Mamoru Takanashi - Academia.edu (original) (raw)

Papers by Mamoru Takanashi

Geophysical Journal International, 2001

To study crustal rock seismic anisotropy and its effect on seismic wave propagation, we measure t... more To study crustal rock seismic anisotropy and its effect on seismic wave propagation, we measure the seismic velocity anisotropy of two amphibolites, one biotite gneiss and one biotite schist from the Hidaka metamorphic belt in central Hokkaido, Japan, under confining pressures up to 150 MPa. The rock microstructures show foliation and lineation characterized by lattice preferred orientation (LPO) of hornblende or biotite. P-and two S-wave velocities are measured along the direction perpendicular to the foliation plane and two directions in the foliation plane: perpendicular and parallel to the lineation. We assume orthorhombic symmetry based on the rock microstructures and obtain Tsvankin's anisotropic parameters (an extension of Thomsen's parameters for orthorhombic symmetry). P-and S-wave phase velocity surfaces are calculated from anisotropy parameters and compared with the measured velocities along particular directions and with the velocity contour maps calculated from the Voigt averages of singlecrystal elastic constants based on the orientation of measured LPO data. Qualitatively, the measured velocity anisotropy agrees with the velocity contour calculated from the LPO data, although large quantitative differences exist between them. All anisotropy patterns can be approximated as transverse isotropy or its modification, appearing as orthorhombic symmetry. Biotite schist (containing 30 per cent volume ratio biotite) shows strong S-wave anisotropy, and the phase velocity surfaces of P waves show a large deviation from ellipticity in the plane perpendicular to the foliation and parallel to the lineation. In the same plane, S waves show a singularity due to a large bulge of the SV velocity surface.

ASEG Extended Abstracts, 2007

Summary For contributing to precise depth delineation in the Ichthys giant gas field offshore nor... more Summary For contributing to precise depth delineation in the Ichthys giant gas field offshore northwest Australia, we investigated shallow velocity heterogeneity effects using forward modelling and seismic data review. We discovered that their removal through applications of high-resolution tomographic velocity modelling and prestack depth migration (PSDM) enabled correct representation of the target reservoir structure. Analysis of synthetic seismic pre-stack gathers generated from forward modelling demonstrated that small velocity anomalies, such as channels, in shallow overburdens could give rise to apparent seismic RMS velocity artefacts at deeper target levels. Delineation of “true” velocity anomalies and implementation of PSDM using precise shallow velocity model were required for solving these problems. In accordance with the phenomenon predicted by forward modelling, examination of coinciding patterns of timethickness, amplitude and deep prestack time migration (PSTM) velocity allowed determination of the shallow heterogeneous layers that caused target velocity undulation. We employed two iterations of tomographic velocity model updating for PSDM velocity model building. First, the shallow heterogeneous velocity patterns were successfully identified by utilising the dense residual moveout picking and the layer-based 3D high-resolution finite-offset tomography. Subsequent grid-based global tomography with constraints was used for updating the entire velocity field and delivered stable velocity pattern at the deep. This two-step approach successfully eliminated deep velocity artefacts.

Robust estimation of anisotropy parameters is one of the most important problems in seismic imagi... more Robust estimation of anisotropy parameters is one of the most important problems in seismic imaging and fracture characterization. However, lateral heterogeneity (LH) on the scale smaller than spreadlength, often associated with velocity lenses in the overburden, can significantly distort anisotropy parameters even for gently dipping structures. In this thesis, I present both time- and depth-domain algorithms designed to estimate anisotropy parameters and perform seismic imaging in the presence of small-scale lateral heterogeneity. I analyze the influence of a thin LH layer on the NMO ellipse (azimuthally vary-ing normal-moveout velocity) for horizontally stratified anisotropic media and show that the distortion of the medium parameters is caused primarily by quadratic lateral velocity variation and the errors increase with the distance between the target and the LH layer. Numerical testing confirms that the developed analytic expression accurately removes the lens-induced distortio...

Moveout analysis of wide-azimuth reflection data is usually performed for smoothly varying veloci... more Moveout analysis of wide-azimuth reflection data is usually performed for smoothly varying velocity fields. However, velocity lenses in the overburden can cause significant laterally-varying errors in the moveout parameters and distortions in data interpretation. The main goal of this paper is to develop a correction for the influence of an elongated velocity lens on the NMO ellipse and nonhyperbolic reflection moveout. We show that an analytic expression for the NMO ellipse in stratified media with lateral velocity variation, introduced in our previous paper, adequately represents lens-induced distortions. The correction for lateral heterogeneity is controlled by the second derivatives of the interval vertical traveltime. This analytic correction provides a quick estimate of the contribution of velocity lenses and substantially mitigates lens-induced distortions in the effective and interval NMO ellipses. To remove the influence of velocity lenses on nonhyperbolic moveout inversion...

One of the most serious problems in anisotropic velocity analysis is the tradeoff between anisotr... more One of the most serious problems in anisotropic velocity analysis is the tradeoff between anisotropy and lateral heterogeneity, especially if velocity varies on a scale smaller than spreadlength. Here, we develop a P-wave MVA (migration velocity analysis) algorithm for transversely isotropic (TI) models that include layers with small-scale lateral heterogeneity. Each layer is described by constant Thomsen’s parameters ɛ and δ and the symmetry-direction velocity V0 that varies as a quadratic function of the distance along the layer boundaries. For tilted TI media (TTI), the symmetry axis is taken orthogonal to the reflectors. We analyze the influence of lateral heterogeneity on image gathers obtained after prestack depth migration and show that quadratic lateral velocity variation in the overburden can significantly distort the moveout of the target reflection. If such errors are not corrected, the medium parameters beneath the heterogeneous layer(s) are estimated with significant er...

Moveout analysis of wide-azimuth reflection data is usually performed for smoothly varying veloci... more Moveout analysis of wide-azimuth reflection data is usually performed for smoothly varying velocity fields. However, ve-locity lenses in the overburden can cause significant laterally-varying errors in the moveout parameters. Here, we derive an analytic expression for the NMO ellipse in stratified media with lateral velocity variation and show that the correction for lateral heterogeneity is controlled by the second derivatives of the interval vertical traveltime. This equation provides a quick estimate of the contribution of velocity lenses and substantially mitigates the lens-induced distortions in the effective and inter-val NMO ellipses. To remove the influence of velocity lenses on nonhyper-bolic moveout inversion of wide-azimuth data, we propose a prestack correction algorithm that involves computation of the lens-induced traveltime distortion for each recorded trace. The method is designed for a horizontally-layered overburden but can handle laterally heterogeneous target lay...

One of the most serious problems in anisotropic velocity anal-ysis is the trade-off between aniso... more One of the most serious problems in anisotropic velocity anal-ysis is the trade-off between anisotropy and lateral hetero-geneity, especially if velocity varies on a scale smaller than spreadlength. Here, we develop a P-wave MVA (migration ve-locity analysis) algorithm for transversely isotropic (TI) mod-els that include layers with small-scale lateral heterogeneity. Each layer is described by constant parameters ε and δ and the symmetry-direction velocity V0 that varies as a quadratic function of the distance along the layer boundaries. For tilted TI media (TTI), the symmetry axis is taken orthogonal to the reflectors. We analyze the influence of lateral heterogeneity on image gathers obtained after prestack depth migration (PSDM) and show that quadratic lateral velocity variation in the overburden can significantly distort the moveout of the target reflection and the parameters of the deeper part of the section. Since the residual moveout is highly sensitive to lateral heterogene-...

Nonhyperbolic moveout analysis plays an increasingly important role in velocity model building be... more Nonhyperbolic moveout analysis plays an increasingly important role in velocity model building because it provides valuable information for anisotropic parameter estimation. However, lateral heterogeneity associated with stratigraphic lenses such as channels and reefs can significantly distort the moveout parameters, even when the structure is relatively simple. Here, we discuss nonhyperbolic moveout inversion for 2D models that include a low-velocity isotropic lens embedded in a VTI (transversely isotropic with a vertical symmetry axis) medium. Synthetic tests demonstrate that a lens can cause substantial, laterally varying errors in the normal-moveout velocity (Vnmo) and the anellipticity parameter η. The area influenced by the lens can be identified using the residual moveout after the nonhyperbolic moveout correction and the dependence of errors in Vnmo and η on spreadlength. To remove lens-induced traveltime distortions from prestack data, we propose an algorithm that involves ...

Analysis of azimuthally varying normal-moveout velocity (i.e., of the NMO ellipse) is often perfo... more Analysis of azimuthally varying normal-moveout velocity (i.e., of the NMO ellipse) is often performed under the assumption that the overburden is laterally homogeneous on the scale of spreadlength. However, small-scale lateral heterogeneity can significantly influence both effective and interval NMO ellipses. Here, we derive an analytic expression for the NMO ellipse in a horizontally layered anisotropic medium with lateral velocity variation. The equation demonstrates that the distortion of the effective NMO ellipse is caused primarily by the quadratic lateral variation of the vertical velocity, which can be represented through the curvature of the vertical traveltime surface. The magnitude of the distortion rapidly increases with the distance between the target and a laterally heterogeneous layer in the overburden. Application of Dix-type differentiation without correcting for the lateral velocity variation can strongly amplify the false elongation or compression of the effective ...

Proceedings of the 14th SEGJ International Symposium, Online, 18–21 October 2021

Earth, Planets and Space

Owing to the strict restrictions on the use of air guns in marine seismic surveys due to concerns... more Owing to the strict restrictions on the use of air guns in marine seismic surveys due to concerns about their potential impact on the marine ecosystem, there have been several cases where seismic surveys were not permitted. This tendency has been particularly significant in coastal waters where fishing activity is flourishing, which creates blank zones in seismic surveys. The authors, therefore, adopted underwater speakers as environment-friendly seismic sources that can be used under such restrictions. In December 2017, the applicability of underwater speakers as a seismic source was tested in a seismic reflection experiment in the northern part of Tokyo Bay. As a result, shallow subsurface structures were successfully imaged, and a low-velocity zone was detected 7-8 m below the seafloor. In this paper, the concept of environment-friendly seismic survey using underwater speakers is reported. In addition, the potential presence of a methane gas layer that was detected in the low-velocity zone is discussed. If the methane gas is widely distributed near the seafloor in the northern part of Tokyo Bay, a large amount of gas might be released into the water and then into the air when, for example, a large-scale earthquake occurs directly underneath the Tokyo Bay area. Given the high flammability of methane, the features and volume of its distribution must be precisely investigated from the perspective of earthquake occurrences in the metropolitan area.

SEG Technical Program Expanded Abstracts 2016

The 13th SEGJ International Symposium, Tokyo, Japan, 12-14 November 2018

International Petroleum Technology Conference

Journal of Geophysical Research: Solid Earth

SEG Technical Program Expanded Abstracts 2017

Journal of the Japanese Association for Petroleum Technology, 2014

As the deep water drilling technology continues to evolve, sub-salt basins has become one of the ... more As the deep water drilling technology continues to evolve, sub-salt basins has become one of the most prospective targets for hydrocarbon reserves. Even though sub-salt is a dif cult target for seismic imaging, various advanced technologies have been developed in the lost decade and dif culties encountered in the new prospect have been reduced. New seismic acquisition technologies such as broadband seismic, full-azimuth and ultra-long offset acquisition mitigate the problems from the presence of salt. Sub-salt imaging techniques has also evolved with the aid of the advanced new acquisition technology, high oil price, and low cost high performance computing (HPC). Anisotropic RTM has became a usual technique in the Gulf of Mexico, and FWI grows in use and popularity nowadays.

GEOPHYSICS, 2015

Onshore hydrocarbon exploration in the back-arc region of Japan suffers from seismic imaging chal... more Onshore hydrocarbon exploration in the back-arc region of Japan suffers from seismic imaging challenges. Seismic data are typically acquired in a difficult environment: along a crooked line over a rough surface topography underlain by severe near-surface weathering layers. The area is geologically complex, and prestack depth processing is desirable. However, the data quality is suboptimal at near offsets, and it hinders the migration velocity analysis required for depth processing. Thus, a geologic interpretation needs to rely on time-processing results. Nevertheless, some data sets are rich in low-frequency components and contain clear refracted and wide-angle reflected waves, both of which are favorable conditions for the application of waveform tomography. We have used one such data set, and we determined the applicability of waveform tomography to estimate the P-wave velocity distribution. To mitigate difficulties in the data, we carefully optimized waveform tomography strategie...

Geophysical Journal International, 2001

To study crustal rock seismic anisotropy and its effect on seismic wave propagation, we measure t... more To study crustal rock seismic anisotropy and its effect on seismic wave propagation, we measure the seismic velocity anisotropy of two amphibolites, one biotite gneiss and one biotite schist from the Hidaka metamorphic belt in central Hokkaido, Japan, under confining pressures up to 150 MPa. The rock microstructures show foliation and lineation characterized by lattice preferred orientation (LPO) of hornblende or biotite. P-and two S-wave velocities are measured along the direction perpendicular to the foliation plane and two directions in the foliation plane: perpendicular and parallel to the lineation. We assume orthorhombic symmetry based on the rock microstructures and obtain Tsvankin's anisotropic parameters (an extension of Thomsen's parameters for orthorhombic symmetry). P-and S-wave phase velocity surfaces are calculated from anisotropy parameters and compared with the measured velocities along particular directions and with the velocity contour maps calculated from the Voigt averages of singlecrystal elastic constants based on the orientation of measured LPO data. Qualitatively, the measured velocity anisotropy agrees with the velocity contour calculated from the LPO data, although large quantitative differences exist between them. All anisotropy patterns can be approximated as transverse isotropy or its modification, appearing as orthorhombic symmetry. Biotite schist (containing 30 per cent volume ratio biotite) shows strong S-wave anisotropy, and the phase velocity surfaces of P waves show a large deviation from ellipticity in the plane perpendicular to the foliation and parallel to the lineation. In the same plane, S waves show a singularity due to a large bulge of the SV velocity surface.

ASEG Extended Abstracts, 2007

Summary For contributing to precise depth delineation in the Ichthys giant gas field offshore nor... more Summary For contributing to precise depth delineation in the Ichthys giant gas field offshore northwest Australia, we investigated shallow velocity heterogeneity effects using forward modelling and seismic data review. We discovered that their removal through applications of high-resolution tomographic velocity modelling and prestack depth migration (PSDM) enabled correct representation of the target reservoir structure. Analysis of synthetic seismic pre-stack gathers generated from forward modelling demonstrated that small velocity anomalies, such as channels, in shallow overburdens could give rise to apparent seismic RMS velocity artefacts at deeper target levels. Delineation of “true” velocity anomalies and implementation of PSDM using precise shallow velocity model were required for solving these problems. In accordance with the phenomenon predicted by forward modelling, examination of coinciding patterns of timethickness, amplitude and deep prestack time migration (PSTM) velocity allowed determination of the shallow heterogeneous layers that caused target velocity undulation. We employed two iterations of tomographic velocity model updating for PSDM velocity model building. First, the shallow heterogeneous velocity patterns were successfully identified by utilising the dense residual moveout picking and the layer-based 3D high-resolution finite-offset tomography. Subsequent grid-based global tomography with constraints was used for updating the entire velocity field and delivered stable velocity pattern at the deep. This two-step approach successfully eliminated deep velocity artefacts.

Robust estimation of anisotropy parameters is one of the most important problems in seismic imagi... more Robust estimation of anisotropy parameters is one of the most important problems in seismic imaging and fracture characterization. However, lateral heterogeneity (LH) on the scale smaller than spreadlength, often associated with velocity lenses in the overburden, can significantly distort anisotropy parameters even for gently dipping structures. In this thesis, I present both time- and depth-domain algorithms designed to estimate anisotropy parameters and perform seismic imaging in the presence of small-scale lateral heterogeneity. I analyze the influence of a thin LH layer on the NMO ellipse (azimuthally vary-ing normal-moveout velocity) for horizontally stratified anisotropic media and show that the distortion of the medium parameters is caused primarily by quadratic lateral velocity variation and the errors increase with the distance between the target and the LH layer. Numerical testing confirms that the developed analytic expression accurately removes the lens-induced distortio...

Moveout analysis of wide-azimuth reflection data is usually performed for smoothly varying veloci... more Moveout analysis of wide-azimuth reflection data is usually performed for smoothly varying velocity fields. However, velocity lenses in the overburden can cause significant laterally-varying errors in the moveout parameters and distortions in data interpretation. The main goal of this paper is to develop a correction for the influence of an elongated velocity lens on the NMO ellipse and nonhyperbolic reflection moveout. We show that an analytic expression for the NMO ellipse in stratified media with lateral velocity variation, introduced in our previous paper, adequately represents lens-induced distortions. The correction for lateral heterogeneity is controlled by the second derivatives of the interval vertical traveltime. This analytic correction provides a quick estimate of the contribution of velocity lenses and substantially mitigates lens-induced distortions in the effective and interval NMO ellipses. To remove the influence of velocity lenses on nonhyperbolic moveout inversion...

One of the most serious problems in anisotropic velocity analysis is the tradeoff between anisotr... more One of the most serious problems in anisotropic velocity analysis is the tradeoff between anisotropy and lateral heterogeneity, especially if velocity varies on a scale smaller than spreadlength. Here, we develop a P-wave MVA (migration velocity analysis) algorithm for transversely isotropic (TI) models that include layers with small-scale lateral heterogeneity. Each layer is described by constant Thomsen’s parameters ɛ and δ and the symmetry-direction velocity V0 that varies as a quadratic function of the distance along the layer boundaries. For tilted TI media (TTI), the symmetry axis is taken orthogonal to the reflectors. We analyze the influence of lateral heterogeneity on image gathers obtained after prestack depth migration and show that quadratic lateral velocity variation in the overburden can significantly distort the moveout of the target reflection. If such errors are not corrected, the medium parameters beneath the heterogeneous layer(s) are estimated with significant er...

Moveout analysis of wide-azimuth reflection data is usually performed for smoothly varying veloci... more Moveout analysis of wide-azimuth reflection data is usually performed for smoothly varying velocity fields. However, ve-locity lenses in the overburden can cause significant laterally-varying errors in the moveout parameters. Here, we derive an analytic expression for the NMO ellipse in stratified media with lateral velocity variation and show that the correction for lateral heterogeneity is controlled by the second derivatives of the interval vertical traveltime. This equation provides a quick estimate of the contribution of velocity lenses and substantially mitigates the lens-induced distortions in the effective and inter-val NMO ellipses. To remove the influence of velocity lenses on nonhyper-bolic moveout inversion of wide-azimuth data, we propose a prestack correction algorithm that involves computation of the lens-induced traveltime distortion for each recorded trace. The method is designed for a horizontally-layered overburden but can handle laterally heterogeneous target lay...

One of the most serious problems in anisotropic velocity anal-ysis is the trade-off between aniso... more One of the most serious problems in anisotropic velocity anal-ysis is the trade-off between anisotropy and lateral hetero-geneity, especially if velocity varies on a scale smaller than spreadlength. Here, we develop a P-wave MVA (migration ve-locity analysis) algorithm for transversely isotropic (TI) mod-els that include layers with small-scale lateral heterogeneity. Each layer is described by constant parameters ε and δ and the symmetry-direction velocity V0 that varies as a quadratic function of the distance along the layer boundaries. For tilted TI media (TTI), the symmetry axis is taken orthogonal to the reflectors. We analyze the influence of lateral heterogeneity on image gathers obtained after prestack depth migration (PSDM) and show that quadratic lateral velocity variation in the overburden can significantly distort the moveout of the target reflection and the parameters of the deeper part of the section. Since the residual moveout is highly sensitive to lateral heterogene-...

Nonhyperbolic moveout analysis plays an increasingly important role in velocity model building be... more Nonhyperbolic moveout analysis plays an increasingly important role in velocity model building because it provides valuable information for anisotropic parameter estimation. However, lateral heterogeneity associated with stratigraphic lenses such as channels and reefs can significantly distort the moveout parameters, even when the structure is relatively simple. Here, we discuss nonhyperbolic moveout inversion for 2D models that include a low-velocity isotropic lens embedded in a VTI (transversely isotropic with a vertical symmetry axis) medium. Synthetic tests demonstrate that a lens can cause substantial, laterally varying errors in the normal-moveout velocity (Vnmo) and the anellipticity parameter η. The area influenced by the lens can be identified using the residual moveout after the nonhyperbolic moveout correction and the dependence of errors in Vnmo and η on spreadlength. To remove lens-induced traveltime distortions from prestack data, we propose an algorithm that involves ...

Analysis of azimuthally varying normal-moveout velocity (i.e., of the NMO ellipse) is often perfo... more Analysis of azimuthally varying normal-moveout velocity (i.e., of the NMO ellipse) is often performed under the assumption that the overburden is laterally homogeneous on the scale of spreadlength. However, small-scale lateral heterogeneity can significantly influence both effective and interval NMO ellipses. Here, we derive an analytic expression for the NMO ellipse in a horizontally layered anisotropic medium with lateral velocity variation. The equation demonstrates that the distortion of the effective NMO ellipse is caused primarily by the quadratic lateral variation of the vertical velocity, which can be represented through the curvature of the vertical traveltime surface. The magnitude of the distortion rapidly increases with the distance between the target and a laterally heterogeneous layer in the overburden. Application of Dix-type differentiation without correcting for the lateral velocity variation can strongly amplify the false elongation or compression of the effective ...

Proceedings of the 14th SEGJ International Symposium, Online, 18–21 October 2021

Earth, Planets and Space

Owing to the strict restrictions on the use of air guns in marine seismic surveys due to concerns... more Owing to the strict restrictions on the use of air guns in marine seismic surveys due to concerns about their potential impact on the marine ecosystem, there have been several cases where seismic surveys were not permitted. This tendency has been particularly significant in coastal waters where fishing activity is flourishing, which creates blank zones in seismic surveys. The authors, therefore, adopted underwater speakers as environment-friendly seismic sources that can be used under such restrictions. In December 2017, the applicability of underwater speakers as a seismic source was tested in a seismic reflection experiment in the northern part of Tokyo Bay. As a result, shallow subsurface structures were successfully imaged, and a low-velocity zone was detected 7-8 m below the seafloor. In this paper, the concept of environment-friendly seismic survey using underwater speakers is reported. In addition, the potential presence of a methane gas layer that was detected in the low-velocity zone is discussed. If the methane gas is widely distributed near the seafloor in the northern part of Tokyo Bay, a large amount of gas might be released into the water and then into the air when, for example, a large-scale earthquake occurs directly underneath the Tokyo Bay area. Given the high flammability of methane, the features and volume of its distribution must be precisely investigated from the perspective of earthquake occurrences in the metropolitan area.

SEG Technical Program Expanded Abstracts 2016

The 13th SEGJ International Symposium, Tokyo, Japan, 12-14 November 2018

International Petroleum Technology Conference

Journal of Geophysical Research: Solid Earth

SEG Technical Program Expanded Abstracts 2017

Journal of the Japanese Association for Petroleum Technology, 2014

As the deep water drilling technology continues to evolve, sub-salt basins has become one of the ... more As the deep water drilling technology continues to evolve, sub-salt basins has become one of the most prospective targets for hydrocarbon reserves. Even though sub-salt is a dif cult target for seismic imaging, various advanced technologies have been developed in the lost decade and dif culties encountered in the new prospect have been reduced. New seismic acquisition technologies such as broadband seismic, full-azimuth and ultra-long offset acquisition mitigate the problems from the presence of salt. Sub-salt imaging techniques has also evolved with the aid of the advanced new acquisition technology, high oil price, and low cost high performance computing (HPC). Anisotropic RTM has became a usual technique in the Gulf of Mexico, and FWI grows in use and popularity nowadays.

GEOPHYSICS, 2015

Onshore hydrocarbon exploration in the back-arc region of Japan suffers from seismic imaging chal... more Onshore hydrocarbon exploration in the back-arc region of Japan suffers from seismic imaging challenges. Seismic data are typically acquired in a difficult environment: along a crooked line over a rough surface topography underlain by severe near-surface weathering layers. The area is geologically complex, and prestack depth processing is desirable. However, the data quality is suboptimal at near offsets, and it hinders the migration velocity analysis required for depth processing. Thus, a geologic interpretation needs to rely on time-processing results. Nevertheless, some data sets are rich in low-frequency components and contain clear refracted and wide-angle reflected waves, both of which are favorable conditions for the application of waveform tomography. We have used one such data set, and we determined the applicability of waveform tomography to estimate the P-wave velocity distribution. To mitigate difficulties in the data, we carefully optimized waveform tomography strategie...