EXTENSIVE‐DILATANCY ANISOTROPY: RELATIVE INFORMATION IN VSPs AND REFLECTION SURVEYS¹

Abstract

YARDLEY, G. and CRAMPIN, S. 1990. Extensive-dilatancy anisotropy : relative information in VSPs and reflection surveys. Geophysical Prospecting 39,337-355.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, shearwaves 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. known as extensive-dilatancy anisotropy or EDA (Crampin, Evans and Atkinson 1984;Crampin 1985Crampin ,1987a.Since analysis of the detailed waveforms of the shear-waves is important for estimating the internal structure of the rock mass, it is desirable to minimize any disturbance to the behaviour of the split shear-waves as they travel through the reservoir to the recording instrument. Shear-waves have complicated interactions with the free surface (Booth and Crampin 1985), particularly in the presence of low-velocity surface layers, topographic irregularities and the expected near-surface crack-and stress-anomalies (Crampin 1990). Consequently, it has been suggested that shear-waves should be recorded subsurface in VSPs, rather than in surface reflection lines, in order to preserve the valuable information about crack and stress geometries contained in the shear wavetrain (Crampin 1987a;Crampin, Lynn and Booth 1989). VSPs, however, are expensive, especially in the high-cost production environments where information about crack and stress geometries would be particularly valuable.We compare the relative information in synthetic shear-wave seismograms from reflection surveys and VSPs in the presence of near-surface layering. We also examine the viability of matrix rotations of multicomponent-multisource data to extract the anisotropic parameters from recorded shear-wave data.Note that the terminology used for seismic anisotropy is given by Crampin (1989). In general, the faster and slower split shear-waves are re...

Waveform modeling of shear wave splitting from anisotropic models in Iceland

“…A shear wave splits into a fast and slow wave when it encounters an anisotropic boundary. Theoretically, as the wavefield enters the top anisotropic layer, the fast and slow waves should split again and form four phases [ Yardley and Crampin , 1991]. To better view the interference of fast and slow waves generated at each anisotropic boundary, we produce synthetic waveforms for a wave with an initial polarization of N10°E and an artificially short period of 1 s. Figure 4 clearly reveal that waveform changes with depth.…”

Fracture detection using crosshole surveys and reverse vertical seismic profiles at the Conoco Borehole Test Facility, Oklahoma

“…The range of angles of emergence is small, and the ray paths are close to vertical for nearly all source positions. This means that the arrivals at the surface are within the shear-wave window for nearly all source depths deeper than about 10 m, so that the effect of interactions of the shear waves with the free surface will be minimized (Yardley & Crampin 1991). It is expected that the orientations of fluid-filled inclusions and fractures may be distorted by near-surface stress anomalies (Crampin 1990b).…”

Delineating fracture zones using surface‐tunnel‐surface seismic data, P‐S, and S‐P mode conversions

“…Fractured media have strong effects on seismic wave propagation, such as causing shear wave birefringence, scattering, and attenuation or changes in the elastic parameters [ Crampin , ; Hudson , ; Eaton et al ., ]. In addition to these, oriented fractures are considered to be a common cause for seismic anisotropy [ Thomsen , ; Yardley and Crampin , ; Thomsen , ]. Apart from the influence on seismic properties, fractures in crystalline rock environments act as conduits for gas and fluid migration, hence affecting the local stress field, the hydrogeological regime, underground infrastructures, and drilling and mining activities, among others.…”