HIGH-RESOLUTION IMAGES OF THE LOWER CRUST - DEEP SEISMIC REFLECTIONS FROM 15-HZ TO 180-HZ

We report the results of an unconventional, broad-band deep-seismic re flection survey undertaken on land in southwest England. Previous mari ne seismic lines in this area have shown strong layered lower-crustal reflectivity. The broad-band survey shows near-normal-incidence P-wave reflections from the lower continental crust and from the Moho with f requencies that range from 15 to 180 Hz. High-frequency lower-crustal S-wave reflections were also observed. These high frequencies were obt ained by placing both sources and receivers in boreholes up to 2 km de ep, drilled within a large homogeneous granite pluton. Recording down- hole avoids loss of high frequency energy during transmission through heterogeneous and highly attenuative near-surface layers. The high fre quencies were generated using explosive sources, and were recorded on sub-surface accelerometers and hydrophones, and on a conventional arra y of surface geophones. Seismic attenuation within the granite is low; the quality factor, Q, lies above 2000. These new, high-frequency obs ervations show that the lower crust does not preferentially reflect pa rticular frequencies. The data are most readily explained as resulting from the combination of a frequency-independent Q in the upper crust with a white reflectivity response in the lower crust. The data are no t consistent with crustal models which are highly tuned to reflect pre ferentially the frequencies employed in conventional deep seismic refl ection surveys. The data require relatively sharp boundaries within th e lower crust; significant variation in physical properties must occur over vertical distances of not more than about 10 m. Sharp boundaries are most easily explained if the reflections result from lithological contrasts.