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S U M M A R Y During the North Atlantic transect (NAT) experiment 11 expanding spread profiles (ESPs) were acquired using a 3300 cubic inch, 30-element tuned airgun array (S/V Prospekta) and a &-channel seismic streamer (R/V Moore). Extremely high-density seismograms were obtained that have allowed us to transform the observed data to the domain of intercept time and ray parameter (tau-p) with high precision. Velocity structures have been obtained by tau-sum inversion, ray tracing, and synthetic WKBJ and reflectivity seismogram methods both in tau-p and x-t domains.A detailed analysis of ESP 5 suggests the presence of a low-velocity zone within the lower oceanic crust at the location of magnetic anomaly MO (118 Ma). It is apparently 2.86 km thick, has a P-wave velocity of 6.5 km s-', S-wave velocity of 3.6 km s-', and occupies much of the depth interval normally associated with oceanic layer 3. Converted shear wave arrivals have been used to obtain the shear velocity structure. We find that the Vp/Vs ratio in the uppermost part of the crust is about 1.9, and much of the crustal section has a VJV, ratio greater than ~. Attenuation of P-and S-waves has been studied by amplitude analysis, and we find that in most of the lower crust Q, is equal to Qp. Comparison with solutions from ESPs in younger and older parts of the basin suggests that ESP 5 is unique in displaying a low-velocity zone. Despite this the total thickness at ESP 5 is not significantly different than at other locations. We suggest that this provides strong evidence that the low-velocity zone occurs within the lower crust, and does not result from serpentinization of the upper mantle beneath thin oceanic crust.

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Results of a 5-year program to measure the seismoacoustic properties of unlithified sediments in the seafloor are presented. A field technique utilizing an impulsive source and a geophone array located on the seafloor was used to obtain detailed travel-time curves which permit a number of different kinds of analysis. A new signal source powered by electrically detonated, 8-gauge shotgun shells was used to focus energy into the bottom and eliminate much of the high-frequency water-borne noise that accompanies the usual explosive sources. Both interface waves and wide-angle refractions were analyzed. A variety of data inversion techniques including slant stacking and cross multiplication were used to obtain dispersion curves, and constrained, least-squares inversion using partial derivatives was used to develop an iterative inversion method to obtain velocity and attenuation as a function of depth. Examples of data from a wide angle of different kinds of sediment ranging from soft Holocene mud to stiff overconsolidated clay are presented. The resulting geoacoustic models reflect the influlence of geologic history, which largely determines the in situ porosity, and the additional effect of the present overburden pressure.