Sensitivity Analysis Of Amplitude Variation With Offset (Avo) In Fractured Media

The variation in seismic P to P reflection amplitude with offset (AVO) caused by a system of fractures embedded in an isotropic background is investigated. Additionally. a sensitivity analysis of AVO parameters with respect to the fracture system parameters is made. The fracture system is assumed to be aligned vertically or horizontally and can be gas filled or fluid filled. Elastic constants are calculated by using formulations of Schoenberg (1988). From the elastic constants, the reflection amplitude as a function of angle is calculated using equations from Ruger (1997). Theoretical results for a single interface between fractured and unfractured media, both with and without lithology change, show opportunities for extraction of crack density information from seismic P-wave data collected in fractured geothermal or hydrocarbon reservoirs. For vertically oriented fractures, wide angle data (> 30°) is crucial for the estimation of fracture parameters.Massachusetts Institute of Technology. Borehole Acoustics and Logging ConsortiumMassachusetts Institute of Technology. Earth Resources Laboratory. Reservoir Delineation Consortiu

Subsurface Imaging with Reverse Vertical Seismic Pro les

A novel imaging process, referred to as vector image isochron (VII) migration, is specifically designed to reduce artifacts caused by arrays with limited apertures. By examining the assumptions behind generalized Radon transform (GRT) migration, a new approach is found which identities and suppresses array artifacts, based on the array geometry and the migration earth model. The new method works in four steps: 1) The conventional image is broken down according to the orientation of imaged planes within the image space, forming a vector image of the earth; 2) the earth model and the geometry of the arrays are used to derive vector image isochrons, which define the shape of reflection events in the vector image space; 3) the vector image is transformed by summing along the isochrons so that it depends on subsurface location and reflector orientation, rather than imaged plane orientation. This process is referred to as vector image isochron (VII) transformation; and 4) the transformed vector image is collapsed to a scalar image by summing over reflector orientations. The VII imaging method is derived in both 2D and 3D with the assumption that at least one of the arrays, source or receiver, is oriented horizontally. The surface array can have any distribution along the surface. The other array can have any orientation, although in this paper it will be assumed to be either another surface array or a vertically oriented downhole array. Downhole surveys in deviated wells, or in multiple wells, can be imaged with VII migration, at the likely cost of more computation time. The VII imaging method is tested on field data acquired in 1998 by MIT and several industry partners. The dataset is a 3D reverse vertical seismic profile (RVSP) over a hydrocarbon-bearing pinnacle reef in the northern Michigan reef trend. The survey exhibited two features of note: 1) A new, strong, downhole vertical vibrator, and 2) a random distribution of surface receiver locations. Due to adverse conditions, a large portion of the surface spread had to be abandoned. The reduced spatial coverage presents a challenge to the new migration method, but also limits the extent of the migrated image, precluding an evaluation of the reflectiveness of the random receiver spread. The limited nature of the receiver array also causes artifacts in the image which resemble migration "smiles". These are partially suppressed by limiting the dip aperture of the migration, but this also limits the reflector dips that can be imaged. The new VII imaging scheme, on the other hand, removes the artifacts without diminishing dipping reflectors. The VII images show more continuity along reflectors than images made with the conventional method

Subsurface imaging with reverse vertical seismic profiles

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2001.Includes bibliographical references (p. 147-152).This thesis presents imaging results from a 3D reverse vertical seismic profile (RVSP) dataset measured at a hydrocarbon bearing pinnacle reef in northern Michigan. The study presented many challenges in seismic data processing and imaging, as the survey geometry was unique in several ways. Reverse VSP, which uses seismic sources in a borehole and receivers on the earth's surface, is fairly rare. RVSP in 3D with a random distribution of surface geophones is unprecedented. At the time this data was collected, no commercially available processing tools existed to address this geometry, so a processing scheme had to be developed. The data processing sequence presented in this thesis, which includes amplitude corrections, first break picking, deconvolution, wavefield separation, and application of statics, takes advantage of the repeatible signature of the new downhole source (Paulsson et al., 1998). Since the data can be handled in common-receiver gathers instead of the usual common-source gathers, it can be treated like several single offset VSPs during the processing sequence. Issues related to the 3D geometry and the random distribution of the receiver array need not be addressed until the imaging step. The generalized Radon transform (GRT) migration method of Miller et al. (1987) provides a high resolution image of a portion of the target reef at 4600 feet (1400 meters) depth. The high resolution of the image is largely due to the downhole source, which generated a high powered signal at frequencies up to several hundred Hertz. Another factor in the high resolution of the image is the success of receiver consistent model-based Wiener deconvolution (Haldorsen et al., 1994), possible because the source signature was repeatable. Due to adverse conditions and power system failure, a large portion of the surface array did not record data.(cont.) The reduced spatial coverage limits the extent of the migrated image, precluding an evaluation of the effectiveness of the random receiver spread. The limited nature of the receiver array also caused artifacts resembling migration smiles in the image. These artifacts are partially suppressed by limiting the aperture of the migration, but this also removes dipping reflectors from the image. To maximize the imaging capibilities of the data, a second approach complimenting the GRT method is developed. This approach, termed vector image isochron (VII) migration, removes array artifacts from the image without losing energy from dipping reflectors. This allows artifacts in the conventional image to be identified, aiding interpretation of the GRT images. VII images also show more even illumination than conventional images, although an effect similar to NMO stretching reduces the resolution of the VII image as compared to the GRT image. The VII scheme is an extension of the GRT migration process of Miller et al. (1987), but involves forming an image which depends on the imaged plane orientation, transforming the image based on the array geometry, then finishing the GRT summation over plane orientations. The VII imaging method is derived in both 2D and 3D with the assumption that the ray paths are straight and that at least one of the arrays, source or receiver, is horizontally oriented. The surface array can have any distribution, regular or random. The other array can have any orientation in general, although this thesis assumes that it will be either another surface array or a vertically oriented borehole array. ...by Mary L. Krasovec.Ph.D

Finite Difference Modeling of Attenuation and Anisotropy

A nite difference scheme which includes the effects of attenuation and anisotropy is tested for seismic reflection and borehole acoustic models. The validity of the scheme is established using a 3D homogenous isotropic model to compare results to the discrete wavenumber method. Three models are then investigated. First, reflections from a 3D at layered model are analyzed for o set and azimuthal dependence of attenuation. Second, discrete fractures are included in a 2D at layered model to examine their effect on reservoir top and bottom reflections. Third, a 3D borehole in both hard and soft formations is modeled to test the effect of attenuation on guided waves.Massachusetts Institute of Technology. Earth Resources LaboratoryUnited States. Dept. of Energy (Grant DE-FC26-02NT15346)Eni S.p.A. (Firm

Frequency Dependence Of Seismic Data From Nigeria: Preliminary Results

Seismic data from the Niger Delta is used to test processing sequences involved in prestack and poststack amplitude and frequency analysis of marine seismic data. Water bottom reverberations are found to present a formidable challenge in poststack frequency and amplitude analysis. However, reflectors with anomalously high amplitudes show low frequency content both in deconvolved poststack data and in the near offsets of prestack data with no deconvolution, which agrees with results in the literature. Lack of detailed knowledge of the lithology prevents investigation of the physical nature of the amplitude and frequency variations.Massachusetts Institute of Technology. Borehole Acoustics and Logging ConsortiumMassachusetts Institute of Technology. Earth Resources Laboratory. Reservoir Delineation Consortiu

3-D Finite Di erence Modeling for Borehole and Reservoir Applications

ERL's in-house nite difference code (Krasovec et al., 2003) has undergone several upgrades in the past year. Most notably, a stretched grid can now be used to greatly reduce the amount of RAM memory needed by certain types of models. Improvements have been made in the GUI front end, allowing more freedom and ease in building the model, source or source array, and receiver array. The finite difference code has contributed to several different research projects at ERL in the past year. A few of these projects, including borehole seismics, reservoir delineation, and source mechanics, are shown in this report.Massachusetts Institute of Technology. Earth Resources LaboratoryMassachusetts Institute of Technology. Borehole Acoustics and Logging Consortiu

Fracture Detection using Amplitude versus Offset and Azimuth Analysis of a 3D P-wave Seismic Dataset and Synthetic Examples

Amplitude versus offset (AVO) analysis of seismic reflection data has been a successful tool in describing changes in rock properties along a reflector. This method is extended to azimuthal AVO (AVOA) in order to characterize vertically aligned fractures within a reservoir, which can be important fluid migration pathways. AVOA analysis is performed on synthetic data using a least squares inversion method to investigate the effects of varying acquisition geometry, amount of noise, and fracture properties. These tests show that it is possible to detect the fractured layer and determine the fracture strike orientation under typical acquisition conditions. This method is also applied to field data collected during an Ocean Bottom Cable (OBC) survey. These data include a broad offset-azimuth range, which is important for the AVOA analysis. The fracture location and strike orientation recovered from the field data analysis are well correlated with borehole information from this area. Based on an understanding of AVOA behavior under synthetic conditions, this technique provides an effective methodology for describing the spatial variability of a fractured reservoir using 3D seismic data.Eni S.p.A. (Firm)United States. Dept. of Energy (Grant number DE-FC26-02NT15346)Massachusetts Institute of Technology. Earth Resources Laborator