Robert Garotta

Combined interpretation of PP and PS data provides direct access to elastic rock properties

Multicomponent seismic has been around for more than 30 years but is still struggling to gain widespread acceptance. Although the benefits of shear-wave exploration are countless, only imaging through gas clouds is regularly implemented—a significant niche perhaps, but a niche nevertheless.

Detection of Azimuthal Anisotropy

The effect of azimuthal anisotropy on conventional P wave data is not a traditional subject for discussion in geophysical conventions. As a considerable number of 3D surveys, most of them involving a wide azimuth range, have been recorded and processed over a number of years, this situation is surprising: whenever accuracy and resolution are required, the effects of azimuthal anisotropy should be examined. To be realistic, the phenomenon is most often not very important, and it is not easy to produce evidence. As of now, one difficulty is in the very small size of the measurable effects, for example azimuth-dependent travel-time differences. More precisely, this size is most often below the level of uncertainty concerning velocity functions, especially in the presence of structures. It seems that real 3D tomography, which is not yet in use, would be the solution. However, in cases of very gentle and constant dip, detection is possible using simple investigations. In such cases, reservoir engineers will no doubt be interested in some of the attributes of azimuthal anisotropy, for example the orientation of the natural coordinates, and maybe some estimate of the intensity of the phenomenon.

Improving resolution and seismic quality assurance through field preprocessing

The constant need to improve the resolution of seismic surveys has led to a continuous increase in the number of acquired channels which has, in turn, resulted in a decrease in the station interval and the spatial filtering of the receiver groups. Over a 40-year period, the average channel number has doubled every seven years (an annual rate of 12%). Advances in processing and large dynamic range recording have made it possible to tolerate noisy traces and thus respect the high‐frequency components of the signal. If acquisition technology improves at a similar rate over the next decade, the recording of every individual receiver is realistic.

About compressional, converted mode, and shear statics

Strong near-surface lateral variations, the lack of a smooth shallow refraction marker, and poor signal-to-noise ratio, can, either alone or in combination, induce residual static problems that affect the seismic result in different ways. These depend on the amplitude and wavelength characteristics of the residual anomalies. In addition, as this article will describe, different types of waves—i.e., compressional or noncompressional—require different statics corrections.

Elastic parameter derivations from multi-component data.

The estimation of elastic parameters from seismic data provides lithologic information for reservoir characterisation. In theory, elastic parameters can be derived from conventional seismic, based upon the amplitude versus offset relationship of compressional mode reflectivity. In practice, this approach is difficult to use, because it requires uncommonly favorable conditions: • accurate move-out corrections at long offsets, • high signal-to-noise ratio. Multi-component technology increases the robustness of elastic inversion: • long-offset considerations are no longer mandatory since more data from P and S (or PS) wave modes are available at shorter offsets, • P and S reflectivities are involved in better balanced conditions, allowing a somewhat lower signal-to-noise ratio than for single-wave mode. The price to pay to access this bimodal inversion is the accurate association of P and S (or PS) propagation times, which correspond to the same depth. The proposed approach to reach this accurate association consists in equating two possible derivations of the Vp/Vs ratio: • first, through the P and S (or PS) transit time ratio, • second, through the P and S (or PS) velocity contrasts obtained from P and S (or PS) reflection coefficients. Non-linear optimisation techniques are used to minimize the differences. Once P and S (or PS) times are accurately associated, the side product (Vp/Vs ratio or Poisson’s ratio) is a piece of information in itself. Moreover, elastic inversion can be obtained from a system of Zoeppritz equations involving P and S (or PS) reflectivities, instead of P-mode reflectivity alone.

3D PP-PS joint Stratigraphic Elastic Inversion through combined AVA attributes

Multi-component seismic is increasingly being recognized not only as a solution to imaging problems, particularly where conventional seismic fails, but also as a tool for reservoir characterization. But geologists are often unenthusiastic to extract earth information from multi-mode seismic images because of their visual differences due to their own physical responses, and when the seismic images are equivalent the PS data does not provide any additional information by itself. The stratigraphic inversion is considered as a tool to transform seismic amplitude variations into petrophysical variations. With multicomponent acquisition, the AVA attributes of PP and PS modes are usually accessible, providing the additional shear wave information for a more constrained elastic inversion. Our inversion method simultaneously reconciles amplitude and time information from both PP and PS-wave data by equating the value of γ = Vp/Vs derived from the velocities or transit time ratio (γt ) and the value derived from the reflectivity ratio (γa), providing not only the high-resolution of the Vp/Vs = γr or the Poison ratio but also eventually Vp, Vs and densities (Garotta et al., 2000). A trace by trace inversion using simulated annealing process was described by (Dariu et al., 2003). In this paper we propose to examine some new possibilities and an extension of the method to 3D constraints in order to decrease the sensitivity to the noise and take into account the geological structure.

About Gamma ratios and their combinations

The ratio between the compressional to shear velocities (Gamma or Γ = Vp/Vs) is a key parameter in the combination of P and S (or PS) data. It can be derived in several ways. The most obvious are the ratio between the S to P propagation times between associated events (ΓT) and the ratio between P to S normal moveout velocities (ΓV). Comparing P and S (or P and PS) seismic amplitudes also gives access to Gamma ratio (ΓA). The different derivations have different properties involving or not anisotropy effects: for example, the ratio ΓV / ΓT detects the effects of anisotropy and can be a lithology indicator. Some other combinations of Gamma ratios are already in use, such as Γeff, defining the location of the conversion point in PS propagation.

Long-Wavelength Static Definition Through Partial Common-Midpoint Stacks

Modeling and spatial scanning of stacking velocity anomalies have usually been used to define long-wavelength residual statics. Basically, such methods involve differences between reflection times of different offset races. Direct comparison of partial common-midpoint stacks leads to equivalent results. Moreover, this approach facilitates the definition of the wavelength range that can really be covered. The response of the procedure is not constant within that range; it may be corrected in two ways: (1) Compensation of amplitude response can be computed and apIlied in the wavelength domain, provided the signal-to-noise ‘S/N) ratio is high enough. (2) Reiteration of the procedure is nore cumbersome, but can be applied to eliminate the remaining momalies, even when the S/N ratio is only fair. The affordable wvelength range depends on the offset difference of the partial tacks. In any case, some criteria must be defined to dispatch ime differences between very long wavelength statics and residal normal moveout.

Advances in the Detection and Compensation of Birefringence

Summary The effects of birefringence or shear wave splitting on a single raypath recorded with a good signal-to-noise ratio can be easily described and formalized, offering a way of deriving birefringence attributes: natural orientation, percentage of azimuthal anisotropy and differential attenuation. To benefit from a good signal, attributes are most often derived from between the main seismic horizons, limiting the resolution. A brief description of the basic principles and procedures used to define birefringence attributes is presented. A global optimization approach that is able to deliver the birefringence attributes within a 3D block for thinner intervals is proposed.

Shear-wave Splitting Analysis Using Simulated Annealing

By using PS-wave multi-component seismic data, it is possible to obtain robust estimates of several subsurface parameters. A global optimization approach is proposed to automatically estimate high-resolution birefringence (splitting) attributes versus depth. Estimated quantities, such as the natural direction of the fast S-wave and the time lag between the fast and slow S-waves, can be directly related to fracture orientation and their density. Birefringence effects of the overburden must be determined and removed prior to estimation of the fracture properties at target horizons. Tests of this global optimization method on synthetic and real case studies were successful.