Over the past 10 years, ray-based postmigration grid tomography has become the standard model-building tool for seismic depth imaging. While the basics of the method have remained unchanged since the late 1990s, the problems it solves have changed dramatically. This evolution has been driven by exploration demands and enabled by computer power. There are three main areas of change. First, standard model resolution has increased from a few thousand meters to a few hundred meters. This order of magnitude improvement may be attributed to both high-quality, complex residual-moveout data picked as densely as 25 m to 50 m vertically and horizontally, and to a strategy of working down from long-wavelength to short-wavelength solutions. Second, more and more seismic data sets are being acquired along multiple azimuths, for improved illumination and multiple suppression. High-resolution velocity tomography must solve for all azimuths simultaneously, to prevent short-wavelength velocity heterogeneity from being mistaken for azimuthal anisotropy. Third, there has been a shift from predominantly isotropic to predominantly anisotropic models, both VTI and TTI. With four-component data, anisotropic grid tomography can be used to build models that tie PZ and PS images in depth.

A decade of tomography

The present invention provides a system for drilling boreholes having a downhole subassembly which contains an acoustic measurement-while-drilling system It uses the acoustic velocity through the formations surrounding the borehole and an acoustic transmitter and a set of receivers for determining the bed boundaries surrounding the borehole formation An acoustic isolator on the tool may be used to attenuate body waves traveling between the transmitter and the receivers Additional attenuation of the body waves is provided by a threshold filter based upon the absolute maximum of the received signals Semblance of the data is determined in a slowness/intercept-time domain Coherence and semblance filtering methods are used to differentiate between reflection signals and noise The position and orientation of the bed boundary relative to the tool are determined A further processing step uses the relative position and orientation determined for a number of tool positions to further discriminate against noise and obtain an absolute position and depth of the bed boundaries

Semblance processing for an acoustic measurement-while-drilling system for imaging of formation boundaries

1A predrill estimate of pore pressure can be obtained from seismic velocities using a velocity‐to–pore‐pressure transform, but the seismic velocities need to be derived using methods having sufficient resolution for well planning purposes. For a deepwater Gulf of Mexico example, significant differences are found between the velocity field obtained using reflection tomography and that obtained using a conventional method based on the Dix equation. These lead to significant differences in the predicted pore pressure. Parameters in the velocity‐to–pore‐pressure transform are estimated using seismic interval velocities and pressure data from nearby calibration wells. The uncertainty in the pore pressure prediction is analyzed by examining the spread in the predicted pore pressure obtained using parameter combinations which sample the region of parameter space consistent with the available well data. If calibration wells are not available, the ideas proposed in this paper can be used with measurements made whi...

Predrill pore-pressure prediction using seismic data

Predrill pore-pressure prediction using seismic dataSeismic Pore-Pressure Prediction

A method, a map and an article of manufacture for the exploration of hydrocarbons. In one embodiment of the invention, the method comprises the steps of: accessing 3D seismic data; dividing the data into an array of relativey small three-dimensional cells; determining in each cell the semblance/similarity, the dip and dip azimuth of the seismic traces contained therein; and displaying dip, dip azimuth and the semblance/similarity of each cell in the form of a two-dimensional map. In one embodiment, semblance/similarity is a function of time, the number of seismic traces within the cell, and the apparent dip and apparent dip azimuth of the traces within the cell; the semblance/similarity of a cell is determined by making a plurality of measurements of the semblance/similarity of the traces within the cell and selecting the largest of the measurements. In addition, the apparent dip and apparent dip azimuth, corresponding to the largest measurement of semblance/similarity in the cell, are deemed to be estimates of the true dip and true dip azimuth of the traces therein. A color map, characterized by hue, saturation and lightness, is used to depict semblance/similarity, true dip azimuth and true dip of each cell; true dip azimuth is mapped onto the hue scale, true dip is mapped onto the saturation scale, and the largest measurement of semblance/similarity is mapped onto the lightness scale of the color map.

Method and apparatus for seismic signal processing and exploration

In seismic exploration, seismic reflections from subsurface formations are recorded in the form of a time-distance array of seismic traces. This array is transformed into a frequency-distance domain. Frequencies at which coherent noise resides within the frequency-distance domain are determined. Wavenumbers corresponding to such frequenices are removed by filtering. The wavenumber-filtered, frequency-distance domain of seismic traces are then transformed back into a time-distance array of seismic traces.

Method for removing coherent noise from seismic data through f-x filtering

In the past few years, significant progress has been made on new velocity analysis algorithms. In the first part of this paper, we will briefly summarize recent advances on velocity analysis. Then we describe a new model-based globally-optimized residual curvature analysis algorithm we have just developed. Like conventional residual curvature analysis, the algorithm is based on the principle that after prestack migration with a correct velocity model, an image in the common image point (CIP) gather is aligned horizontally regardless of structure. Unlike conventional residual curvature analysis, this algorithm uses not only the interpreted CIP gathers, but also the interpreted migrated depth section as input. The algorithm is model-based, and uses modelbased CIP ray tracing to relate residual moveouts in CIP gathers to errors in the velocity model. Residual moveouts measured in CIP gathers are globally used in the optimization process for updating the whole velocity model. Also model-based normal incident ray tracing is used for updating the reflector boundaries.

Macro Velocity Model Estimation Through Model-based Globally-optimized Residual-curvature Analysis

Primary reflections in seismic records are often obscured by coherent noise making processing and interpretation difficult. Trapped water modes, surface waves, scattered waves, air waves, and tube waves to name a few, must be removed early in the processing sequence to optimize subsequent processing and imaging. The authors have developed a noise canceling algorithm that effectively removes many of the commonly encountered noise trains in seismic data. All currently available techniques for coherent noise attenuation suffer from limitations that introduce unacceptable signal distortion sand artifacts. Also, most of those techniques impose the dual stringent requirements of equal and fine spatial sampling in the field acquisition of seismic data. Their technique takes advantage of characteristics usually found in coherent noise such as being localized in time, high aliased, nondispersive (or only mildly so), and exhibit a variety of moveout patterns across the seismic records. When coherent noise is localized in time, a window much like a surgical mute is drawn around the noise. The algorithm derives an estimate of the noise in the window, automatically correcting for amplitude and phase differences, and adaptively subtracts this noise from the window of data. This signal estimate is then placed back in the record.more » In a model and a land data example, the algorithm removes noise more effectively with less signal distortion than does f-k filtering or velocity notch filtering. Downgoing energy in a vertical seismic profile (VSP) with irregular receiver spacing is also removed.« less

A procedure for optimally removing localized coherent noise

Method for removing coherent noise from a series of seismic traces. Each seismic trace included in the series is Fast Fourier Transformed into the frequency domain. For each frequency of the resultant f-x representation, a series of N complex numbers, one corresponding to each trace, is selected. The selected series of complex numbers corresponding to each frequency are processed to produce autoregressive model coefficients from which a polynomial rooting provides complex roots related to wavenumbers and decay constants. Amplitudes and phases for complex non-stationary spatial sinusoids associated with the estimated wavenumbers are then determined. For all frequencies f, the collection of determined wavenumbers, decay constants, amplitudes and phases constitute an f-k representation of the seismic traces. The produced f-k representation is examined to identify components corresponding to seismic data and the f-k components which correspond to seismic data are summed to reconstruct frequency components of an f-x representation of the acquired seismic traces with coherent noise removed.

Method for removing coherent noise from seismic data

Method for restoring seismic data to missing or severely noise contaminated traces included in a seismic section. The seismic section is transformed into a data localizing space and muted to retain only the localized data components of the seismic section. The muted transform space is inverse-transformed into the x-t domain to produce first estimates of the seismic data to be restored. The first estimates of the restored data are substituted for the corresponding traces in the original seismic section. Fully restored traces corresponding to the missing or severly noise contaminated traces are determined from the single estimate of the restored traces.

Method of restoring seismic data

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