Alexander Droujinine

Research Note: Sparsity-Aware Multiple Microseismic Event Localization Blind to the Source Time-Function

We consider the problem of simultaneously estimating three parameters of multiple microseimic events, i.e., the hypocenter, moment tensor, and origin time. This problem is of great interest because its solution could provide a better understanding of reservoir behavior and can help to optimize the hydraulic fracturing process. The existing approaches employing spatial source sparsity have advantages over traditional full-wave inversion-based schemes; however, their validity and accuracy depend on the knowledge of the source time-function, which is lacking in practical applications. Thisbecomes evenmorechallenging whenmultiplemicroseimicsourcesappear simultaneously. To cope with this shortcoming, we propose to approach the problem from a frequency-domain perspective and develop a novel sparsity-aware framework that is blind to the source time-function. Through our simulation results with synthetic data, we illustrate that our proposed approach can handle multiple microseismic sources and can estimate their hypocenters with an acceptable accuracy. The results also show that our approach can estimate the normalized amplitude of the moment tensors as a by-product, which can provide worthwhile information about the nature of the sources.

Regularized inversion of 3D full tensor gradient (FTG) data for dynamic reservoir monitoring

Summary Three-Dimensional Full Tensor Gradiometry (3D FTG) acquires ultra-sensitive measurements of the Earth’s (vector) gravity gradient field. Departures from simple weakening of the field in the vertical direction are due to subsurface variations in density. We have undertaken a numerical examination of the feasibility of using this system for detecting lateral density contrasts in subsurface layers during reservoir monitoring. Our gravity modeling focuses on the additional value added by taking account of the horizontal components of gravity gradient in imaging local targets. A regularized inversion algorithm that can describe and predict the dynamic behavior of a hydrocarbon reservoir has been developed and tested on synthetic FTG data based on realistic petrophysical models. Our approach also yields estimates of uncertainty in hydrocarbon production data. Results show that the technique is particularly useful for direct monitoring of gas-oil contact or temperature front expansion during CO2 injection in heavy-oil reservoirs at shallow or moderate depths.

WAZ wavefield de‐remigration illumination analysis and sub‐salt model building

Essential for wide-azimuth (WAZ) migration is to define a cost-effective acquisition and to apply adequate prestack migration for imaging a sub-salt target. To address this, we have developed and tested a special WAZ quality control (QC) depth processing sequence that has been incorporated into the sub-salt prestack depth migration (PSDM) model building workflow. The crucial point is the cost-effective wavefield de-remigration. It involves modeling synthetic data over a reference model and migrating the synthetic data over the reference or perturbed model. This yields the full de-remigrated volume of common image gathers as well as stacked amplitudes (subsurface fold) rendered along key horizons. To detect spurious residual moveout (RMO) picks associated with migration artifacts and picking errors, we apply the RMO QC analysis by thresholding stacked de-remigrated amplitudes at pick locations. Our studies show that this analysis is vital for successful tomographic velocity updating.

Event‐consistent volume dip‐azimuth‐curvature estimation with complex‐trace beamforming

A robust algorithm for the simultaneous estimation of dip, azimuth and curvature in the depth migrated domain is developed. The algorithm makes use of instantaneousattribute beamforming in small windows aligned with locally coherent migrated events. This is the following two-step procedure: inline and cross-line linear beam stacks to compute the two local dips at image points; 360-degree azimuthal parabolic beamforming to estimate azimuthally dependent curvatures with respect to the plane tangent to the instantaneous-phase surface at the image point. Amplitude and phase content of migrated traces are processed independently using complex trace analysis in order to facilitate this procedure. The instantaneous phase is of central importance since it describes the lateral continuity of events and defines instantaneous dips corresponding to instantaneous apparent phase velocity and group velocity. This attribute is used to compute the instantaneous frequency that defines the beam wavelet at the peak of waveform envelope. The instantaneous amplitude or envelope represents an input to model-based wavefield (Born-Kirchhoff) de-remigration of picked events, useful for dip QC. The QC analysis involves thresholding of modeled migrated amplitudes predicted by Kirchhoff remigration of the Born demigrated wavefield. This also includes simple thresholding of the instantaneous frequency. The analysis is shown to be capable of eliminating undesired events. The algorithm was tested using a 3-D marine streamer dataset with high noise. The success with field migrated data demonstrates that the algorithm is particularly useful for slope-limited tomographic velocity updates and structural interpretation when the signal-to-noise ratio (SNR) is low.

A study of sub-basalt depth imaging using the local radon attributes on the Erlend Tertiary volcanic complex : north of Shetland, UK

I Acquiring conventional 3 km towed streamer data along a 2D profile in the North of Shetland (UK) enables us to use the local Radon-attributes within the context of depth processing methodology for accurate delineation of volcanic units and imaging beneath high-velocity layers. The objective is to map the radially-dipping structure of the Erlend pluton and to investigate the potential existence of relatively soft Cretaceous sediments underneath volcanic units. Success in the Erlend Volcano study requires strict attention to the separation between different groups of events. The crucial point is the generalized discrete Radon transform formulated in terms of local wavefront (dip and curvature) characteristics. This transform is utilized during pre-CMP processing and migration to minimize event-coupling artefacts. These artefacts represent cross-talk energy between various wave modes and include the unwanted part of the wavefield. We show how to produce detailed subsurface images within the region of interest (exploration prospect only) by applying the closely tied processes of prestack event enhancement and separation, well-driven time processing for velocity model building, and final event-based prestack depth imaging. Results show enhanced structural detail and good continuity of principal volcanic units and deeper reflections, suggesting a faulted 0.6 0.9 km thick layer of Cretaceous sediments in the proximity of well 209/09-1. Our interpretation complements existing low-resolution geophysical models inferred from gravity and wide-angle seismic data alone.

Velocity Updating with Wavefield-Based Residual Moveout QC

P052 Velocity Updating with Wavefield-Based Residual Moveout QC A.B. Droujinine* (Shell International Exploration & Production BV) F. Kuiper (Shell International Exploration & Production BV) P. Milcik (Shell International Exploration & Production BV) & D. van Daalen (Shell International Exploration & Production BV) SUMMARY We have developed an inexpensive wavefield-based technique for automatic QC analysis of residual moveout (RMO) on common image gathers and dip attributes within the context of tomographic velocity updating. The crucial step is the unified Born-Kirchhoff demigration of RMO picks followed by thresholding of migrated amplitudes as predicted by Kirchhoff migration of the demigrated wavefield. Local coherency

Full-wave long offset seismic imaging and velocity analysis with gravity and well constraints - a case study from NW Corrib, offshore Ireland

Summary Long offset data from a recent marine streamer survey acquired in the Slyne Basin (NW Corrib, offshore Ireland) were reprocessed using advanced data pre-processing (source deconvolution, complex multiple attenuation and Radon-based wavefield separation), anisotropic velocity analysis and prestack time/depth full-wave imaging algorithms. Results of velocity analysis were constrained by borehole and gravity data to assist in the validation of interpretations. The focusing capabilities of symmetric wave modes have been analyzed and compared. Methods were incorporated into a single workflow tailored to identify key horizons below the top-basalt boundary.