Velocity and attenuation in young oceanic crust: New downhole log results from DSDP/ODP/IODP Holes 504B and 1256D

To characterize the acoustic properties of oceanic crust and investigate the nature of the oceanic layer 2/3 seismic boundary, we analyze sonic logging waveforms recorded in the two deepest holes ever drilled in intact oceanic crust: Ocean Drilling Program (ODP)/Integrated Ocean Drilling Program (IODP) Hole 1256D and Deep Sea Drilling Program (DSDP)/ODP Hole 504B. Hole 1256D penetrated 15 Ma old crust on a superfast spreading segment of the Cocos plate, while Hole 504B was drilled in a similar to 6 Ma old section of an intermediate spreading segment of the Nazca plate. Careful reprocessing of different sets of waveforms allows us to draw depth profiles of the primary parameters controlling wave propagation in the crust: compressional and shear velocities (V(p) and V(s)), and attenuation. The results underline the importance of proper filtering and processing parameters for the acquisition of acoustic logs in hard rocks. While the reprocessed V(p) logs improve previous data, particularly at the bottom of Hole 1256D, the V(s) logs represent significant enhancements in quality and in extent of previous profiles in the two holes. Attenuation results provide the first measure of intrinsic attenuation in these holes. The velocity and attenuation logs confirm that Hole 504B had reached oceanic layer 3 without recovering gabbros. Conversely, while gabbros were recovered in Hole 1256D, no layer 3 velocity values were recorded in the interval logged. However, the reprocessed data suggest that the transition to layer 3 might occur within the deepest 87 m of the hole that were not logged or immediately below. Accordingly, the layer 2/3 seismic boundary does not appear to be related to the top of the plutonic crust. Despite different ages, spreading rates, and mineralogy, however, the two sites display remarkably similar velocity and attenuation profiles through the entire crustal sections logged, suggesting that they can be reliably considered representative of the acoustic properties of the Nazca/Cocos spreading system and more generally of undisturbed oceanic crust.