A key requirement for collecting good data is a suitable seismic source; one which is powerful enough to generate detectable waves all along the survey spread and to the required depth. The frequency signature of the source is also important: something which generates more high-frequency energy will result in greater detail, but the signal will be attenuated (absorbed) much faster; a low frequency source will provide good depth penetration, but resolution is compromised. With vibration sources (as opposed to impact sources like a hammer or weight-drop) it is possible to define the specific vibration frequency, or even choose a “sweep” of frequencies from high to low. Use of a vibration or impact source defines “active” methods, but in “passive methods” the source is background vibrations originating from both natural and human motion (such as tides, distant tectonic activity, construction, traffic). These are very low frequency but very powerful vibrations, typically used in surface wave studies.
For more information, please visit Types of Seismic Survey.
Geophones will need to be matched to the task in hand and are normally defined by their orientation and their “Hz” rating, which defines the frequencies they are most sensitive to. As with sources, low frequency units respond well to energy that has likely travelled further into the ground, whilst their higher frequency counterparts will be able to deliver better detail. Some methods need lower frequency units, such as any surface wave analysis, whilst others like reflection can benefit from those with a higher rating. As for orientation, refraction studies normally require vertically mounted sensors, reflection surveys often use horizontally orientated units, and some methods use geophones that have multiple sensors mounted in opposing orientations. For hydrophones, which measure pressure change rather than motion, the only choice is active or passive. Passive units simply send the signals generated by the vibrations passing through them, whilst active systems amplify the signal at the sensor before transmitting back along the spread cable.
The layout of the investigation will determine whether the result is a sounding (showing velocity variation only with depth – “1D”) or something more comprehensive which also reveals velocity variations along a profile (“2D”) or across an area (“3D”).
Kate Lowery
Posted November 8, 2016
With a series of public hearings on proposed federal rules for offshore seismic testing scheduled to begin this week, let’s take a look at the basics on the safe technologies and procedures used to survey for oil and natural gas reserves. These are key as the Bureau of Ocean Energy Management (BOEM) conducts hearings in New Orleans (Nov. 9), Gulfport, Miss. (Nov. 10), Fort Walton Beach, Fla. (Nov. 14), Mobile, Ala. (Nov. 15) and Houston (Nov. 17).
Seismic surveys use acoustic waves to create images of the earth through analysis of vibrations from those waves. Some seismic waves can penetrate solid rock and fluids into the deep inner layers of the earth, while others can only travel along the earth’s surface, like ripples on water. According to the International Association of Geophysical Contractors (IAGC), offshore surveys are conducted by sending acoustic waves into different layers of rock beneath the sea floor, then recording how long it takes each wave to bounce back and measuring the various characteristics of those waves.
Seismic surveys can be conducted on land and in the ocean. The oil and gas industry uses both types of surveys – to determine where oil and gas reserves are on land and in the sea. Both surveys use the same process to complete seismic imaging of earth’s subsurface layers. For offshore testing, a vessel with long floating cables, or streamers, and specialized microphones, called hydrophones, attached to the cables sits on the water’s surface. Air guns located behind the vessel and in front of the streamers, shoot blasts of air into the water at regular intervals, which creates acoustic wavelets. These wavelets travel through subsurface layers of rock then travel back through the water to the hydrophones.
Data is recorded based on distance the wavelets had to travel and then analyzed to produce an image to mirror the subsurface layers. Onshore seismic testing uses trucks rather than ships to conduct the surveys, but all steps in the process are the same.
Seismic testing is utilized by many industries for many different tasks. These methods are used to locate groundwater or potentially recoverable mineral deposits or to check foundations for roads and buildings. They also are commonly used by the U.S. Geological Survey, the National Science Foundation, and the offshore wind industry.
In the oil and gas industry, seismic surveys are used to locate and estimate offshore oil and gas reserves. These surveying activities have been used by the industry, and around the world, for more than 30 years. “Seismic surveys are temporary and transitory and are the least intrusive and most cost-effective means to understanding where recoverable oil and gas resources likely exist,” according to the IAGC.
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Seismic testing is vital to our nation’s and our world’s energy supplies. Most of the world’s oil and natural gas is produced offshore, and global and domestic energy demand is expected to grow for many years. Oil and gas are two main sources of energy for our country and the world, and are necessary to meet this growing demand. Seismic surveys allow our industry to continue to discover and produce more energy to meet these growing needs.
Andy Radford, API upstream senior policy advisor, noted during a recent press call that “Advances in seismic imaging technology and data processing over the last decade have dramatically improved the industry’s ability to locate oil and natural gas offshore. And those energy sources – especially in the Gulf – can be harnessed to create hundreds of thousands of jobs, help American consumers and strengthen our national energy security.”
He also stated that “a rigorous environmental analysis and permitting process ensures that seismic surveys are properly managed and conducted so they have minimal impact on the marine environment.”
Before any seismic testing can be conducted, marine animal behavior and movement habits are analyzed. Areas of concern for impacts on marine life are strictly off limits to testing vessels so that those populations and ecosystems are left undisturbed. In order to make sure that marine animals are given time to leave an offshore testing area, vessels begin the survey process with a “soft start”. This includes low level air blasts to introduce the seismic activity and noise to marine species. Gradually, the sound is increased to full operational levels for the full survey to begin.
If any visual or acoustic disturbances to marine animals are spotted or any sensitive marine life is detected in the survey area, all activity ceases immediately and are restarted only when the area is clear.
The compressed air released by the survey ships is local and transient in nature. Fish may temporarily swim away from the seismic air source, but they often return after the vessel has left that area. To ensure the safety and economic viability of fishing operations, survey operators will work with regulatory bodies and local fishing communities to avoid sensitive spawning areas.
Further, surveyors are required to have monitoring and mitigation practices in place that are proportionate to the potential risks associated with their activities, meaning that industry will avoid sensitive biological areas where marine species forage, breed, and seasonally congregate, including dolphins and whales.
In fact, William Brown, BOEM’s chief environmental officer has recognized that offshore seismic survey technology has been used in the United States with “no known detrimental impact to marine animal populations or to commercial fishing.” In the United States these surveying methods have been utilized in the Gulf of Mexico and along the Atlantic coast.
The oil and gas industry is committed to improving the scientific understanding of the impacts of our operations on marine life, while responsibly producing oil and gas resources to meet our nation’s energy needs.
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For more information about offshore seismic surveys, visit the IAGC's website here.
Kate Wallace is an associate of research and content development for the American Petroleum Institute. Before joining API she was a researcher and policy analyst at America’s Natural Gas Alliance, and worked on pollinator conservation programs and state wildlife conservation policies before entering the energy industry. Kate graduated from the University of Connecticut with a bachelor’s degree in Resource Economics, and earned her Master of Public Administration from George Mason University. She loves taking her dogs on hikes, travelling and navigating the northern Virginia/DC craft beer and wine scenes with her friends and family.
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