Owing to improvement of the signal/noise ratio, large airgun arrays consisting of 2 to 4 × 466 in.3 (7.64 ℓ) guns fired at 12- to 18-s intervals provide more refracted arrivals from the upper crust than can be obtained with a small 25 in.3 (.41 ℓ) airgun. In particular, sonobuoy data obtained previously with small airguns indicated a 1.5-km-thick layer with a refraction velocity of 3.6 km/s overlying a major refractor with a velocity of about 5.1 km/s near the Mid-Atlantic Ridge. However, large-airgun-sonobuoy results reveal intermediate velocities between the 3.6 and 5.1 km/s refractors and commonly achieve penetration down to the mantle.
Large-airgun data have been used to contour acoustic basement refraction velocities in the North Atlantic; these show a clear increase of velocity with age. A contour map of mantle depths is shown for the North Atlantic; it is based on 100 measurements, more than half of which are from high-quality, two-ship refraction data (solutions that include layers 2 and 3). A comparison between mantle depths determined from sonobuoy and two-ship solutions shows that the two-ship solutions have three times the variation of sonobuoys from the same geographical region.
Solutions are plotted as velocity-depth points to show variations in the entire crust as a function of age. As the crust ages, the uppermost basement velocity increases and total crustal rock thickness increases rather abruptly on Upper Jurassic crust. These effects combine to decrease the gross crustal velocity gradient (exclusive of water and sediment) from about 1.0 sec−1 near the Mid-Atlantic Ridge to about 0.5 sec−1 near the continental margins.