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

Laboratory data are reported for ultrasonic compressional wave attenuation (α p ) as a function of pore pressure in a Juan de Fuca Ridge dredge basalt. Four experiments have been made to determine the relationships between attenuation and quality factor (Q p ) and confining and pore pressures in the shallow ocean crust. Attenuation was measured at 1) a constant differential pressure of 40 MPa; 2) confining pressures to 120 MPa and atmospheric pore pressures; 3) a constant confining pressure of 50 MPa while varying pore pressures; 4) a constant confining pressure of 100 MPa while varying pore pressures. For atmospheric pore pressures, α p ranges from 1.64 dB/cm to 7.08 dB/cm (Q p = 24 to 10). In addition, attenuation increases systematically with increasing pore pressure and decreasing differential pressure (confining pressure ‐ pore pressure). Results from DSDP and ODP downhole packer experiments suggest that the hydrostatic pore pressure regime may best approximate in situ conditions for young oceanic crust. Hydrostatic pore pressures (@ 5000 meters depth) reduce Q p as much as 35% from normal atmospheric 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 well with seismic Q p data.