Effects of pore pressure on compressional wave attenuation in a young oceanic basalt

Laboratory data are reported for ultrasonic compressional wave attenuation (alpha(p)) as a function of pore pressure in a Juan de Fuca Ridge dredge ba salt. Four experiments have been made to determine the relationships betwee n attenuation and quality factor (Q(p)) and confining and pore pressures in the shallow ocean crust. Attenuation was measured at 1) a constant differe ntial pressure of 40 MPa; 2) confining pressures to 120 MPa and atmospheric pore pressures; 3) a constant confining pressure of 50 MPa while varying p ore pressures; 4) a constant confining pressure of 100 MPa while varying po re pressures. For atmospheric pore pressures, alpha(p) ranges from 1.64 dB/ cm to 7.08 dB/cm (Q(p) = 24 to 10). In addition, attenuation increases syst ematically with increasing pore pressure and decreasing differential pressu re (confining pressure - pore pressure). Results from DSDP and ODP downhole packer experiments suggest that the hydrostatic pore pressure regime may b est approximate in situ conditions for young oceanic crust. Hydrostatic por e pressures (@ 5000 meters depth) reduce Q(p) as much as 35% from normal at mospheric pressure conditions; therefore, pore pressures. generated in the upper oceanic crust may be responsible in part for the observed low seismic Q(p) within layer 2A. Q(p) measurements at elevated pore pressures agree w ell with seismic Q(p) data.