Figure 3.57b: The Albatross expedition led by Hans Petterson is a famous Swedish oceanographic research trip that between summer 1947 and autumn 1948 sailed around the world covering 45,000 nautical miles. The purpose of the expedition was to explore the depths of the Atlantic, Pacific and Indian Oceans near the equator by studying aspects of physical, chemical, biological, and geological oceanography. It retrieved core- samples from the bottom of the ocean, took water samples, made temperature recordings, carried out deep-sea trawling, took continuous echo soundings, and carried out seismic reflection measurements of the sediment thickness using ‘sink bombs’ as sources with Weibull’s method. In the Atlantic the echograms revealed that in some places the deep-sea floor was essentially flat except for narrow, steep-sided valleys. These plains were interpreted as evidence of thick sediment and the valleys as grabens caused by faulting. In fact, the Swedish expedition had discovered the first deep-sea channels caused by turbidity currents (Menard, 1987).
surface and the bottom (sea bottom multiples). In addition, and importantly, one could also observe much weaker and deeper echoes, due to sound waves which had penetrated through the sediment layer and become reflected against transition surfaces within it, or from the underlying rock bed. He went on to publish a paper on the propagation of
pressure waves from underwater explosions, and he refined his new method to investigate the depth of sediments using explosive sources (Weibull, 1947, 1954). He observed among other things the bubble pulse phenomenon which has since troubled geophysicists. He measured the maximum thickness of the sedimentary layer in the centre of the Tyrrhenian Sea, part of the Mediterranean Sea off the western coast of Italy, where, below a water layer of 3,600m, he estimated the sediment to have a thickness of nearly 3,000m (Weibull, 1947). Wallodi Weibull next participated in the first part of the
famous Swedish Albatross expedition (1947–48) which crossed the equator 18 times and covered 45,000 nautical miles (83,000 km), where the main task was sediment coring of the ocean floor in the Atlantic, Pacific and Indian Oceans near the equator. Sediment soundings with depth charges were carried out at 75 different positions, where in most cases, two depth charges were set to explode at different depths (500 and 2,500, 4,500, or 6,500m). Te thickness of what was assumed to be sediment ranged up to 3,500m in the North Atlantic but was no more than a few hundred metres in the Pacific. Weibull also noted that acoustic waves were impenetrable to layers of lava. Tis observation is valid still today as little progress indeed has been made to look for sediments beneath thick lavas. In Petterson (1954) Weibull’s results were interpreted: “… if
we accept Weibull’s maximum sediment thickness in the central Atlantic of about 12,000 feet, and if we further assume that the whole of this sediment is of the same type as its surface layer, namely Atlantic red clay, we can make an approximate estimate of the time required for accumulating a layer of this thickness. Taking 0.3 inches in 1,000 years as a reasonable value for the rate of sedimentation of Atlantic red clay, we arrive at a time span of nearly 500 million years.” Petterson of course realised that this estimate had major uncertainties due to compaction which would reduce thickness and possible variations in the rate of sedimentation. However, he found it evident that the
Figure 3.57c: A reproduction of one of Weibull’s oscillograms, obtained in the central Atlantic Ocean between Madeira and the Mid- Atlantic Ridge. We can see three breaks in the record, due to deep echoes thrown back by three different reflecting layers in the bottom, the uppermost of sediment thickness 1,550m and the middle one of thickness 2,200m. The deepest, which is presumably reflected from the bedrock beneath the sediment carpet, indicates a total thickness of the latter of 3,475m, which was a record for the cruise. It was assumed that the wave velocity was 1,500 m/s. On the image, the scribblings refer to: Station no 315, shot no 341, where the depths were e=20m for the hydrophone, a=3,730m for the charge, and b=4,360m for the sea bottom. The charge consisted of a hydrostatic fuse (4 g) + 3x170 g of TNT = 514 g.
age of the particular part of the Atlantic Ocean where these great thicknesses were found must be counted in many millions of years. Tis, he stated, appears to knock the bottom out of Wegener’s famous ‘continental drift’ theory, according to which the Atlantic Ocean should have begun to open up in Cretaceous time, that is, merely 60 to 80 million years ago. Te sedimentary thicknesses found in the Pacific and
Indian Oceans were surprising. Petterson (1954) wrote: “Te explanation for the much lower figures found in the other oceans is even more far-fetched. Tey cannot well indicate a lesser age of the oceans in question, but may instead be due to a much lower rate of sedimentation. In fact there are certain indications that at least in the central parts of the Pacific Ocean the accumulation of red clay proceeds at a rate about ten times slower than in the central Atlantic.” Weibull’s technique was later taken up by Scripps Institution
of Oceanography scientists who made numerous thickness measurements in the Pacific.
Maurice Ewing (1906–1974), American geophysicist, used seismic methods to make funda- mental contributions to the understanding of marine sediments and ocean basins. Making seismic refraction measurements along the Mid-Atlantic Ridge, and in the Mediterranean and Norwegian seas, Ewing took the first seismic measurements in open seas in 1935. During World War II he did pioneering work on the transmission of sound in seawater. His work with Frank Press on interpreting surface waves led to the first well-founded estimate of the depth of the Mohorovičić discontinuity under the ocean floor. To honour the memory of Maurice Ewing and his contributions to geophysics, SEG established in 1978 the Maurice Ewing Medal as the highest honour given by SEG. Maurice Ewing is quoted as saying: “Imagine millions of square miles of a tangled jumble of massive peaks, saw-toothed ridges,
Figure 3.58: Maurice Ewing. 123
John T. Chiarella
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