Modern Land Methods
For the most part, newer methods of creating vibrations in the earth have replaced dynamite. Engineers have developed various mechanical impactors and vibrators to create seismic waves on the earth's surface that penetrate down into the rock layers. One of the first nondynamite sources of surface energy was the Thumper, developed by Petty-Ray Geophysical (fig. 2.8). This impactor drops a heavy steel slab from as high as 9 feet (2.7 metres) onto the ground to create shock waves. Later, Sinclair Oil and Gas Company developed the Dinoseis, which uses amixture of propane and oxygen in an expandable chamber to create an explosion. The explosion chamber is mounted under a truck and is lowered to the ground for use. The most popular seismic device used today for land exploration is the Vibroseis, developed by Conoco (fig. 2.9).
Figure 2.8 The Thumper drops a 6,000-pound steel slab (surrounded by safety chains to warn personnel) 9 feet to strike the earth and create shock waves.
Figure 2.9 The Vibroseis truck has a vibrator mounted underneath it that creates low-frequency sound waves. Geophones pick up the sonic reflections for recording (inset). (Courtesy of Trapp and Eskew Geophysical Consultants)
The Vibroseis generates continuous low-frequency sound waves whose reflections are picked up and changed into electrical impulses by the geophones. These recorded impulses are sent to a computer for analysis, then printed in the form of a seismic reflection profile. Geologists then analyze the profile to determine subsurface structures.
Although explorationists usually use geophones on the surface of the earth to gather seismic data, they may also use them in the borehole of an existing well. In this method, technicians run geophones into a well and attach them to the wall of the hole at intervals of 20 to 100 feet (6 to 30 metres). With the receivers deep in the earth, surface noise is less likely to distort the signal from the seismic source.Another advantage of this method is that it can gather information about the geological structures in the immediate vicinity of the hole.