A-team Seminar - Fall, 2009

Date

Speaker

Subject

Notes

9/10

Xiaoxiang

How to compute the Frechet matrix in TTI tomography

One critical step in gridded TTI tomography is to compute the partial derivative of traveltime with respect to a medium parameter defined on a grid vertex. I will show the computation based on ray tracing in the initial model.

9/17

Mamoru

Understanding and correction of lateral heterogeneity effects on nonhyperbolic moveout analysis for VTI layers

Lateral heterogeneity causes substantial, laterally varying errors of the NMO velocity and eta in the nonhyperbolic inversion. To correct the lens-induced distortions, we propose a correction approach based on computing time shifts as functions of the location, offset and target depth. The prestack data and nonhyperbolic parameters after the correction represent background values of traveltime, Vnmo and eta.

9/24

Alex

Inversion of converted-wave data for anisotropic parameter estimation and reservoir characterization at Rulison Field, Colorado

The first part of the talk will focus on the PS-wave traveltime equations for anisotropic parameter estimation in Rulison Field, Colorado. The upper-part of the reservoir will be modelled as a VTI over HTI media and the strong UMV Shale reflector will be analysed. The influence of the Thomsen parameters on the P- and PS-wave reflection coefficients will be studied. The P- and PS-wave amplitudes will finally be inverted in order to delineate the gas-saturated sand-bodies. Vp/Vs volumes will be generated in order to characterize the lithology and the overpressure intervals. The effects of shear-wave splitting on PS-amplitudes will helps us determine the fractured intervals.

10/1

Praj

3D Shear Wave VSP Depth Imaging of Fractured Sandstones in Rulison Field, Colorado

Reservoir rocks in the gas producing interval of Rulison Field, Colorado comprises of tight fractured sandstones that evade detection by P-waves. These sandstones, however, have better shear impedance contrast with the encasing shale. In the current study I use an uncommon, yet very effective procedure that uses non-converted shear waves recorded in a borehole in a 3D VSP survey to image the reservoir interval and beneath. The 3D depth imaging results tie with the well logs and with the existing surface seismic data. The recovered images not only provide a significant uplift in terms of sandstone detection and structure delineation but a quick-look interpretation of images also indicates anisotropy resulting from fractures.

10/8

Round table discussion

10/15

Mamoru

Analytical expression of lateral heterogeneity effects in nonhyperbolic moveout parameters for a VTI layer

Overburden lateral heterogeneity causes substantial, laterally varying errors in the nonhyperbolic parameter inversion.

Here, I present the analytical expression about the relationship between lateral heterogeneity and nonhyperbolic parameter distortions. The result shows that the magnitude of the nonhyperbolic parameter distortions depends strongly on depth of the laterally heterogeneous layer, in addition to the 2^nd and 4^th lateral derivatives of velocity (or time).

One advantage of this analytical expression is that we can consider the possible parameter errors just by using seismic image and nonhyperbolic parameters, rather than using prestack seismic data.

10/22

Jim Gaiser

(ION Geophysical Corp)

Elastic-wavefield Interferometry Using P-wave Source VSPs: Wamsutter Field, Wyoming

In this study we investigate methods of elastic-wavefield interferometry to obtain P- and S-wave velocity information from the 3D P-wave vibrator VSPdata at Wamsutter field in Wyoming. Wamsutter field consists of naturally fractured tight gas sand reservoirs where geophysical objectives are lithology discrimination and fracture identification. We examine P-wave pseudo-source VSP data to eliminate strong tube-wave noise generated by sources near the borehole, and converted P- to S-wave (C-wave) pseudo-source VSPs for S-wave splitting analysis. However, data at the stationary-phase point is not conducive to conventional interferometry due to tube-wave noise and C-wave polarity reversals.

Removing physical sources at the stationary-phase point, perturbing the integration path of the Green's function and applying a radiation condition effectively recovers P- and C-wave pseudo-source VSPs, reduces tube-wave noise and separates wavefields. The retrieved wavefields correspond to vertical incidence and can be used for conventional P- and S-wave velocity analyses. Four-component C-wave VSPs retrieve S-wave splitting in transmitted and reflected converted waves that provide calibration for C-wave azimuthal anisotropy measurements from surface-seismic data.

10/29

No seminar (SEG)

11/5

Yongxia

Gaussian beam modeling for SV-wave in TI media

In certain anisotropic media, SV waves have cuspoidal wavefront. The ray tracing method can not provide a good approximation for the cusps because the ray amplitude at caustic points goes to infinite. Here, the Gaussian beam method is applied to handle the caustics. We talk about the factors for the Gaussian beam summation. The Gaussian beam summation provides a good approximation for the wavefield if we use proper parameters including the beam width, the beam interval and the range of the beam for summation.

11/12

Subhashis Mallic (Wyoming Univ)

Full-waveform inversion of reflection data

11/19

Bharath

Estimation of shear wave interval traveltimes

We have used velocity independent layer stripping together with the "PP + PS" method to estimate shear wave interval traveltimes using pure P and mode converted (PS) data. The interval traveltimes will be the input for the problem of the estimation of interval anisotropic attenuation parameters.

11/26

No seminar (Thanksgiving)

12/3

Steve

12/10

Lorenzo