Comparison of seismic reflection data to a synthetic seismogram in a volcanic apron at Site 953

The volcanic apron of Gran Canaria at Site 953 is characterized by numerous, closely spaced reflectors, allowing a high-resolution stratigraphic correlation. The calibration of the presite survey seismic data (during the Meteor Cruise 24) with regard to the lithology and stratigraphy found at the drill site was achieved by computing a synthetic seismogram serving as the link between seismic and borehole data. Because logging data were available for only 53% of the hole, velocity and density measurements taken from the recovered cores were used in the missing intervals to obtain a complete synthetic seismogram. Most reflectors in the upper ~900 m of the sequence (lithologic Units I-V) turned out to be thin volcaniclastic layers intercalated to the nonvolcanic background sediments. Their thicknesses are generally <2 m, and the reflections from their tops and bases overlap, forming a single reflection. The limit of the seismic detection of such interbeds is on the order of several decimeters and thus requires special care for the processing of the velocity and density data to avoid destruction of the signal from these thin layers.