abstract

It is found that ocean bottom seismometers (O.B.S.) deployed in sedimented areas produce markedly different seismograms from surface hydrophones. These differences are found to be due to ringing on the O.B.S. records produced by converted shear waves trapped in the sediment layer. These shear waves do not propagate into the water and hence the hydrophone record is much “cleaner” than the O.B.S. record. It is also shown that the presence of refracted shear waves like P-S-P and P-S-S may be related to the presence of a sedimentary layer in some areas. It is suggested that the disappearance of refracted S waves in some areas without sediment is related to high attenuation and/or very low shear velocities caused by cracks and inhomogeneities in the crust. Under sedimented areas the cracks may be sufficiently filled so as to substantially reduce the attenuation and/or increase the bulk shear velocity.

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