Two expanding spread profiles (ESPs) were analyzed in detail to investigate the deep structure of the volcanoes and the underlying oceanic crust near Oahu. One ESP was located on the southeastern submarine flank of Kauai (ESP 10), and the other was located directly over the ridge axis between Oahu and Molokai (ESP 5). Two‐dimensional models of the entire crust were obtained by iterative comparison of both travel times and relative amplitudes of arrivals on the ray‐theoretical synthetic seismograms with those in the data. A 2.4‐km vertical offset in the preexisting crust, inferred to be a large fault zone, was found near the midpoint of ESP 10. This fault may form the southern wall of a graben or half graben in which Kauai sits. The layering and seismic velocities of the preexisting oceanic crust near Kauai have remained unmodified by the load of the volcanic pile. ESP 5 is located over a shallow 4‐km‐thick basin filled with volcaniclastics and possibly volcanic flows bordered by the islands of Oahu and Molokai, Penguin Bank, and two buried rift zones. Under this basin the main volcanic edifice is 4 to 5 km thick. The top of the preexisting oceanic crust is 8 to 9 km below sea level and is 3 to 4 km lower than the crust away from the islands. The underlying oceanic crust is 6 to 7 km thick excluding the Moho transition zone, which is of normal thickness and at a depth from 15 to 16 km. The existence of a large subcrustal plutonic body is refuted.

An expanding spread profile (ESP) over 80 Ma Pacific Ocean crust near Hawaii was modeled with a one‐dimensional solution for travel times, relative amplitudes, and sometimes frequency content. The resulting profiles of compressional and shear wave velocities and Poisson's ratio are similar to those determined by other ocean crustal studies except that the shear wave Q in the sediments is very low (10) and some unusual high‐frequency, high‐phase‐velocity arrivals require a laminated structure within the mid‐crustal section at about 3 km below the base of the sediment. The laminations are probably alternating layers of gabbro and ultramafic cumulates. The cumulates may be fractionation products from an upper level magma chamber that was present at the time of crustal formation. Reflections which may correspond to these laminations have been observed in reflection records both near this ESP and in the North Atlantic. Comparison of different ophiolite models for ocean crust reveal that the most commonly used velocity‐lithology model is probably in error for the pillow and dike basalts, resulting in an interpretation of overly thick layers in previous studies of refraction lines. The preferred interpretation is that the average total thickness of the pillow and dike sequences is between 1.0 and 1.5 km.

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A multichannel seismic survey has resulted in greatly improved resolution of structural details of deep sea gas hydrates off the west coast of Canada, revealing numerous geological features not before evident. The survey using the Naval Research Laboratory deep‐towed acoustic/geophysics system (DTAGS), provided high‐resolution images and layer velocities more than 10 times better than those obtained in the past using conventional systems. Vertical resolution within 2 m and horizontal resolution within 20 m were achieved. The work demonstrates that high‐resolution seismic surveys with deep towed multichannel systems can provide important new information about pathways of fluid and gas migration that control the formation of gas hydrates. Conventional surface‐towed seismic systems are unable to do this.

Using vector acoustic sensors for marine geoacoustic surveys instead of the usual scalar hydrophones enables one to acquire three-dimensional (3D) survey data with instrumentation and logistics similar to current 2D surveys. Vector acoustic sensors measure the sound wave direction directly without the cumbersome arrays that hydrophones require. This concept was tested by a scaled experiment in an acoustic water tank that had a well-controlled environment with a few targets. Using vector acoustic data from a single line of sources, the three-dimensional tank environment was imaged by directly locating the source and all reflectors.