A fixed receiver for recording multichannel wide-angle seismic data on the seabed

Previous experiments to record seismic data at wide angle on the continental shelf have generally been unsuccessful in determining velocity structure in the lower crust; either the lines were too short or shot-receiver density too sparse to identify lower crustal arrivals. In contrast, deep normal incidence profiles show good structural resolution in the crust and uppermost mantle. A sea-bottom multichannel instrument has been developed to record datasets containing closely spaced traces, in order to improve the resolution of reversed wide-angle experiments on the continental shelf.The Pull-up Multichannel Array (PUMA) is a 1200 m, 12-channel hydrophone array for remotely recording seismic data on the seabed. It consists of 12 short hydrophone sections linked by 100 m-long passive sections. A pressure case is attached to the array at one end, in which recording electronics, cassette tape recorders and a battery power supply are housed. The PUMA is designed for deployment in water depths less than 200 m from a research ship and is moored to buoys for recovery.The instrument, which was successfully used in an experiment west of Lewis, Outer Hebrides, UK (Powell and Sinha, 1987) was specifically designed to provide a reliable determination of the velocity structure of the crust and uppermost mantle over part of the BIRPS WINCH deep normal incidence profile. Because the traces are closely spaced it is easy to correlate phases across the record section and to monitor changes in amplitude. A velocity structure for the continental crust and uppermost mantle has been devised from these data, using amplitude modelling.