AMPLITUDE BLANKING IN SEISMIC PROFILES FROM LAKE-BAIKAL

Imaging of the deepest sedimentary section in Lake Baikal using multic hannel seismic profiling was hampered by amplitude blanking that is re gionally extensive, is associated with water depths greater than about 900 m and occurs at sub-bottom depths of 1-2 km in association with t he first water-bottom multiple. Application of a powerful multiple sup pression technique improved the quality of occasional discontinuous, d ipping primary reflections, but failed to substantially alter the non- reflective character of the blanking zone. Detailed analysis of amplit udes from original data and synthetic models show that the threshold f or detecting primary energy in deep water of Lake Baikal occurs when t he primary is about 14-20 dB less than the multiple energy. The blanki ng occurs because of anomalously low reflectivities of the deep sedime nts coupled with this 20 dB limitation in real data processing. The bl anking cuts across seismic stratal boundaries, and is therefore probab ly unrelated to depositional lithologies. The deepest, early rift depo sits, inferred to come from a mixed fluvial and lacustrine setting, do not easily explain the widespread and uniform character of the blanke d deposits. More likely, blanking occurs because of processes or pheno mena that physically alter the deposits, causing them to be non-reflec tive and/or highly attenuating. No single process explains all the obs ervations, but a combination of diagenesis, overpressure, and the pres ence of dispersed free gas at sub-bottom depths of 1-2 km, offer plaus ible and possible conditions that contribute to blanking. (C) 1996 Pub lished by Elsevier Science Ltd.