BACKSCATTERED COHERENT NOISE AND SEISMIC-REFLECTION IMAGING OF THE OCEANIC-CRUST - AN EXAMPLE FROM THE RIFT-VALLEY OF THE MID-ATLANTIC RIDGE AT 23-DEGREES-N

Seismic reflection surveys shot over oceanic crust are contaminated by source-generated coherent noise that limits the identification and in terpretation Of subbasement reflections. Coherent noise arrivals are g enerated around the seismic line by scattering from the upper surface of the basaltic layer and by backscattering of turning waves from the underside of the same interface; both types of arrival may: be enhance d by conventional common midpoint (CMP) stacking. Some turning arrival s also exhibit negative moveout in a CMP gather due to the increase of crustal velocity with depth. In unsedimented areas, coherent arrivals scattered from the top of the seafloor can be aligned by dip moveout, with or without a subsequent zero-offset migration prestack, and supp ressed, but turning arrivals will usually remain. Turning arrivals gen erated by linear features in the seafloor, such as fault scarps, appea r on a stack section as nearly linear coherent arrivals, the apparent dip of which is related to the features' angle to the seismic line. Af ter migration, these turning wave arrivals can extend close to the top ography that produced them and continue to late times, leading to the possibility that such arrivals might be incorrectly interpreted as int rusions that crosscut, but do not offset. the Moho. Careful identifica tion of crustal reflections on a seismic profile along the MARK area r ift valley indicates that the Snake Pit hydrothermal field marks the n orthern limit of identifiable crustal reflections, at least some of wh ich appear to be related to magma injection. The recent extensive volc anism of the northern MARK area may well be a result of faulting cause d by the stong northward growth of the southern spreading center, perm itting melt to rise more easily to shallow crustal levels.