Seismic Studies of Subsurface Structure in the Ewa Coastal Plain, Oahu, Hawaii

Abstract

Seismic studies using well-logging, refraction, and reflection methods were carried out in 1965 in conjunction with a core-sample drilling project in the Ewa Coastal Plain, Oahu, Hawaii. The seismic well-logging technique gave a complicated velocity-depth profile, with higher velocities associated with reef limestone and lower velocities associated with mud deposits. The seismic refraction method showed a simpler velocity-depth profile with only a few distinct layers. The seismic reflection method corroborated the simpler profile obtained with the refraction method. The two profiles were reconciled, as the complicated profile can be averaged out into the simpler profile. The averaging-out process can be applied to the whole sedimentary column so that a P-wave velocity value may represent the sedimentary layer at any given locality. However, no single value can be assigned as typical for sedimentary layers for the entire Hawaiian area. The velocity values depend upon the composition of the layer, which is made up of varying proportions of mud, reef limestone, and weathered basalt. Layer 2 of the oceanic crust in the Hawaiian area has a rather uniform character, with seismic velocities ranging from 4.8 to 5.1 km/sec, and thicknesses from 4 to 8 km.