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

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 krn / sec, and thicknesses from 4 to 8 km. BECAUSE STUDIES to date of the upper crust in the Central Pacific Basin have been largely geophysical in nature, actual samples of the crust were needed to correlate published geophysical data with suggested lithologic and stratigraphic interpretations. With this objective in mind, drilling proj ects to recover core samples were carried out in 1965 in the Ewa Coastal Plain on the island of Oahu, Hawaii . Following the drilling, and after the stratigraphy was roughly determined, seismic well-logging and seismic refraction were conducted in the vicinity of the holes to correlate seismic parameters with the drilling data. This paper deals with the results of the seismic work. The Ewa Plain was chosen for the drilling project because it is the widest coastal plain on any island in the Central Pacific Basin. Stearns and Chamberlain (1967) have reported preliminary results of the drilling and descriptions of core samples, and Resig (1969) has reported 1 Hawaii Institute of Geophysics Contribution N o. 349. Manuscript received January 6, 1970. 2 Hawaii Institute of Geophysics, University of Hawaii, Honolulu 96822. the paleontology of the core samples. Early results from a similar drilling project carried out on Midway Islands have been reported by Ladd, Tracey, and Gross (1967). The seismic program was conducted in four stages: (1) acoustic logging of the well designated as Ewa I (see Fig. 1) , (2) an east-west seismic refraction traverse of about 1,800 meters (about 5,900 feet) in the vicinity of Ewa I, (3) two seismic refraction traverses at sea off the Ewa Coastal Plain, and (4) seismic reflection probing at the Ewa I site.