FRACTURE DETECTION USING CROSSHOLE SURVEYS AND REVERSE VERTICAL SEISMIC PROFILES AT THE CONOCO BOREHOLE TEST FACILITY, OKLAHOMA

Shear waves in combined crosshole surveys (CHSs) and reverse vertical seismic profiles (RVSPs) at the Conoco Borehole Test Facility, Oklahoma, are analysed to examine the relationship between shear-wave propagation and natural fracture systems. The surveys cover a range of azimuths and angles of incidence, and give the possibility of studying fracture-induced azimuthal anisotropy in some detail. The data display several features characteristic of seismic anisotropy. Shear-wave splitting, displaying substantial time delays up to 7 ms, for source-geophone distances of less than 50 m, suggest large fracture densities. Estimates of fast shear-wave polarizations and time delays confirm the N75-degrees-E +/- 10-degrees fracture strike deduced from other geological and hydrological observations. Analyses of CHS data including modelling with synthetic seismograms suggest that, at least the dominant fracture set, is rotated by 20-degrees from the vertical. One of the most important results of this study is the positive identification of guided waves propagating between wells along interfaces with sufficient velocity constrast in CHS profiles. It appears that combinations of CHSs and RVSPs offer unique possibilities in evaluating fracture parameters at shallow depths. This could be important for the study of fluid flow at shallow depths in hydrological investigations.