The use of seismic waves to detect the presence of underground oil and gas reservoirs has been practiced for more than half a century. Seismic waves are also used in mining applications, tunnel detection, imaging of local hydrogeological features, and building construction. Basically, seismic exploration techniques involve generating seismic waves with one or more seismic sources, coupling those waves into the earth, and measuring the perturbed seismic waves at pre-selected locations from the source(s) with one or more seismic receivers. Analysis of the information from the measurement of the perturbed seismic waves allows for the determination of the location of, for instance, oil and gas reservoirs.
The current seismic exploration methods consist of four general configurations. The first technique, referred to as Surface Seismic Exploration, uses a configuration where the source(s) and receiver(s) are all on the surface of the earth. The second approach, referred to as Vertical Seismic Profiling (VSP), uses a configuration where the source(s) are on the surface, but the receiver(s) are beneath the surface in one or more boreholes. Borehole depths can be anywhere from a few hundred feet to over 10,000 ft. The third approach, referred to as Reverse VSP, places the source(s) in the borehole(s) but has the receiver(s) on the surface. The fourth configuration, referred to as Cross Borehole, places both source(s) and receiver(s) in different boreholes.
Clamped single station borehole seismic receivers are available from a variety of companies including: E G & G, Sunnyvale, Calif.; Western Atlas; Shlumberger, Paris, France; Teledyne Geotech; and Halliburton, Houston, Tex. These receivers are long (typically 3 ft. in length) and heavy (typically on the order of 150 lbs), and include a variety of mechanisms for clamping the receiver to the borehole including, hydraulic devices, spring-loaded devices, magnetic devices, and motor-driven mechanisms. All of these single-station instruments were originally designed for VSP applications and, as such, were designed primarily to respond to frequencies below 150 Hz. Additionally, because these receivers are large and heavy, they do not readily accommodate stringing of multiple receivers in a single borehole. A further limitation of some is that they are not capable of high temperature operation.
Some prior art receivers utilize hydraulically-driven clamp arm mechanisms, in which the arms extend either perpendicular to the receiver housing or extend outward therefrom in an angular rotation fashion. The use of hydraulics generally requires larger receivers which at higher frequencies result in distortion of the signal(s) to be measured. Alternately, the receiver can be divided into two subparts (a receiver unit and a hydraulic pump unit), coupled together by hydraulic hoses with both parts lowered in the borehole together. While not receivers, U.S. Pat. No. 4,569,412 to Bouyoucos, et al. and U.S. Pat. No. 4,702,343 to Paulsson disclose seismic sources which utilize hydraulic clamp mechanisms which drive one or more clamp arms perpendicular to the borehole wall.
In all known prior art receivers which used electric motor-driven clamp mechanisms, one or more clamp arms extend outward from the receiver housing in an angular rotation fashion. This results in a clamp force which is not entirely perpendicular to the borehole wall. This results in the creation of a large angular moment about the location where the clamp arm or arms connect to the tool which, in turn, causes low resonant receiver frequencies. Such angular-rotation-type clamp arm designs also inhibit high frequency receiver operation because the design itself results in clamp arm resonances. E G & G Geometrics borehole seismometer (model VLP-N785) and OYO Geospace borehole shuttle 170 are believed to be typical of this type of design.
U.S. Pat. No. 4,805,727 to Hardee et al. discloses a long and heavy down hole seismic generator in which the clamping apparatus includes a pair of oppositely disposed and transversely extendable shoe members, which are designed to center the generator in the borehole. The shoe members are connected to linkage arms which, in turn, are carried by an elongated screw drive rod. The linkage arms are set at an angle with respect to the drive rod in a scissor-jack type arrangement. The rod is coupled to the shaft of a reversible servo motor for bi-directional linear movement and locking in any desired position. In operation, rotation of the drive rod 146 in a counter-clockwise direction as shown in FIG. 15 causes the sleeve member 174 to start moving upwardly thereby forcing the lower end of the clamping shoes 166 and 168 outwardly. When the lower portion of the shoes can no longer be extended or bind up, then continued rotation of the drive rod 146 causes the drive rod itself and the upper sleeve member 184 to move downwardly thereby forcing the upper portion of the shoe members 166 and 168 to move outwardly. Rotation of the drive rod 146 is continued until the shoes are fully extended. While the shoes themselves have essentially linear movement, the scissor-jack linkage arms form an angular transmission mechanism, much like the clamp arms which extend outward in angular rotation fashion. Since the linkage arms are never perpendicular to the borehole and since the linkage arms are coupled to pins, this design results in low resonate frequencies. Further, centering the generator in the borehole has the disadvantage of creating a rocking motion of the generator about the location it is clamped to the borehole.
It is an object of the present invention to provide an advanced seismic receiver which is compact (approximately 16 inches in length), lightweight (approximately 30 lbs.), incorporates an improved electric motor driven borehole clamp design, which overcomes the limitations of the prior art, and which is capable of flat frequency response from 0 Hz to 2,000 Hz. The receiver according to the present invention:
1. has a simple electric motor driven clamp mechanism, that mechanically drives a single clamp or shoe in a linear fashion perpendicular into the borehole wall and, simultaneously, drives the receiver housing into direct contact with the opposite side of the borehole wall;
2. utilizes state of the art low noise piezoelectric accelerometers as the preferred sensing elements or, alternately, geophones;
3. is designed with finite element modeling techniques to achieve a mechanical design (clamp arm, housing, mechanical inter-connects, and accelerometry use and mounting) that yields a structure whose resonant frequencies are in excess of 2,000 Hz;
4. provides simple and reliable operation in a small lightweight package;
5. utilizes standard wireline interface if so desired;
6. incorporates multi-station interconnect capability, so that multiple receivers can be interconnected and deployed in a single well or borehole; and
7. enables the receiver to be deployed in boreholes with a wide variety of inside diameters, ranging from about 4 inches to 8 inches.
Using the electric motor-driven clamp of the present invention has several significant advantages over hydraulically-driven clamps, including:
1. use of a linear (as opposed to angular) transmission mechanism;
2. electric motors can operate in well-bores as deep as 20,000 ft., whereas hydraulic clamp means are typically limited to about 5,000 ft. depth (unless equipped with a down-hole hydraulic pump);
3. electric motors can operate up to temperatures of 200.degree. C., whereas the use of hydraulic clamps has never been demonstrated at temperatures above 125.degree. C.;
4. the electric motor-driven clamp can be mounted in a small lightweight package, whereas hydraulic clamp mechanisms require larger, heavier packages (or a separate hydraulic pump unit); and
5. a motor-driven clamp enables the receiver to be connected by only a standard wireline interface, whereas the hydraulic clamps require a special custom interface or a hydraulic interface.
The use of a smaller, lighter package with the motor-driven clamp of the present invention enables the receiver to couple to the borehole over a much wider range of vibrations and avoid the low resonant frequencies inherent in U.S. Pat. No. 4,805,727 to Hardee, et al., as explained below.
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