Jaime Ramos-Martínez

3-D seismic modeling for cracked media: Shear‐wave splitting at zero‐offset

Splitting of zero‐offset reflected shear‐waves is measured directly from three‐component finite‐difference synthetic seismograms for media with intersecting vertical crack systems. Splitting is simulated numerically (by finite differencing) as a function of crack density, aspect ratio, fluid content, bulk density, and the angle between the crack systems. The type of anisotropy symmetry in media containing two intersecting vertical crack systems depends on the angular relation between the cracks and their relative crack densities, and it may be horizontal transverse isotropy (HTI), tetragonal, orthorhombic, or monoclinic. The transition from one symmetry to another is visible in the splitting behavior. The polarities of the reflected quasi‐shear waves polarized perpendicular and parallel to the source particle motion distinguish between HTI and orthorhombic media. The dependence of the measured amount of splitting on crack density for HTI symmetry is consistent with that predicted theoretically by the shea...

Source-Parameter Estimation by Full Waveform Inversion in 3D Heterogeneous, Viscoelastic, Anisotropic Media

A source parameter estimation algorithm that incorporates Green9s functions for 3D heterogeneous viscoelastic anisotropic media is developed. The algorithm is based on the construction of synthetic seismograms for a double-couple point source by linear combination of the five fundamental sources that correspond to the moment tensor components. Construction of the Green9s functions for such complex media is performed numerically by eighth-order finite differencing. For the source inversion, the model search procedure of Zhao and Helmberger (1994) is used. Results for a synthetic event in a realistic 3D model of the San Fernando basin show that incorporation of the 3D structure reduces the residual errors of the waveform fitting by more than 50% compared to those for a 1D layered model; azimuthal anisotropy produced by vertical cracks reduces the errors about 30%; and attenuation accounts for the other 20%. The algorithm is applied to real data from two 1994 Northridge aftershocks. The source parameters obtained from high-frequency seismograms at local distances are similar to those obtained using traditional approaches at regional distances. In the source estimations from real data, the largest improvement in the residuals is observed in going from a 1D to the 3D model. The most significant contribution of the 3D structure is a considerable improvement in the uniqueness of solution. Incorporation of anisotropy and attenuation to the 3D model into estimations do not improve the solutions for the real data and implies that these aspects of the model are not yet sufficiently well defined to contribute to improving source-parameter estimations.

A comparison of inversion results for two full-waveform methods that utilize the lowest frequencies in dual-sensor recordings

Summary In this abstract, we test the capability of full-waveform inversion to exploit the lowest frequencies available from dual-sensor, single-streamer recordings. Two different, time-domain methods are tested. The first method follows the “conventional” approach of iteratively updating interval velocity by using the gradient obtained from back-projected residuals between modelled and recorded data. The second method constructs perturbations in impedance from the gradient, which is then used to update the velocity. These methods are tested on 2-D and 3-D, synthetic data, using a wavelet with realistic bandwidth. They are also applied to field data for one cable from a dual-sensor field survey. Both this inversion and the synthetic inversion study indicate that features ~ 0.5 km can be accurately recovered at depths of a few kilometres using maximum offsets of only 8 km.

True geometry tomography for velocity model building with applications to WATS seismic data

Tomography has been widely employed for velocity model building. The typical work flow starts with the migration of an initial velocity model. This is followed by picking residual moveouts and then updating the velocity through tomography. The migration process provides common image gathers and a stack. At the tomography stage, a ray tracer is used to trace specular rays from the image points to the surface to set up the system of linear equations for the tomographic inversion by linking valid ray pairs to their corresponding residuals. For narrow azimuth (NAZ) surveys, searching for valid ray pairs is usually limited to a narrow azimuth band. Since neither the gathers nor the stack contain acquisition geometry information, the selected specular ray pairs may not reflect the true ray paths, resulting in inaccurate rays being used in the inversion process. In addition to the problem of “which rays to choose”, we also have the problem of “how many rays to choose”. These problems are even more difficult to handle with acquisition configurations other than NAZ, such as the wide azimuth towed-streamer (WATS) surveys, multi-azimuth (MAZ) surveys, and ocean bottom cable (OBC) surveys. To overcome these problems, we propose a tomography method that incorporates the acquisition geometry information and uses vector offsets to account for both offsets and azimuths. To address the ill-posed nature of the system of equations, we developed an anisotropic Laplacian regularization operator that allows different smoothing along different directions. We validate the method with tests on both synthetic and field data with a WATS geometry.

Estimation of quality factors from converted‐wave PS data

Quality factors for compressional and shear waves QP and QS measured in laboratory and in situ, have been related to rock type and fluid saturation. The ratio QP/QS, for instance, has been shown to discern water or brine from gas saturated sandstones. In theory, quality factors QP and QS can be estimated from multicomponent seismic data. A composite quality factor for the P to S or PS reflected mode, QPS, similar to the interpreted PS velocity VPS, can be estimated in theory from converted wave prestack data. Furthermore, the parameters needed to retrieve QPS correspond to the converted wave processing parameters VPS and γ= VP/VS. The converted-wave quality factor can be used as a lithology and fluid content indicator since this parameter depends on the VP/VS and QP/QS ratios.

Numerical modeling of seismic amplitudes for non-vertical fractured media

Reflectivity and finite-difference modeling are used to analyze the effect of fracture dip on reflection amplitudes in homogeneous and heterogeneous azimuthal anisotropic media. Analysis of the amplitudes suggests that fracture dip, along with fracture density and orientation, should be considered in azimuthal AVO analysis. Particularly, fracture angles measured with respect to the vertical axis that are greater than ±15° can have an important effect in the amplitudes. Lateral variations of fracture dip within the length of a shot gather have also an important effect on the seismic amplitudes.

Full wavefield inversion to estimate impact-source orientation from multicomponent land seismic data

Orientation of an impact source is estimated from multicomponent land data. The seismic wavefield produced by an arbitrarily oriented impact source is represented by the linear combination of the nine independent wavefield components (Greens functions) for a full vector source. Coefficients of this linear combination are expressed in terms of the parameters that define source orientation. The parameters are an azimuthal angle, a vertical angle, and source strength, which are the unknowns of the inverse problem. Once the Greens functions are computed, source parameters are estimated by full wavefield inversion using a grid search over the source parameters. This inverse procedure also provides information about the uniqueness and resolution of the solution. Information on the source orientation is dominated by the relative amplitudes between components, so the inversion can be performed even when the source-time wavelet is not well known. The approach is tested using a synthetic common-source gather. Res...