Seismic Trails

Seismic exploration involves sending sound waves into the ground, recording how they reflect back and interpreting the results to provide an image of subsurface geology to determine whether oil may exist.

Seismic exploration, authorized by the U.S. Congress, was conducted on the coastal plain of the Arctic Refuge during the winters of 1984 and 1985. Exploration during winter causes less damage to tundra vegetation and soils than in summer, but damage does occur. Snow cover on the Refuge coastal plain is normally shallow, usually less than one foot deep. Strong winds blow the snow into depressions, leaving higher areas with thinner snow cover, making them more susceptible to impacts from vehicles.

As a result of the 1984-85 seismic exploration, known as 2-D (two-dimensional) seismic, 1250 miles of trails - made by drill, vibrator and recording vehicles - crossed the coastal plain tundra (see map above). Additional trails were created by D-7 Caterpillar tractors that pulled ski-mounted trailer-trains between work camps.

Refuge staff have monitored recovery of the seismic trail damage on the Refuge by collecting vegetation data at 100 permanent plots every 4 years. To determine how much trail is still disturbed they rate another 200 points for disturbance level. While 90% of all trails recovered well during the first 10 years after exploration, 6% of trails had still not recovered by 2002, 18 years after the disturbance. This indicates that about 150 miles of disturbed trail remained in 2002, based on a total length of about 2500 miles of original trails, both seismic lines and camp-move trails. Some of the trails have become troughs visible form the air. Others show changes in the amount and type of tundra plants. In some areas, permafrost (permanently frozen ground) melted and the trails remain wetter than they were previously. Some of these impacts are expected to persist for decades.

The National Research Council's committee on Cumulative Environmental Effects of Alaska North Slope Oil and Gas Activities estimated that 32,000 miles of trails were made by seismic exploration between 1990 and 2001 on Alaska's North Slope. The committee wrote that "nearly all of our knowledge about long-term recovery from seismic exploration comes from a single U.S. Fish and Wildlife Service study" - the Arctic Refuge study. Since no comparable research has been initiated in the development areas of the North Slope, Refuge staff will continue to monitor recovery of trails in the Refuge into the future.

A new technology used to create three-dimensional subsurface images (known as 3-D seismic) is currently used across Alaska's North Slope and within a half mile of the Refuge's western border. This technique requires a much denser grid of trails than did 2-D seismic. While the 1984-85 trails on the Refuge were about four miles apart, 3-D seismic trails are one half mile or less apart.

Example of a seismic trail that remained visible in 2002:

Seismic trail near Marsh Creek (red dot on map above).
(a) During exploration in winter 1984.
(b) Showing ruts of crushed sedge tussocks in July 1984.
(c) In July 2002, 18 years after disturbance.

Why was this trail still visible in 2002? Detailed plant sampling in permanent plots can provide an answer. In the surrounding undisturbed tundra, the evergreen shrubs labrador tea and cranberry covered 30% of the ground, but only covered 5% of the area in the trail. In addition, because the trail became a wetter trough, the amount of willow and sedge almost doubled and the amount of moss decreased.

Information about Arctic Refuge seismic trail monitoring can be found in:

Emers M. and J. C. Jorgenson. 1997. Effects of winter seismic exploration on the vegetation and soil thermal regime of the Arctic National Wildlife Refuge. In Crawford, R. M. M. Ed., 1997. Disturbance and recovery in Arctic lands: an ecological perspective. Kluwer Academic Publishers, Dordrecht, the Netherlands.

Information about winter seismic exploration, including 3-D seismic techniques, can be found in the partial bibliography of scientific research pertaining to the Refuge, and specifically in:

Bureau of Land Management. 1998. "Northeast National Petroleum Reserve-Alaska, Final Integrated Activity Plan/Environmental Impact Statement, Volume 1" prepared in cooperation with the Minerals Management Service, August, 1998.

National Research Council. 2003. Cumulative environmental effects of oil and gas activities on Alaska's North Slope. The National Academies Press, Washington D.C.