High-Resolution, Multichannel, Marine Seismic Surveying Using A Small Airgun Source

Abstract

A high-resolution, multichannel, CMP, marine seismic reflection survey was carried out in Lake Simcoe, southern Ontario, Canada, in June 1998, to examine large channel features within the glacial sediments beneath the lake bottom. Over 300 line-kilometers were obtained at close line spacings with an oil-filled hydrophone receiver array and a 1 cu. in. airgun fired accurately at 5 meter horizontal intervals. Firing on distance was accomplished by integrating a real-time DGPS (Differential Global Positioning System) and a computer-driven triggering system which adjusts the firing rate depending on the ship’s velocity. The survey was conducted with an average boat speed of approximately 4 knots (7 km/hour), and multichannel records were recorded on a Geometrics R24 seismograph at 2.2-3 second intervals. Preliminary results show remarkable subbottom detail. Reflections within the glaciolacustrine sequence on the final stacked sections (6-fold) have dominant frequencies in excess of 1 kHz. Simultaneous recording of single-channel seismic data with a Datasonics Chirp II profiler allow comparison of the multichannel stacked sections with this higher-resolution system. Even with the use of a small chamber (1 cu. in.), the airgun data have the advantage of greatly enhanced depth penetration. The bedrock surface can be observed at a depth of -40 m below bottom except in areas where overlying, coarse-grained sediments (including high-velocity tills) are thick (>-20 m). The high resolution of the stacked sections depends on careful processing, which includes spiking deconvolution to whiten the amplitude spectrum, and accurate, closely-spaced velocity determinations. Relative amplitudes were preserved during processing (no automatic gain control was applied), allowing analyses of impedance contrasts and estimates of lithologies to be made. This survey dramatically demonstrates the resolution potential of multichannel marine data obtained with a small airgun source.