Thomas A. Dickens

RTM angle gathers using Poynting vectors

Angle-domain common image gathers have been shown to provide several advantages for the analysis of migrated images. In this abstract we describe an efficient new method for creating angle gathers for reverse time migration (RTM). The method relies on the computation of the direction of energy flux (known in electromagnetics as the Poynting vector) in the space-time domain. Given the Poynting vectors for source and receiver wavefields, we may readily derive reflection angle and azimuth, and reflector dip and azimuth if desired. Unlike some previously considered angle-gather algorithms, the method depends only on local information at the reflection point, thereby avoiding the loss of resolution inherent in mixing quantities from different horizontal locations. In addition, we derive the weight function needed to produce correct relative amplitudes for these angle gathers.

Massively parallel electrical-conductivity imaging of hydrocarbons using the IBM Blue Gene/L supercomputer

Large-scale controlled source electromagnetic (CSEM) three-dimensional (3D) geophysical imaging is now receiving considerable attention for electrical conductivity mapping of potential offshore oil and gas reservoirs. To cope with the typically large computational requirements of the 3D CSEM imaging problem, our strategies exploit computational parallelism and optimized finite-difference meshing. We report on an imaging experiment, utilizing 32,768 tasks/processors on the IBM Watson Research Blue Gene/L (BG/L) supercomputer. Over a 24-hour period, we were able to image a large scale marine CSEM field data set that previously required over four months of computing time on distributed clusters utilizing 1024 tasks on an Infiniband fabric. The total initial data misfit could be decreased by 67 percent within 72 completed inversion iterations, indicating an electrically resistive region in the southern survey area below a depth of 1500 m below the seafloor. The major part of the residual misfit stems from transmitter parallel receiver components that have an offset from the transmitter sail line (broadside configuration). Modeling confirms that improved broadside data fits can be achieved by considering anisotropic electrical conductivities. While delivering a satisfactory gross scale image for the depths of interest, the experiment provides important evidence for the necessity of discriminating between horizontal and vertical conductivities for maximally consistent 3D CSEM inversions.

Integrated velocity model estimation for improved positioning with anisotropic PSDM

There are many geologic settings where anisotropic migration is necessary to obtain accurate seismic images. While this is well known, stable anisotropic parameter estimation has posed a serious challenge. Seismic data, though extensive in coverage, cannot constrain the anisotropy parameters alone (Tsvankin and Thomsen, 1995). The set of parameters is better constrained by integrating the seismic information with certain types of well data. However, the well data are generally sparse, so the parameters are only constrained at a few locations. Nonuniqueness is obviously a fundamental issue in our estimation problem.

Diffraction tomography for crosswell imaging of nearly layered media

Diffraction tomography is a high‐resolution imaging technique applicable to the mapping of formation velocities away from the borehole, achieving a spatial resolution of better than one acoustic wavelength when used to image synthetic model data. However, traditional filtered back‐propagation diffraction tomography algorithms are based on weak‐scattering and constant‐background velocity assumptions, which limits their applicability to models of realistic structural complexity. Results are obtained using a new, computationally efficient single‐mode (P‐wave) diffraction tomography algorithm that is applicable to models, including geologically realistic ones, whose strongest variations can be approximated as a set of horizontal layers. The algorithm starts with a layered model of the subsurface velocity structure, which may be constructed from well logs or by using traveltime tomography. Application of layered diffraction tomography updates this model to reveal 2-D structures such as faults, pinch‐outs, and ...

Inversion study of a large marine CSEM survey

The Brazil RC Marine CSEM survey was collected in April of 2004 for the ExxonMobil Remote Reservoir Resistivity Mapping (RM) Project. The portion of the survey reported here consisted of a total of 735 km of transmitter towlines arranged approximately on a 5 km x 5 km rectangular grid (see Figure 1). Vertical and horizontal electric field measurements were recovered at a total of 23 seafloor locations from a deployment of 36 seafloor instruments. Imaging of these CSEM data into full three-dimensional conductivity volumes represents a formidable challenge due to the subtle effects of reservoir targets, the volume of data and its large dynamic range. In this presentation, we report on an initial round of inversion results obtained using both isotropic and anisotropic (VTI) imaging methods. Our results support the need for an anisotropic model to accurately represent subsurface resistivity.

Stack‐and‐Denoise: A new method to stack seismic datasets

Stacking combines a collection of noisy seismic gathers, such as NMO-corrected CMP gathers or migrated gathers, into a single less noisy seismic section. Existing techniques stack each gather independently, and in the process, ignore the tremendous structure of seismic signals. We propose a novel technique called Stack-andDenoise (SAD) that exploits the structure of seismic signals to obtain an enhanced stack. SAD comprises the following steps: (1) Estimate the signal-to-noise ratios (SNRs) for each trace in the gather using a new iterative algorithm called Leave-Me-Out; (2) Use the SNRs to perform a weighted stack of the noisy gathers (3) Attenuate the residual noise in the weighted stack using denoising in the wavelet, Fourier, or curvelet transform domain, for example. Though simple, SAD enjoys some desirable optimality properties. During synthetic and real data experiments, SAD significantly attenuated both multiples and random noise in the stacked estimate.

Spatial resolution of diffraction tomography

Diffraction tomography is an imaging technique applicable to crosshole seismic data and aimed at achieving optimal spatial resolution away from the borehole. In principle the method can form acoustic images equivalent to extending acoustic well logs away from the wellbore and into the formation with a spatial resolution less than one wavelength of the radiation employed to gather the crosshole data. This paper reports on the capability of diffraction tomography to produce high-resolution reconstructions of simple targets from limited-view-angle data. The goal is to quantify the resolution and velocity-reconstruction capability of diffraction tomography with realistic source–receiver geometries. Simple targets (disks and low-contrast sequences of layers) are used for this study. The scattering from these targets can be calculated without approximation, making them ideal test cases for the algorithm. The resolution capability of diffraction tomography is determined to be on the order of one wavelength for s...

Ray tracing in tilted transversely isotropic media: a group velocity approach

Efficient computation of traveltimes in anisotropic media has become increasingly important given the wide use of anisotropic Kirchhoff prestack depth migration. Several methods have been presented for computing traveltime maps yielding first-arrival times for anisotropic media, based on finite-difference eikonal solutions or the direct-mapping Fermat’s principle (FP) approach. While the first-arrival techniques are useful for computing traveltime maps in areas where multipathing is not important, e. g. in sedimentary basins, or for performing velocity updates in such areas, accurate migrations in complex areas require ray-trace solutions. This abstract presents an approach to qP-wave ray tracing in tilted transversely isotropic (TTI) media that is based upon the same group velocity approximation used in the Fermat’s principle algorithms. The method is simple to implement and accurate for the range of anisotropy values encountered in practice. Its use of a slowness parameterization identical to that of the FP method implies that it can be employed to calculate traveltime maps in areas where the faster FP technique has been used to perform velocity analysis; the two techniques provide consistent traveltimes, independent of anisotropy strength.

Effects of Uncertainty In Rock-physics Models On Reservoir Parameter Estimation Using Marine Seismic AVA And CSEM Data

Summary This study investigates the effects of uncertainty in rockphysics models on estimates of reservoir parameters from joint inversion of seismic AVA and CSEM data. The reservoir parameters are related to electrical resistivi ty using Archie’s law, and to seismic velocity and density u sing the Xu-White model. To account for errors in the rock-p hysics models, we use two methods to handle uncertainty: ( 1) the model outputs are random functions with modes or means given by the model predictions, and (2) the paramet ers of the models are themselves random variables. Using a sto chastic framework and Markov Chain Monte Carlo methods, we obtain estimates of reservoir parameters as well as of the uncertainty in the estimates. Synthetic case studie s show that uncertainties in both rock-physics models and their associated parameters can have significant effects on estimates of reservoir parameters. Our method provi des a means of quantifying how the uncertainty in the est imated reservoir parameters increases with increasing unce rtainty in the rock-physics model and in the model parameters. We find that in the example we present, the estimation of w ater saturation is relatively less affected than is the estimation of clay content and porosity.

Incorporating non‐seismic information for improved positioning with anisotropic PSDM

Theoretical and numerical studies have shown that surface seismic data alone do not adequately constrain VTI anisotropy parameter estimation (Tsvankin and Thomsen, 1995). The integration of sonic log, vertical checkshot, offset checkshot (OCS), and horizon-based constraints into our migration velocity analysis procedure allows us to develop models for the VTI parameters V0, e, and δ (Thompson, 1985) that are consistent with geology, measured traveltimes, and moveout of the seismic data. We show anisotropic PSDM (APSDM) examples from three marine areas, each of which exhibits marked P-wave anisotropy. Our velocity analysis procedure leads to models that are physically reasonable, and in each case we observe improved positioning accuracy compared to isotropic PSDM.