Robert H. Tatham

V p V s and lithology

Published laboratory investigations suggest an association exists between the ratio of seismic compressional and shear‐wave velocities (Vp/Vs) and sedimentary rock lithology. Comparisons of some theoretical models with these laboratory studies suggest that crack, or pore, geometry has a stronger effect on observed Vp/Vs values than elastic constants of the minerals comprising the matrix. Further, it can be inferred that the observed association between lithology and Vp/Vs is a result of an association between lithology and distribution of pore and crack shapes. Direct observation of crack shapes for a variety of lithologies is a next step in strengthening these inferences. The present study reviews the empirical relations between Vp/Vs and lithology and examines two published theoretical models of cracked elastic media. The models suggest that seismic velocities of sandstones may be controlled by cracks and pores with aspect ratios in the range of 10-1 to 1, dolomite in the range of 10-2 to 10-1, and dens...

V p /V s ; a potential hydrocarbon indicator

Theoretically and experimentally, the shear‐wave velocity of a porous rock has been shown to be less sensitive to fluid saturants than the compressional wave velocity. Thus, observation of the ratio of the seismic velocities for waves which traverse a changing or laterally varying zone of undersaturation or gas saturation could produce an observable anomaly which is independent of the regional variation in compressional wave velocity. One source of shear‐wave data in reflection seismic prospecting is mode conversion of P waves to shear waves in marine areas of high water bottom P-wave velocity. A relatively simple interpretative technique, based on amplitude variation as a function of the angle of incidence, is a possible discriminant between shear and multiple compressional arrivals, and data for a real case are shown. A normal moveout velocity analysis, carefully coupled with this offset discriminant, leads to the construction of a shear‐wave reflection section which can then be correlated with the usua...

A PHYSICAL MODEL STUDY OF SHEAR-WAVE SPLITTING AND FRACTURE INTENSITY

In a series of physical model experiments, fractured media are simulated by stacks of thin Plexiglas sheets clamped together tightly to form blocks. The plates are assembled underwater, and a very thin water layer between the sheets prevents formation of an effectively welded interface between them. Thus, the stacked material is not a series of welded plates but rather a truly fractured medium simulating a potential petroleum reservoir with only fracture porosity and permeability. Sheets of constant thickness are used, but the intensity of fracturing between the different models is simulated by using different thicknesses of Plexiglas for each model. Observation of direct shear‐wave arrivals through the stack, with propagation parallel to the sheets and polarization of particle motion allowed to be parallel to, normal to, or in any arbitrary angle to the sheets, definitively demonstrate the existence of shear‐wave splitting and hence anisotropy. For Plexiglas sheets 1/16 in thick (0.16 cm) representing a ...

Separation of S-wave and P-wave reflections offshore western Florida

Hard water‐bottom marine environments, such as offshore western Florida, have presented particular problems in the acquisition and processing of seismic reflection data. One problem has been the limited angle of incidence (less than critical) available to P‐wave penetration into the subsurface. Mode conversion from P‐wave to S‐waves (SV), however, is quite efficient over a broad range of angles of incidence. After the success of a previously reported physical model experiment, an experimental line was acquired offshore western Florida. The 19 mile line, located approximately 100 miles west of Key West, Florida, was shot and processed. Three key factors have contributed to the successful recording of mode‐converted S‐wave reflections: (1) recognition of the effect of the group length on attenuation of energy arriving at large angles of incidence; (2) tau‐p processing techniques that allow separation of energy by angle of incidence; and (3) velocity filtering over a range of hyperbolic normal‐moveout (NMO) ...

Seismic shear-wave observations in a physical model experiment

The observation and common‐depth‐point (CDP) processing of mode‐converted shear waves is demonstrated for real data collected in a physical model experiment. The model, submerged in water, represented water depth scaled to 250 ft, the first subsea reflector at 4000 ft, and the last reflector at 7000 ft below the sea floor with a structural wedge at the center. Very efficient mode conversion, from P to SV and back to P, is anticipated for angles of incidence at the liquid‐solid interface (sea floor) between 35 and 80 degrees. The model, constructed of Plexiglas and 3180 resin, will support elastic shear‐wave propagation. One anticipated problem, internal reflections from the sides of the model, was solved by tapering the sides of the model to 45 degrees off vertical. The P wave reflection coefficient at an interface between Plexiglas and water is 35 percent for vertical incidence, but it diminishes to very nearly zero between 43 and 75 degrees. Thus, by tapering the sides of the model, any undesired intern...

Sensitivity analysis of seismic reflectivity to partial gas saturation

We analyze the sensitivity of seismic reflectivity to contrasts in density, seismic propagation velocities, Poisson’s ratio, and gas saturation using the complete Zoeppritz equations. Sensitivities of reflection coefficients to each bulk elastic parameter are computed as the partial derivatives of the seismic reflectivities, relative to each parameter. The sensitivity of reflectivity to gas saturation is then calculated as the full derivative of the reflectivities with respect to gas saturation, assuming both a homogeneous and a patchy distribution of gas in the pore fluids. We compute sensitivities for a sealing shale/gas-sand interface and a gas-sand/wet-sand (gas-water contact, GWC) interface. For the SH-SH reflectivity, the effect of density contrast is strongest in the 30°–50° range of incidence angles for the fluid-fluid interface and at nearer offsets for the shale/gas-sand interface. P-SV reflectivity forthe fluid-fluid interfaces is more sensitive to density contrast in the range of angles of inc...

Surface-wave dispersion applied to the detection of sedimentary basins

Seismic surface-wave velocities are greatly affected by crustal structure. Because there is a strong contrast in the physical properties of clastic sediments and underlying basement materials, surface-wave dispersion provides a fast, convenient, and inexpensive means of detecting sedimentary basins and estimating their thickness. Model calculations and published reports of explosion studies indicate that sedimentary thicknesses as shallow as 500 m ( approximately 1650 ft) should be detectable by analysis of routinely recorded earthquake seismograms.This study demonstrates the use of seismic surface-wave dispersion to detect sedimentary basins and to estimate their thickness. The technique is used first for the Mississippi embayment region of the U.S. Gulf Coast, where the crustal structure is known and the results can be verified, and then applied to offshore Greenland, where the crustal structure is unmapped but a sedimentary basin is suspected. The data used are available seismograms of natural earthquakes and, for the Gulf Coast area, an underground nuclear explosion.Because this technique requires only existing, readily available data and may be applied to many regions of the world, it offers an attractive reconnaissance tool in petroleum exploration. In the present study, surface-wave dispersion and the effects of shallow crustal structure are reviewed in light of this application, and the advantages and limitations of the technique are explored.

Tau-p Filtering

Examination, evaluation and processing of seismic data in the two-dimensional τ-p domain offers many advantages over analysis and processing in the originally recorded X-t domain or other transformed domains. Since the slowness p (1) is the reciprocal the horizontal phase velocity, (2) represents the apparent angle of emergence at the surface, and (3) in a flat-layered earth it is also a constant conserved quantity (the ray parameter) along the raypath, we can readily address (1) antenna and array problems, (2) angle of incidence effects, and (3) problems (such as multiples) associated with the ray geometry. Examples of these applications include attenuation of ground roll, separation of P and S-waves where the angle of incidence defines the efficiency of mode conversion, and more efficient attenuation of multiples where they are exactly periodic in the τ-p domain.

Time-lapse CSEM Analysis of a Shaly Sandstone Simulated By Comprehensive Petro-electric Modeling

We simulate a poorly consolidated shaly sandstone reservoir model representing a prograding near-shore zone geological pattern. To account for the spatial distribution of petrophysical properties, an effective porosity model is first simulated by Gaussian geostatistics. Dispersed clay and dual water models are then efficiently combined with other well-known theoretical and experimental petrophysical correlations to consistently simulate reservoir model parameters. Next, the constructed reservoir model is subjected to numerical simulation of multi-phase fluid flow to replicate a waterflooding scenario on a black oil reservoir and to predict the spatial distributions of fluid pressure and saturation. Finally, a modified Archie’s equation for shaly sandstones is utilized to simulate rock resistivity. As a result, the comprehensive petro-electric model developed in this paper can be efficiently utilized in sensitivity analyses of CSEM (controlled-source electromagnetic) data to petrophysical properties and, ultimately, applied to reservoir characterization and monitoring research. As an example, we choose to present a time-lapse frequency domain CSEM feasibility study over the 2D reservoir model embedded in a 1D background resistivity model. Three sets of marine 2.5D CSEM data are simulated by a parallel adaptive finite element algorithm. Our analysis demonstrates that a detectable time-lapse signal after 5 years and a strong time-lapse signal after 10 years of waterflooding are attainable using the current CSEM technology.

Thin layers and shear-wave splitting

The near‐surface weathering layer is considered by many to be strongly anisotropic. Any shear‐wave signal passing through this low‐velocity layer will inherit, to some degree, the anisotropic response of this layer. For thin weathering layers, information about previous anisotropic events may be distorted; when the thickness of this layer approaches some physically defined limit, however, a previous layer’s anisotropic signature is completely overwritten. Hodograms and Alford rotations are typically used to analyze shear‐wave splitting in the presence of azimuthal anisotropy. When the time‐delay generated by an azimuthally anisotropic layer is ⩾τ/8, where τ = one period of the wavelet’s dominant frequency, distortion of a shear‐wave signal is great enough to degrade the accuracy of the interpretation in hodogram analysis. We found that Alford rotations are superior to visual hodogram analysis when the time delay between the fast and slow shear‐waves is less than τ/8. When two azimuthally anisotropic layer...