The Downhole Orbital Vibrator (DOV) provides a highly stable, acoustically-coupled, linear-physics, cylindrically-symmetric 50 ～ 350Hz swept-frequency far-field seismic wavelet with angular power-band at angles < 20° ～ 30° from the radial plane normal to the host borehole. DOV acoustic waves have a uniform displacement amplitude of order 1 ～ 3μ across the DOV axial cross-section. The resulting acoustic wave-front acts as a plane wave diffracted at a slit with DOV length and width. A slit-diffraction spectrum is observed in DOV far-field seismic spectrum; the far-field spectrum is shifted by factor f relative to the sweep-monitor spectrum. The DOV equivalent mechanical size predicted by the diffraction-slit model a-grees with observed radiation amplitudes to within 20 % . Tube waves are negligible. DOV sourcing promises to enable crosswell seismics to exploit wave-guide reservoir structures such as that of the Liaohe Oil Field, NE China, to achieve two maior observational benefits: interwell offsets of order 1km, and time-lapse monitoring of oil-water contacts. (1) DOV correlated-wavelet wave-guide signals with S/N ≈ 5 are observed at 650m offsets at Liaohe. Source energy concentrated near the radial plane makes the DOV efficient for transmitting seismic energy along sand-shale wave-guide formations and avoids exciting tube waves. The 50 ～ 350Hz frequency range is compatible with 1km crosswell transmission in Q ≈ 40 ～ 50 sedimentary rock at Liaohe; higher frequencies are largely absorbed. With waveguide energy propagating in 2 rather than 3 dimensions , it is likely that DOV wave-guide signals can be observed at offsets of 1km or greater. At 1km inter-well offsets, crosswell seismic wave-guide surveys can be expected to encompass multiple well-pairs. For N randomly located wells with 1km mean offset, 1km-offset crosswell data yields integrated wide-sector data ∝ N~2 well-pairs rather than isolated narrow-sector data ∝ N well-pairs available with 300 ～ 400m offsets. (2) Suitably edited stacks of DOV auto-correlated monitor wavelets are stable to a few parts in 10,000. Dedicated borehole-to-borehole travel-time monitoring data over & 300m offset is accurate to 100 μs for period of a few hours and has resolved 3ms tectonically-in-duced travel-time variations over a period of 13 days with S/N ≈ 30. This level of source-wavelet stability promises to permit suitably sensitive travel-time and amplitude monitoring of wave-guide trapped seismic energy to target movement of oil-water interfaces in production sands such as present at the Liaohe oil field.
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