Rishi Bansal

Wavelet distortion correction due to domain conversion

Converted-wave (PS) data, when converted to PP time, develop time- and location-varying compression of the seismic wavelet due to a variable subsurface γ (γ=Vp∕Vs) . The time-dependent compression distorts the wavelet in a seismic trace. The lack of a consistent seismic wavelet in a domain-converted PS volume can eventually lead to an erroneous joint PP/PS inversion result. Depth-converted seismic data also have wavelet distortion due to velocity-dependent wavelet stretch. A high value of seismic velocity produces more stretch in a seismic wavelet than a low value. Variable wavelet stretch renders the depth data unsuitable for attribute analysis. A filtering scheme is proposed that corrects for distortion in seismic wavelets due to domain conversions (PS to PP time and time-to-depth) of seismic data in an amplitude-preserving manner. The method uses a Fourier scaling theorem to predict the seismic wavelet in the converted domain and calculates a shaping filter for each time/depth sample that corrects for ...

Correction for NMO stretch and differential attenuation in converted-wave data: A key enabling technology for prestack joint inversion of PP and PS data

In many parts of the world, P- or S-wave impedance (I P or I S or seismic velocities ( V P or V S ) are not good indicators of lithology. In these cases, estimation of density or V P / V S is essential for reservoir characterization. Conventional inversion of PP-mode data is known to yield I P and I S , but a reliable estimate of density requires prestack inversion of very large-angle PP data (Roy et al., 2008). Such large-angle PP data sets necessitate acquisition and true-amplitude processing of unusually large offsets.

Joint PP-PS angle-stack analysis and AVA inversion in Grane Field, offshore Norway

Eleven years ago, publication of the UK North Sea Alba Field OBC converted-wave (PS) image examples by MacLeod et al. caused quite a stir. The authors provided convincing evidence that “the Alba survey is the first 3D OBC survey in which successful converted-wave imaging of the reservoir has created significant economic benefit.” Most of us with an interest in multicomponent seismology know it well, as Alba has become one of the classic examples of a “stealth” reservoir (Rape et al., 2005), having low P- but high S-impedance contrast. The Alba reservoir and others like it are difficult to recognize on conventional P-wave images, but they stand out quite nicely when viewed on their PS equivalents.

Diffraction velocity analysis by path‐integral seismic imaging

We propose a framework for diffraction imaging and velocity analysis that combines the methods of velocity continuation and path-integral imaging. Zero-offset diffractions are separated from reflection data and efficiently imaged with a suite of constant-velocity models using a spectral implementation of velocity continuation. An unweighted path-integral image is constructed by summing all of these constant-velocity images, which amplifies events that are stationary between images and attenuates those that change position. Diffraction apexes remain stationary and are thus amplified relative to diffraction flanks and migration artifacts, yielding an estimate of the migrated image. The path-integral image is spatially blurred relative to the ideal result, and the amount of spatial blurring is related to the range of velocities used to construct it. Therefore, we use the path-integral image as an intermediate result—with which we compare each of the velocity continuation test images using local similarity. At each t-x location, the test image with the highest similarity to the path-integral indicates a likely effective migration velocity. The local velocity estimates are combined to yield a heterogeneous effective-medium migration velocity model, and its corresponding diffraction image.

A novel approach to estimating near‐surface S‐wave velocity and converted‐wave receiver statics

Summary We present a workflow for estimating near-surface S-wave velocity and deri ving long- and short-wavelength receiver statics for converted-wave (PS) data. The estimated S-wave velocity can also be used for depth migration of convertedwave data. The novelty of our method lies in detecting refracted S-wave arrivals present in the data, and using a depth-dependent weighting function to combine the S-wave velocities inverted from Scholte-wave and turning-ray tomography, respectively. We note considerable improvement in the quality of time-migrated convertedwave data after applying the receiver statics solution that was derived using our method.

Pre‐conditioning angle stack input for joint PP‐PS inversion

We present a workflow for pre-conditioning convertedwave (PS) and P-wave (PP) angle stacks prior to joint inversion. The process includes the following steps in order to improve the resolution of PS data and the registration of PP and PS reflections: 1) bandwidth enhancement of the far-angle data via Qand waveletstretch compensation, 2) native-domain (PP and PS) AVA inversions or attribute calculations, and 3) interval VP/VS derived from the initial inversion passes is used to further refine data alignment prior to final joint inversion. Examples using data from current project work are shown.