Robert R. Stewart

Converted‐wave seismic exploration: Methods

Multicomponent seismic recording (measurement with vertical‐ and horizontal‐component geophones and possibly a hydrophone or microphone) captures the seismic wavefield more completely than conventional single‐element techniques. In the last several years, multicomponent surveying has developed rapidly, allowing creation of converted‐wave or P‐S images. These make use of downgoing P‐waves that convert on reflection at their deepest point of penetration to upcoming S‐waves. Survey design for acquiring P‐S data is similar to that for P‐waves, but must take into account subsurface VP/VS values and the asymmetric P‐S ray path. P‐S surveys use conventional sources, but require several times more recording channels per receiving location. Some special processes for P‐S analysis include anisotropic rotations, S‐wave receiver statics, asymmetric and anisotropic binning, nonhyperbolic velocity analysis and NMO correction, P‐S to P‐P time transformation, P‐S dip moveout, prestack migration with two velocities and wa...

Converted-wave seismic exploration: Applications

Converted seismic waves (specifically, downgoing P‐waves that convert on reflection to upcoming S‐waves are increasingly being used to explore for subsurface targets. Rapid advancements in both land and marine multicomponent acquisition and processing techniques have led to numerous applications for P‐S surveys. Uses that have arisen include structural imaging (e.g., “seeing” through gas‐bearing sediments, improved fault definition, enhanced near‐surface resolution), lithologic estimation (e.g., sand versus shale content, porosity), anisotropy analysis (e.g., fracture density and orientation), subsurface fluid description, and reservoir monitoring. Further applications of P‐S data and analysis of other more complicated converted modes are developing.

Exploration Seismic Tomography: Fundamentals

This publication encompasses seismic tomography from the earliest work to current exploration and development imaging. Applications and case histories are presented.

Tomographic determination of three-dimensional seismic velocity structure using well logs, vertical seismic profiles, and surface seismic data

A tomographic technique (traveltime inversion) has been developed to obtain a two‐ or three‐dimensional velocity structure of the subsurface from well logs, vertical seismic profiles (VSP), and surface seismic measurements. The earth was modeled by continuous curved interfaces (polynomial or sinusoidal series), separating regions of constant velocity or transversely isotropic velocity. Ray tracing for each seismic source‐receiver pair was performed by solving a system of nonlinear equations which satisfy the generalized Snell’s law. Surface‐to‐borehole and surface‐to‐surface rays were included. A damped least‐squares formulation provided the updating of the earth model by minimizing the difference between the traveltimes picked from the real data and calculated traveltimes. Synthetic results indicated the following conclusions. For noise‐free cases, the inversion converged closely from the initial guess to the true model for either surface or VSP data. Adding random noise to the observations and performin...

Joint PP and PS seismic inversion

We present a case history of joint inversion of PP and PS reflection seismic data using a weighted stacking technique. Our example comes from Blackfoot Field, owned and operated by PanCanadian Petroleum, in southeastern Alberta, Canada. The exploration target at Blackfoot is a Lower Cretaceous channel system approximately 1.4 km deep (Figure 1). Figure 1. The Glauconitic channel system at Blackfoot Oil Field, Alberta, is a sequence of sand and shale filled valleys incised into Lower Cretaceous and Mississippian carbonates. The Blackfoot interpretation has an upper and lower channel that are prospective and separated by a nonporous lithic channel. These Glauconitic channels, with sand or shale fill, are found throughout the region, and, as there were many episodes of channel formation, can be stacked on top of one another. At Blackfoot, the channel interval is about 40 m thick and 100 m wide. There tends to be good porosity in an upper channel and a lower channel that are separated by a tight, lithic channel. The upper channel, where present, is usually gas-prone, while the lower channel is generally oil-prone. When the pore fluid in the channel sands is a compressible hydrocarbon instead of incompressible water, the bulk compressibility is reduced and this modifies the signature of seismic reflection data. Because pressure waves and shear waves sense different rock and pore-fluid properties, joint use of PP and PS data can provide superior lithologic discrimination. The conversion of one elastic wave, either P or S , into another upon reflection or transmission at an interface is described by the Zoeppritz equations. These equations are algebraically quite complex and it is not practical to reproduce them here. Instead, we will present useful concepts and approximate forms. (We invite the reader to visit our Web site, http://www.crewes.org, and interactively examine the equations using our …

Interpreting channel sands with 3C-3D seismic data

A 3C-3D seismic survey was acquired over the Blackfoot Field (near Strathmore, Alberta, Canada) in 1995. The survey, sponsored by a group of exploration companies, was planned and conducted by the CREWES Project (Department of Geology and Geophysics, The University of Calgary) and Boyd Exploration Consultants. Simultaneously with the surface data acquisition, a five‐level 3-C downhole tool (from Western Atlas International) was deployed in a well, and a 3C-3D VSP was recorded.

Field data comparisons of MEMS accelerometers and analog geophones

Geophones have been the motion sensor of choice in oil and gas exploration surveys for many years and for good reason: they require no electrical power to operate, are lightweight, robust, and able to detect extremely small ground displacements. However, the seismic industry recently has developed considerable interest in microelectro mechanical systems (MEMS) accelerometers, which are similar to those used to sense accelerations for airbag deployment and missile guidance (among many other uses). The MEMS element is tied to an application specific integrated circuit (ASIC) that includes sensor signal conditioning, feedback control, and digitization blocks. The result is a custom-built unit for seismic applications that requires a power supply to operate.

Rapid map and inversion of P-SV waves

Multicomponent seismic recordings are currently being analyzed in an attempt to improve conventional P-wave sections and to find and use rock properties associated with shear waves (e.g. Dohr, 1985; Danbom and Dominico, 1986). Mode-converted (P-SV) waves hold a special interest for several reasons: They are generated by conventional P-wave sources and have only a one-way travel path as a shear wave through the typically low velocity and attenuative near surface. For a given frequency, they will have a shorter wavelength than the original P wave, and thus offer higher spatial resolution; this has been observed in several vertical seismic profiling (VSP) cases (e.g., Geis et al., 1990). However, for surface seismic data, converted waves are often found to be of lower frequency than P-P waves (e.g., Eaton et al., 1991).

An algebraic reconstruction technique for weakly anisotropic velocity

Laboratory measurements and field experiments indicate that many sedimentary rocks display velocity anisotropy (Robertson and Corrigan, 1983; White et al., 1983; Thomsen, 1986; Winterstein, 1986; Gaiser et al., 1987). Accounting for anisotropy is proving important in several areas including the processing and interpretation of shear‐wave data (Lynn and Thomsen, 1986; Bush and Crampin, 1987) and the integration of vertical seismic profile (VSP) data and surface seismic data (Chiu and Stewart, 1987a).

Poststack migration of P-SV seismic data

The exploding‐reflector model is only satisfied for zero‐offset P-SV data when VP/VS is depth‐invariant. P-SV diffractions in a vertically‐inhomogeneous medium are approximately hyperbolic, and an expression for their migration velocity is derived. The resulting migration velocities are 6–11 percent less than the corresponding P-SV rms velocities. Migration of DMO‐corrected synthetic P-SV stacked data, using a conventional phase‐shift algorithm and the derived migration velocities, is found to adequately collapse diffractions, whereas migration using the rms velocity function gives significant overmigration.