Don C. Lawton

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.

Imaging structures below dipping TI media

Seismic anisotropy in dipping shales causes imaging and positioning problems for underlying structures. We developed an anisotropic depth-migration approach for P-wave seismic data in transversely isotropic (TI) media with a tilted axis of symmetry normal to bedding. We added anisotropic and dip parameters to the depth-imaging velocity model and used prestack depth-migrated image gathers in a diagnostic manner to refine the anisotropic velocity model. The apparent position of structures below dipping anisotropic overburden changes considerably between isotropic and anisotropic migrations. The ray-tracing algorithm used in a 2-D prestack Kirchhoff depth migration was modified to calculate traveltimes in the presence of TI media with a tilted symmetry axis. The resulting anisotropic depth-migration algorithm was applied to physical-model seismic data and field seismic data from the Canadian Rocky Mountain Thrust and Fold Belt. The anisotropic depth migrations offer significant improvements in positioning and reflector continuity over those obtained using isotropic algorithms.

Orthorhombic anisotropy: A physical seismic modeling study

An industrial laminate, Phenolic CE, is shown to possess seismic anisotropy. This material is composed of laminated sheets of canvas fabric, with an approximately orthogonal weave of fibers, bonded with phenolic resin. It is currently being used in scaled physical modeling studies of anisotropic media at The University of Calgary. Ultrasonic transmission experiments using this material show a directional variation of compressional- and shear-wave velocities and distinct shear-wave birefringence, or splitting. Analysis of group-velocity measurements taken for specific directions of propagation through the material demonstrates that the observed anisotropy is characteristic of orthorhombic symmetry, i.e., that the material has three mutually orthogonal axes of two-fold symmetry. For P waves, the observed anisotropy in symmetry planes varies from 6.3 to 22.4 percent, while for S waves it is observed to vary from 3.5 to 9.6 percent.From the Kelvin-Christoffel equations, which yield phase velocities given a set of stiffness values, expressions are elaborated that yield the stiffnesses of a material given a specified set of group-velocity observations, at least three of which must be for off-symmetry directions.

Image mispositioning due to dipping TI media: A physical seismic modeling study

A scaled physical model was constructed to investigate the magnitudes of imaging errors incurred by the use of isotropic processing code when there is seismic velocity anisotropy present in the dipping overburden. The model consists of a block of transversely isotropic (TI) phenolic material with the TI axis of symmetry dipping at an angle of 45°. Its scaled thickness is 1500 m, and it is intended to simulate the dipping clastic sequences found in many fold‐thrust belts. A piece of isotropic Plexiglas, affixed to the underside of the anisotropic block, has a step function in it to simulate a target reef edge or fault. The anisotropy parameters of the material are δ = 0.1 and e = 0.24. On zero‐offset data the imaged position of the target is shifted laterally 320 m in the updip direction of the beds, whereas on time‐ and depth‐migrated multichannel sections the shift is 300 m. The lateral shift is offset dependent, with the amount of shift in any common‐midpoint gather decreasing from 320 m on the near off...

Ground penetrating radar: 2-D and 3-D subsurface imaging of a coastal barrier spit, Long Beach, WA, USA

Abstract The ability to effectively interpret and reconstruct geomorphic environments has been significantly aided by the subsurface imaging capabilities of ground penetrating radar (GPR). The GPR method, which is based on the propagation and reflection of pulsed high frequency electromagnetic energy, provides high resolution (cm to m scale) and shallow subsurface (0–60 m), near continuous profiles of many coarser-grained deposits (sediments of low electrical conductivity). This paper presents 2-D and 3-D GPR results from an experiment on a regressive modern barrier spit at Willapa Bay, WA, USA. The medium-grained sand spit is 38 km long, up to 2–3.5 km wide, and is influenced by a 3.7-m tidal range (spring) as well as high energy longshore transport and high wave energy depositional processes. The spit has a freshwater aquifer recharged by rainfall. The GPR acquisition system used for the test was a portable, digital pulseEKKO™ system with antennae frequency ranging from 25 to 200 MHz and transmitter voltages ranging from 400 to 1000 V. Step sizes and antennae separation varied depending on the test requirements. In addition, 100-MHz antennae were used for conducting antennae orientation tests and collecting a detailed grid of data (50×50 m sampled every meter). The 2-D digital profiles were processed and plotted using pulseEKKO™ software. The 3-D datasets, after initial processing, were entered into a LANDMARK™ workstation that allowed for unique 3-D perspectives of the subsurface. To provide depth, near-surface velocity measurements were calculated from common midpoint (CMP) surveys. Results from the present study demonstrate higher resolution from the 200-MHz antennae for the top 5–6 m, whereas the 25- and 50-MHz antennae show deeper penetration to >10 m. For the study site, 100-MHz antennae provided acceptable resolution, continuity of reflections, and penetration. The dip profiles show a shingle-like accretionary depositional pattern, whereas strike profiles show a horizontal and subhorizontal, nearly continuous reflection pattern. Results from the GPR experiment reveal upper shoreface reflections with dip towards the ocean at about 1–2°. The loss of signal from below a depth of 6–8 m indicates a lithofacies change because of the storm wave base. The parallel broadside and perpendicular broadside antennae orientation tests show detailed stratigraphy, continuity, and depth of penetration. The cross-polarization test exhibits reduced continuity of reflections and less depth of penetration, but dipping reflections are apparent. The grid pattern data provided a detailed view of 3-D geometry of individual reflections. High quality data were obtained, processed, and directly exported into a LANDMARK™ workstation for interpretation. The resulting interpretations of the upper shoreface beds from the test cube (50×50 m; total 2600 traces) are shown as vertical sections (slices), horizontal sections (time slices), contour maps, 3-D representations of individual beds, and an isopach map. The 3-D depositional framework allows a more detailed interpretation than widely spaced 2-D profiles.

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.

Computation of refraction static corrections using first-break traveltime differences

Differences in first-arrival traveltimes between adjacent records in multifold reflection surveys can be used to compute the depth and velocity structure of near-surface layers. The procedure uses the redundancy of first-break data in multifold surveys to enable a statistically reliable refraction analysis to be undertaken for either end-on or split-spread recording geometries. The traveltime differences as a function of source-receiver offset provide a direct indication of the number of refractors present, with each refractor being defined by an offset range with a constant time difference. For each refractor, the time-difference value at a common receiver from two shotpoints is used to partition the intercept time into the delay time at each shotpoint. This procedure is repeated until the delay times at all shotpoints and for all refractors have been computed. Refractor depths and velocities are evaluated from this suite of delay times. A surface-consistent static correction to a selected datum level is then calculated at each surface station, using a replacement velocity equal to that of the deepest refractor.In a case history from the Canadian Rocky Mountain foothills, short- and intermediate-wavelength weathering static anomalies were resolved successfully. Elevation and weathering static corrections of up to 40 ms were computed, with an estimated error of less than + or -3 ms.

Amplitude responses of thin beds : sinusoidal approximation versus Ricker approximation

The amplitude response of a thin bed with arbitrary upper and lower normal incidence reflection coefficients is studied. Two analytical expressions for the normal incidence amplitude response as a function of the thickness are derived and are both valid for weak reflectivities and for thicknesses below (1/8)λd, where λd is the dominant wavelength. The first expression is based on the substitution of a cosine wave for the source wavelet, and the second is based directly on the analytical expression for a Ricker wavelet. The results calculated from these two expressions are compared to numerical modeling results for a Ricker wavelet for several models. We found that the differences between the two expressions are small, and both are good approximations. Above the (1/8)λd thickness, the percentage differences increase rapidly for both expressions, implying that the thin‐bed assumptions in both derivations break down rapidly beyond the (1/8)λd thickness. Below the (1/8)λd thickness, except in the case where t...

A practical method for estimating effective parameters of anisotropy from reflection seismic data

The location of any event imaged by P‐wave reflection seismic data beneath a tilted transversely isotropic (TTI) overburden is shifted laterally if isotropic velocities are used during data processing. The magnitude of the shift depends on five independent parameters: overburden thickness, angle of tilt, symmetry‐axis velocity, and the Thomsen anisotropy parameters and δ. The shift also varies with source–receiver offset.We have developed a procedure to estimate these five parameters when the tilt of the symmetry axis from the vertical is equal to the dip of the TTI layer (except in the special cases transverse isotropy with vertical or horizontal axis of symmetry). We observe three attributes of seismic data processed using isotropic velocities: the zero‐offset arrival time of a selected reflection, the difference in arrival time between a near‐offset and a far‐offset arrival, and the difference in imaged location (smear) of this target event between the same offsets. We then perform a cascaded scan of t...