We investigate the fine structure of the D″ layer beneath the southwestern Pacific using short-period data from 7 deep events in the Fiji-Tonga region recorded at the J-Array in Japan. We use a double array migration method which integrates source array beam-forming with receiver array beamforming. The phase-weighted stacking technique, which reduces incoherent noise by employing complex trace analysis, is also applied. This combination greatly enhances small phases common to the source and receiver beams. We present coherent phases originating from the lowermost mantle using these new array techniques. The results indicate that seismic energy is reflected at two negative discontinuities near 2550 and 2650 km. The migration shows only weak evidence for scattering objects which may be an indication that scattering in the D″ layer beneath the southwestern Pacific is weak. ¿ 2001 American Geophysical Union