Seismic Exploration
The term "seismic exploration" can be a little misleading. It doesn't necessarily mean looking for literal earthquake faults, although it is used for that purpose. For the oil and gas industry it means looking for the types of strata that hold fossil fuels. This technology was developed so that oil and gas reserves could be detected hundreds and thousands of feet below the surface of the ocean, thus eliminating the need and expense to construct oil platforms to conduct test drilling. Seismic exploration is also used for mapping the ocean floor's topography.
Here's how it works.
A ship towing an array of airguns aims the cylinders downward and sets off "blasts" that are created by the sudden release of compressed air. These blasts are set off every few seconds and run round the clock as the ship travels a specific grid, or transect, pattern. The same ship (and sometimes another ship) is towing an array of hydrophones that will detect and record how the sounds are reflected back from the ocean floor. Because different types of rock and other substrate reflect sound differently, how long the sound takes to return can tell the scientist what lies beneath the sea floor and, more importantly, where. There are many different ways airgun arrays can be designed and used, and many ways sensors and hydrophones are deployed depending on the features of the site being surveyed. This is simply a general look at how the technology is used.
Explosives were once used when underwater seismic exploration first started, but the need to be able to fire more rapidly and more accurately drove the development of using compressed air instead. Also, the need to control the noise of the air bubbles as they rose to the surface (thus interfering with the received signal) led to the use of multiple airguns of varying sizes. It's a very complex technology, and every seismic project is different depending on where it operates and the requirements of the survey.
Why Seismic Exploration is a Problem
Most seismic surveying work is conducted round the clock for weeks and sometimes months at a time. The airguns are blasted every few seconds, 24 hours a day. The sound generated by each airgun array blast is extremely intense and can range as high as 255 dB re 1µ Pa-m. (Compare to the intensity and duration of the LFA signal that is broadcast at 215-240 dB re 1µ Pa-m for up to 100 seconds.) This means that any marine organism that lives in that area or nearby will be driven out by the sound, or injured or killed if it stays within the blasting area, or within what is called the "zone of influence" surrounding the source of sound. While the "peak" sound is acute for only a fraction of a second, the "echo" of that sound repeats as it's reflected first off the seabed, then off the water's surface, back down to the bottom, and so on, decreasing in intensity each time it's reflected. But within a few seconds, another blast is released, starting another reflective process.
In the case of oil and gas exploration, in the Gulf of Mexico for instance, multiple surveys can be taking place at the same time, and dozens of surveys can be conducted back-to-back, continuously ensonifying an entire region for years at a time with high levels of one of the loudest sounds produced by man. There are literally thousands of "blocks" of oil leases being granted in this area, an area that is host to at least 29 species of marine mammals and is a "biologically important area" for a discrete stock of the highly endangered sperm whale, not to mention being an important economic area for many commercial species of fish.
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