Keynote Address: Joint geophysical imaging for fractured reservoirs
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Seismic and electromagnetic wave fields in the accessible (drillable) earth both respond to changes in rock properties and structure, yet are not usually combined into a single subsurface map that reflect these common changes. Variations in layer thicknesses, folds, faults, fault-related offsets, porosity, fluids, and saturation create anomalies in both fields. The presence of oriented fractures and fabric adds anisotropic responses to both as well. Ideally, both fields would be used to create a map that combines their responses to a sought after property, say porosity, in a single ?joint geophysical image?. The members of the Institute of Earth Science and Engineering are working toward such JGI maps, progress in which is reported on in this presentation. A simple example of JGI is the inversion of high-resolution seismic refraction and magnetotelluric data collected over a simple layer-over-basement structure. Here the common factor is the layer thickness, the value of which is most accurately found forcing the seismic velocity and apparent resistivity models to give the same number. A less simple example is the combined use of seismic travel times and MT resistivity converted to seismic velocity to locate microearthquakes. An even more complicated example is the inversion of shared S-wave-splitting and MT-polarization effects from zones of oriented and fluid-filled fractures. Some of theoretical and practical aspects of these three cases will be discussed, including: (a) data gathering techniques, (b) physical models of the shared properties, especially in the case of fractures and anisotropy, and (c) quantitative methods for combining measurements.Keywords:
Magnetotellurics
Exploration geophysics
Seismic refraction
Seismic Tomography
Vertical seismic profile
Geophysical Imaging
Seismic migration
Salt movement often results in steeply-dipping complex structures, which pose significant challenges for model building and migration. In recent years, advances in seismic imaging algorithms have permitted imaging of steep structures by exploiting the two-way wave equation via the introduction of reverse time migration (RTM). With such imaging algorithms, double bounces and turning wave reflections can be imaged, thereby enabling the imaging of vertical and overturned salt flanks. However, despite advances in the migration algorithms, the derivation of a suitable earth model incorporating the anisotropic behaviour of the velocity field remains a significant challenge, requiring tight integration of geological interpretation, and geophysical skills.
Geophysical Imaging
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A limited seismic investigation of Ball Mountain Dam, an earthen dam near Jamaica, Vermont, was conducted using multiple seismic methods including multi‐channel analysis of surface waves (MASW), refraction tomography, and vertical seismic profiling (VSP). The refraction and MASW data were efficiently collected in one survey using a towed land streamer containing vertical‐displacement geophones and two seismic sources, a 9‐kg hammer at the beginning of the spread and a 40‐kg accelerated weight drop one spread length from the geophones, to obtain near‐ and far‐offset data sets. The quality of the seismic data for the purposes of both refraction and MASW analyses was good for near offsets, decreasing in quality at farther offsets, thus limiting the depth of investigation to about 12 m. Refraction tomography and MASW analyses provided 2D compressional (Vp) and shear‐wave (Vs) velocity sections along the dam crest and access road, which are consistent with the corresponding VSP seismic velocity estimates from nearby wells. The velocity sections helped identify zonal variations in both Vp and Vs (rigidity) properties, indicative of material heterogeneity or dynamic processes (e.g. differential settlement) at specific areas of the dam. The results indicate that refraction tomography and MASW methods are tools with significant potential for economical, non‐invasive characterization of construction materials at earthen dam sites.
Geophone
Seismic refraction
Vertical seismic profile
Hammer
Rayleigh Wave
Passive seismic
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Prestack Kirchhoff depth migration is commonly used in borehole seismic imaging, where there is uneven illumination due to the limitations of the source-receiver geometry. A new vertical seismic profile (VSP) migration/imaging workflow has been established that incorporates the structure-dip information derived from a newly developed structure tensor analysis into the existing VSP Kirchhoff migration/imaging technique. This allows us to better image the structures in the vicinity of a borehole and the far-field dipping events away from the borehole. We tested the workflow with the HESS salt model. The results were compared with those from reverse time migration, which found that Kirchhoff migration combined with structure-dip information not only reduced ambiguities of the imaging result but also allowed for imaging dip structures (e.g., fault) in the far region from the borehole. This allows for imaging dip structures and provides a useful extension of existing VSP imaging capabilities using Kirchhoff migration.
Seismic migration
Geophysical Imaging
Vertical seismic profile
Optoacoustic imaging
Prestack
Magnetic dip
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Both vertical well seismic imaging and deviated well seismic imaging are needed in oilfield development.At the same time,it is not possible for us to obtain detailed reservoirs and interlayers with high resolution when only surface seismic data are used.In this research,we investigated cross well staggered-grid finite-difference reverse-time migration algorithm and absorbing boundary conditions.In the end,we obtained reverse-time migration image from the theory model and field seismic data.The result shows that the migration algorithm is correct,and the signal-to-noise ratio and resolution ratio of reverse-time migration imaging profile are higher than that of surface seismic imaging profile.What is more,for the reverse-time imaging profile,the details of layers are clearer,and it coincides well with the actual strata,which makes it much more credible.This means we can use the above method to get detailed reservoirs and interlayers with high resolution.
Seismic migration
Geophysical Imaging
Seismic exploration
SIGNAL (programming language)
Passive seismic
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Summary Imaging seismic waves beneath near surface hard rock have a complex challenge. The seismic waves will be scattered. Many seismic wave imaging targets, for example, hydrocarbon potentials, are under near surface hard rock environment such as: under reef or vulcanic rock. Reflection seismic method, however, has some constraints to obtain good images beneath near surface hard rock environment. Irregularities of volcanic and carbonates rock create scattering and nonlinearity in the reflected wave. Then, the reflected wave propagates in caustics or non-linear paths. Therefore, many seismic migration methods fail in providing accurate image beneath near surface hard rock environment. We present imaging cases beneath hard rock environment using reverse time migration based on inverse scattering. The migration uses individual shot record data and migrated by the reverse time migration of Born’s approximation. The inverse scattering reverse time integrates the whole individual migrated shot record data sequentially. The results show the layers and faults beneath hard rock environment can be revealed clearly. The longer far offset of split spread configuration produce better quality image than shorter far offset of one. The seismic imaging beneath near surface hard rock environment are useful for unlocking the new potential area beneath near surface hard rocks.
Seismic migration
Geophysical Imaging
Reflection
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Cross-well seismic is a geophysical exploration method, where the seismic source is triggered in one well and the data are accepted in another well, in order to obtain the geological structure between the two wells. In this paper, we mainly study the algorithm of elastic wave reverse time migration imaging with cross-well seismic models. The forward simulations, pre-processing of received data, and reverse time migration imaging are presented. To overcome the problem of large amounts of memory costs in reverse time migration, we develop a saving boundary scheme into cross-well seismic to reduce the memory consumption. The feasibility of our algorithm is verified by a horizontally layered model.
Seismic migration
Geophysical Imaging
Seismic exploration
Vertical seismic profile
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This paper presents and analyzes results from a Finite Difference Modeling (FDM), processing, and imaging study of a Walkaway Vertical Seismic Profiling (WVSP) survey, and discusses how the images from WVSP enhance those from the surface seismic data. It is shown that the results from the WVSP integrate well with the image from the surface seismic performed in the same line. For the study, a seismic model with vertically and horizontally varying velocities was built and a WVSP data set was generated. The surface seismic had difficulty to show clear images from the layers with steep dips and near vertical displacements on the model due to the lack of ray coverage. The study demonstrates that the WVSP geometry can record reflections from near vertical layers facing the borehole, which help with imaging the parts of the subsurface structure which were missing in the surface seismic. With proper combination of the images from the WVSP and the surface seismic, a more complete image profile of the subsurface can be constructed around the borehole. However, while contributing to surface seismic, it is also seen that the WVSP introduces more migration artifacts related to source interval distance and interbed multiples than the surface seismic data.
Vertical seismic profile
Geophysical Imaging
Seismic survey
Seismic to simulation
Profiling (computer programming)
Seismic migration
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Abstract In complex geological environments encountered in today's exploration practice there is a need for model building and imaging schemes based on an as accurate a description as possible of wave propagation in the earth. In this paper we discuss the most advanced members of a family of wave equation based imaging and model building schemes, namely Reverse Time Migration and Full Waveform Inversion. We illustrate the potential of these schemes by results obtained on an OBS survey in the Gulf of Mexico. Introduction Seismic imaging consists of two steps. One starts by building a subsurface velocity model and follows this by creating the actual reflection image by a depth migration in this velocity model. Of these two steps depth migration is well understood. It is based on a single scattering description of wave propagation in the earth. This description provides a linear relationship between the reflectivity in the subsurface and the reflection data measured at the surface of the earth. Depth migration boils down to constructing the inverse of this relationship on the reflection data and thus constructs an image of the reflectivity in the subsurface. Obviously, because of the single scattering assumption, this does not deal with multiple reflections, which therefore have to be attenuated before migration. The most advanced depth migration algorithm in use today is Reverse Time Migration. The velocity model building problem is much harder, as it is inherently a non-linear problem. The standard approach is to exploit the redundancy in the seismic data, for example the offset between source and receiver. One migrates subsets of the data with fixed values of the redundant coordinates (" minimal datasets??) and requires that at each horizontal location (x,y) the depth of an imaged reflector does not vary with the redundant coordinate. If it does, the variations are used to update the velocity model iteratively. This leads to a family of well known reflection tomography schemes, which go under the name of migration velocity analysis. Although most of them are based on some form of raytracing, these methods do have a natural generalization towards the wave equation domain, see e.g. Symes 2008. It is important to stress that they are based on primary reflections only. These methods therefore all rely on effective multiple attenuation and/or interpreter based identification of the primaries. The latter is still one of the most important bottlenecks of migration velocity analysis, especially in poor signal to noise situations, such as subsalt. Full Waveform Inversion is another, completely different method for estimating the velocity model. In this approach one tries to find the model by requiring that the data calculated by solving the wave equation in this model optimally resemble the measured data in a seismic survey. The great advantage of this formulation is that it should work for all wave types, not only for primary reflections. In this paper we will take a closer look at Reverse Time Migration and Full Waveform Inversion.
Seismic migration
Geophysical Imaging
Reflection
Model building
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Walkaway vertical seismic profiling (VSP) acquisition with three‐component geophones allows for direct measurement of compressional as well as shear energy. This makes full elastic reverse time migration an attractive alternative for imaging data. We present results from elastic reverse time migration of a marine walkaway VSP acquired offshore Norway. The reverse time migration scheme is based on a high‐order finite‐difference solution to the two‐way elastic wave equation. Depth images of the subsurface are constructed by correlation of forward‐ and back‐propagated elastic wavefields. In the walkaway VSP configuration, the number of shots is much larger than the number of geophone levels. Using processing methods operating in the shot/receiver domain, it is advantageous to use the reciprocal relationship between the walkaway VSP and the reverse VSP configurations. We do this by imaging each component of each geophone level as a reverse VSP common shot gather. The final images are constructed by stacking partial images from each level. The depth images obtained from the vertical components reveal the major characteristics of the geological structure below geophone depth. A graben in the base Cretaceous unconformity and a faulted coal layer can be identified. The horizontal components are more difficult to image. Compared to the vertical components, the horizontal component images are more corrupted by migration artifacts. This is because the horizontal component images are more sensitive to aperture effects and to the shear‐wave velocity macromodel. When converted to two‐way time, the migration results tie well with the surface seismic section. Comparison of fully elastic and acoustic reverse time migration shows that the vertical component is dominantly PP-reflected events, whereas the horizontal components get important contributions from PS-converted energy. The horizontal components also provide higher resolution because of the shorter wavelength of the shear waves.
Geophone
Vertical seismic profile
Seismic migration
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Pre-stack reverse-time migration shows great superiority in dealing with steep dip angle structures and complex velocity models,but the low frequency noise seriously affects the quality of imaging. The reflect angle can be calculated by the Poynting vector,the direction of which indicates that of the propagation of seismic wave, and meanwhile seismic wave field can be separated into up-going one and down-going one.This paper proposes a new imaging condition,by using which the imaging noise caused by the cross-correlation of the up-going wavefield and the down-going one in the same direction,as well as wide angle noise can be well suppressed. Compared with the conventional crosscorrelation imaging conditions,the new imaging condition is easy to realize,the extra calculation and storage are both very small.The new imaging condition is applied in Marmousi model and the migration results shows that the low frequency noise is well suppressed.
Seismic migration
Geophysical Imaging
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