Jeffrey Shragge

Image segmentation for tracking salt boundaries

SUMMARY Image segmentation can be used to track salt boundaries when the salt boundary amplitude is greater than any other local reections. We apply a modied version of the normalized cut image segmentation method to partition seismic images along salt boundaries. In principle our method should work even when the boundaries are not continuous, and conventional horizon tracking algorithms may fail. Our implementation of this method calculates a weight connecting each pixel in the image to each pixel in a local neighborhood. The weight is made weak where the negative amplitude of the complex trace along the shortest path between the two pixels has a minimum and is less than a threshold value. This method is demonstrated to be effective on synthetic 2D seismic sections and can easily be modied to be applied to 3D data. To overcome the formidable computational expense and storage requirements, three cost saving approaches are proposed. Firstly, pixels are sampled from windows centered at powers of 2, this greatly increases the sparseness of the weight matrix. Secondly, initial solutions are provided to subsequent segmentations for multiple segmentation passes in iterative velocity analysis. Thirdly, an iterative multiscale approach should allow the tracking of the bright salt events in large 3D cubes.

Riemannian wavefield extrapolation: Nonorthogonal coordinate systems

Riemannian wavefield extrapolation (RWE) is used to model one-way wave propagation on generalized coordinate meshes. Previous RWE implementations assume that coordinate systems are defined by either orthogonal or semiorthogonal geometry. This restriction leads to situations where coordinate meshes suffer from problematic bunching and singularities. Nonorthogonal RWE is a procedure that avoids many of these problems by posing wavefield extrapolation on smooth, but generally nonorthogonal and singularity-free, coordinate meshes. The resulting extrapolation operators include additional terms that describe nonorthogonal propagation. These extra degrees of complexity, however, are offset by smoother coefficients that are more accurately implemented in one-way extrapolation operators. Remaining coordinate mesh singularities are then eliminated using a differential mesh smoothing procedure. Analytic extrapolation examples and the numerical calculation of 2D and 3D Green’s functions for cylindrical and near-spher...

Prestack wave-equation depth migration in elliptical coordinates

We extend Riemannian wavefield extrapolation (RWE) to prestack migration using 2D elliptical-coordinate systems. The corresponding 2D elliptical extrapolation wavenumber introduces only an isotropic slowness model stretch to the single-square-root operator. This enables the use of existing Cartesian finite-difference extrapolators for propagating wavefields on elliptical meshes. A poststack migration example illustrates advantages of elliptical coordinates for imaging turning waves. A 2D imaging test using a velocity-benchmark data set demonstrates that the RWE prestack migration algorithm generates high-quality prestack migration images that are more accurate than those generated by Cartesian operators of the equivalent accuracy. Even in situations in which RWE geometries are used, a high-order implementation of the one-way extrapolator operator is required for accurate propagation and imaging. Elliptical-cylindrical and oblate-spheroidal geometries are potential extensions of the analytical approach to 3D RWE-coordinate systems.

Nonconfocal all-optical laser-ultrasound and photoacoustic imaging system for angle-dependent deep tissue imaging

Abstract. Biomedical imaging systems incorporating both photoacoustic (PA) and ultrasound capabilities are of interest for obtaining optical and acoustic properties deep in tissue. While most dual-modality systems utilize piezoelectric transducers, all-optical systems can obtain broadband high-resolution data with hands-free operation. Previously described reflection-mode all-optical laser-ultrasound (LUS) systems use a confocal source and detector; however, angle-dependent raypaths are lost in this configuration. As a result, the overall imaging aperture is reduced, which becomes increasingly problematic with depth. We present a reflection-mode nonconfocal LUS and PA imaging system that uses signals recorded on all-optical hardware to create angle-dependent images. We use reverse-time migration and time reversal to reconstruct the LUS and PA images. We demonstrate this methodology with both a numerical model and tissue phantom experiment to image a steep-curvature vessel with a limited aperture 2-cm beneath the surface. Nonconfocal imaging demonstrates improved focusing by 30% and 15% compared to images acquired with a single LUS source in the numerical and experimental LUS images, respectively. The appearance of artifacts is also reduced. Complementary PA images are straightforward to acquire with the nonconfocal system by tuning the source wavelength and can be further developed for quantitative multiview PA imaging.

Illumination Compensation for Image-domain Wavefield Tomography

Wavefield tomography and waveform inversion are related techniques that share the need to simulate accurate wavefields in the subsurface. In both cases, models are updated iteratively using gradients computed by, for example, the adjoint state method. The major difference between these techniques is the domain in which one formulates the objective function that compares the observed and simulated wavefields. For image-domain wavefield tomography, the objective function is defined based on a residual evaluated using migrated images. The role of the penalty function is to highlight image defocusing caused by imperfect velocity. This is accomplished by defining an operator which annihilates an image corresponding to the correct velocity. Conventional techniques, like differential semblance optimization, assume that correctly migrated images are completely focused, which is not the case when illumination is poor. In this paper, we address this problem by defining an alternative penalty operator which takes illumination into consideration and leads to more robust and accurate inversion results. This technique is particularly relevant for imaging in complex areas, e.g. sub-salt.

Extended imaging conditions for passive seismic data

Seismic monitoring at injection sites (e.g., CO2 sequestration, hydraulic fracturing) has become an increasingly common tool amongst oil and gas producers. The information obtained from these data is often limited to seismic event properties (e.g., location, initiation time, moment tensor), the accuracy of which greatly depends on the assumed or estimated elastic velocity models. However, estimating accurate 3D velocity models from passive array data remains a challenging problem. Extended imaging conditions (eICs) for passive wave-equation imaging algorithms represent a key step towards generating - and verifying - elastic velocity models. By extending imaging conditions away from zero-lag in time and space we can better evaluate the focusing of a given event based on the principle that waves focus at zero lag only when the velocity models are “correct”. We demonstrate that given an elastic medium and multi-component recordings, we can propagate and correlate microseismic P- and S-wavefield modes to compute eICs for P- and S- velocity perturbations. We observe that the maximum correlation deviates from the zero-lag in time and space for a P/S cross-correlation imaging condition when using an incorrect P- and/or S-wave velocity, and thus there is sensitivity to velocity error not observable when using individual wavefield components.

Student-based archaeological geophysics in northern Thailand

Summary As part of the 2010 near-surface geophysics workshop in Chiang Mai, Thailand, local archaeological targets were used as a basis for teaching geophysical data collection, processing, and interpretation techniques. By addressing local issues and interests, the workshop was able to demonstrate to participants and the local community how near-surface geophysics can be applied using simple survey methods and low-cost processing techniques.

Estimation of reservoir fluid saturation from 4D seismic data: effects of noise on seismic amplitude and impedance attributes

Time-lapse (4D) seismic data sets have proven to be extremely useful for reservoir monitoring. Seismic-derived impedance estimates are commonly used as a 4D attribute to constrain updates to reservoir fluid flow models. However, 4D seismic estimates of P-wave impedance can contain significant errors associated with the effects of seismic noise and the inherent instability of inverse methods. These errors may compromise the geological accuracy of the reservoir model leading to incorrect reservoir model property updates and incorrect reservoir fluid flow predictions. To evaluate such errors and uncertainties we study two time-lapse scenarios based on 1D and 3D reservoir model examples, thereby exploring a number of inverse theory concepts associated with the instability and error of coloured inversion operators and their dependence on seismic noise levels. In the 1D example, we show that inverted band-limited impedance changes have a smaller root-mean-square (RMS) error in comparison to their absolute broadband counterpart for signal-to-noise ratios 10 and 5 while for signal-to-noise ratio (S/N) = 3 both inversion methods present similarly high errors. In the 3D example we use an oilfield benchmark case based on the Namorado Field in Campos Basin, Brazil. We introduce a histogram similarity measure to quantify the impact of seismic noise on maps of 4D seismic amplitude and impedance changes as a function of S/N levels, which indicate that amplitudes are less sensitive to 4D seismic noise than impedances. The RMS errors in the estimates of water saturation changes derived from 4D seismic amplitudes are also smaller than for 4D seismic impedances, over a wide range of typical seismic noise levels. These results quantitatively demonstrate that seismic amplitudes can be more accurate and robust than seismic impedances for quantifying water saturation changes with 4D seismic data, and emphasize that seismic amplitudes may be more reliable to update fluid flow model properties in the presence of realistic 4D seismic noise.

Geophysical remote sensing of a historical aboriginal gravesite in Quairading, Western Australia

Burial sites have extreme cultural significance to societies around the world. Until recently, insufficient recognition of Aboriginal heritage in Australia has led to a very poor understanding and documentation of many culturally significant locations, including burial sites. In some cases, sites have been preserved through the efforts of local people; however, others were subsequently redeveloped or even completely destroyed. Local Aboriginal people are usually the best source of information regarding these locations and can identify broad regions with historical significance, but seldom do they provide precise details about individual grave locations. There are still many Aboriginal gravesites throughout Australia where the exact burial locations are unknown. Locating gravesites - and doing so in a way that minimises site disturbance - is paramount to any investigation and preservation program. For efficient investigation of large areas, geophysical remote sensing provides practical and non-invasive tools for investigation of large poorly documented burial areas. The UWA Society of Exploration Geophysicists Student Chapter, in conjunction with the South West Aboriginal Land and Sea Council, acquired several near-surface geophysical surveys over a known aboriginal burial site near Quairading, Western Australia. Multiple techniques were used to delineate possible grave locations, including ground penetrating radar (GPR), magnetics and conductivity. While work is ongoing with the data processing and integration, and future surveys are planned, early indications show anomalies that may be related to burial locations.

A Major Geophysical Experiment in the Capricorn Orogeny, Western Australia

A major geophysical experiment has begun in the Capricorn Orogen in Western Australia. Orogen-scale passive seismic and magnetotelluric surveys are on-going and preliminary results suggest have successfully delineated the base of the crust and major structures and tectonic boundaries. Airborne electromagnetic data have successfully mapped features in the near-surface such as palaeovalleys. The integration of the different geophysical datasets with each other and with parallel geological studies are intended to lead to a better understanding the Capricorn Orogen and develop exploration approaches and appropriate toolkits that significantly improve our ability to prospect under cover.