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 Volume 45, Issue 1, 1997
Geophysical Prospecting  Volume 45, Issue 1, 1997
Volume 45, Issue 1, 1997


Observations of borehole‐source amplitude reduction due to casing
Authors Richard L. Gibson, Walter R. Turpening, Andrea Born and Roger M. TurpeningTheoretical results show that the amplitude of a borehole source is reduced when the well in which it is operated is cased and cemented. This reduction is a strong function of the formation velocity and is more weakly dependent on the direction of propagation of the wave travelling from source to receiver and on the diameter of the borehole itself. We have tested these predictions with data gathered in a cross‐hole seismic experiment conducted in two stages in 1990 and 1991. The source and receivers were located in the MIT/Stech 1‐21A and MIT/Burch 1‐20B wells at the Earth Resources Laboratory (ERL) test site in Michigan (USA). Though the source well (MIT/Stech 1‐21A) was uncased in 1990, a steel casing and cement were added prior to the collection of data in 1991. Several receiver positions were reoccupied to compare data collected with the source in open and cased holes. Using a velocity model for this area and borehole diameter measurements from a calliper log, a compensation factor can be computed that will adjust the data collected in 1991 to have amplitudes comparable to that collected in the first stage of the experiment. The accuracy of the results demonstrates the validity of the theory, which can be very useful in better understanding seismic waveforms recorded in cross‐hole experiments.



Direct interpretation of magnetotelluric sounding data based on the frequency‐normalized impedance function
Authors Ahmet T. Başokur, Cemal Kaya and Emin U. UlugergerliAn important aspect of any non‐linear inversion method is the generation of a suitable or good initial model as this controls the rate of convergence and accuracy of the result. To overcome the problem, a numerical method is presented for direct interpretation of magnetotelluric sounding data based on the frequency‐normalized impedance (FNI) function. The expressions used to calculate the parameters are developed, first for a two‐layer case under the assumption that deeper layers do not contribute to the early part of the FNI curve, and they are then generalized for an n‐layer situation. The parameters of the first layer are computed by using successive sample values and the final estimate is obtained from the arithmetic mean of selected values by excluding unacceptable results in the logarithmic space. The top layer is then removed using a reduction equation. The repetition of the procedure on successive branches of the FNI function gives successive layer parameters, the resistivity of the substratum being obtained at the final step, when the reduction equation becomes equal to the square root of that resistivity. The proposed method can be used as a complementary method for iterative inversion as it creates an initial guess which is close to the optimal solution. The solution produced by the direct interpretation may also be modified by the interpreter to incorporate prior geological information before being input to iterative interpretation schemes.



Fractured reservoir delineation using multicomponent seismic data
More LessThe characteristic seismic response to an aligned‐fracture system is shear‐wave splitting, where the polarizations, time‐delays and amplitudes of the split shear waves are related to the orientation and intensity of the fracture system. This offers the possibility of delineating fractured reservoirs and optimizing the development of the reservoirs using shear‐wave data. However, such applications require carefully controlled amplitude processing to recover properly and preserve the reflections from the target zone. Here, an approach to this problem is suggested and is illustrated with field data. The proposed amplitude processing sequence contains a combination of conventional and specific shear‐wave processing procedures. Assuming a four‐component recording (two orthogonal horizontal sources recorded by two orthogonal horizontal receivers), the split shear waves can be simulated by an effective eigensystem, and a linear‐transform technique (LTT) can be used to separate the recorded vector wavefield into two principal scalar wavefields representing the fast and slow split shear waves. Conventional scalar processing methods, designed for processing P‐waves, including noise reduction and stacking procedures may be adapted to process the separated scalar wavefields. An overburden operator is then derived from and applied to the post‐stacked scalar wavefields. A four‐component seismic survey with three horizontal wells drilled nearby was selected to illustrate the processing sequence. The field data show that vector wavefield decomposition and overburden correction are essential for recovering the reflection amplitude information in the target zone. The variations in oil production in the three horizontal wells can be correlated with the variations in shear‐wave time‐delays and amplitudes, and with the variations in the azimuth angle between the horizontal well and the shear‐wave polarization. Dim spots in amplitude variations can be correlated with local fracture swarms encountered by the horizontal wells. This reveals the potential of shear waves for fractured reservoir delineation.



A joint inversion algorithm to process geoelectric and surface wave seismic data. Part II: applications
More LessSeismic and geoelectric methods are often used in the exploration of near‐surface structures. Generally, these two methods give, independently of one other, a sufficiently exact model of the geological structure. However, sometimes the inversion of the seismic or geoelectric data fails. These failures can be avoided by combining various methods in one joint inversion which leads to much better parameter estimations of the near‐surface underground than the independent inversions. In the companion paper (Part I: basic ideas), it was demonstrated theoretically that a joint inversion, using dispersive Rayleigh and Love waves in combination with the well‐known methods of DC resistivity sounding, such as Schlumberger, radial dipole‐dipole and pole‐pole arrays, provides a better parameter estimation. Two applications are shown: a five layer structure in Borsod County, Hungary, and a three‐layer structure in Thüringen, Germany. Layer thicknesses, wave velocities and resistivities are determined. Of course, the field data sets obtained from the ‘real world’ are not as complete and as good as the synthetic data sets in the theoretical Part I. In both applications, relative model distances, in percentages, serve as quality control factors for the different inversions; the lower the relative distance, the better the inversion result. In the Borsod field case, Love wave group slowness data and Schlumberger, radial dipole‐dipole and pole‐pole (i.e two‐electrode) data sets are processed. The independent inversion performed using the Love wave data leads to a relative model distance of 155%. An independent Schlumberger inversion results in 41%, a joint geoelectric inversion of all data sets in 15%, a joint inversion of Love wave data and all geoelectric data sets in 15% and the robust joint inversion of Love wave data and the three geoelectric data sets in 10%. In the Thüringen field case, only Rayleigh wave group slowness data and Schlumberger data were available. The independent inversion using Rayleigh wave data results in a relative model distance of 19%. The independent inversion performed using Schlumberger data leads to 34%, the joint and robust joint inversion of Rayleigh wave and Schlumberger data gave results of 18% and 20%, respectively.



Multichannel estimation of frequency‐department Q from VSP data^{1}
Authors P. E. Harris, C. Kerner and R. E. WhiteAbstractA Method of estimating attenauation from the first arrivals of VSP data is presented. The motivation is the desire to investigate the effects of scattering on wave propagation, and particularly the apparent attenuation and associated phase delay due to fine layering (the O'Doherty‐Anstey effect).
In order to take account of the frequency dependence of the predicted scattering attenuation, and to provide robust statistics for the estimates, a beam‐forming method is used to measure the attenuation. This simularaneously estimates the slowness and polarization angle of the different wave modes, and results in attenuation measurements which are largely free of interference from reflected and mode‐converted energy. By working in the frequency domain and measuring amplitude decay with depth, the frequency dependence of the attenuation is also accounted for. The beam‐forming algorithm works in two passes, the first of which estimates slownesses and polarization angles over a small depth range, while the second uses the information from the first pass over a larger depth range to estimate attenuation.
An approximate error analysis of the method shows that the standard variance of the estimated Q values is proportional to Q^{2} and the data quality (measured by its spectral coherence), and inversely proportional to the square of the analysis depth range and the square of the frequency. Hence the depth resolution is traded against the stability of the results.
The method is applied to a zero‐offset three‐component VSP. The data are of good quality, with a bandwidth ranging from 180 Hz in the shallow part to 100 Hz in the deepest part. Stable results were obtained using a 450 m depth range. Above about 50 Hz, there is little evidence of frequency dependence in the attenuation. There is a clear division in depth into layers of higher and lower attenuation, with values of Q typically between 50 and 200. Below 50 Hz, however, attenuation increases rapidly with decreasing frequency throughout the depth range, with values of Q of less than 10 at 15 Hz. This behaviour appears anomalous since on physical grounds we expect very high values of Q at low frequency, and we have no explanation for these observations.



THE IMPORTANCE OF TOPOGRAPHIC CORRECTIONS ON MAGNETOTELLURIC RESPONSE DATA FROM RUGGED REGIONS OF ANATOLIA
Authors Aysan Gürer and O. Metin ÌlkışıkTopographic irregularities cause some distortions of magnetotelluric (MT) fields. In the vicinity of a topographic feature, the TM‐mode distortion increases with the height and inclination of the slope. It is well‐known that TM‐mode (E⊥) topographic effects are much greater than TE‐mode (Eı) distortions. We have made a study of MT anomalies in TM‐mode due to two‐dimensional topography. In order to reduce these effects, the distortion tensor stripping technique was used. After corrections, the resulting data can be interpreted as if they were obtained over a flat surface and depend only on the subsurface structure. However, this technique sometimes causes some geometrical distortions of the real subsurface structure. One of our aims is to overcome this failure. We have modified the correction coefficients by considering the actual one‐dimensional geology. Model studies showed that our approach is especially useful in removing the terrain effects on complex 2D subsurface structures. The other purpose of this study is to emphasize the importance of a proper terrain correction for data from sites having mountainous topography over complex geology, e.g. strike‐slip faults, suture zones and rift valleys. Some examples of MT data sets collected from the North Anatolian Fault Zone and from the thrust regions of the Western Taurides will be presented.



Resistivity and IP modelling of an anisotropic body located in an isotropic environment
Authors L. Eskola and H. HongistoA solution based on Tabarovskii's coupled pair of surface integral equations is given for the potential of a direct current flowing in an electrically anisotropic body and within the enclosing isotropic surroundings. The sources of the secondary potential exterior and interior to the body are fictitious surface charge distributions. The equations are solved numerically using point matching with pulse functions as subsectional basis functions. The model used in the applications is a long prism, excited by long line current electrodes aligned parallel to the strike. The strike length is set at a length sufficient to guarantee 2D behaviour of the model. Comparisons of computation results indicate that for the models, electrode arrays and numerical procedures applied, the solutions based on fictitious surface sources converge faster and behave more regularly than those based on real surface charges. When compared with previously published integral equation solutions, the present solution seems to be relatively efficient, even in the case of purely isotropic models. The model experiments also showed that at moderate resistivity contrasts, the anomaly shapes are strongly dependent on the directions of the principal axes of the body resistivity. However, when the external resistivity is more than 100 times that of the geometric mean of the principal resistivities in the body, with the principal resistivities differing from each other by at most one order of magnitude, the contribution of the anisotropy to the anomaly diminishes as a result of electrical saturation.



FRACTURE CHARACTERIZATION FROM NEAR‐OFFSET VSP INVERSION
Authors Steve Horne, Colin MacBeth, J. Queen, W. Rizer and V. CoxA global optimization method incorporating a ray‐tracing scheme is used to invert observations of shear‐wave splitting from two near‐offset VSPs recorded at the Conoco Borehole Test Facility, Kay County, Oklahoma. Inversion results suggest that the seismic anisotropy is due to a non‐vertical fracture system. This interpretation is constrained by the VSP acquisition geometry for which two sources are employed along near diametrically opposite azimuths about the well heads. A correlation is noted between the time‐delay variations between the fast and slow split shear waves and the sandstone formations.



Azimuthal variation in AVO response for fractured gas sands
Authors Colin M. Sayers and James E. RickettNatural fractures in reservoirs play an important role in determining fluid flow during production, and hence the density and orientation of fractures is of great interest. In the presence of aligned vertical fractures, the reflection amplitude at finite offset varies with azimuth. The effect of natural fractures on the azimuthal AVO response from a gas‐sandstone reservoir encased within shale is investigated. A simple expression for the difference in P‐wave reflection coefficient from the top of the reservoir parallel and perpendicular to the strike of the fractures is obtained in terms of the normal and tangential compliances, ZN and ZT, of the fractures. This expression is valid for small anisotropy and material contrasts and is compared with the results of numerical modelling. For a given value of ZT, the azimuthal variation in reflection coefficient at moderate offsets is found to increase with decreasing ZN/ZT. For gas‐filled open fractures ZN/ZT ≈ 1, but a lower ratio of ZN/ZT may result from the presence of cement or clay within the fractures, or from the presence of a fluid with non‐zero bulk modulus. For ZN/ZT = 1 and moderate offsets, the variation with offset of the reflection coefficient from the top of the fractured unit is dominated by the contrast in Poisson's ratio between the gas sand and the overlying shale, the effect of fractures only becoming noticeable as the critical angle for the unfractured sandstone is approached. However, for reflections from the base of the fractured unit, the variation in reflection amplitude with azimuth is much greater at conventional seismic offsets than for the reflection from the top. Azimuthal variations in the strength of the reflection from the top of the reservoir depend only on the variation in reflection coefficient, whereas the raypath is also a function of azimuth for reflections from the base of the fractured unit, leading to stronger, more visible, variations of AVO with azimuth. It follows that an azimuthal variation in AVO due to fractures in the overburden may be misinterpreted as due to the presence of aligned fractures in the reservoir.

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