HIGHLY SENSITIVE ACCELEROMETER

Abstract: A highly sensitive accelerometer for determining the acceleration of a structure includes a mass within a housing suspended by opposing support members. The support members are alternately wound around a pair of fixed mandrels and the mass in a push pull arrangement. At least a portion of one of the support members comprises a transducer capable of measuring the displacement of the mass within the housing. An embodiment of the invention employs optical fiber coils as support members for use in interferometric sensing processes. Arrays of such interferometer based accelerometers may be multiplexed using WDM or similar methods.

Description: Highly Sensitive Accelerometer Technical Field This invention relates to highly sensitive accelerometers, and more particularly to a fiber optic based accelerometer. Background Art It is known to monitor the physical characteristics of structures and bodies using sensors. One such application is the monitoring of oil wells to extract such information as temperature, pressure, fluid flow, seismic and other physical characteristics. The monitoring of oil wells presents certain challenges for conventional sensors because of the harsh environment in terms of high pressures and temperatures and has historically been dominated by the use of electronic sensors and optical sensors to a lesser degree. The presently used electrical sensors are limited for several reasons. The on-board electronics of such sensors must operate in a very hostile environment, which includes high temperature, high vibration and high levels of external hydrostatic pressure. Such electrical sensors also must be extremely reliable, since early failure entails a very time consuming and expensive well intervention. Electronics, with its inherent complexity, are prone to many different modes of failure. Such failures have traditionally caused a less than acceptable level of reliability when these electrical sensors are used to monitor oil wells. There are numerous other problems associated with the transmission of electrical signals within well bores. In general, there are many problems encountered in providing an insulated electrical conductor for transmitting electrical signals within well bores. Such electrical conductors are extremely difficult to seal against exposure to well bore fluids, which are at high temperatures, high pressures, and present a very corrosive environment. Such electrical conductors, once damaged by the fluids that penetrate the insulating materials around the electrical conductors, will typically short electrical signals. Additionally, electrical transmissions are subject to electrical noises in some production operations. It is typical to use an accelerometer to measure downhole seismic disturbances to determine the acoustic wave characteristics of underground layers in proximity of the well bore. In general, it is typical to consider an accelerometer as a mass-spring transducer housed in a sensor case with the sensor case coupled to a moving body, the earth, whose motion is infened from the relative motion between the mass and the sensor case. Such accelerometers are analyzed by considering the relative displacement of the mass as being directly related to the acceleration of the case and therefore the earth in proximity of the well bore. An array of accelerometers is typically placed along the length of a well bore to determine a time dependant seismic profile. One prior art accelerometer is a piezoelectric based electronic accelerometer. The piezoelectric based electronic accelerometer typically suffers from the above identified problems common to electrically based sensors. In particular, most higher performance piezoelectric accelerometers require power at the sensor head. Also, multiplexing of a large number of sensors is not only cumbersome but tends to occur at significant increase in weight and volume of an accelerometer array, as well as a decrease in reliability. It is also known to use optical interferometers for the measurement of acceleration of certain structures. It is also well known that fiber optic interferometric accelerometers can be designed with fairly high responsivities and reasonably low threshold detectability. Some prior art types of fiber optic accelerometers include interferometric fiber optic accelerometers based on linear and nonlinear transduction mechanism, circular flexible disks, rubber mandrels and liquid- filled-mandrels. Some of these fiber optic accelerometers have displayed very high acceleration sensitivity (up to 10 4 radians/g), but tend to utilize a sensor design that is impractical for many applications. For instance, sensors with a very high acceleration sensitivity typically tend to have a seismic mass greater than 500 grams which seriously limits the frequency range in which the device may be operated as an accelerometer and are so bulky that their weight and size renders them useless in many applications. Other fiber optic accelerometers suffer either from high cross-axis sensitivity or low resonant frequency or require an ac dither signal and tend to be bulky (>10 kg), expensive and require extensive wiring and electronics. Even optical interferometers designed of special material or construction are subject to inaccuracies because of the harsh borehole environment and because of the very tight tolerances in such precision equipment. For many applications, the fiber optic sensor is expected to have a flat frequency response up to several kHz (i.e., the device must have high resonant frequency), high sensitivity, must be immune to extraneous measurands (e.g., dynamic pressure), must have a small foot print and packaged volume that is easily configured in an array (i.e., easy multiplexing). Summary of the Invention Objects of the present invention include provision of a fiber optic accelerometer for use within a harsh environment. The invention may be used in harsh environments (high temperature, and/or pressure, and/or shock, and/or vibration), such as in oil and/or gas wells, engines, combustion chambers, etc. In one embodiment, the invention may be an all glass fiber optic sensor capable of operating at high pressures (>15 kpsi) and high temperatures (>150 °C). The invention will also work equally well in other applications independent of the type of environment. It is an object of the present invention to provide a highly sensitive linear accelerometer for sensing acceleration in a predetermined direction. The accelerometer is comprised of a rigid housing a mass suspended therein by at least two elastic support members which are axially aligned in the predetermined direction and attached to opposite ends of the housing and further attached to the mass. At least a portion of one of the elastic support members comprises a transducer capable of measuring a displacement of the mass within the housing in response to an acceleration along the predetermined direction. Certain embodiments include a pair of fixed mandrels rigidly attached to opposite ends of the housing and the mass comprises at least one floating mandrel where the elastic support members are each wrapped around one of the fixed mandrels and the floating mandrel. It is another object of the present invention to provide a linear accelerometer where the mass comprises a pair of floating mandrels and wherein each elastic support member is wrapped about one of the fixed mandrels and one the floating mandrels. In another embodiment the mandrels and the mass of the accelerometer comprise a toroidal shape. It is yet another object of the present invention to provide a linear accelerometer where at least one of the elastic support members comprises an optical fiber the movement of the mass induces in the fiber a variation in length conesponding to the movement for interferometric measurement of the variation in length of the fiber. It is still another object of the present invention to provide a linear accelerometer having an axial alignment assembly attached to the mass limiting movement of the mass in a direction perpendicular to the predetermined direction where the axial alignment assembly comprises a flexure member attached to the mass and the housing allowing axial movement of the mass in the predetermined direction and limits non-axial movement of the mass. In an embodiment a pair of alignment assemblies are employed where the flexure member is a diaphragm positioned on an alignment rod and the diaphragm is captured within a bore in the housing about their outer periphery. Another embodiment provides for a bore positioned in the fixed mandrels for capturing the diaphragms. In another embodiment the flexure member comprises a thin flexible plate and at least one pair of the flexure members are attached to the mass and to the housing. It is still further an object of the present invention to provide the linear accelerometer where the transducer comprises a strain sensing element including a fiber optic strain sensor, a piezo electric device, a PVDF material or a resistive strain gage. It is another object of the presenting invention to provide a highly sensitive linear accelerometer for sensing acceleration in a predetermined direction having a rigid housing and a mass having an elongated body and rounded ends, and a pair of fixed mandrels rigidly attached to the housing a predetermined distance apart, and two pairs of elastic support members axially aligned in the predetermined direction and wrapped around the fixed mandrels and the rounded ends in a continuous fashion to suspend the mass within the housing. At least a portion of one of the elastic support members comprises a transducer capable of measuring a displacement of the mass within the housing in response to an acceleration along the predetermined direction further includes a pair of axial alignment assemblies attached to the mass limiting movement of the mass in a direction peφendicular to the predetermined direction. It is yet another object to provide a linear accelerometer where the fixed mandrels and the mass are comprised of a toroidal shape. It is still another object of the present invention to provide an apparatus for vertical seismic profiling of an earth borehole having an x-direction, a y- direction and a z-direction orthogonal to each other, where the apparatus includes an optical fiber transmission cable and a plurality accelerometers coupled to the borehole and in optical communication with the optical fiber transmission cable and positioned in each of the three orthogonal directions. The accelerometer is a highly sensitive linear accelerometer for sensing acceleration in a predetermined one of the directions, the accelerometer includes a rigid housing, a mass, at least two elastic support members comprised of optical fiber axially aligned in the predetermined direction and attached to opposite ends of the housing and further attached to the mass, the elastic support members suspending the mass within the housing wherein at least a portion of one of the elastic support members comprises a transducer capable of measuring a displacement of the mass within the housing in response to an acceleration along the predetermined direction and providing a respective sensing light signal indicative of static and dynamic forces at a respective accelerometer location. The apparatus also includes an optical signal processor connected to the optical transmission cable providing seismic profile information based on the respective sensing light signal. Brief Description of the Drawings Fig. 1 is a schematic diagram of a acceleration monitoring system incorporating a highly sensitive accelerometer in accordance with the present invention; Fig. 2 is a cross-sectional view of an earth borehole having an anay of accelerometers of the invention deployed therein for vertical seismic profiling; Fig.3 is a schematic diagram of a spring mass acceleration model of the prior art; Fig. 4 is a side view of a schematic representation of an accelerometer of in accordance with the present invention; Fig. 5 is a top view of a schematic representation of an accelerometer of in accordance with the present invention; Fig. 6 is a perspective view of an embodiment of the accelerometer of the present invention; Fig. 7 is an exploded perspective view of the accelerometer of Fig. 6 showing the axial alignment assemblies; Fig. 8 is a perspective view of an embodiment of the accelerometer of the present invention; Fig. 9 is a perspective view of the mass and axial alignment assemblies of the accelerometer of Fig. 8; Fig. 10 is a perspective view of an embodiment of the accelerometer of the present invention comprised of toroidal shaped members; Fig. 11 is a perspective view of an embodiment of the accelerometer of Fig. 6 having an alternative axial alignment assembly; Fig. 12 is a graphical representation of the response of an embodiment of the present invention to a test signal; Fig. 13 is a graphical representation of the phase response of the embodiment of Fig. 12; Fig. 14 is a graphical representation of the amplitude response of the embodiment of Fig. 12; Fig. 15 is a side view of an elastic support member comprising an optical fiber wrap having a pair of Bragg gratings around each optical wrap, in accordance with the present invention; Fig. 16 is a side view of optical fiber wrap with a pair Bragg gratings within each wrap, in accordance with the present invention; Fig. 17 is a side view of optical fiber wrap interferometer, in accordance with the present invention; Fig. 18 is a top view in partial section of an elastic support member having an optical fiber with a pair of Bragg gratings, in accordance with the present invention; Fig. 19 is a top view in partial section of an alternative geometry of an elastic support member having an optical fiber with a pair of Bragg gratings, in accordance with the present invention; Fig. 20 is a top view in partial section of an elastic support member having an alternative geometry optical fiber in the form of a radiator coil; Fig. 21 is a top view in partial section of an elastic support member having an alternative geometry optical fiber in the form of a race track; Fig. 22 is a top view of three alternative strain gauges, in accordance with the present invention; and Fig. 23 is a top view in partial section of an elastic support member showing a strain gage. Best Mode for Carrying Out the Invention Referring to Fig. 1, a structure 10 subjected to a hostile environment, such as an oil or gas well borehole, building, bridge, aircraft, pump or other structure or component subjected to acceleration and wishing to be intenogated has coupled to it at least one highly sensitive accelerometer 22 as will be more fully described herein below. Highly sensitive accelerometer 22 is part of transmission cable string 20 connected by transmission cable 28 to a signal converter 40 and signal processing equipment 35. The acceleration of structure 10 in any of the three axes 30, 32, 34 is detected by accelerometer 22, depending on the orientation of the accelerometer, as will be more fully described herein after. The signal processing equipment may comprise any known instrumentation for processing the electrical, electro-optic, or optical signal of the various embodiments of the present invention. In a particular embodiment of the present invention, accelerometer 22 is mounted within a hermitically sealed vessel (not shown) and is disposed in a harsh environment having a high temperature (up to about 175 degrees C), high pressure (up to about 20 ksi), a high EMI environment or any non-harsh environment where a highly sensitive accelerometer is needed. Also in certain embodiments, accelerometer 22 may comprise a fiber optic based device and transmission cable 28 may comprise an environmentally hardened capillary tube such as that disclosed in commonly owned, copending US Patent Application serial number 09/121,468, title Optical Fiber Cable for Use in Harsh Environments, to Bonja, filed July 23, 1998, the disclosure of which is incorporated herein in its entirety. The transmission cable 28 is routed to accelerometer 22 and provides for the delivery of communication signals between the signal processing equipment 35 and is connected therebetween either directly or via interface equipment (not shown) as required. The accelerometer is closely coupled to the structure by bolting, clamping or other known methods. Accelerometer 22 of the present invention may be used, for example, as a single device to monitor structure 10 directly, in an array of similar such accelerometers to monitor structure 10. In one alternative an array of accelerometers 22 may be coupled to a structure 10 to determine the structure's response to the sunounding environment such as, for example for performing vertical seismic profiling, and as are distributed over a known length. Refening to Fig. 2, structure 10 maybe any structure, such as a casing or production pipe, coupled to a borehole within an oil or gas well, and penetrates various earth layers 12, 14, 16. Such a borehole may be fifteen to twenty thousand feet or more in depth. As is known in the art, the borehole is filled with a high temperature and pressure drilling fluid 18 which presents an extremely conosive and hostile environment. Transmission string 20 includes an anay of accelerometers 22, 23, 24, 25 as described herein above connected by transmission cable 28 which may comprise an optical fiber positioned within a capillary tube. The accelerometers 22, 23, 24, 25 may comprise a single accelerometer or may comprise two or three linear accelerometers 22 of the present invention positioned in any of the three axes 30, 32, 34 (Fig. 1) and transmit a respective sensing light signal indicative of static and dynamic forces at the respective accelerometer location. The anay of accelerometers 22, 23, 24, 25 is useful for performing the vertical seismic profiling of the invention, with the optical fiber sensors distributed over a known length, such as 5000 feet. Over the known length, the accelerometers 22, 23, 24, 25 are evenly spaced at a desired interval, such as every 10 to 20 feet, for providing the desired vertical seismic profiling. As described in greater detail herein, each accelerometer includes fiber optic sensors that reflect a nanow wavelength band of light having a central wavelength. Each accelerometer operates at a different wavelength band and central wavelength such that the signals may be easily detected using Wavelength Division Multiplexing (WDM) techniques. The entire optical fiber, positioned within the transmission cable 28, is lowered to a desired depth, for example as measured from the upper most sensor, such as 1 ,000 feet. An acoustic wave source, such as a small charge of dynamite 42 (a seismic shot), is detonated by a blaster 45 in a shallow shothole 50 that is offset from the borehole 10 by a selected distance, such as 3,000 feet. Still referring to Fig. 2, acoustic waves radiate from the shot along a direct path 52 and a reflected path 54. The reflected waves 54 are reflected off of the various earth layers 12, 14, 16. As will be described in greater detail hereinafter, the direct seismic waves 52 and reflected seismic waves 54 cause the sunounding various earth layers 12, 14, 16 to react and the motion of the earth is detected by the accelerometers 22, 23, 24, 25 through structure 10 coupled to the earth. Resulting data signals are transmitted through the transmission cable 28 to the demodulator 40 and optical signal processing equipment 35. In one embodiment of the invention, after the seismic shot, the transmission cable string 20 is repositioned within the borehole for additional seismic profiling. In another embodiment of the invention, the accelerometers 22, 23, 24, 25 are distributed over the entire length of the transmission cable 28 such that the entire borehole 10 is characterized in a single shot. In an anay of accelerometers of the present invention, each accelerometer operates at a different wavelength band and central wavelength such that the signals may be easily detected using Wavelength Division Multiplexing (WDM) techniques. Signal processing equipment 35 and signal converter 40, which may comprise one or more demodulators, interpret the wavelength phase change from the return signals. A number of prior art performance deficiencies are addressed by accelerometer 22 in accordance with the present invention. For instance, for fiber optic based embodiments, the lowest resolvable or measurable acceleration will be limited by the detection noise floor of the interferometer which is configured around the optical fiber coils 80, 82 (Fig. 4), in conjunction with the phase measurement scheme and the scale factor of the accelerometer mechanism. In, for instance, seismic applications, though the present invention is not limited to such, accelerometer 22 is required to detect accelerations as low as 10-lOOG/rtHz. Furthermore, it is well known that high performance interferometers and phase measurement systems can detect phase shifts as low as 10 to 100 microad/rtHz or even better. The optical fiber coils of the support members of an interferometer with an associated phase measurement system, yield an accelerometer sensitivity or scale factor of about 1 krad/G, or higher, to achieve measurements with the indicated noise floor (Fig. 12 is an example of a typical test signal relative to the noise floor of an embodiment of the present invention). With reference to Fig. 4, accelerometer 22 maybe fabricated with scale factors of between 500 and 5000 krad/G, that covers the range of scale factors, as detailed herein below, necessary to use this accelerometer in seismic applications. As previously noted, interferometer measurement systems exhibit scale factors that increase with increasing fiber length. The fixed 86, 88 and floating mandrels 90, 92 are used to create multiple coil turns of fiber 66 in each support member, thereby enabling a small package for an accelerometer with high scale factor. In this accelerometer 22, the effective scale factor can be described in terms of the strain applied to the fiber by the moving mass of the floating mandrels. It is interesting to note that the scale factor will be proportional to the mass of the design and inversely proportional to the cross sectional area of the supporting fiber. Normally, as the length of the fiber of an interferometer 62 (Fig. 3) increases, the sensitivity increases. However, the supporting fiber consists of a number of turns in the suspension coil. As the fiber length increases, the number of turns increases and the total fiber cross sectional area of the suspension bands increases. The effect is to make the scale factor approximately independent of total fiber length. The range of accelerometer 22 can be limited by one of 2 factors. For instance, if the phase measurement system has a limited range, then large accelerations cannot be interpreted. However, cunent phase demodulator technology, as typified by an Optiphase model OPD-200, produced and sold by Optiphase, that can track phase changes over many 2π cycles, for example, removes this aspect as a limitation. The other potential limitation might be the mechanical strength of the fiber. The present invention has been reviewed with respect to the mechanical implications of large acceleration changes imposed on the suspension coils. It is useful to realize that even at very high shock conditions, for example as high as 200 G's, that the transient load is shared by all of the fibers in the coil. In such a situation, the maximum load applied to any filament in the coil can be much less than 10% of the ultimate strength of the glass filament. This load sharing ability is a benefit of the accelerometer of the present invention demonstrating inherent durability and large acceleration range capability. 5 A typical approach for accelerometer design is to define the operating bandwidth to be the flat signal response spectral region below the first structural resonance of the suspended mass. In the case of accelerometer 22, it is important to keep in mind that the stiffness of the coils will has an impact on the resonant frequency and the total glass cross sectional area of the coil relative to the o accelerometer mass must be considered when designing the fundamental resonant frequency. We have discovered that an adequate scale factor can be achieved while maintaining the system resonance above 1 kHz. This discovery enables the present invention to satisfy many seismic transducer application requirements. Examples of both the amplitude and phase response functions of a typical device 5 are shown in Figs. 13 and 14, verifying the ability to achieve high resonant frequencies while achieving good sensitivity. Certain embodiments of accelerometer 22 make it practically insensitive to position with respect to gravity as will be shown in greater detail herein below. In practice it is generally not practical to use long fiber length / in a single 0 strand as shown in Fig. 3. As such the present invention uses multiple windings 80, 82 of fiber 66 to obtain a long effective fiber length as best shown with reference to Figs. 3 and 4. The windings 80, 82 of fiber optic accelerometer 22 each comprise N turns of fiber 66 coiled around a fixed mandrel 86, 88 and around a second active mandrel 90, 92 that is free and used to strain the fiber by 5 its own mass. The fixed mandrels may be grounded to a housing represented by 98 and the active mandrels may be restrained from movement normal to the axial direction represented by anow 70. When housing 98 is subjected to motion in the axial direction 70 the acceleration associated with that motion is detected by the transducers, or sensor coils 94, 96 in a manner similar to the mass/spring system 0 of Fig. 3. A single sensor coil 94 or 96, could be used to measure acceleration in the axial direction, however the push-pull or differential anangement of the pair of sensor coils (in an interferometer for example) 94, 96 of fiber optic accelerometer 22 provides mechanical symmetry which lowers total harmonic distortion and cross axis sensitivity. Mechanical symmetry could also be achieved by replacing one of the sensor coils with another material having a substantially similar spring rate. In essence the active mandrels 90, 92 are suspended between at least one pair of springs or elastic support members, at least a portion of one of which is a strain sensing element, preferably comprised of optical fibers 66. In alternative embodiments, one of the pair of sensor coils 94, 96 may be either be used as a dummy anangement to create mechanical symmetry in the axial direction 70, or as a back-up anangement in the event that one of the sensor coils fails, or as a secondary sensor coil in a push-pull or differential anangement. The latter effectively doubles the accelerometer scale factor. Any known optical fiber may be used having various diameters, however the fiber diameter is important to the performance as well as the durability and reliability of the accelerometer. For example, an optical fiber having a relatively large diameter has a minimum bend radius to ensure a predictable lifetime with failure. If a large diameter fiber is used, a commensurately large mandrel diameter should be used to accommodate the fiber for reliability reasons. However, as mandrel diameter grows so too does the overall volume of accelerometer 22. Referring to Fig. 6 there is shown an embodiment of accelerometer 22 in accordance with the present invention as described herein above including 3 elastic support members 150, 152, 154 comprised of windings of optical fibers, although other elastic support members could be employed without deviating from the present invention. Elastic support members 150, 152 combined are comprised of the same length of fiber as elastic support member 154 and cooperate in a push-pull anangement to suspend mass 156 within housing 158. The wraps of support 154 are wound in a continuous fashion about fixed mandrel 160 rigidly attached to housing 158 and mandrel end 162 of mass 156. Similarly the wraps of support members 150, 152 are wound in a continuous fashion about fixed mandrel 164 rigidly attached to housing 158 and mandrel end 166 of mass 156. The support member 150, together with support member 152, comprise one sensor coil and support member 154 comprises a second sensor coil, both being similar to sensor coil 94, 96 described with reference to Figs. 3 and 4. Support members 150, 152 act as a spring to bias known proof mass 156 against the spring action of support member 154 and cooperate to suspend the mass within housing 158. The fixed mandrels 160, 162 are positioned within the housing 158 to produce a predetermined initial bias in each of the elastic support members 150, 152, 154. Support members 150, 152, 154 are axially aligned, in the direction indicated by anow 168, with each other, housing 158 and mass 156. Fixed mandrels 160, 164 and mandrel ends 162, 166 include grooves 170 positioned thereon to facilitate assembly and maintain the axial positioning of the support members. Accelerometer 22 accurately detects acceleration in the axial direction 168 as will be more fully explained herein below. Mass 156 is comprised of central portion 171 between mandrel ends 162, 166, however embodiments of the present invention include those wherein a single cylindrical floating mandrel comprises the total mass and around which both interferometers are wound. Mass 156 of accelerometer 22 further includes alignment assemblies 172, 174 as best shown in Figs. 6 and 7 for limiting the movement of mass 156 peφendicular to the axial direction 168. Alignment assemblies 172, 174 are comprised of alignment rods 176, 178 that slidably pass through holes 180, 182 in mandrel ends 162, 166 respectively and are attached to diaphragms 184, 186 by, for example, threaded nuts 188, 190. Diaphragms 184, 186 are captured within bores 192, 194 in housing 158 by end plates 196, 198 installed on the ends of the housing by screws (not shown) for example. Boss 200 on end plates 196, 198 cooperate with lip 202 within bores 192, 194 to capture diaphragms 184, 186 about their outer edges within the bore to allow for flexure of the diaphragms in the axial direction 168. Diaphragms 184, 186 are comprised of a thin flexible material, metal for example, which provides for a highly flexible member along the axial direction 168 but is quite rigid in the plane of the diaphragms (peφendicular to the axial direction). This allows relatively unimpeded movement of mass 156 in the axial direction 168 while virtually eliminating movement of the mass assembly in non-axial directions. By limiting the movement of the mass 156 in non-axial directions, alignment assemblies 172, 174 of accelerometer 22 greatly reduce cross-axis response. Alternative embodiments of the alignment assemblies include ones wherein the holes 180, 182 cooperate with alignment rods 176, 178 in a close tolerance anangement to preclude the need for diaphragms 184, 186. In this particular embodiment the alignment rods 176, 178 limit movement of the mass 156 in non-axial direction by interference with the walls of the holes 180, 182. In operation accelerometer 22 may be mounted to a structure, such as an oil production tube 10 (Fig. 2) for example, by rigid attachment of housing 158 by any method such as bolting, welding or other known methods. As the structure experiences acceleration due to changes in movement direction, or relative velocity, mass 156 will shift in the axial direction 168 within housing 158 in a magnitude related to the acceleration of the structure in the axial direction. Elastic support members 150, 152 154 will respond by elongating or relaxing, the action of which will lengthen or shorten the optical fibers and produce a signal conesponding to the acceleration. For example, when the structure, or housing 158 thereby, is accelerated in the direction indicated by anow 210 mass 156 will be displaced within the housing in the opposite direction indicated by anow 21 1. In this particular case the tension in support member 154 will increase (and the fiber length therein will therefore increase) and the tension in support members 150, 152 will decrease (and the fiber length therein will decrease). Similarly, when the structure, or housing 158 thereby, is accelerated in the direction indicated by anow 211 mass 156 will displaced within the housing in the opposite direction indicated by anow 210. In this particular case the tension in support members 150, 152 will increase (and the fiber length therein will therefore increase) and the tension in support member 154 will decrease (and the fiber length therein will decrease). The change in phase angle of the light within the fibers as inteφreted by the processing equipment 35 (Fig. 1) caused by the change in length of the fibers conesponds to a known acceleration level as described herein above. The support members are independent coil systems and their output can be manipulated accordingly in known a manner such as differential or in an independent mode as a single coil in a sensor leg of an interferometer. Other methods of determining a conesponding change in length of the support members are included in the present invention and will be more fully described herein below. In an alternative embodiment only one of the interferometers, either the one comprised by support member 154 or the one comprised by support members 150, 152 is used for outputting a signal responsive to the acceleration of the accelerometer 22. Referring to Figs. 8 and 9 there is shown another embodiment of accelerometer 22 in accordance with the present invention as described herein above including two pairs of elastic support members 150, 152, 154, 155 comprised of windings of optical fibers, although other elastic support members could be employed without deviating from the present invention. Elastic support members 150, 152 are comprised of the same length of fiber as elastic support members 154, 155 and cooperate in a push-pull anangement to suspend mass 156 within housing 158. The wraps of supports 154, 155 are wound in a continuous fashion about fixed mandrel 160 rigidly attached to housing 158 and mandrel end 162 of mass 156. Similarly the wraps of support members 150, 152 are wound in a continuous fashion about fixed mandrel 164 rigidly attached to housing 158 and mandrel end 166 of mass 156. Each of the support members 150, 152, 154, 155 comprise a sensor coil for use in an interferometer, with all being similar to sensor coils 94, 96 described with reference to Figs. 3 and 4. Support members 150, 152 act as a spring to bias mass 156 against the spring action of support members 154, 155 and cooperate to suspend the mass within housing 158. The fixed mandrels 160, 162 are initially positioned within the housing 158 to produce a predetermined initial bias in each of the elastic support members 150, 152, 154, 155. Support members 150, 152, 154, 155 are axially aligned, in the direction indicated by anow 168, with each other, housing 158 and mass 156. Fixed mandrels 160, 164 and mandrel ends 162, 166 include grooves 170 positioned thereon to facilitate assembly and maintain the axial positioning of the support members. Accelerometer 22 accurately detects acceleration in the axial direction 168 as will be more fully explained herein below. Accelerometer 22 as shown in Fig. 8 is small enough to fit within a tube 91 , having end caps 93 for use in sealing and protecting the device from the environment. Tube 91, in one embodiment, is comprised of Inconel material and has outside dimensions of approximately one inch in diameter and approximately 3.5 inches in length. At least one of end caps 93 further includes an exit hole 97 including any known sealing feature for routing the transmission cable 28 (Fig. 1) from the housing. The mandrel diameters 100 are approximately 11-13 mm and the distance between fixed mandrels 160, 164 and floating mandrels 162, 166 respectively is about 44 mm in a 0.0 g state. Mass 156 is comprised of a metallic material and is approximately 60 grams. Support members 150, 152, 154, 155 are comprised of an 80 micron optical fiber and a total length of between about 10 m and about 20 m is used with the number of wraps varying from about 39 to about 105. The housing 158, mass 156 and mandrels are may all be comprised of metal materials. In embodiments where the support members are comprised of optical fibers, the use of an all metal configuration with glass fibers yields an extremely stable and reliable accelerometer 22 even at elevated temperatures. Referring to Fig. 9, mass 156 is includes cylindrically shaped mandrel ends 162, 166, however embodiments of the present invention include those wherein a single cylindrical floating mandrel comprises the total mass and around which both sensor coils are wound. Mass 156 of accelerometer 22 further includes alignment assemblies 172, 174 for limiting the movement of mass 156 peφendicular to the axial direction 168. Alignment assemblies 172, 174 are comprised of alignment rods 176, 178 respectively and are attached to diaphragms 184, 186 by, for example, welding or gluing. Diaphragms 184, 186 are captured within bores 192, 194 (not shown) in housing 158 about their outer edges within the bore to allow for flexure of the diaphragms in the axial direction 168. Diaphragms 184, 186 are comprised of a thin flexible material, metal for example, which provides for a highly flexible member along the axial direction 168 but is quite rigid in the plane of the diaphragms (peφendicular to the axial direction). This allows relatively unimpeded movement of mass 156 in the axial direction 168 while virtually eliminating movement of the mass assembly in non- axial directions. By limiting the movement of the mass 156 in non-axial directions, alignment assemblies 172, 174 of accelerometer 22 greatly reduce cross-axis response. In operation accelerometer 22 may be mounted to a structure, such as an oil well casing, or an oil production tube 10 (Fig. 2) for example, by rigid attachment of housing 158 by any method such as bolting, welding or other known methods. As the structure experiences acceleration due to changes in movement direction, or relative velocity, mass 156 will shift in the axial direction 168 within housing 158 in a magnitude related to the acceleration of the structure in the axial direction. Elastic support members 150, 152 154, 155 will respond by elongating or relaxing, the action of which will lengthen or shorten the optical fibers and produce a signal conesponding to the acceleration. For example, when the structure, or housing 158 thereby, is accelerated in the direction indicated by anow 210, mass 156 will be displaced within the housing in the opposite direction indicated by anow 211. In this particular case the tension in support members 154, 156 will increase (and the fiber length therein will therefore increase) and the tension in support members 150, 152 will decrease (and the fiber length therein will decrease). Similarly, when the structure, or housing 158 thereby, is accelerated in the direction indicated by anow 211, mass 156 will displaced within the housing in the opposite direction indicated by anow 210. In this particular case the tension in support members 150, 152 will increase (and the fiber length therein will therefore increase) and the tension in support members 154, 155 will decrease (and the fiber length therein will decrease). Referring to Fig. 10 there is shown yet another embodiment of the present invention wherein fixed mandrels 160, 164 are both in the form of a torus having an internal bore 161, 163 and wherein mass 156 is in the form of an elongated torus having a bore 165. Fixed mandrels are attached to a housing represented by 158 and similar to that described hereinabove by any known method. In accordance with the present invention and as described herein above four pairs of elastic support members 150, 151, 152, 153 bias mass 156 toward fixed mandrel 160 and four pairs of elastic support members 154, 155, 157, 159 bias mass 156 toward fixed mandrel 164. Although the embodiment in Fig. 10 is shown with reference to four pairs of supports members, the present invention includes more pairs. Also, although shown as a torus, the mass and fixed mandrels may comprise any shape which permits placement of support members in a 360 degree distributed fashion about the mandrels and mass. Elastic support members 150, 151, 152, 153 are comprised of the same length of fiber as elastic support members 154, 155, 157, 159 and cooperate in a push-pull anangement to suspend mass 156 within housing 158. The wraps of supports 154, 155, 157, 159 are wound in a continuous fashion about fixed mandrel 160 through bore 161 rigidly attached to housing 158 and through bore 165 of mass 156 around and mandrel end 162. Similarly the wraps of support members 154, 155, 157, 159 are wound in a continuous fashion about fixed mandrel 164 through bore 163 rigidly attached to housing 158 and through bore 165 and mandrel end 166 of mass 156. Each of the support members may comprise a coil for use in an interferometer, with all being similar to sensor coils 94, 96 described with reference to Figs. 3 and 4. Support members 150, 151, 152, 153 act as a spring to bias mass 156 against the spring action of support members 154, 155, 157, 159 and cooperate to suspend the mass within housing 158. The fixed mandrels 160, 162 are initially positioned within the housing 158 to produce a predetermined initial bias in each of the elastic support members. Support members 150-159 are axially aligned, in the direction indicated by anow 168, with each other, housing 158 and mass 156 and are preferably evenly distributed in the radial direction. Referring to Fig. 11 there is shown an alternative embodiment of axial alignment assemblies 172, 174 comprised of flexure members 184-187 are attached to the mass 156 and the housing 158 near their outboard ends by, for example, welding or gluing, to allow for flexure of the flexure members in the axial direction 168. Flexure members 184-187 are comprised of a thin flexible material, metal for example, which provides for a highly flexible member along the axial direction 168 but is quite rigid in the plane of the flexure members (peφendicular to the axial direction). This allows relatively unimpeded movement of mass 156 in the axial direction 168 while virtually eliminating movement of the mass assembly in non-axial directions. By limiting the movement of the mass 156 in non-axial directions, alignment assemblies 172, 174 of accelerometer 22 greatly reduce cross-axis response. The performance of the accelerometer 22 of the present invention is best shown with reference to Fig. 12 that shows a plot of the relative response of the accelerometer of Fig. 8 to an excitation force on a calibration test shaker. The test shaker set up is known in the industry and is comprised of standard input and output components as well as a known reference accelerometer. The specific accelerometer 22 was designed to operate with a bandwidth from about 5 Hz up to about 500 Hz. Accelerometer 22 of the present invention was subjected to a test signal of approximately 126 μg in the axial direction 168 at a frequency of 25 Hz. Line 101 represents the performance of accelerometer 22 when the axial direction was parallel to the z-axis as represented by anow 34 (Fig. 1) and shows an extremely sensitive 65 dB signal to noise ratio response represented by point 104 at the 25 Hz test signal frequency with very little spurious response on either side of the test signal. Similarly line 103 represents the performance of accelerometer 22 when the axial direction and the test force are parallel to the x- axis as represented by anow 30 (Fig. 1) and the same 25 Hz test signal. Line 103 shows an almost exact level of response at the test signal frequency of 25 Hz. In addition, the orientation of the accelerometer did not adversely affect the relatively low spurious signals on either side of the test signal. The relatively low noise is further demonstrated in the figure with the largest such peak being less than 28 dB at 60 Hz. The 60 Hz signal is due to ground loops in the calibration system and is not considered an accelerometer enor signal. Such signals, once their cause is identified, can in most instances be isolated and eliminated. It is an important feature of the present invention that orientation of the accelerometer with respect to gravity has little effect on its performance. Therefore, anays of accelerometers 22 in the three orthogonal directions 30, 32, 34 (Fig. 2) can be used to measure the vector directions of seismic detected waves. Referring to Figs. 13 and 14 the bandwidth of the accelerometer is shown. The accelerometer was tested as described herein above with reference to Fig. 12 and the phase response was checked against the reference accelerometer (Fig. 13) and the amplitude response was checked relative to the reference accelerometer for a frequency range up to about 500 Hz. The phase response represented by line 105 in Fig. 13 is relatively flat which demonstrates that the accelerometer 22 is operating well away from the resonant frequency of the device. Line 105 further shows the accelerometer lacks spurious signals within the bandwidth that could otherwise result in enors within the desired operating bandwidth. Likewise, the relative amplitude response represented by line 107 in Fig. 14 is relatively flat and free of spurious signals. This further demonstrates that the accelerometer 22 is operating well away from the resonant frequency of the device and behaves predictably in the frequency range of 5 Hz to 500 Hz. In an embodiment of the present invention that utilizes fiber optics as the elastic support members, they may be connected individually or may be multiplexed along one or more optical fibers using wavelength division multiplexing (WDM), time division multiplexing (TDM), or any other optical multiplexing techniques (discussed more hereinafter). Referring to Fig. 15 the support member comprising a wrap 302, may have a pair of gratings 310, 312 on opposite ends of the wrap 302. The wrap 302 with the gratings 310, 312 may be configured in numerous known ways to precisely measure the fiber length L or change in fiber length ΔL, such as an interferometric, Fabry Perot, time-of-flight, or other known anangements. An example of a Fabry Perot technique is described in US Patent. No. 4,950,883 "Fiber Optic Sensor Anangement Having Reflective Gratings Responsive to Particular Wavelengths", to Glenn. One example of time-of-flight (or Time- Division-Multiplexing; TDM) would be where an optical pulse having a wavelength is launched down the fiber 66 and a series of optical pulses are reflected back along the fiber 66. The length of each wrap can, at any point in time, then be determined by the time delay between each return pulse and the related acceleration of the mass 156 (FIG. 8) thereby. Alternatively, a portion or all of the fiber between the gratings (or including the gratings, or the entire fiber, if desired) may be doped with a rare earth dopant (such as erbium) to create a tunable fiber laser, such as is described in US Patent No. 5,317,576, "Continuously Tunable Single Mode Rare-Earth Doped Laser Anangement", to Ball et al or US Patent No. 5,513,913, "Active Multipoint Fiber Laser Sensor", to Ball et al, or US Patent No. 5,564,832, "Birefringent Active Fiber Laser Sensor", to Ball et al, which are incoφorated herein by reference. Referring to Fig. 19, another type of tunable fiber laser that may be used in an accelerometer 22 of the present invention is a tunable distributed feedback (DFB) fiber laser, such as that described in V.C. Lauridsen, et al, "Design of DFB Fibre Lasers", Electronic Letters, Oct. 15, 1998, Vol.34, No. 21, pp 2028-2030; P. Vanning, et al, "Erbium Doped Fiber DGB Laser With Permanent π/2 Phase- Shift Induced by UV Post-Processing", IOOC'95, Tech. Digest, Vol. 5, PD1-3, 1995; US Patent No. 5,771,251, "Optical Fibre Distributed Feedback Laser", to Kringlebotn et al; or US Patent No. 5,51 1,083, "Polarized Fiber Laser Source", to D'Amato et al. In that case, a grating 316 is written in a rare-earth doped fiber and configured to have a phase shift of λ/2 (where λ is the lasing wavelength) at a predetermined location 318 near the center of the grating 316 which provides a well defined resonance condition that may be continuously tuned in single longitudinal mode operation without mode hopping, as is known. Alternatively, instead of a single grating, the two gratings 310,312 may be placed close enough to form a cavity having a length of (N + Vι)X, where N is an integer (including 0) and the gratings 310,312 are in rare-earth doped fiber. Referring to Fig. 16, instead of positioning the gratings 310,312 outside the wrap 302, they may be placed along the wrap 302. In that case the grating reflection wavelength may vary with acceleration changes, and such variation may be desired for certain configurations (e.g., fiber lasers) or may be compensated for in the optical signal instrumentation 35 (Fig. 1) for other configurations, e.g., by allowing for a predetermined range in reflection wavelength shift for each pair of gratings. Alternatively, instead of each of the wraps being connected in series, they may be connected in parallel, e.g., by using optical couplers (not shown) prior to each of the wraps, each coupled to the common fiber 66. Referring to Fig. 17, alternatively, the accelerometer 22 may also be formed as a purely interferometric sensor by wrapping the mandrels (86, 88, 90, 92, of Fig. 4, and similar) with the wrap 302 without using Bragg gratings where each wrap has a separate fiber 66. In this particular embodiment, known interferometric techniques may be used to determine the length or change in length of the fiber 66 between the mandrels due to movement of the mass 156 (Fig. 8), such as Mach Zehnder or Michaelson Interferometric techniques, such as that described in US Patent 5,218,197, entitled "Method and Apparatus for the Non-invasive Measurement of Pressure Inside Pipes Using a Fiber Optic Interferometer Sensor" to Canoll. The interferometric wraps may be multiplexed such as is described in Dandridge, et al, "Fiber Optic Sensors for Navy Applications", IEEE, Feb. 1991, or Dandridge, et al, "Multiplexed interferometric Fiber Sensor Anays", SPIE, Vol. 1586, 1991, pp 176-183. Other techniques to determine the change in fiber length may be used. Also, reference optical coils (not shown) may be used for certain interferometric approaches and may also be located in or around the accelerometer 22 but may be designed to be insensitive to axial accelerations. Also, for any geometry of the wraps described herein, more than one layer of fiber may be used depending on the overall fiber length desired. It is further within the scope of the present invention the wrap 302 may comprise the optical fiber 66 disposed in a helical pattern (not shown) about the mandrels. Other geometries for the wraps may be used if desired. The desired axial length of any particular wrap is set depending on the characteristics of the ac sensitivity, and other parameters, desired to be measured, for example the magnitude of the acceleration to be measured. Referring to Figs. 18 and 19, embodiments of the present invention include configurations wherein instead of using the wrap 302, the fiber 66 may be disposed on or within an elastic member 300 similar to those described herein above with reference to the various figures. In that case, the fiber may have shorter sections 314 that are disposed on the elastic support members that optically detect strain in the members. The orientation of the strain sensing element will vary the sensitivity to strain on the member caused by acceleration. Referring to Figs. 20 and 21, alternatively the optical strain sensor 320, 322 on the support member 300 may have a longer length with various alternative geometries, such as a "radiator coil" geometry 320 (Fig 20) or a "race-track" geometry 322 (Fig. 21), which would be disposed along the support member to measure strain. In this particular embodiment, the length will be set long enough to optically detect the changes to the strain on the elastic member and the acceleration thereby as described herein above. Referring to Fig. 18, in particular, the pairs of Bragg gratings (310,312), may be located along the fiber 66 with at least a section 314 of the fiber 66 between each of the grating pairs located on the elastic members 300. In that case, known Fabry Perot, interferometric, time-of-flight or fiber laser sensing techniques may be used to measure the change in length of at least a section of the elastic support member 300, in a manner similar to that described in the aforementioned references. Referring to Fig. 18, alternatively, the gratings 310, 312 may be individually disposed on the support members 300 and used to sense the strain on the members (and thus displacement of the mass 156). When a single grating is used support member, the grating reflection wavelength shift will be indicative of changes in strain on the member. Any other technique or configuration for an optical strain gage may be used. The type of optical strain gage technique and optical signal analysis approach is not critical to the present invention, and the scope of the invention is not intended to be limited to any particular technique or approach. For any of the embodiments described herein, the strain sensors, including electrical strain gages, optical fibers and/or gratings among others as described herein, may be attached to the elastic support members by adhesive, glue, epoxy, tape or other suitable attachment means to ensure suitable contact between the strain sensor and the elastic member. The strain gages, optical fibers or sensors may alternatively be removable or permanently attached via known mechanical techniques such as mechanical fastener, spring loaded, clamped, clam shell anangement, strapping or other equivalents. Alternatively, the strain gages, including optical fibers and/or gratings, may be embedded in the elastic members. Also, for any of the embodiments described herein the support member may also comprise any strain sensitive material, such as a PVDF. Referring to Figs. 22, 23 it is also within the scope of the present invention that any other strain sensing technique may be used to measure the variations in strain on the elastic member, such as highly sensitive piezoelectric, electronic or electric, strain gages attached to or embedded in the elastic support members. Refening to Fig. 22, different known configurations of highly sensitive piezoelectric strain gages are shown and may comprise foil type gages 340. Referring to Fig. 23, an embodiment of the present invention is shown wherein the strain sensors comprise strain gages 330. In this particular embodiment strain gages 340 are disposed about a predetermined portion of the elastic member 300. It should be understood that any of the embodiments described herein may comprise elastic support members in the form of discrete strips of material that are merely attached to the housing 158 and the mass 156 by any known method. It should be further understood that although description of the embodiments has been given with reference to the mass 156 moving, it is within the scope of the present invention that the housing 158 may move and the mass remain stationary, the relative motion between the two features being detected by the change in length of the support member. It should be understood that, unless otherwise stated herein, any of the features, characteristics, alternatives or modifications described regarding a particular embodiment herein may also be applied, used, or incoφorated with any other embodiment described herein. Also, the drawings shown herein are not drawn to scale. Although the invention has been described and illustrated with respect to exemplary embodiments thereof, the foregoing and various other additions and omissions may be made therein and thereto without departing from the spirit and scope of the present invention.

Claims: Claims What is claimed is: 1. A highly sensitive linear accelerometer for sensing acceleration in a predetermined direction, said accelerometer comprising: a rigid housing; a mass; at least two elastic support members axially aligned in said predetermined direction and attached to opposite ends of said housing and further attached to said mass, said elastic support members suspending said mass within said housing; and at least a portion of one of said elastic support members comprising a transducer capable of measuring a displacement of said mass within said housing in response to an acceleration along said predetermined direction. 2. The accelerometer of claim 1 further comprising: a pair of fixed mandrels rigidly attached to opposite ends of said housing; and wherein said mass comprises at least one floating mandrel and wherein said elastic support members comprise a wrap and wherein said elastic support members are disposed about one said fixed mandrel and said floating mandrel. 3. The accelerometer of claim 2 wherein said mass comprises a pair of floating mandrels and wherein each said elastic support member is disposed about one of said fixed mandrels and one of said floating mandrels. 4. The accelerometer of claim 2 wherein said mandrels and said mass comprise a toroidal shape. 5. The accelerometer of claim 1 wherein at least one of said elastic support members comprises an optical fiber coil. 6. The accelerometer of claim 5 wherein said and wherein said movement induces in said fiber a variation in length conesponding to said movement for interferometric measurement of said variation in length. 7. The accelerometer of claim 1 further comprising an axial alignment assembly attached to said mass limiting movement of said mass in a direction peφendicular to said predetermined direction. 8. The accelerometer of claim 7 wherein said axial alignment assembly comprises a flexure member attached to said mass and said housing allowing axial movement of said mass in said predetermined direction and limits non-axial movement of said mass. 9. The accelerometer of claim 7 wherein said flexure member comprises a diaphragm. 10. The accelerometer of claim 9 further comprising: a pair of said axial alignment assemblies further comprising an alignment rod wherein said diaphragm is disposed on an end of said rod ; a bore positioned in each end of said housing; and wherein said diaphragms are captured within said bore about the periphery of said diaphragms. 11. The accelerometer of claim 9 further comprising: a pair of said axial alignment assemblies, wherein said axial alignment assemblies further comprise an alignment rod wherein said diaphragm is disposed on an end of said alignment rod; a bore positioned in each end of said fixed mandrels; and wherein said diaphragms are captured within said bore about an outer periphery of said diaphragms. 12. The accelerometer of claim 8 wherein said flexure member comprises a thin flexible plate wherein at least one pair of said flexure members are attached to said mass at a first end and to said housing at second end. 13. The accelerometer of claim 1 wherein said transducer comprises a strain sensing element. 14. The accelerometer of claim 13 wherein said strain sensing element comprises a fiber optic sensor, a piezo electric device, a PVDF material or a resistive strain gage. 15. The accelerometer of claim 1 further comprising an outer housing having end caps wherein said accelerometer is disposed within said outer housing. 16. A highly sensitive linear accelerometer for sensing acceleration in a predetermined direction, said accelerometer comprising: a rigid housing; a mass having an elongated body and a first and second rounded ends; a first fixed mandrel and a second fixed mandrel, both rigidly attached to said housing a predetermined distance apart; a first pair of elastic support members axially aligned in said predetermined direction and wrapped around said first fixed mandrel and said first rounded end in a continuous fashion; a second pair elastic support members axially aligned in said predetermined direction and wrapped around said second fixed mandrel and said second rounded end in a continuous fashion; at least a portion of one of said elastic support members comprising a transducer capable of measuring a displacement of said mass within said housing in response to an acceleration along said predetermined direction; and a pair of axial alignment assemblies attached to said mass limiting movement of said mass in a direction peφendicular to said predetermined direction. 17. The accelerometer of claim 15 wherein said first and second fixed mandrels and said mass are comprised of a toroidal shape. 18. An apparatus for vertical seismic profiling of the earth having an x- direction, a y-direction and a z-direction orthogonal to each other, said apparatus comprising: an optical fiber transmission cable; and a plurality of linear accelerometers coupled to the earth and in optical communication with said optical fiber transmission cable and positioned in each of the three orthogonal directions, each said linear accelerometer comprising highly sensitive accelerometer for sensing acceleration in a predetermined one of said directions, said accelerometer comprising: a rigid housing; a mass; and at least two elastic support members comprised of optical fiber axially aligned in said predetermined direction and attached to opposite ends of said housing and further attached to said mass, said elastic support members suspending said mass within said housing wherein at least a portion of one of said elastic support members comprises a transducer capable of measuring a displacement of said mass within said housing in response to an acceleration along said predetermined direction and providing a respective sensing light signal indicative of static and dynamic forces at a respective accelerometer location. 19. The apparatus of claim 18 further comprising an optical signal processor connected to said optical transmission cable providing seismic profile information based on said respective sensing light signal. 20. The apparatus of claim 18 further comprising an anay of said linear accelerometers coupled to the earth at a plurality of predetermined positions. 21. The apparatus of claim 18 wherein the plurality of accelerometers are coupled to the earth via an oil well casing, a bore hole, or an oil production tube.

Inventors: BERG ARNE, DUNPHY JAMES R, KNUDSEN SVERRE, WOO DANIEL, BERG, ARNE, DUNPHY, JAMES, R., KNUDSEN, SVERRE, WOO, DANIEL
Applicants: BERG ARNE, CIDRA CORP, DUNPHY JAMES R, KNUDSEN SVERRE, WOO DANIEL, BERG, ARNE, CIDRA CORPORATION, DUNPHY, JAMES, R., KNUDSEN, SVERRE, WOO, DANIEL
Application year: 2000
Main classification: G01P15
IPC codes: G01P15, G01V1