In-situ seismic attenuation was measured through the upper basalt series using vertical seismic profile (VSP) data collected on ODP Leg 104 at the Vdring Plateau, located along the Norwegian continental margin. Assuming a constant Q model, measurements were made using the spectral ratio method over the bandwidth 7.8 to 62.5 Hz. An effective attenuation of 2.7 x 10 _4 dB-s/m ± 0.6 x 10~4 (Q-effective = 25) was measured over a 525-m interval. Apparent attenuation due to intrabed multiple reflections was computed using synthetic VSP data generated from a well log-derived impedance model. The contribution of apparent attenuation can account for all of the observed attenuation in the upper basalt series. Further, the computed apparent attenuation was found to vary with frequency /as/ 09 over the bandwidth 7.8 to 62.5 Hz, indicating that the initial assumption of constant Q was not valid. Q-apparent was computed at two frequencies: at 30 Hz Q-apparent = 55, and at 60 Hz Q-apparent = 29. Within our range of error, intrinsic attenuation in the upper basalt series is less than 0.6 x 10 ~4 dB-s/m (Q-intrinsic > 115).
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