Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2002-11-01
2004-10-19
McElheny, Jr., Donald (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
Reexamination Certificate
active
06807489
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for absolute preserved amplitude processing of data obtained by means of the seismic prospecting technique known as VSP, wherein seismic waves received by one or more multi-axis pickups coupled with the formations surrounding a well, coming from a seismic source arranged at the surface, either under direct arrival conditions, or after reflection on discontinuities of the underlying formation, are recorded.
2. Description of the Invention
The VSP technique is conventionally used to measure propagation times and velocities, and to obtain a zero-phase reference of the series of reflections on the reflectors encountered by the well (the stacking domain located immediately below the VSP measuring points is commonly referred to by specialists as corridor stack or VSP log, a designation that is used in the description hereunder). However, this series is produced by means of processing tools which modify the amplitude of the reflected signals: multiplication by a constant gain, rough spherical divergence compensation, dynamic time equalization and spectral equalization, etc. In fact, conventional methods allow recovery of the amplitude contrasts of the reflections in relation to one another according to the processing procedure used, but in practice they fall to recover the absolute amplitude ratio of the reflected waves in relation to the direct waves reaching the receivers. As a consequence, on the one hand, diffractions of very high amplitude may be mistaken for reflections, which leads the interpreter to be mistaken in the assessment of the structure in the vicinity of the well and, on the other hand, the real amplitude of the reflections cannot be exploited or interpreted.
The prior art in the field of seismic attenuation measurement, in particular by means of the Vertical Seismic Profiling method, and its consideration during processing, is illustrated by many publications, and notably by the following publications:
Gardner, G. H. F., L. W. Gardner, and A. R. Gregory: Formation Velocity and Density;
The Diagnostic Basics for Stratigraphic Traps. Geophysics Vol. 39, No. 6, 1974, pp. 770-780;
Hauge, P. S.: Measurements of Attenuation from Vertical Seismic Profiles, Geophysics, Vol. 46, 1981, pp. 1548-1558;
Kan, T. K., et al.: Attenuation Measurement from Vertical Seismic Profiling, SEG Expanded Abstracts, LA meeting, October 1981, pp. 338-350;
Lee, M. W., et al.: Computer Processing of Vertical Seismic Profile Data, Geophysics, Vol. 48, No. 3, March 1983, pp. 282-287;
Newman Paul: Divergence Effects in a Layered Earth, Geophysics, Vol. 38, No. 3, June 1973, pp. 481-488;
Newman, P. J., et al.: In Situ Investigation of Seismic Body Wave Attenuation in Heterogeneous Media, Geophysical Prospecting 30, pp. 377-400, 1982;
Payne, M. A.: Looking Ahead with Vertical Seismic Profiles, Geophysics Vol. 59, No. 8, August 1994, pp. 1182-1191;
Pujol et al.: Interpretation of a Vertical Seismic Profile Conducted in the Columbia Plateau Basalts, Geophysics, Vol. 54, No. 10 (October 1989), pp. 1258-1266;
Pujol & Smithson: Seismic Wave Attenuation in Volcanic Rocks from VSP Experiments, Geophysics, Vol. 56, No. 9 (September 1991), pp. 1441-1445;
Spencer T. W. et al.: Seismic Q—Stratigraphy or Dissipation, Geophysics, Vol. 47, No. 1 (January 1982), pp. 16-24;
Spencer, T. W., 1985: Measurements and Interpretation of Seismic Attenuation in Fitch, A. A., Ed. Developments in Geophysical Exploration Methods, 6, Elsevier Science Publ. Co. Inc., pp. 71-109;
Stainsby S. D. et al.: Q Estimation from Vertical Seismic Profile Data and Anomalous Variations in the Central North Sea, Geophysics, Vol. 50, No. 4 (April 1985), pp. 615-626;
Rainer Tonn: The Determination of the Seismic Quality Factor Q from VSP Data. A Comparison of Different Computational Methods, Geophysical Prospecting, April 1990;
Ross, W. S., et al.: Vertical Seismic Profile Reflectivity. Ups Over Downs, Geophysics, Vol. 52, No. 8 (August 1987), pp. 1149-1154;
Rutledge, J. T., and Winkler, H., Attenuation Measurements in Basalts Using Vertical Seismic Profile Data from the Eastern Norwegian Sea: SEG, Expanded Abstracts, pp. 711-713, New Orleans, 1987;
Sokora, W. L., 1996, Predicting Formation Target Depth Ahead of the Bit with High Accuracy: A case Study from the Arun Field for a Deviated Well: Proceedings of the indonesia Petroleum Association, IPA96-2.5-028;
Wu R. and K. Aki, Scattering Characteristics of Elastic Waves by an Elastic Heterogeneity, Geophysics, Vol. 50, No. 4, April 1985, pp. 582-595;
Yuehua Zeng, Feng Su and Keiiti Aki, Scattering Wave Energy Propagation in Random Isotropic Scattering Medium, JGR, Vol. 96, No. B1, pp. 607-619, January 1991.
The aforementioned publications describe methods of measuring the attenuation of seismic waves in transmission for vertical seismic profile data (VSP). These measurements are sometimes performed too roughly but, unfortunately, none of these publications provides a solution concerning the way to use these measurements so as to more exactly recover by processing the amplitude of the reflected events observed on the VSPs, for any distance between the position of the well pickups and of the reflectors, including reflectors located below the well bottom, which is the major object of the method according to the invention.
The spherical divergence, which is the most important factor in the amplitude decrease of a spherical seismic wave, is often compensated by an approximate law of Z=Vt type (Newman and Worthington, 1982), or by an exponential law of exp(&pgr;f &tgr;/Q) type for the events reflected below the well bottom (Payne, 1994), or by a rough time power law T
n
, superscript n being adjusted by guesswork typically between 1 and 2, as it is generally done by well survey service companies. In a stratified medium close to a one-dimensional model, the spherical divergence can be taken into account more accurately by a t.V
2
law (Newman, 1973), but this relation is rarely used (Pujol, 1991). The local impedance is never taken into account in the aforementioned publications, and the amplitude of the reflections is never examined. The 1D hypothesis is always made, but never verified in the literature. Many authors use a method of studying the evolution of the amplitude spectrum ratio of the direct arrival of the VSP taken at different depths (Kan, 1981) to determine the attenuation and the quality factor Q which characterizes it; others (for example Stainby, 1985) use the widening of the direct arrival pulse width: these methods may therefore be very sensitive to the reflected or diffracted waves that interfere with the direct arrival. Some authors, such as Rainer Tonn (1990), have successfully compared various measuring methods.
All the methods used assume the stationarity of the signal of the VSP downgoing wave, and this hypothesis is unfortunately not always verified in real cases. In effect, the fact that a spherical wave is propagated in a 1D stratified medium implies that part of the energy transmitted in P wave is converted to an S wave, even for low propagation incidences, and therefore the attenuation measured on the direct wave is often overestimated.
However, the order of magnitude of the measured attenuations is 1 to 13 dB per 1000 m (Pujol, 1989) for heterogeneous sedimentary or volcanic rocks.
The velocity variation function of the frequency is often insignificant between 10 and 100 Hz, even when considering a dispersive model of intrinsic attenuation, and the inner multiples can generate by themselves a not insignificant fraction of the total attenuation, for example 30% or 2 dB for 1000 m (Kan, 1981).
Any velocity heterogeneity close to the well can produce interferences which in most cases attenuate direct arrivals, but sometimes amplify them. This also depends on the way the amplitude is measured on the direct arrival (on the peak, the trough or the spectrum, therefore with a windowing and an amplitude variation linked with the apodization of the signal selected, etc.). Besides, propaga
Naville Charles
Serbutoviez Sylvain
Institut Francais du Pe'trole
McElheny Jr. Donald
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