1. Field of the Invention
This invention relates to improvements in methods and apparatuses for seismic exploration, and more particularly, to improvements in methods and apparatuses for generating seismic waves.
2. Description of the prior art
In carrying out seismic exploration of subterranean formations, beneath, for example, the surface of the earth or the floor or bed of a body of water or the like, many types of apparatus for generating seismic or acoustic waves have been advanced. Presently, and for many years in the past, seismic waves have been generated by detonating an explosive charge, such as dynamite or the like, placed in a hole drilled at the desired wave initiating point. The waves generated travel outwardly from the point of explosion into the earth to be reflected by subterranean formations, the reflections being then detected on the surface at one or more geophones or other seismic wave detectors, and recorded for subsequent analysis. The use of an explosive charge, however, encompasses several undesirable limitations. First, the particular explosive charge used requires strict adherence to safety precautions to prevent catastrophic results from premature or accidental detonation. Secondly, ordinarily a large number of "shots" or explosions are required to achieve a sufficient record for analysis, and a large number of holes within which the explosive charges are located must be drilled. The drilling of such holes is not only expensive, but is time-consuming as well. Nevertheless, the dynamite or explosive charge method has continued to be popular in use, especially since the resulting acoustic or seismic waves they generate resemble a mathematical "impulse," which may be easily mathematically analyzed to determine useful information, such as the type, depth, and location of the subterranean subsurfaces, and so forth.
In recent years, mechanical apparatuses have been receiving increased interest for replacing the traditional use of dynamite or other explosive charge. One relatively uncomplex system involves simply dropping a large weight to impart its kinetic energy to the earth, thereby generating seismic waves for detection.
Other recently advanced apparatuses include gas explosion devices, many of which, in general, employ a large diameter striker plate forming a portion of a combustion chamber. The large striker plates are typically on the order of 24, 36, and sometimes 70 inches in diameter with a relatively large piston disposed above it within the chamber. The striker plate is oridinarily placed directly onto the surface of the ground at the point from which the seismic waves are to be generated, and a large weight exerted on the piston. (In many instances, the apparatus is disposed or located directly on the ground without a large mass resting on the piston to enable it to "bounce" into the air after the explosion has occurred.) The piston characteristically is sealed against pressure leaks between the cylinder and the piston by a large "O" ring. An explosion producing gas, such as propane or the like, is introduced into the small volume combustion chamber between the striker plate and the piston and ignited, causing the striker plate to rapidly move downwardly to impart energy into the ground. At the same time, the combustion explosion forces the piston upwardly, typically 1/4 to 1-1/2 inches, or more, to overcome the weight, if any, resting upon it.
In both the falling weight system and the gas explosion system, catching devices are sometimes employed to prevent secondary impacts against the earth which may generate unwanted noise signals. Thus, for example, in combination with weight dropping apparatuses, systems may be employed which secure and raise the mass after the initial impact of the mass onto the ground to prevent it from bouncing or otherwise reimpacting upon the ground. Likewise, in the gas explosion devices, mechanisms are frequently incorporated which "catch" the striking plate and reacting mass which, if otherwise unrestrained, would "bounce" after the explosion.
Ordinarily, the weight dropping and explosion seismic wave generators are of relatively large size, overall weights of 2,000 to 8,000 pounds not being uncommon. Since the generators necessarily must be transported among the sites of interest, they frequently are suspended in pairs from the sides or end of a bed of a truck, which may be driven to remote locations desired for testing. The generators, being of such large size and weight, are usually symmetrically balanced about a common axis of the truck to equalize the load presented. Thus, for example, frequently two or more gas explosion generators are symmetrically mounted on brace structures off opposite sides of the truck, but nevertheless, present significant impediments to the truck's maneuverability.
In addition, another of the inherent characteristics of the presently used gas explosion generators and weight dropping apparatuses is that their use in swampy, muddy, or mucky areas is difficult because of the maneuverability problem caused by their large weight, and because after firing, the large plate becomes embedded in the soft surface upon which it has been fired, and is difficult to remove because of the suction generated under its surface area. One possible solution to this problem is to provide an elongated structure which does not present a large surface area in contact with the ground. Several elongated structures have been proposed, although apparently none heretofore has recognized the advantage in using an elongated structure in mud or other such mucky areas. One of the elongated structures advanced is the recoil seismic impulse gun of R. A. Peterson, U.S. Pat. No. 3,283,844. In that embodiment, two pistons are disposed within an upstanding vertical cylinder, and a gas explosion is generated between them forcing one up in reaction to the force of the other piston which is forced downwardly onto an anvil and striker plate. The pistons are relatively heavy, being 3,700 pounds and 2,000 pounds, and the anvil is approximately 1,000 pounds. The striker plate upon which the anvil is driven is approximately 4,000 pounds. Thus, the total weight of the Peterson gun is in the neighborhood of 11,000 pounds. It should be noted that the stroke of the 3,700 pound piston is on the order of nineteen inches and the stroke of the 2,000 pound piston is in the neighborhood of 22 inches.
A similar recoilless impulse gun has been proposed by R. A. Kirby in U.S. Pat. No. 3,215,223, which incorporates active and reaction pistons, but rather than presenting an anvil against which one of the pistons is driven, the gun includes a piston rod attached to one of the driven pistons, the piston rod carrying an impacting plate external to the piston cylinder. The Kirby gun also is relatively heavy, the piston and shaft alone being on the order of 1,900 pounds.
Many of the seismic wave generating apparatuses used heretofore employ valve arrangements or assemblies through which the gas or fuel injected into the apparatus is controlled. Commonly, the valves are one-way valves which allow the fuel to flow in a single direction into the combustion chamber of the apparatus, but do not allow the gas to flow out once in the compression chamber. The valves used are relatively complicated, require maintenance, and are not trouble free.
In addition to a significant maintenance requirement for apparatuses employing inlet fuel valves, considerable maintenance is ordinarily otherwise required for most of the apparatuses heretofore advanced. Because of the tremendous shocks generated by the explosions within the compression chamber, not all of which are transferred into the ground, a portion of the energy is absorbed by the apparatus itself and its surrounding elements. Even in apparatuses having very short piston strokes, on the order of one-quarter inch or more, the rapid relative movement between the piston and the wall of the compression chamber generates considerable frictional force and heat and considerable wear and tear on the parts. This situation is further aggravated by the large weights of the components involved.