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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. B8, PAGES 16,119–16,133, 2001

Modeling seismic reflection patterns from Ocean Drilling Program Leg 168 core density logs: Insight into lateral variations in physical properties and sediment input at the eastern flank of the Juan de Fuca Ridge

Lars Zühlsdorff

Department of Geosciences, University of Bremen, Bremen, Germany


Volkhard Spiess

Department of Geosciences, University of Bremen, Bremen, Germany


Abstract

At the eastern flank of the Juan de Fuca Ridge, single-channel and multichannel very high resolution seismic data were recorded with a water gun and two Generator-Injector Guns, which provided different source frequencies. To ground truth the seismic records, traces were visually correlated with reflection patterns of synthetic seismograms calculated from closely spaced density measurements on Ocean Drilling Program Leg 168 drill cores. The quality of both core logging and seismic data is sufficiently high to reveal a relation between density variations and seismic reflections at higher source frequencies. This suggests that density approximates subbottom variations in seismic impedance. The core density data are also sufficient to model a frequency-dependent decrease in seismic reflection amplitudes, which is observed above a buried basement ridge. This indicates that low reflectivity in recorded traces is independent of processing artifacts, rough topography, or seafloor inclination and can be related to locally low in situ sediment density and high porosity, which appear to be associated with fluid flow. Further, the part of the amplitude decrease, which is due to an overall reduction of density contrasts, appears to be frequency-dependent. In addition, the comparison between seismic records and core data reveals distinct depositional environments, which are divided by the buried basement ridge. In the western part of the study area a transition from an older to a younger part of the sediment section is observed. The former is characterized by focused turbiditic input in the vicinity of ODP Site 1024 and a gradual decrease in grain size toward ODP Site 1025. In the younger part, however, much thinner turbidites were deposited more evenly between both sites. Only one seismic reflector can be traced throughout the study area, suggesting that only major turbiditic events are capable of affecting both sides of the ridge. The sediment thickness above this reflector indicates that average accumulation rates for the youngest part of the sediments east and west of the ridge are rather similar.

Received 3 March 2000; accepted 22 December 2000.

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Citation: Zühlsdorff, L., and V. Spiess (2001), Modeling seismic reflection patterns from Ocean Drilling Program Leg 168 core density logs: Insight into lateral variations in physical properties and sediment input at the eastern flank of the Juan de Fuca Ridge, J. Geophys. Res., 106(B8), 16,119–16,133.