Changes in attenuation with depth in an ocean carbonate section: Ocean Drilling Program sites 806 and 807, Ontong Java Plateau

Abstract

Changes of in situ seismic attenuation with depth are estimated from full waveform acoustic logs in a soft formation (rock with shear velocity less than borehole fluid velocity). The relative attenuation Qp−1 is computed from the variation with depth of the P wave amplitude spectrum of a single source receiver pair. The method is applicable when unknown source and receiver responses of the sonic tool make other methods difficult to apply. Tests with synthetic data generated by a full waveform method verify that in a soft formation the decay of the P wave amplitude spectrum depends mainly on attenuation. The method is applied to Ocean Drilling Program data from holes 806B and 807A on the Ontong Java Plateau in the western Pacific Ocean. Four attenuation logs were computed independently using data from four source receiver pairs. Agreement of the four logs at each site and agreement of results from the two sites suggest that the method is robust and practical. The attenuation logs show a maximum attenuation between 300 and 500 m below the seafloor. They agree well with compilations of high‐frequency attenuation versus porosity and frequency in marine sediments but are somewhat greater than results from seismic experiments, possibly owing to the presence of sedimentary microbeds and the three‐decade difference between the seismic and sonic frequency bands.