Oil and Natural Gas - Energy Policy Analysis: A Conceptual Framework - page 81

Seismic surveys are the most common geophysical methods and seem to give the most reli-
able results (USFS 1979). Methods used in seismic exploration vary, but the premise basic to
all involves imparting an artificially generated shock wave into the ground so that the reflected
waves from layers of different density rock can be recorded. Methods of producing the shock
wave vary from using explosives to dropping a heavy object on the ground. Each formation
reflects the shock wave back to the surface, where sensitive instruments record and measure
the intensity of reflections. The resulting data are analyzed to predict where fossil fuels may
have collected (API 1978).
One method uses a thumper unit consisting of one or more large trucks from which a suspended
3-ton slab is repeatedly dropped to the ground from a height of several feet. Additional support
vehicles for equipment and crew are required. The vibroseis method involves use of three or four
large trucks, under each of which is mounted a platform. The platform can be lowered and the
truck hydraulically raised above it to provide the necessary weight. When a motor is turned on,
vibrating the entire unit, a shock wave is generated. In the dinoseis method, a large bell-shaped
metal chamber is mounted beneath a truck and lowered to the ground at test sites. Propane and
oxygen are ignited inside the chamber with a spark to create an explosion, which, in turn, drives
the required shock wave into the ground. A subsurface explosion method may utilize a large truck
mounted with equipment used to drill a two- to six-inch diameter hole from twenty-five to 200
feet deep, into which five to fifty pounds of explosives are placed. When detonated, the explosion
generates a shock wave (API 1978).
Liquid releases from oil and gas extraction include accidental spills of crude oil from blowouts
of drilling rigs, surface spills of crude from drilling pads, and slower chronic releases associated
with disposal of water produced from oil- or gas-bearing formations (“produced water”) and oil-
bearing cuttings created during the drilling process.
Access Road Construction
Exploration for oil and gas produces environmental effects on roadless areas not regularly visited
by humans similar to those discussed for exploration of coal resources in Chapter 2 . In some
terrain, road construction is necessary to provide access to drill sites for large equipment, with
attendant disruption of the natural environment. Each of the four seismic survey techniques
described above requires various vehicles such as jug trucks, equipment trucks, recording trucks,
and personnel carriers. Shot hole drilling also may require water trucks. A typical seismic opera-
tion may use ten to fifteen men and five to seven trucks (USDOI 1981b). The seismic sensors
and energy source are located along lines on a one- to two-mile grid. Existing roads are used if
possible. Lines may require clearing of vegetation and loose rock to improve access for trucks.
Each mile of line cleared to a width of eight to fourteen feet represents disturbance of about
one acre of land. After initial testing, road spacing may be reduced if results are favorable, and
the process is repeated. There can be little modification of the road system to accommodate
topography, since a line should not deviate more than eleven degrees from straight (USDOI
1981b) to get accurate data.
When rugged terrain prevents use of ground vehicles, portable drills may be backpacked or
flown by helicopter onto location. A series of twenty-five-foot-deep holes is drilled and loaded with
explosives; then all are detonated simultaneously. Another technique involves a surface explosion
that consists of draping explosive packs over stakes driven into the ground. Five to fifty pounds
of explosives in each pack are detonated simultaneously (MDFWP 1983).
An area may be explored with seismic methods several times by the same or different com-