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Seismic survey, method of investigating subterranean structure, particularly as related to exploration for petroleum, natural gas, and mineral deposits. The technique is based on determining the time interval that elapses between the initiation of a seismic wave at a selected shot point (the location where an explosion generates seismic waves) and the arrival of reflected or refracted impulses at one or more seismic detectors. Seismic air guns are commonly used to initiate the seismic waves. This technique has largely replaced the practice of exploding dynamite underground. Electric vibrators or falling weights (thumpers) may also be employed at sites where an underground explosion might cause damage—e.g., where caverns are present. Upon arrival at the detectors, the amplitude and timing of waves are recorded to give a seismogram (record of ground vibrations).
Seismic Surveys: Advantages And Controversy
Generally, the density of rocks near the surface of Earth increases with depth. Seismic waves initiated at a shot point at or near the surface may reach the receiving point by reflection, refraction, or both. When the shot point is close to the receiving point, reflected waves usually reach the receiving point first. At greater distances, however, the seismic pulse travels faster by the refraction path because its velocity is greater along the top of the lower, denser layer than it is through the upper layer; in this case, the refracted wave arrives first.
Interpretation of the depths and media reached by seismic waves thus depends on the distance between shot points and receiving points and the densities of the strata. The results of a seismic survey may be presented in the form of a cross-sectional drawing of the subsurface structures as if cut by a plane through the shot point, the detector, and Earth’s centre. Such drawings are called seismic profiles.Seismic surveys are used to locate and estimate the size of offshore oil and gas reserves. To carry out such surveys, ships tow multiple airgun arrays that emit thousands of high-decibel explosive impulses to map the seafloor. The auditory assault from seismic surveys has been found to damage or kill fish eggs and larvae and to impair the hearing and health of fish and marine mammals, making them vulnerable to predators and leaving them unable to locate prey or mates or communicate with each other. These disturbances can disrupt and displace important migratory patterns, pushing marine life away from suitable habitats like nurseries and foraging, mating, spawning, and migratory corridors. This is especially dangerous for critically endangered species like the North Atlantic right whale, with an extremely population (only 411 individuals left in the world), where just one death can have population-wide impacts. In addition, seismic surveys have been implicated in whale beaching and stranding incidents. Sadly, in November 2018, as part of the America First Energy Strategy, the Trump administration authorized five seismic surveys in the Atlantic. This is the first step in expanding new offshore oil drilling off the US coasts. Learn more here.
Seismic surveys utilize arrays of airguns to produce powerful sound waves. Sudden releases of pressurized air bubbles create the sound source, with up to 20 guns fired at the same time, while “streamers” of hydrophones listen for echoes. Using sophisticated acoustic processing, these echoes can provide information about geological structures up to 40 kilometers below the sea floor. Seismic surveys are used by the oil and gas industry in its search for new hydrocarbon deposits and the monitoring of reservoirs as they are emptied. The “source level” of most airgun arrays can be 200 to 240 decibels (dB) (in water). There is a difference of about 60 dB when converting the sound level from water to air, so in air, the airgun sound level would be about 140 to 180 dB. For comparison, a loud rock concert is about 120dB and a jet engine at 100 feet is about 140dB. A typical seismic air gun array pulled by a ship might fire its compressed air bubbles into the ocean five or six times a minute — more than 7, 000 shots in 24 hours.
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Because sound can travel hundreds or even thousands of miles under water, it’s not surprising that seismic airguns can be heard at great distances. In 2004, bioacousticians began reporting that airgun noise from distant surveys along the coast of South America (and perhaps Africa) can be the dominant sounds in some mid-Atlantic study sites, at times making it difficult or impossible to hear the whales or seaquakes they are trying to study. Airgun noise is over 200dB (often 230db) at the source, drops quickly to under 180dB (usually within 50-500 meters, depending on source level and local conditions), and continues to drop more gradually over the next few kilometers, until leveling off at somewhere near 100dB. At this level, the sound can still travel for hundreds or thousands of kilometers. In many or most locations, 100dB is significantly louder than the existing ambient background noise, so when the airguns raise the background noise to this level, it potentially masks local biological calls and signals. Such effects have been noted at ranges from 1, 300 to 3, 000 km from active surveys. These sounds are primarily low frequency, so at long distances, the effects are most pronounced for larger species such as the great whales and some fish that use low-frequency sounds.
At the International Whaling Commission 2004 meetings and at 2004 meetings of the US Marine Mammal Commission’s Advisory Committee on Sound, research was presented that suggests human noise can shrink the area in which whales can communicate with each other by two to four orders of magnitude (that is, when the sea is especially loud, their effective communication area is one hundredth to one ten-thousandth the size that it would be in the absence of human noise). See Resolutions Adopted during the 56th Annual Meeting of the International Whaling Commission and Marine Mammals and Noise - A Sound Approach to Research And Management from the US Marine Mammal Commission.
While seismic surveys have been taking place for decades, changes in industry practices (including exploration on the continental slopes where sound may bounce into long-range sound channels, and increasing use of 4D (repeat) surveys to monitor reservoir depletion during the life of active reservoirs) are increasing their use and the resultant concern about harm to sea life. The North Sea has been the site of many 4D surveys, and the industry considers the Gulf of Mexico fields now “mature” enough to “benefit” from surveys there.
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The passage of an energy bill in the United States in the summer of 2005 increased attention on airguns, as the bill called for a comprehensive inventory to be made of oil and gas reserves on the Outer Continental Shelf. The Minerals Management Service (MMS) (now Bureau of Ocean Energy Management (BOEM)) plans to review existing data and research, then to conduct new surveys as needed to increase geologic knowledge in areas of high reservoir potential. Each year there are about 20 permitted 3-D seismic surveys in the U.S. Gulf of Mexico. That work is facilitated by a disputed 2004 Environmental Assessment (pdf) that concluded “geological and geophysical activities evaluated in the EA will not significantly affect the quality of the human environment.” The document does set a standard 500 m safety radius for seismic surveys. More recently, plans been announced for an environmental review followed by seismic surveys in the Chukchi and Beaufort Seas, Alaska and off the Atlantic coast. There will now be a supplemental EIS for the Arctic region.
Public hearings were conducted in April 2012 to receive public input on seismic surveys for the mid-Atlantic and south-Atlantic regions. Click here for more info and to hear what seismic testing sounds like underwater. According to a report issued by Oceana in April 2013, seismic testing in the mid-Atlantic and south-Atlantic regions would cause:
As mentioned above, there is growing concern that sound introduced into the sea by human activities has detrimental effects on marine mammals. For example, mounting evidence suggests that high-intensity anthropogenic sound from sonar and airguns leads to strandings and subsequent mortality of beaked whales. Although the mechanisms of injury in these events are unclear, the species affected and the implicated sound levels follow a consistent pattern.
Seismic Surveys Market Size: Swot Analysis By Forecast To 2030
A well-established body of research indicates that marine mammals try to avoid active seismic survey vessels, often exhibiting avoidance behavior at ranges of 5-30km; however, it is not uncommon for whales or dolphins to approach closer to operating airguns, whether out of curiosity or because of a biological need to be where they are. In 2002, two beaked whales (the species of whales that has proven
The passage of an energy bill in the United States in the summer of 2005 increased attention on airguns, as the bill called for a comprehensive inventory to be made of oil and gas reserves on the Outer Continental Shelf. The Minerals Management Service (MMS) (now Bureau of Ocean Energy Management (BOEM)) plans to review existing data and research, then to conduct new surveys as needed to increase geologic knowledge in areas of high reservoir potential. Each year there are about 20 permitted 3-D seismic surveys in the U.S. Gulf of Mexico. That work is facilitated by a disputed 2004 Environmental Assessment (pdf) that concluded “geological and geophysical activities evaluated in the EA will not significantly affect the quality of the human environment.” The document does set a standard 500 m safety radius for seismic surveys. More recently, plans been announced for an environmental review followed by seismic surveys in the Chukchi and Beaufort Seas, Alaska and off the Atlantic coast. There will now be a supplemental EIS for the Arctic region.
Public hearings were conducted in April 2012 to receive public input on seismic surveys for the mid-Atlantic and south-Atlantic regions. Click here for more info and to hear what seismic testing sounds like underwater. According to a report issued by Oceana in April 2013, seismic testing in the mid-Atlantic and south-Atlantic regions would cause:
As mentioned above, there is growing concern that sound introduced into the sea by human activities has detrimental effects on marine mammals. For example, mounting evidence suggests that high-intensity anthropogenic sound from sonar and airguns leads to strandings and subsequent mortality of beaked whales. Although the mechanisms of injury in these events are unclear, the species affected and the implicated sound levels follow a consistent pattern.
Seismic Surveys Market Size: Swot Analysis By Forecast To 2030
A well-established body of research indicates that marine mammals try to avoid active seismic survey vessels, often exhibiting avoidance behavior at ranges of 5-30km; however, it is not uncommon for whales or dolphins to approach closer to operating airguns, whether out of curiosity or because of a biological need to be where they are. In 2002, two beaked whales (the species of whales that has proven
