MODELING GUIDED-WAVES IN CROSS-HOLE SURVEYS IN UNCRACKED AND CRACKED ROCK

The theoretical behaviour of guided waves in cross-hole seismic survey s is examined with dispersion curves and synthetic seismograms. The mo delling suggests, and field observations confirm, that the dominant en ergy in many cross-hole seismic surveys propagates as guided waves, as well as body waves, with the energy tied to an interface, or combinat ion of interfaces, whenever there are interfaces with significant velo city contrasts continuous between the wells. The dispersion and wavefo rms of such modal solutions carry detailed information about the inter nal structure, particularly the stress-aligned structure of fluid-fill ed inclusions in the immediate neighbourhood of the waveguide. Since t he information content can be controlled to some extent by the choice of frequency and source type and the level of source and receiver in t he two wells, such guided waves in cross-hole surveys may be important for monitoring changes in nearly horizontal reservoirs during hydroca rbon production processes. Initially, we investigate the basic propert ies of propagation in isotropic strata, and then examine the behaviour of guided waves in the stress-aligned fluid-filled inclusions.