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Shake, Rattle & Roll: Understanding Seismic Testing
The number of seismic tests for oil and gas conducted on public lands in the United States is increasing, and this should concern anybody interested in conserving wildlife, plants and vegetation, soil, and the very character of these lands. As domestic oil and natural gas exploration increases, geophysicists seek to map and understand underground deposits that may hold valuable hydrocarbon resources.
Seismic testing is their preferred method, and one result is a proliferation of linear barriers across the landscape Seismic testing evolved from the discovery that when earthquakes occur, it is possible to capture the sound waves created and use the data to map geophysical features that lie underground. Much of what we know about the Earthøs core, mantle, and crust is the result of this discovery, and it follows that man-made seismic, or sound, waves can also be used to map subsurface geologic formations and locate stores of oil and natural gas. However, as seismic tests become more prevalent, there is growing concern about their impacts -- seismic testing requires intensive cross-country travel, often with vehicles that weigh 60,000 to 80,000 pounds.
How Itøs Done
To conduct a test using the preferred Three-dimensional (3-D) seismic method, long cables are first laid along a îreceiver line." Next, dynamite blasts or Thumper trucks (also called vibrasise trucks) are used along a îsource line" to create what is essentially a man-made earthquake, sending energy into the earth. The energy waves bounce off of the subsurface formations and back to the surface where they are captured by îgeophones," which are connected by the receiver lines to a îdoghouse," or data receiving truck. Knowing the frequency at which the energy is created, it is possible to analyze the frequency of the returning waves and create a map of the subsurface area.
An earlier testing method, 2-Dimensional (2-D) seismic, is conducted by placing a receiver line across an area of land, and creating energy along that same line. In other words, the receiver line and the
3-D seismic yields a picture that shows a volume of earth, which is much more valuable. In conducting a 3-D test, a number of receiver lines are placed parallel to each other across a landscape. The lines run at an angle (often perpendicular) to the source line1, in a îbrick pattern." To create the necessary energy, two methods are generally employed. In the vibrasise method, four trucks move in tandem along the source line, stop at a predetermined point, lower a self-contained platform, and vibrate in unison, sending energy into the earth. This is repeated hundreds or thousands of times in the course of one test. If dynamite is used, a drill rig creates a îshot hole" (50-100 feet deep) along the source line, into which a charge is placed. The charge is set off to create energy in what is called the îshot hole" method, and this process is repeated over the entire testing area. In addition to source vehicles, ATVs are also used during each test. These are driven along the receiver lines to troubleshoot problems.
Clearly, seismic testing is a vehicle-intensive process. In order to collect the most valuable data, it is not possible for the source lines to run along existing roads. With both methods (shothole and vibrasise) it is necessary for a number of vehicles to drive cross-country, causing potentially severe ecological impacts.
Under the National Environmental Policy Act (NEPA), federal agencies are required to analyze the potential impacts of proposed activities. In all seismic projects studied for this review, Environmental Assessments (EAs) were conducted, which are less thorough than Environmental Impact Statements (EISs). These documents outline the potential impacts to wildlife, soil, and vegetation, among others, and offer mitigation methods to minimize the effects.
A review of EAs conducted by the Bureau of Land Management (BLM) for projects in the Moab, UT and Green River Basin, WY areas reveals that BLM findings of no significant impact draw largely on anecdotal evidence and do not rely on verified science or cited references. In these EAs, interviews, internal agency documents, and observations from past projects are used to conclude that the impacts from seismic testing will be temporary and non-severe. In contrast, the documented science on linear disturbances like ORVs and roads is quite extensive, and suggests that long-term damage is occurring.
Use of ORVs, for example, destroys habitat and forage for wildlife and disturbs threatened and endangered species. The impacts to soil and vegetation include compaction, which causes erosion and reduced plant growth, as less water is able to penetrate the surface. Ruts may also be caused if vehicles operate when the ground is wet, which can cause even greater problems with compaction and water runoff. The heavy vehicles used in 3-D seismic testing create twotracks that run across the landscape. If these tracks are not eliminated, unauthorized use of recreational ORVs may occur once testing has ended, and the impacts to wildlife, soil, and vegetation will be even more pronounced.
ORV use is known to destroy vegetation that serves as natural soil-protective elements, even after one pass of a vehicle (Wilshire 1983). This impact becomes more significant as the number of trips over an area increases (Payne et al. 1983). In arid regions, ORVs have been found to increase water runoff and erosion (Hinckley et al. 1983), a result of soil compaction and decreases in soil porosity and infiltration capacity (Webb 1983). Impacts were found even when use of such vehicles was slight, and the first passes of a vehicle over a landscape were found to be the most damaging (Iverson et al. 1981). It is estimated that recovery from soil compaction and a natural return to bulk density, strength and infiltration capacity make take a century to occur. In addition, invasive vegetative species were found in compacted areas within a few years, but native species were much slower to return (Webb & Wilshire 1980).
The impacts of roads, seismic lines and other linear disturbances have a number of impacts on wildlife populations, including individual disruption, habitat avoidance, social disruption, habitat disruption or enhancement, direct and indirect mortality, and effects on population. These impacts have received substantial treatment and attention (Jalkotzy, et al. 1997). Studies also show the dramatic effects that roads have on the movement and mortality of wildlife (Forman & Alexander 1998; Trombulak & Frissell 2000), and the balance of this evidence is so strong that policies have been enacted to reduce road densities in national forests to protect wildlife (Hourdequin 2000).
Impacts of seismic testing have been largely ignored. This is starting to change, however, as more attention is paid in areas where the testing is most prevalent, especially Utah and Wyoming. It is important that the lack of understanding and information about the processes and impacts of these projects be solved, and greater public pressure placed on the federal agencies conducting these reviews. Given the documented impacts of ORV use on wildlife, soils, and vegetation, it is imperative that the BLM and other federal agencies pay greater attention to these projects and the effects they are having on our federal lands. It also needs to be determined whether the two-tracks created during seismic testing are used for recreational purposes once testing is completed.
The absence of scientific research on these issues is disturbing, and until more study is done, it is hard to justify that these impacts are short-term and unimportant. One way this might be corrected is to force the BLM to conduct EISs when considering seismic projects, which would result in much greater scrutiny and require a higher threshold of scientific evidence before determining that a project will have no impact.
1. Actual cables are placed along a receiver line, but a source line is a theoretical line along which vibrasise trucks will drive or dynamite blasts will be placed to create the energy necessary.
--- Erich Zimmermann is a graduate student in Environmental Studies at the University of Montana.
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Hinckley, B.S., R.M. Iverson, and B. Hallet. Accelerated water erosion in ORV-use areas. In
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