Michael C. Mueller

Coarse-layer stripping of vertically variable azimuthal anisotropy from shear-wave data

Alford rotation analysis of 2C × 2C shear‐wave data (two source components, two receiver components) for azimuthal anisotropy is valid only when the orientation of that azimuthal anisotropy is invariant with depth. The Winterstein and Meadows method of layer stripping vertical seismic profiling (VSP) data relaxes this restriction for coarse‐layer variation of the orientation of the anisotropy. Here we present a tensor generalization of the conventional convolutional model of scalar wave propagation and use it to derive generalizations of Winterstein and Meadows layer stripping, valid for 2C × 2C data and for the restricted 2C-only case, in the VSP and reflection contexts. In the 2C × 2C VSP application, the result reduces to that of Winterstein and Meadows in the case where both fast and slow shear modes have the same attenuation and dispersion; otherwise, a balancing of mode spectra and amplitudes is required. The 2C × 2C reflection result differs from the 2C × 2C VSP result since two applications of the...

Using shear waves to predict lateral variability in vertical fracture intensity

During the last five years, the use of shear‐wave splitting in geophysical exploration has made the transition from research project to very promising field technique. The measurement of the azimuth of the fast shear‐wave and analysis of the S-wave splitting (or birefringence) are now commonplace. Furthermore, a very timely practical application is suggested by the recent proposal that reflectivity on post‐rotation S1 and S2 data sets should indicate lateral variation of anisotropy. This could be of great value because the determination of lateral variation in fracture intensity in a vertically fractured sedimentary layer is necessary to insure high efficiency in a horizontal drilling program designed to drain the fractures.

Pitfalls of using conventional and discrete Radon transforms on poorly sampled data

The least‐square discrete Radon transform (DRT) is currently one of the most popular methods used in the suppression of multiples and other coherent noise events on irregularly sampled data gathers used for prestack true amplitude analysis. Unfortunately, in the absence of a priori information, this technique suffers from the same aliasing problems as Fourier and conventional (τ, p) methods. Although the DRT is able to reconstruct the original image more accurately than conventional (τ, p) transforms, a harmful by‐product is an increase in the amplitude of aliased events in the transform domain. In particular, the DRT will boost the amplitude of the aliases of true events that fall outside the p analysis window to help reconstruct the input data. These amplified aliases degrade signal periodicity in the (τ, p) domain. If muted, they can destroy subtle amplitude changes necessary for amplitude variation with offset (AVO) analysis. At the very least, one should carefully evaluate the choice of analysis wind...

Compensating for the effects of gas clouds on C-wave imaging : a case study from Valhall

P -to- S converted waves (or C -waves) have been successfully used to image beneath gas clouds in many areas. However, C -waves often suffer from severe diodic effects due to the gas clouds; that is, the C -wave amplitude and traveltime may be different in the forward and reverse shooting directions, giving rise to different C -wave stacking velocities ( diodic V C ) and velocity ratios ( diodic γeff). In some cases, whether a horizon (a geologic target) beneath a gas cloud can be undershot also depends on the shooting directions ( diodic illumination). These effects, compounded with the asymmetric raypath of the C -wave and the uncertainties in the P -wave data due to the gas clouds, will further increase the difficulties and costs of processing C -wave data. In this article, using the 2-D Valhall data as an example, we examine these effects and discuss ways to compensate for them during processing for improving the C -wave imaging. The 2-D Valhall data were acquired in 1996. In the survey area, the overburden layers are highly charged with gas. These gas clouds are not thought to be present in economic quantities, but they attenuate the P -wave energy substantially and reduce the P -wave velocity, inducing dimming and push-down effects on the P -wave data (Figure 1a). Figure 1. (a) The final prestack-migrated P -wave imaging from the vertical geophone of the Valhall data. Note the dimming and push-down effects due to gas clouds. The vertical axis is two-way P -wave time. (b) The initial poststack-migrated image of the C -wave from the in-line horizontal geophone of the Valhall data. The processing involves asymptotic common-conversion-point (ACCP) binning, NMO, DMO, stack, and poststack migration. The vertical axis is two-way C -wave time. The picks shown are examples of P -wave to C -wave …

Case studies of the dipole shear anisotropy log

Crossed dipole acoustic logs represent a new class of measurement of subsurface formations which can provide detailed information on anisotropy not previously available. As with any new class of data, departures from previous conceptions may occur. In this case, the detail inherent in sonic-scale measurements provide information on anisotropy at sub-formation scale whereas seismic investigations of anisotropy are normally restricted to sequence-scale. When borehole conditions allow formation properties to be measured, the tool provides detailed information interpretable in terms of fracturing. Crossed dipole acoustic logs were acquired using the multisource/multireceiver technique, and Alford rotation (Alford, 1986; Thomsen, 1988 and Esmersoy, 1994) was employed to resolve observed flexural or shear-wave mode splitting. The data show indications of sensitivity to natural fracturing and, secondarily, to borehole ovality. The azimuth of the leading shear-wave polarization, qS1, compare favorably with independent data on fracture azimuth. Thomsen anisotropy parameter {gamma} as high as 10--20% is observed in certain anisotropic intervals. Such intensities, and their spatial occurrence on log data, are new information on anisotropy.

Effective VTI anisotropy for consistent monitoring of microseismic events

The monitoring of induced or triggered microseismic events increasingly is being used to inform the efficient production of unconventional reservoirs. A key aspect of economic production in these low-permeability rocks is hydraulic fracture stimulation, usually in horizontal wells. To evaluate the success of the stimulation, engineers rely on monitoring the induced (or triggered) microseismic events that are then interpreted to map the stimulated reservoir volume and likely drainage area of the well. These microseismic events can be mapped either from downhole or surface monitoring arrays. In this study, we discuss a newly developed methodology that allows economic and consistent mapping of microseismic events from multiple stimulated wells across an entire field. This approach allows better comparison of stimulation techniques between wells in order to optimize long-term development of the reservoir. As well, the method enables a relatively robust observation of velocity anisotropy that leads to better w...

Reducing Structural Uncertainty On the Azeri Field Using Ocean Bottom Seismic: Offshore Azerbaijan

The interpretation of 3D Ocean Bottom Seismic survey over part of the Azeri field, offshore Azerbaijan has delivered significant reduction in structural uncertainty. The crest and South/Central flank of the field was poorly imaged on towed streamer data; this led to “no data areas” with large depth uncertainty and a poorly resolved structural model. The depth migrated PZ products from the OBS survey have facilitated a choice between alternative structural interpretations and provided improvements in depth prediction for development drilling.

Layer‐stripping of azimuthal anisotropy from reflection shear‐wave data

A common technique for analyzing shear-wave splitting is the tensor rotation (Alford, 1986) or vector rotation (Thornsen, 1988) of multicomponent shear data. These techniques, as originally formulated, require that the principal directions of azimuthal anisotropy do not vary with depth, so that the shear modes split only once. They commonly succeed, to a good approximation (cf, eg, Willis et al (1986), but rarely succeed perfectly, raising doubts as to the validity of this assumption.

Interpretation of a dipole shear anisotropy log in a thrust belt setting

A dipole shear anisotropy log taken in the region of the northern Oman thrust belt reveals complexity in the stress state and natural fracturing. This complexity would not be predicted from a simple assumption of lithology or formation bounded behavior. Three anisotropic signatures are contained in this dataset: high magnitude and consistent azimuth due to natural fracturing, low magnitude and inconsistent azimuth due to isotropy, and medium to high magnitude and consistent azimuth due to borehole ovality caused by tectonic stress. These three signatures, however, do not correspond to formational or lithologic boundaries in the logged interval.