Elastic wave velocities within oceanic layer 2 from sonic full waveform logs in Deep Sea Drilling Project Holes 395A, 418A, and 504B

Abstract

Multichannel full waveform acoustic logs were recorded in Deep Sea Drilling Project (DSDP) hole 418A during Ocean Drilling Program (ODP) leg 102, DSDP hole 395A during ODP leg 109, and DSDP hole 504B during ODP leg 111, to provide nearly continuous measurements of elastic wave velocities as a function of depth within oceanic layer 2 for different spreading rates and crustal ages. The velocities depend primarily on the morphology of the basalt. Massive units have Vp above 5 km/s, Vs above 2.8 km/s, and Vp/Vs below 1.9. Their velocities increase with depth as expected from crack closure due to confining stress and are lower where fractures are present. Vp and Vs within pillows are quite variable but in general are lower than in massive units, and Vp/Vs is higher. Velocities within breccias are similar to or lower than those of pillows. Within a given morphology the elastic properties do not depend on spreading rate. Within pillows, velocity variations depend primarily on the degree of alteration infilling and the mineralogy of the infilling matedhal, and secondadhly on conditions at emplacement. Coherent shear arrivals are sometimes not found within shallow pillows, and in extreme cases the compressional arrival is also incoherent, because of scattering resulting from the similarity between the sonic wavelength and the pillow size. This "sampling bias" in the sonic log is the most likely cause of somewhat higher average sonic than seismic velocities measured within extrusive basalts at shallow depths. Other causes include the fact that laterally finite massive units may be oversampled by borehole measurements and that voids between pillows and fractures, whose size is within an order of magnitude of the sonic wavelength, have less effect on sonic than on seismic velocities. The crustal section can be separated only roughly into seismic layers on the basis of the sonic velocities. Changes in velocity with depth within the extrusive section at a single site giving rise to previously defined seismic layers 2A and 2B are due to a change with depth in the properties of the pillows, associated with differences in the amount and type of infilling matedhals, rather than to a change with depth in the relative proportion of pillows and massive units. Because alteration occurs early and its history is different at different depths, depth-dependent variations in pillow properties are (1) established at young age and (2) persist even in old crust. Within the intrusive section of DSDP hole 504B, velocities are similar to or higher than those measured in saturated minicores at atmospheric pressure, and seismic and sonic velocities are approximately equal, suggesting that little large-scale porosity is present. 47, 1215-1228, 1982. Pezard, P. A., Electrical properties of mid-ocean ridge basalt and implications for the structure of the oceanic crust at Deep SeaDrilling Project site 504, J. Geophys. Res., this issue.Purdy, G. M., New observations of the shallow seismic structure of young oceanic crust, J. Geophys. Res., 92, 9351-936...

Downhole Measurements and Electrical Images in Hole 896A, Costa Rica Rift

“…These low values probably reflect the extent of groundmass alteration and associated neoformation of conductive minerals in fracture and veins. Compressional velocities average 5.0 km/s in the hole and are never less than 3.5 km/s, consequently higher than that obtained in the upper part of Hole 504B (Moos et al, 1990). This also reflects the importance of crack sealing at Hole 896A, in relation to intense hydrothermal circulation and alteration at this basement high and local heat-flow maximum.…”

“…Also, the acoustic velocity is almost greater than 4.0 km/s throughout in Hole 896A, which supports the idea of pervasive pore space filling by secondary minerals. On the other hand, Moos et al (1990), for example, reported numerous sections with values less than 3.5 km/s in the shallow basement of Hole 504B. Physical property measurements in Hole 896A consequently indicate that the shallow crust at Site 896 has been altered to a large extent by hydrothermal circulation.…”

Physical properties of upper oceanic crust: Ocean Drilling Program Hole 801C and the waning of hydrothermal circulation

“…Original first‐break travel time logs were not available for Sites 396 and 417, so their velocity logs could not be reprocessed. For Sites 418 and 504, we used velocity logs calculated by semblance analysis of waveforms [ Moos et al , 1990]. Velocity and resistivity are both controlled primarily by porosity, but resistivity measurements are less affected by poor borehole conditions.…”

“…These data, acquired and published over a period of three decades, have not been combined in a systematic study. Johnson and Semyan [1994] have compiled and analyzed physical properties of basalt cores, and two studies [ Carlson and Herrick , 1990; Moos et al , 1990] have compared results from the three deepest‐penetration sites (Holes 395A, 418A, and 504B). This paper focuses only on the top 300 m of basalt, thereby permitting us to document and compare temporal changes among 4 times as many logged sites (Figure 1).…”

Anisotropic seismic scattering in old Atlantic crust

“…Volume heterogeneities, while also capable of producing significant secondary scattered energy, do not have such a great attenuating affect on the primary transmitted S diving wave [Dougherty and Stephen, 1988]. However, the seismic velocity structure of the upper crust surrounding Hole 418A was found to be indicative of relatively sealed older crust [Swift et al, 1988;Moos et al, 1990]. Subbasement features in the form of aligned crustal heterogeneities or cracks are the most likely cause of the scattering seen here.…”