ALBERT

Notes: The accuracy of estimating crack-strike from the algebraic equivalent of a popular technique, the dual source cumulative technique (DCT), for analysing shear-wave splitting in seismic experiments is evaluated for earth models permeated by different alignments of micro-cracks. A complementary analysis is performed using another analysis procedure, the dual-independent source-geophone technique (DIT), to investigate any benefits of the alternative formulation. The investigation considers synthetic vertical seismic profile (VSP) and reflection data for an earth model with two layers over a half-space, and three different classes of crack-strike variation with depth: uniform crack-strike, an abrupt change of crack-strike between the upper and lower layer, and a continuous increase over both layers. The synthetic data for zero-offset and near-offset VSPs and a reflection profile are computed using a full-wave modelling package in which equivalent anisotropic media simulate distributions of aligned vertical, parallel, water-filled microcracks. Estimates from the two techniques agree for the constant crack-strike model, but differ for the VSP data with crack-strike changes. The asymptotic behaviour of the two angular parameters θG and θS from DIT suggest that it may be used to determine crack-strike under appropriate circumstances in these VSPs, when the time-delay between the split shear-waves for the layer of interest exceeds the peak period of the wavelet. In this limit, θG tends to follow the crack-strike change with θS tending to a constant value, whereas DCT will give a misleading value between the upper and lower crack-strike. Although the behaviour of DIT is not understood in all cases, θG and θS values from the VSP data always appear to diverge near the point where an abrupt crack-strike change takes place. This could be used as a qualitative indicator for layer stripping. Both techniques agree for the reflection data as the recorded data matrix is necessarily symmetric, but still give misleading results for deeper layers in the presence of crack-strike changes. This study suggests that more care should be taken when designing and analysing experimental configurations for detecting crack properties in reservoir rocks, to consider the response and resolution limits of the analysis techniques. A note of caution is offered to those who directly interpret polarization estimates as crack-strike.

Notes: Shear-waves have complicated interactions with the free surface, particularly in the presence of low-velocity surface layers, topographic irregularities, and the expected near-surface crack and stress anomalies. Consequently, it has been suggested that shear-waves should be recorded subsurface in vertical seismic profiles (VSPs), in order to extract accurate information about the in situ crack and stress geometry contained in shear-wave splitting. This paper compares the information in synthetic shear-waves in reflection gathers and VSPs, in order to assess the relative merits of the two techniques for investigating shear-wave splitting. Synthetic seismograms demonstrate that in the presence of even very simple surface layers, shear-waves recorded in reflection surveys at the surface have polarizations which may not indicate crack and stress geometry at depth. In contrast, shear-waves recorded in VSPs are relatively unaffected by surface layers and near-surface stress and crack anomalies, and the behaviour of shear-wave splitting is dominated by the structure of the rock mass in the vicinity of subsurface geophones. Matrix rotations of multicomponent-multisource shear-wave reflection data to extract the information contained in the split shear-waves, are found to be directly meaningful only in situations where crack orientations do not change with depth.

Notes: The study of shear-wave splitting can yield information about crack strike, crack density, and other parameters of the cracks and aligned inclusions within the in situ rockmass. Such information is important for reservoir characterization and other hydrocarbon production applications. Crack density is usually inferred from the percentage shear-wave anisotropy which is measured from time delays between the two split shear waves. Reservoirs may be too thin for discernible time delays to build up, and the time delay (and crack density) in a reservoir layer may be unresolvable in conventional arrival-time analysis of reflection surveys or vertical seismic profiles.Shear-wave amplitude versus offset (AVO) techniques are studied to see if they provide a more viable method of determining anisotropic parameters in thin reservoir layers. The behaviour of reflected shear-wave amplitudes with angle of incidence is investigated in simple two-layer models. The variations in shear-wave AVO are calculated for a range of percentage shear-wave anisotropies for water-filled cracks and dry cracks in the second layer (representing a reservoir) to see if characteristic information about reservoir properties can be extracted from the shear-wave AVO signatures. The shear-wave AVO curves, for thin cracks, are sensitive to changes in anisotropy and crack content. Most of the information about crack content is contained in wide offset reflection data which suggests applications in crosshole monitoring of enhanced oil recovery. The variations in shear-wave AVO may be distorted by the acquisition system. Most of the information about the percentage anisotropy is contained in the amplitudes of shear waves reflected at near-vertical angles of incidence. Vertical incidence reflection amplitude methods are reviewed and a simple graphical procedure is suggested for determining their viability in different reservoir environments.