Vertical Seismic Profile into Upper Oceanic Crust in Hole 504B

Abstract

Seismograms from vertical seismic profiles (VSP) on Ocean Drilling Program Legs 111 and 148 were merged. First arrival traveltimes picked on the best quality traces are equivocal in the sediment but suggest an average velocity of about 1.8 km/s for the lower 110 m of section. In basement, VSP velocities follow the trend of the sonic log velocities but are generally equal to the highest sonic velocities measured in an interval. The general trend of VSP velocity follows that of refraction velocities, but the decrease in gradient marking the seismic Layer 2/3 boundary occurs ~500 m shallower at Hole 504B than in the surrounding 1-2 km region and appears to correlate with the top of the sheeted dikes. A downgoing arrival with velocities of ?>-A km/s is interpreted as a shear wave converted either by rough basement surface near the borehole or by shallow-buried lateral velocity variations. Several other arrivals can be most easily explained as coherent energy scattered from basement roughness or velocity inhomogeneities within the upper 500 m of basement. Downhole increases in VSP velocities correlate with events in the upgoing wavefield suggesting internal crustal reflections, but the amplitudes are no greater than side-scattered energy and coherency is generally poor over borehole depths exceeding 100-200 m.

Seismic attenuation in upper ocean crust at Hole 504B

“…Curve-fitting the digital data indicate that the error due to missampling the peak amplitude is <5%. A downward propagating secondary arrival, which originates from a scatterer at or near the top of basement and has amplitudes within an order of magnitude of the direct wave [Swift et al, 1996] closely follows the first pulse in the lower -800 m of the borehole (Figure 1). Vertically polarized shear arrivals, generated by scattering away from the borehole near the top of basement, also propagate downhole after the direct wave [Swift et al, 1996;Swift et al, 1998b].…”

“…A downward propagating secondary arrival, which originates from a scatterer at or near the top of basement and has amplitudes within an order of magnitude of the direct wave [Swift et al, 1996] closely follows the first pulse in the lower -800 m of the borehole (Figure 1). Vertically polarized shear arrivals, generated by scattering away from the borehole near the top of basement, also propagate downhole after the direct wave [Swift et al, 1996;Swift et al, 1998b]. In addition, seismometer resonance on a number of the Leg 111 stations generated ringing in the 70-130 Hz band, but the first peak is not as affected by this ringing as the second, oppositely directed peak (Figures 1 and 3a) [Swift et al, 1996].…”

“…a receiver at a nominal clamping interval of 10 m with seismograms sampled at 1 ms for 3 s. The seismic sources for the two experiments were different, however, producing significantly different direct wave signatures [Swift et al, 1996]. Leg 111 used a 950 cubic inch air gun suspended at 4.5 m depth with 14 MPa air pressure, whereas Leg 148 used a 300 cubic inch air gun at 10 MPa fired at a depth of 6.7 m. These two data sets were merged by Swift et al [1996], who applied a time-invariant factor of 2 to seismograms between 886 and 1386 mbsf to correct an apparent error in amplitude scaling arising during initial processing.…”

“…Vertically polarized shear arrivals, generated by scattering away from the borehole near the top of basement, also propagate downhole after the direct wave [Swift et al, 1996;Swift et al, 1998b]. In addition, seismometer resonance on a number of the Leg 111 stations generated ringing in the 70-130 Hz band, but the first peak is not as affected by this ringing as the second, oppositely directed peak (Figures 1 and 3a) [Swift et al, 1996]. Both ringing and scattered arrivals prohibit using spectral ratios, because they move spectral notches between receiver depths in ways unrelated to attenuation, but neither type of energy significantly affects the peak amplitude of the direct arrival.…”

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“…The loss of high frequency energy in the initial 50ms over just 350m (less than a wavelength at 10Hz) can clearly be seen. [Swift et al, 1991;Swift and Stephen, 1992] Figure 9: Interval velocities from one-way traveltimes to VSP receiver depths [Leg 118 Shipboard Scientific Party, 1989]. The dashed line connecting the squares represents raw velocities, which are sensitive to small subjective errors in the selection of seismic arrivals.…”