Multioffset Vertical Seismic Profile Research Articles

Application of multioffset vertical seismic profiling in fracture mapping : Carswell, A; Moon, W M Geophysics V54, N6, June 1989, P737–746

Application of multioffset vertical seismic profiling in fracture mapping : Carswell, A; Moon, W M Geophysics V54, N6, June 1989, P737–746

Dynamic elasticity of microbedded and fractured rocks

Microbedded rocks have an anisotropic frequency‐dependent sound speed which depends on the intrinsic sound speeds of the individual microbeds and on the O'Doherty‐Anstey effect. Fractured rocks have an anisotropic frequency‐dependent sound speed which depends on the intrinsic sound speed of the unfractured rock, the frequency‐dependent phase shift that occurs during reflection or transmission across a fracture, and the interfracture O'Doherty‐Anstey effect. These effects are neglected by the quasistatic methods presently used to generate elastic constants. Here I introduce a new method for generating elastic constants that contain all the above effects. First, a statistical description of the rock is used to generate a sample of the rock. Then an exact two‐way method is used to propagate just a few plane waves, of frequency ƒ, a distance of several wavelengths from the source. If an equivalent homogeneous medium exists at frequency ƒ, then the computed motions must also satisfy a one‐way elastic wave equation for that equivalent medium. This one‐way wave equation is used to invert for the elastic coefficients. When no equivalent medium exists, perhaps because ƒ is too large, this is indicated by the inversion. Possible applications of the method are prediction of seismic sound speeds from measurements of bed thicknesses in cores; analysis of laboratory data for fracture constitutive relations; and inversion of multioffset vertical seismic profiling data for elastic coefficients comparable with those predicted from cores.

Application of multioffset vertical seismic profiling in fracture mapping

Multioffset vertical seismic profiling (VSP) combines the improved vertical resolution of VSP with the lateral resolution of the conventional seismic method. In this study, the multioffset VSP technique was employed to map the fracture zones in a granite batholith in which the Atomic Energy of Canada Ltd.’s Underground Research Laboratory (URL) is located. With shotpoints along a vertical shaft and receiver arrays on the surface (cemented to outcrops), six 2-D seismic sections were obtained. The upcoming and downgoing events were separated using a Radon transform wave‐field separation method. For the given multioffset experimental configuration, the VSP-CDP transformation converted the VSP section into conventional type seismic sections. The results indicate that the multioffset configuration is an effective method for mapping deep fracture zones, in this case with respect to the URL shaft. However, the VSP-CDP transformation method used in this study tends to stretch the shallow reflection events, resulting in reduced resolution.

Reverse‐time wave‐field extrapolation, imaging, and inversion

The scattered wave field propagated backward in time into an arbitrary background medium is related via a volume integral to perturbations in velocity about the background, which are expressed as a scattering potential. In general, there is no closed‐form expression for the kernel of this integral representation, although it can be expressed asymptotically as a superposition of plane waves backpropagated from the receiver array. When the receiver array completely surrounds the scatterer, the kernel reduces to the imaginary part of the Green’s function for the background medium. This integral representation is used to relate the images obtained by imaging algorithms to the actual scattering potential. Two such relations are given: (1) for the migrated image, obtained by deconvolving the extrapolated field with the incident field; and (2) for the reconstructed image, obtained by applying a one‐way wave operator to the extrapolated field and then deconvolving by the incident field. The migrated image highlights rapid changes in the scattering potential (interfaces), whereas the reconstructed image can, under ideal conditions, be a perfect reconstruction of the scattering potential. “Ideal” conditions correspond to (1) weak scattering about a smoothly varying background medium, (2) a receiver array with full angular aperture, and (3) data of infinite bandwidth. Images obtained from a multioffset vertical seismic profile (VSP) illustrate some of the practical differences between the two imaging algorithms. The reconstructed image shows a much clearer picture of the target (a reef structure), in part because the one‐way imaging operator eliminates artifacts caused by the limited aperture of the receiver array.

Reservoir Development Using Offset VSP Techniques in the Denver-Julesburg Basin

Multioffset vertical seismic profiles (VSP's) have been used successfully to delineate and develop a Sand D field in the Denver-Julesburg basin. Sand D production in Wattenberg field was established by the Sierra Energy and Berge Exploration Well 34-3. Subsequently, three successive wells failed to find the Sand D channel. A stratigraphic model study suggested that offset VSP techniques could be used to determine the extent of the Sand D reservoir accurately and to identify potential offset drilling locations. On the basis of this study, a five-offset VSP was conducted in the discovery well. Interpretation of the data produced an accurate map of the reservoir limits and identified two additional drilling locations. The first of these locations was drilled and completed in Sand D. Subsequently, a VSP was run in this well to complete the delineation of the reservoir and to reconfirm the second offset location.

Multi-Offset Vertical Seismic Profiles: Fracture and Fault Identification for Appalachian Basin Reservoirs--Two Case Examples: ABSTRACT

Many Appalachian basin reservoirs occur in older rocks that are commonly fractured and faulted. These fractures and faults very often act as the reservoir trapping mechanism, especially in lithologies with no log-detectable matrix porosity. Traditional logging techniques, although possibly showing fault or fracture presence in the well bore, seldom provide clues to the extent of fracturing or location of nearby faults. Surface seismic data should show faults and perhaps even fracturing, but showing these features is often not possible in rugged terrain or in areas with thick coverings of unconsolidated surface material. Traditional seismic also has resolutions lower than that needed to detect small faults (less than 70 ft). Two case examples are shown from the northern Appalachian basin. The first example utilizes Schlumberger's slim hole seismic tool in cased holes in an area of thick unconsolidated glacial material along the Bass Island trend of western New York. The second example utilizes Schlumberger's SAT tool in an open-hole environment in an area of northwestern Pennsylvania with disturbed surface bedding and poor conventional surface seismic returns. The slim hole tool provides good data but with only slightly greater resolution than surface Vibroseis data. The SAT tool provides excellent resolution (down tomore » 25 ft) in highly disturbed bedding.« less

Reconstruction of reflecting structures from vertical seismic profiles with a moving source

When a vertical seismic profile (VSP) is recorded, the illuminated part of a reflector depends upon the shape and position of the reflector itself as well as on the seismic velocities and the positions of sources and receivers. A preferable arrangement for the investigation of structures of reflectors is to fix the receiver(s) at constant depth(s) in the well and move the source horizontally along a line at the Earth’s surface, usually called a “multioffset VSP” (MSP) or “walkaway VSP.” As a test of the resolution power of this survey geometry, synthetic records were generated from a subsurface model by inverse Kirchhoff migration. Three different methods were applied for the reconstruction. Wavefront construction leads to the correct shape of the reflectors, thus assuring the validity of the modeling method applied. Reflection‐point mapping delivered a near similarity to the model, but without focusing fault edges. Kirchhoff migration resulted in a detailed image of the reflectors with fault edges focused. Application of reflection‐point mapping and Kirchhoff migration to a real survey delivered results consistent with results from a survey at the Earth’s surface.

Theoretical vertical seismic profiling seismograms

Offset vertical seismic profiling (VSP) theoretical seismograms which include multiples and mode conversions can be computed using a modified “reflectivity” method. In this method, the transformed displacement potentials are first calculated by multiplying the source spectrum by the composite reflectivity function. Integration over wavenumber, followed by inverse Fourier transformation over the frequency range of the signal, yields the synthetic trace. The composite reflectivity function for a buried receiver is derived from Kennett’s matrices (Kennett, 1974, 1979) which are synthesized to form phase‐related reflection and transmission coefficients from a layer stack. Both conventional fixed source‐moving receiver and fixed receiver‐walkaway source (multioffset) VSP geometries can be handled easily. The method can also readily accommodate deviated‐hole VSP. The method is general in that no ray needs to be specified. Because the order of the multiples can be controlled, wraparound problems with the discrete Fourier transform can be avoided. The normal‐incidence VSP seismograms can be rapidly generated as a special case. Several examples illustrate the method. Some classes of laterally varying structures can be approximately handled by restricting the range of ray‐angle integration and by using the principle of superposition.