VELOCITY AND ATTENUATION ANISOTROPY CAUSED BY MICROCRACKS AND MACROFRACTURES IN A MULTIAZIMUTH REVERSE VSP

Previous analyses of crosshole survey (CHS) and reverse vertical seismic profile (RVSP) data sets at the Conoco Borehole Test Facility (CBTF) have shown that fracture parameters can be estimated from seismic data. Continuing this development, we analyse data recorded from shear-wave sources in a shallow multiazimuthal RVSP. The RVSP data cover 160” of azimuth and display symmetry in arrival times and amplitudes about an azimuth of approximately N70”E. They can be interpreted as both velocity anisotropy (azimuthal variation of traveltimes) and attenuation anisotropy (azimuthal variation of amplitudes). Since the direction N70”E has been previously identified as the strike of the dominant macroand microfractures, as well as the direction of maximum horizontal stress at depth in the area, the observed velocity and attenuation variations can be interpreted in terms of stress-aligned fractures and microcracks. The data also show evidence of scattering characteristic of large-scale fractures. These interpretations are confirmed by matching synthetic seismograms to the field data, where the observed symmetry features in the seismograms are well-reproduced in the synthetic data. Nearby surface exposures suggest there are two approximately perpendicular fracture sets at CBTF. In such media, the polarizations of shear waves are expected to be complicated and we show that under appropriate circumstances it is possible to infer multiple fracture sets if sufficient azimuthal coverage is available in either VSPs or RVSPs. The shear-wave polarizations, when plotted in equal-area polar projections, show two maxima in directions which are approximately orthogonal and almost exactly parallel to the strikes of the two fracture sets at a neighbouring surface outcrop. The observed shear-wave polarizations are well-matched by the synthetic shearwave seismograms calculated for media with two fracture sets. The remarkable agreement between measured shear-wave polarizations from RVSP data, CHS data and borehole data and from the synthetic seismograms confirms the usefulness of shear waves for mapping subsurface fractures. There is increasing interest in fracture characterizatior using geophysical methods, particularly shear-wave anisotropy and shear-wave splitting (reviewed by Crampin and Lovell, 199 1; Crampin, 1993). The Conoco Borehole Test Facility (CBTF) in Kay County, Oklahoma, is an excellent laboratory for the study of naturally fractured rock. Over the past few years CBTF has been the site of an extensive programme of integrated geological studies (surface mapping, core analysis and well-log analysis), geophysical studies [crosshole surveys (CHSs), vertical seismic profiles (VSPs) and reverse VSPs (RVSPs)] and hydrological studies (fluid flow measurements) conducted around an array of shallow groundwater boreholes about 50 m deep. Results from various studies have been reviewed by Queen et al. (1992). This paper is part of these integrated studies and is a continuation of our early analyses of CHS, VSP and RVSP data sets (Queen and Rizer, 1990; Queen et al., 1990; Rizer, 1990; Liu et al., 1991a, 1991b; Lines et al., 1992). The RVSP data analysed here provide convincing evidence of fracture-related seismic velocity anisotropy and attenuation anisotropy. This paper makes three principal contributions. Firstly, it shows clear evidence of systematic azimuthal traveltime and amplitude variations due to fractures at shallow depths from a multiazimuthal RVSP at CBTF site. The data also show evidence of the scattering characteristic of intermediatelength fractures where the fracture length and spacing are of the same order as the seismic wavelength. The observed variations in traveltime and amplitude are indicative of fracturerelated velocity and attenuation anisotropy and are modelled with synthetic seismograms, although the scattering thought to be caused by intermediate-length fractures has not yet been adequately modelled mathematically. Secondly, it illustrates the possibility of using multiazimuth VSPs or RVSPs to map both single and multiple subsurface fracture sets. The measured initial shear-wave polarizations show two clear directions which are almost parallel to the strikes of the two fracture sets mapped from surface outcrops. This result is further supported by a comparison with theoretical predictions. Thirdly, it shows that the results obtained from seismic ‘British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA ‘Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW “Exploration Research and Services, Conoco Inc., P.O. Box 1267. Ponca City, Oklahoma 74603 We thank Conoco Inc. for providing the RVSP data sets. The necessary preprocessing (including the rotary source decomposition) of the data was performed at Conoco Inc., where J.B. Sinton, V.D. Cox and P.L. Buller performed most of the earty analyses with J.H. Queen. We thank P. Wild and X.-W. Zeng for help with the data processing and D.C. Booth for useful discussion and comments. We also thank D. Corrigan and another anonymous reviewer for their comments for improving the manuscript. Synthetic seismograms were calculated using the ANISEIS package of Macroc Ltd. and Applied Geophysical Software Inc. This work was supported by Conoco (UK) Ltd. through Contract No. 2.343 and the Natural Environment Research Council and is published with the approval of Conoco Inc. and the Director of the British Geological Survey (NERC).