Pitfalls and limitations in seismic attribute interpretation of tectonic features

Seismic attributes are routinely used to accelerate and quantify the interpretation of tectonic features in 3D seismic data. Coherence (or variance) cubes delineate the edges of megablocks and faulted strata, curvature delineates folds and flexures, while spectral components delineate lateral changes in thickness and lithology. Seismic attributes are at their best in extracting subtle and easy to overlook features on high-quality seismic data. However, seismic attributes can also exacerbate otherwise subtle effects such as acquisition footprint and velocity pull-up/push-down, as well as small processing and velocity errors in seismic imaging. As a result, the chance that an interpreter will suffer a pitfall is inversely proportional to his or her experience. Interpreters with a history of making conventional maps from vertical seismic sections will have previously encountered problems associated with acquisition, processing, and imaging. Because they know that attributes are a direct measure of the seismic amplitude data, they are not surprised that such attributes “accurately” represent these familiar errors. Less experienced interpreters may encounter these errors for the first time. Regardless of their level of experience, all interpreters are faced with increasingly larger seismic data volumes in which seismic attributes become valuable tools that aid in mapping and communicating geologic features of interest to their colleagues. In terms of attributes, structural pitfalls fall into two general categories: false structures due to seismic noise and processing errors including velocity pull-up/push-down due to lateral variations in the overburden and errors made in attribute computation by not accounting for structural dip. We evaluate these errors using 3D data volumes and find areas where present-day attributes do not provide the images we want

Wave equation calculation of most energetic traveltimes and amplitudes for Kirchhoff prestack migration

This work was conceived during a visit by Kurt Marfurt to Seoul National University, sponsored by the Korean Ministry of Science andTechnology.This work was financially supported by the Brain Korea 21 Project of the Ministry of Education of Korea and the National Research Laboratory project of the Ministry of Science and Technology. The authors acknowledge the support of the Korea Institute of Science and Technology Information (KISTI) under the Grand Challenge Support Program and the use of the Supercomputing Center

Efficient calculation of a partial-derivative wavefield using reciprocity for seismic imaging and inversion

Linearized inversion of surface seismic data for a model of the earths subsurface requires estimating the sensitivity of the seismic response to perturbations in the earths subsurface. This sensitivity, or Jacobian, matrix is usually quite expensive to estimate for all but the simplest model parameterizations.We exploit the numerical structure of the finite-element method, modern sparse matrix technology, and source–receiver reciprocity to develop an algorithm that explicitly calculates the Jacobian matrix at only the cost of a forward model solution. Furthermore, we show that we can achieve improved subsurface images using only one inversion iteration through proper scaling of the image by a diagonal approximation of the Hessian matrix, as predicted by the classical Gauss-Newton method. Our method is applicable to the full suite of wave scattering problems amenable to finiteelement forward modeling.We demonstrate our method through some simple 2-D synthetic examples

Traveltime calculations from frequency-domain downward-continuation algorithms

We present a new, fast 3D traveltime calculation algorithm that employs existing frequency-domain waveequation downward-continuation software. By modifying such software to solve for a few complex (rather than real) frequencies, we are able to calculate not only the first arrival and the approximately most energetic traveltimes at each depth point but also their corresponding amplitudes.We compute traveltimes by either taking the logarithm of displacements obtained by the oneway wave equation at a frequency or calculating derivatives of displacements numerically. Amplitudes are estimated from absolute value of the displacement at a frequency. By using the one-way downgoing wave equation, we also circumvent generating traveltimes corresponding to near-surface upcoming head waves not often needed in migration.We compare the traveltimes computed by our algorithm with those obtained by picking the most energetic arrivals from finite-difference solutions of the one-way wave equation, and show that our traveltime calculation method yields traveltimes comparable to solutions of the one-way wave equation. We illustrate the accuracy of our traveltime algorithm by migrating the 2D IFP Marmousi and the 3D SEG/EAGE salt models.This work was financially supported by National Laboratory Project of Ministry of Science and Technology, Brain Korea 21 project of theKorea Ministry of Education, and grant No. R03- 2000-000-00003-0 from the Basic Research Program of the Korea Science & Engineering Foundation

Traveltime and amplitude calculations using the damped wave solution

Because of its computational efficiency, prestack Kirchhoff depth migration remains the method of choice for all but the most complicated geological depth structures. Further improvement in computational speed and amplitude estimation will allow us to use such technology more routinely and generate better images. To this end, we developed a new, accurate, and economical algorithm to calculate first-arrival traveltimes and amplitudes for an arbitrarily complex earth model. Our method is based on numerical solutions of the wave equation obtained by using well-established finite-difference or finite-element modeling algorithms in the Laplace domain, where a damping term is naturally incorporated in the wave equation. We show that solving the strongly damped wave equation is equivalent to solving the eikonal and transport equations simultaneously at a fixed reference frequency, which properly accounts for caustics and other problems encountered in ray theory. Using our algorithm, we can easily calculate first-arrival traveltimes for given models. We present numerical examples for 2-D acoustic models having irregular topography and complex geological structure using a finite-element modeling code.This work was financially supported by National Research Laboratory Project of the Korea Ministry of Science and Technology, Brain Korea 21 project of the Korea Ministry of Education, grant No. R05-2000-00003 from the Basic Research Program of the Korea Science&Engineering Foundation, and grant No. PM10300 from Korea Ocean Research & Development Institute

Traveltime and amplitude calculation using a perturbation approach

Accurate amplitudes and correct traveltimes are critical factors that govern the quality of prestack migration images. Because we never know the correct velocity initially, recomputing traveltimes and amplitudes of updated velocity models can dominate the iterative prestack migration procedure. Most tomographic velocity updating techniques require the calculation of the change of traveltime due to local changes in velocity. For such locally updated velocity models, perturbation techniques can be a significantly more economic way of calculating traveltimes and amplitudes than recalculating the entire solutions from scratch. In this paper, we implement an iterative Born perturbation theory applied to the damped wave equation algorithm. Our iterative Born perturbation algorithm yields stable solutions for models having velocity contrasts of 30% about the initial velocity estimate, which is significantly more economic than recalculating the entire solution.This work was financially supported by National Research Laboratory Project of the Korea Ministry of Science and Technology, Brain Korea 21 project of the Korea Ministry of Education, grant No. R05-2000-00003 from the Basic Research Program of the Korea Science&Engineering Foundation, and grant No. PM10300 from Korea Ocean Research & Development Institute

Multi-spectral Volumetric Curvature Adding Value to 3D Seismic Data Interpretation

Summary Volumetric attributes computed from 3D seismic data are powerful tools in the prediction of fractures and other stratigraphic features. Geologic structures often exhibit curvature of different wavelengths. Curvature images having different wavelengths provide different perspectives of the same geology. Tight (shortwavelength) curvature often delineates details within intense, highly localized fracture systems. Broad (long wavelength) curvature often enhances subtle flexures on the scale of 100-200 traces that are difficult to see in conventional seismic, but are often correlated to fracture zones that are below seismic resolution, as well as to collapse features and diagenetic alterations that result in broader bowls. Such multi-spectral volumetric estimates of curvature are very useful for seismic interpreters and we depict a number of examples demonstrating such applications.