Kirchhoff-Helmholtz modeling of seafloor scattering in a Mid-Atlantic Ridge seismic reflection line: Origin of scatterers and possible masking of a magma chamber reflector

Abstract

In January 1989, multichannel seismic reflection data were collected over the MARK area (23° N) of the slow-spreading Mid-Atlantic Ridge (MAR) in hopes of imaging an axial crustal magma chamber. Unfortunately, the seismic reflection data are dominated by what appears to be scattering from a very rough unsedimented seafloor. No magma chamber reflection could be seen through the scatter. Some stronger coherent events, however, could be interpreted as dipping intracrustal reflectors. Kirchhoff-Helmholtz scattering synthetics from digital SeaBeam bathymetry maps indicate that almost all of the events seen in the reflection data originate from the seafloor and not the subsurface. A back projection algorithm was used to locate the points of origin of these seafloor scattered events. The sources of most of these scattered events are from smaller holes in the bathymetry rather than from large features, implying focusing within a Fresnel zone as the mechanism for the scattering. Combining Kirchhoff-Helmholtz transmission synthetics, using reflectivity-derived starting amplitudes, with the scattering synthetics, show that a magma chamber with an amplitude reflection coefficient as high as 0.10 would be made unobservable because of high-amplitude MAR seafloor scattering.