3.3 A Brief History of Marine Seismic Sources


We know only too well that what we are doing is nothing more than a drop in the ocean. But if the drop were not there, the ocean would be missing something.


3.3.1 Fundamental Work


Mother Teresa (1910–1997), beatified in October 2003 and canonised as a saint in 2016. Shell crews first used driftwood and syrup cans as floats,


In 1917 Rayleigh gave one of the first theoretical descriptions of an underwater air bubble. He was interested in the sounds emitted by water in a kettle as it comes to the boil, and explained these noises as resulting from the partial or complete collapse of bubbles as they rise through cooler water. Te water in Rayleigh’s theory was treated as an incompressible liquid. In 1942 Kirkwood and Bethe derived an


equation of motion for an exploding bubble in a compressible liquid. Later on, Keller and Kolodner (1956) derived a similar equation assuming adiabatic conditions that lead to a damping of the bubble oscillations. During WWII much classified research was done on producing and detecting underwater sound in the ocean, including creating and studying bubble pulses, and many advances in theory and instrumentation were made. Some of this work was subsequently declassified and parts that are of interest to marine seismic surveying were published by Ewing (1948).


3.3.2 The Early Seismic Source


In the early days of offshore seismic exploration, after WWII, the sound source was dynamite or similar high-energy explosives detonated well below the sea surface or at the sea bottom. Aside from the physical dangers involved in using dynamite or TNT, the bubble effect was a major problem in these early surveys. In 1947, Shell ‘solved’ the bubble problem by floating the charges about four feet (1.2m) below the surface, rather than letting them sink to the bottom (Priest, 2007). Te shallow depth explosion then gave a clean pulse since the gas bubble was vented to the atmosphere during its initial expansion, thereby improving the seismic records. Tis solution, however, had been published somewhat earlier by Lay (1945). He found that the bubble oscillations could be prevented by bringing the charge close enough to the surface of the water so that the gas bubble would burst through the surface on first expansion. He also noted the alternative solution: by increasing the charge, the radius of the bubble would increase so that the surface would break before the contraction could occur.


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before advancing to balloons and ‘turkey bags’ inflated by air compressors. Soon it became the practice to fire small charges, each tied to a balloon via a piece of string, fired even as shallow as 2 ft (0.6m) below the surface. Tis procedure fully eliminated the explosion bubble pulse.


Figure 3.17: A seismic source (dynamite) is fired in a Shell Oil seismic survey in the Gulf of Mexico, April 1951. Here, the charge has been detonated at a shallow depth so that the gas bubble vents into the atmosphere very soon after the detonation. The effect looks dramatic and was exciting to capture on photos. When the detonated depth was large, the surface would just slightly rise and after a while vent into a water burst.


DeGolyer Library, SMU


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