall image quality at target horizons.

Research paper thumbnail of On: “Decomposition into minimum and maximum phase components” by E. Eisner and G. Hampson (G EOPHYSICS , 55, 897–901, July 1990)

GEOPHYSICS, 1993

How to locate the zero time in a recorded, noisy waveform? How to quantify the extent of minimum‐... more How to locate the zero time in a recorded, noisy waveform? How to quantify the extent of minimum‐phaseness of a waveform? And how to render a waveform or a seismic trace minimum phase? These are some of the questions with which we approached this paper which describes a new method for locating the time origin in a given waveform and, thereafter, factoring the waveform as a convolution of a causal minimum‐phase component and an anticausal maximum phase component.

Research paper thumbnail of Bound states of the potentialV(r)=−Z(r+β)

Physical Review A

We have studied the bound states of the potential V(r) =-Z/(r + P). The S-wave energy level E(P),... more We have studied the bound states of the potential V(r) =-Z/(r + P). The S-wave energy level E(P),. as a function of the parameter p, has a logarithmic singularity at the origin, The imaginary part of E(p) for P~0 goes as Z + ' P ', where I is the angular momentum.

Research paper thumbnail of Nonperturbative approach to screened Coulomb potentials

Physical Review A

The energy levels of two simple examples of screened Coulomb potentials have been analyzed using ... more The energy levels of two simple examples of screened Coulomb potentials have been analyzed using nonperturbative methods. The analysis indicates that the energy levels as a function of the perturbation parameter) have a branch cut along the negative real axis; starting from the origin. Furthermore, there are singularities on the second sheet, along~X~e '' for~X~0. As a consequence of these singularities, the energy levels have an asymptotic series in X, which means that orie cannot use a power series in X to describe the energy levels to an arbitrary accuracy, The approximate but nonperturbative expression for the energy levels, which has been obtained by using dispersion relations, predicts energy levels which are in good agreement with those obtained from variational calculations.

Research paper thumbnail of Porosity interpretation through seismics

Geoexploration

ABSTRACT This paper describes a procedure for porosity interpretation from seismic data (POISE) b... more ABSTRACT This paper describes a procedure for porosity interpretation from seismic data (POISE) by exploiting merits of several recent advances in seismic data processing and interpretation.Briefly, the procedure is as follows: starting from NMO corrected CDP gathers of P-wave data and applying amplitude versus offset (AVO) analysis, one can separate out zero offset “P-wave” and “Swave” stacks both as a function of P-wave time. Here the term “S-wave” refers to a stack that we would have obtained if we had carried out an S-wave survey and displayed the data in P-wave traveltime. Application of maximum likelihood deconvolution/modelling technique on stacks leads to models for P-wave and S-wave reflection coefficients. Finally, inverting the reflection coefficients for interval transit time, a section can be prepared to display ΔTs-ΔTp, the difference in slowness of the S-wave and P-wave as a function of the P-wave traveltime.The final output of POISE for ΔTs-ΔTp is particularly useful for studying variations in lithology and porosity within a formation. The interpretation is fouSnded on an empirical observation by Ikwuakor that ΔTs-ΔTp from character logs for a large number of carbonate and sandstone samples, is proportional to the core porosity (for a given rock type).The utility of POISE is illustrated with a field example in a carbonate reservoir.

Research paper thumbnail of Seismic Detection of Fractured Zones in Bombay High Basement

With the discovery of oil in fractured / weathered basement in Bombay offshore, it is of consider... more With the discovery of oil in fractured / weathered basement in Bombay offshore, it is of considerable importance to inquire whether such zones can be inferred from using seismic data prior to drilling. The purpose of this paper is to show that with the help of (1) isochron map showing structural highs and fault trends at the basement and (ii) carefully processed PIVT ( Pseudo Interval Velocity Transform) sections, it is possible to locate fractured zones in the basement favorable for hydrocarbon accumulation. A good correlation is observed between the occurrence of hydrocarbons in the basement and seismic features for the data of Bombay High (North).

Research paper thumbnail of Tutorial-Phase Considerations in Deconvolution

Research paper thumbnail of Pre-stack suppression of marine multiples using wave equation

When the velocities of the multiples are close to the primaries, the spread length id short, or t... more When the velocities of the multiples are close to the primaries, the spread length id short, or the multiples have large reverberation time, the conventional techniques such as stacking and de-convolution do not suppress the multiples satisfactorily. To utilize the advantages of these methods optimally, it is desirable if at least some of the multiples are suppressed before stack. This paper presents a technique for pre-stack suppression of marine multiples generated by the water surface and a flat reflector parallel to the water surface. The technique is similar to the recent work of Morely (1982) on water bottom multiples and basically involves two steps: First, the multiples are predicted using wave equation in frequency – wave number domain. The prediction operator is simply exp ( 2 i kz d ) where kz = ( k - kx)1/2 , d is the depth of the reflector which is assumed to generate multiples, k = w/c is the magnitude of the wave vector (kx , kz ), w is the angular frequency, and c is...

Research paper thumbnail of Seismic Tomography - A review

Developments in Seismic Tomography in the last decade are reviewed. Bias is towards applications ... more Developments in Seismic Tomography in the last decade are reviewed. Bias is towards applications to Exploration Geophysics. Utility of velocities obtained through seismic tomography for processing of seismic reflection data is discussed at length.

Research paper thumbnail of Migration Velocity and Depth Migration: A synthetic case study

Research paper thumbnail of On the use of instantaneous amplitude in timing the seismic events

Bulletin of the Seismological Society of America

It has been suggested (Farnbach, 1975; Unger, 1981) that instantaneous amplitude associated with ... more It has been suggested (Farnbach, 1975; Unger, 1981) that instantaneous amplitude associated with a seismic signal may be used for timing the onset of a seismic event. The purpose of this note is to point out that, in principle, this method leads to an advanced timing of the onset. Computations using synthetic waveforms suggest that, for a low-frequency (less than about 0.5 Hz) disturbance, the error in estimating the onset time would be too large to be acceptable in earthquake seismology.

Research paper thumbnail of Minimum-Phase-Needs of air gun signatures

A parameter, which we refer to as "Minimum-phase-ness" of a causal finite length wavefo... more A parameter, which we refer to as "Minimum-phase-ness" of a causal finite length waveform is introduced. Using this parameter we examine the dependence of Minimum-phase-ness of recorded air-gun signatures on the ratio of the primary to bubble amplitude, the sampling interval and location of zero time.

Research paper thumbnail of Long‐offset depth processing for sub‐basalt imaging in west of Bavla area Cambay Basin, Gujarat, India

SEG Technical Program Expanded Abstracts 2009, 2009

Throughout the world there are vast areas of the earth covered by basalt flow that are under-expl... more Throughout the world there are vast areas of the earth covered by basalt flow that are under-explored for hydrocarbons due to poor sub-basalt imaging. The very high reflectivity at the top of the basalt and high energy attenuation within the basalt cause the sub-basalt reflections to be very weak and completely masked by different orders of multiple generated above the basalt. In order to attempt to image below the basalt, seismic data with long maximum offset have been acquired. Sub-basalt reflections do become visible on long-offset data in the cone formed between the basalt top (Trap Top) refraction and the refraction from shallow layers as shown in Figure 1. Processing of long-offset data does have challenges associated with it such as mode conversion and anisotropy (both intrinsic as well as long-offset effect). In the present study a model-based noise attenuation technique has been applied to suppress the different kind of noises and VTI anisotropy has been modeled and accounted for in Kirchhoff Pre-Stack Depth Migration imaging. The results are very encouraging.

Research paper thumbnail of Extraction of Vp & Vs of Sea Bottom Sediments, Directivity Correction, and QCof Amplitudes for Inversion of Marine Seismic Data

Summary Seismic Amplitudes contain information regarding elastic properties of sediments which is... more Summary Seismic Amplitudes contain information regarding elastic properties of sediments which is useful for discriminating lithology and fluids. In particular, knowledge of V p & Vs of sea bottom sediments, in conjunction with the seismic image, can provide pre-drill warning on shallow drilling hazards. This paper provides a method for extracting V p & Vs of sea bottom sediments from

Research paper thumbnail of Are screened coulomb potentials amenable to pertubation?

Physics Letters A, 1978

ABSTRACT

Research paper thumbnail of Simple Factorizable Multi-Regge Model

Physical Review Letters, 1969

ABSTRACT

Research paper thumbnail of Simplified Multi-Regge Model at t≠0

Physical Review D, 1970

ABSTRACT

Research paper thumbnail of The problem of the relation between double pomeron exchange in inclusive and exclusive experiments

Nuclear Physics B, 1973

The pionization region of the inclusive single-particle spectrum is accounted for by double pomer... more The pionization region of the inclusive single-particle spectrum is accounted for by double pomeron exchange in the absorptive part of a six-point amplitude. In this paper a multiperipheral model for the six-point amplitude with double pomeron exchange is used for continuation by crossing and analyticity to the physical region of the exclusive two particle ~ four particle production process. The cross section for n-p-~ n-0r+n-)p in the double-Regge region is then calculated and compared with the experimental analysis of Lipes, Zweig and Robertson which sets an upper bound to the strength of the double pomeron exchange coupling. This upper bound, coupled with the model for continuation to the inclusive cross section, is shown to give too small a magnitude for the double pomeron exchange in the pionization region. Further avenues for investigation are discussed.

Research paper thumbnail of Double-pomeron decoupling and the relation of exclusive to inclusive experiments with the dual resonance model

Nuclear Physics B, 1973

The double-pomeron coupling strength ~'n tile dual resonance model is found in both the inclusive... more The double-pomeron coupling strength ~'n tile dual resonance model is found in both the inclusive and exclusive regions by comparison with experiments. Double-pomeron coupling occurs in inclusive experiments in the Mueller diagram for the central plateau region. Its strength can also be bounded from its non-observation in the two-particle to four-particle exclusive experiments. The dual resonance model is used to perform the analytic continuation of a six-point amplitude between these regions. The results show that the coupling strength for two forward pomerons in the exclusive region must be less than 1 of that in the inclusive region. This is experimental evidence for substantial forward double-pomeron decoupling in exclusive processes.

Research paper thumbnail of Wave Equation Based Deghosting of OBC Data

Towed streamer operations in congested areas lead to gaps in 3D coverage making the OBC (Ocean Bo... more Towed streamer operations in congested areas lead to gaps in 3D coverage making the OBC (Ocean Bottom Cable) method employing detectors on the ocean bottom a necessity. In order to ensure that high quality seismic data is obtained using this method one has to address the problem of water column reverberation at the acquisition stage itself. Every primary arrival at a detector location is followed by secondary arrivals due to reverberation of the seismic energy between the water surface and water bottom. If the water depth is small (typically below 10m), the time gap between the primary and secondary arrivals is small and the undesired secondaries can be removed by deconvolution algorithms.

Research paper thumbnail of Anisotropic PSDM: A case study from Western Offshore, India

SEG Technical Program Expanded Abstracts 2004, 2004

Summary Isotropic pre-stack depth migration (PSDM) in the presence of anisotropy in the subsurfac... more Summary Isotropic pre-stack depth migration (PSDM) in the presence of anisotropy in the subsurface, suffers from positioning errors; vertical as well as lateral. It is well known that layering in shaly formations produces anisotropy which can be modeled as transverse Isotropy (TI). In the present paper we present a case study from shallow offshore India. The subsurface in the area is characterized by massive shale (800-1000 m) underlain by thick limestone ‐ shale alternations. The isotropic PSDM showed a 50 m mismatch at this level with hockey-stick effect visible at far offsets on PSDM gathers. We have estimated the heterogeneity and anisotropy parameters i.e. velocity gradient (G), and Thomson parameters (e and δ) from seismic and well data. These parameters, estimated in a number of wells spread over 1000 sq. km., were found to be practically constant. These parameters were incorporated in depth velocity modeling and PSDM along 2D profiles. The study of PSDM depth image gathers has shown that in this area non-hyperbolic effect due to heterogeneity dominates beyond offsets of 1140 m (offsetdepth ratio of 1.4). The similar effect due to anisotropy is visible only after offsets of 1440 m (offset depth ratio of 1.8). However after Anisotropic PSDM (APSDM) the gathers are flattened up-to 1940 m (offsets depth ratio 2.5). APSDM images match well with well-tops within seismic resolution with accompanying improvements in overall image quality at target horizons.

Research paper thumbnail of On: “Decomposition into minimum and maximum phase components” by E. Eisner and G. Hampson (G EOPHYSICS , 55, 897–901, July 1990)

GEOPHYSICS, 1993

How to locate the zero time in a recorded, noisy waveform? How to quantify the extent of minimum‐... more How to locate the zero time in a recorded, noisy waveform? How to quantify the extent of minimum‐phaseness of a waveform? And how to render a waveform or a seismic trace minimum phase? These are some of the questions with which we approached this paper which describes a new method for locating the time origin in a given waveform and, thereafter, factoring the waveform as a convolution of a causal minimum‐phase component and an anticausal maximum phase component.

Research paper thumbnail of Bound states of the potentialV(r)=−Z(r+β)

Physical Review A

We have studied the bound states of the potential V(r) =-Z/(r + P). The S-wave energy level E(P),... more We have studied the bound states of the potential V(r) =-Z/(r + P). The S-wave energy level E(P),. as a function of the parameter p, has a logarithmic singularity at the origin, The imaginary part of E(p) for P~0 goes as Z + ' P ', where I is the angular momentum.

Research paper thumbnail of Nonperturbative approach to screened Coulomb potentials

Physical Review A

The energy levels of two simple examples of screened Coulomb potentials have been analyzed using ... more The energy levels of two simple examples of screened Coulomb potentials have been analyzed using nonperturbative methods. The analysis indicates that the energy levels as a function of the perturbation parameter) have a branch cut along the negative real axis; starting from the origin. Furthermore, there are singularities on the second sheet, along~X~e '' for~X~0. As a consequence of these singularities, the energy levels have an asymptotic series in X, which means that orie cannot use a power series in X to describe the energy levels to an arbitrary accuracy, The approximate but nonperturbative expression for the energy levels, which has been obtained by using dispersion relations, predicts energy levels which are in good agreement with those obtained from variational calculations.

Research paper thumbnail of Porosity interpretation through seismics

Geoexploration

ABSTRACT This paper describes a procedure for porosity interpretation from seismic data (POISE) b... more ABSTRACT This paper describes a procedure for porosity interpretation from seismic data (POISE) by exploiting merits of several recent advances in seismic data processing and interpretation.Briefly, the procedure is as follows: starting from NMO corrected CDP gathers of P-wave data and applying amplitude versus offset (AVO) analysis, one can separate out zero offset “P-wave” and “Swave” stacks both as a function of P-wave time. Here the term “S-wave” refers to a stack that we would have obtained if we had carried out an S-wave survey and displayed the data in P-wave traveltime. Application of maximum likelihood deconvolution/modelling technique on stacks leads to models for P-wave and S-wave reflection coefficients. Finally, inverting the reflection coefficients for interval transit time, a section can be prepared to display ΔTs-ΔTp, the difference in slowness of the S-wave and P-wave as a function of the P-wave traveltime.The final output of POISE for ΔTs-ΔTp is particularly useful for studying variations in lithology and porosity within a formation. The interpretation is fouSnded on an empirical observation by Ikwuakor that ΔTs-ΔTp from character logs for a large number of carbonate and sandstone samples, is proportional to the core porosity (for a given rock type).The utility of POISE is illustrated with a field example in a carbonate reservoir.

Research paper thumbnail of Seismic Detection of Fractured Zones in Bombay High Basement

With the discovery of oil in fractured / weathered basement in Bombay offshore, it is of consider... more With the discovery of oil in fractured / weathered basement in Bombay offshore, it is of considerable importance to inquire whether such zones can be inferred from using seismic data prior to drilling. The purpose of this paper is to show that with the help of (1) isochron map showing structural highs and fault trends at the basement and (ii) carefully processed PIVT ( Pseudo Interval Velocity Transform) sections, it is possible to locate fractured zones in the basement favorable for hydrocarbon accumulation. A good correlation is observed between the occurrence of hydrocarbons in the basement and seismic features for the data of Bombay High (North).

Research paper thumbnail of Tutorial-Phase Considerations in Deconvolution

Research paper thumbnail of Pre-stack suppression of marine multiples using wave equation

When the velocities of the multiples are close to the primaries, the spread length id short, or t... more When the velocities of the multiples are close to the primaries, the spread length id short, or the multiples have large reverberation time, the conventional techniques such as stacking and de-convolution do not suppress the multiples satisfactorily. To utilize the advantages of these methods optimally, it is desirable if at least some of the multiples are suppressed before stack. This paper presents a technique for pre-stack suppression of marine multiples generated by the water surface and a flat reflector parallel to the water surface. The technique is similar to the recent work of Morely (1982) on water bottom multiples and basically involves two steps: First, the multiples are predicted using wave equation in frequency – wave number domain. The prediction operator is simply exp ( 2 i kz d ) where kz = ( k - kx)1/2 , d is the depth of the reflector which is assumed to generate multiples, k = w/c is the magnitude of the wave vector (kx , kz ), w is the angular frequency, and c is...

Research paper thumbnail of Seismic Tomography - A review

Developments in Seismic Tomography in the last decade are reviewed. Bias is towards applications ... more Developments in Seismic Tomography in the last decade are reviewed. Bias is towards applications to Exploration Geophysics. Utility of velocities obtained through seismic tomography for processing of seismic reflection data is discussed at length.

Research paper thumbnail of Migration Velocity and Depth Migration: A synthetic case study

Research paper thumbnail of On the use of instantaneous amplitude in timing the seismic events

Bulletin of the Seismological Society of America

It has been suggested (Farnbach, 1975; Unger, 1981) that instantaneous amplitude associated with ... more It has been suggested (Farnbach, 1975; Unger, 1981) that instantaneous amplitude associated with a seismic signal may be used for timing the onset of a seismic event. The purpose of this note is to point out that, in principle, this method leads to an advanced timing of the onset. Computations using synthetic waveforms suggest that, for a low-frequency (less than about 0.5 Hz) disturbance, the error in estimating the onset time would be too large to be acceptable in earthquake seismology.

Research paper thumbnail of Minimum-Phase-Needs of air gun signatures

A parameter, which we refer to as "Minimum-phase-ness" of a causal finite length wavefo... more A parameter, which we refer to as "Minimum-phase-ness" of a causal finite length waveform is introduced. Using this parameter we examine the dependence of Minimum-phase-ness of recorded air-gun signatures on the ratio of the primary to bubble amplitude, the sampling interval and location of zero time.

Research paper thumbnail of Long‐offset depth processing for sub‐basalt imaging in west of Bavla area Cambay Basin, Gujarat, India

SEG Technical Program Expanded Abstracts 2009, 2009

Throughout the world there are vast areas of the earth covered by basalt flow that are under-expl... more Throughout the world there are vast areas of the earth covered by basalt flow that are under-explored for hydrocarbons due to poor sub-basalt imaging. The very high reflectivity at the top of the basalt and high energy attenuation within the basalt cause the sub-basalt reflections to be very weak and completely masked by different orders of multiple generated above the basalt. In order to attempt to image below the basalt, seismic data with long maximum offset have been acquired. Sub-basalt reflections do become visible on long-offset data in the cone formed between the basalt top (Trap Top) refraction and the refraction from shallow layers as shown in Figure 1. Processing of long-offset data does have challenges associated with it such as mode conversion and anisotropy (both intrinsic as well as long-offset effect). In the present study a model-based noise attenuation technique has been applied to suppress the different kind of noises and VTI anisotropy has been modeled and accounted for in Kirchhoff Pre-Stack Depth Migration imaging. The results are very encouraging.

Research paper thumbnail of Extraction of Vp & Vs of Sea Bottom Sediments, Directivity Correction, and QCof Amplitudes for Inversion of Marine Seismic Data

Summary Seismic Amplitudes contain information regarding elastic properties of sediments which is... more Summary Seismic Amplitudes contain information regarding elastic properties of sediments which is useful for discriminating lithology and fluids. In particular, knowledge of V p & Vs of sea bottom sediments, in conjunction with the seismic image, can provide pre-drill warning on shallow drilling hazards. This paper provides a method for extracting V p & Vs of sea bottom sediments from

Research paper thumbnail of Are screened coulomb potentials amenable to pertubation?

Physics Letters A, 1978

ABSTRACT

Research paper thumbnail of Simple Factorizable Multi-Regge Model

Physical Review Letters, 1969

ABSTRACT

Research paper thumbnail of Simplified Multi-Regge Model at t≠0

Physical Review D, 1970

ABSTRACT

Research paper thumbnail of The problem of the relation between double pomeron exchange in inclusive and exclusive experiments

Nuclear Physics B, 1973

The pionization region of the inclusive single-particle spectrum is accounted for by double pomer... more The pionization region of the inclusive single-particle spectrum is accounted for by double pomeron exchange in the absorptive part of a six-point amplitude. In this paper a multiperipheral model for the six-point amplitude with double pomeron exchange is used for continuation by crossing and analyticity to the physical region of the exclusive two particle ~ four particle production process. The cross section for n-p-~ n-0r+n-)p in the double-Regge region is then calculated and compared with the experimental analysis of Lipes, Zweig and Robertson which sets an upper bound to the strength of the double pomeron exchange coupling. This upper bound, coupled with the model for continuation to the inclusive cross section, is shown to give too small a magnitude for the double pomeron exchange in the pionization region. Further avenues for investigation are discussed.

Research paper thumbnail of Double-pomeron decoupling and the relation of exclusive to inclusive experiments with the dual resonance model

Nuclear Physics B, 1973

The double-pomeron coupling strength ~'n tile dual resonance model is found in both the inclusive... more The double-pomeron coupling strength ~'n tile dual resonance model is found in both the inclusive and exclusive regions by comparison with experiments. Double-pomeron coupling occurs in inclusive experiments in the Mueller diagram for the central plateau region. Its strength can also be bounded from its non-observation in the two-particle to four-particle exclusive experiments. The dual resonance model is used to perform the analytic continuation of a six-point amplitude between these regions. The results show that the coupling strength for two forward pomerons in the exclusive region must be less than 1 of that in the inclusive region. This is experimental evidence for substantial forward double-pomeron decoupling in exclusive processes.