Patent Publication Number: US-2009219546-A1

Title: Interferometric Gravity Sensor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
     This case claims priority to U.S. Provisional Patent Application Ser. No. 61/033,417, filed Mar. 3, 2008 (Attorney Docket: 711-125US), which is incorporated by reference. 
    
    
     If there are any contradictions or inconsistencies in language between this application and one or more of the cases that have been incorporated by reference that might affect the interpretation of the claims in this case, the claims in this case should be interpreted to be consistent with the language in this case. 
     FIELD OF THE INVENTION  
     The present invention relates to gravity sensors in general, and, more particularly, to interferometric gravity sensors. 
     BACKGROUND OF THE INVENTION  
     An individual gravity sensor can be used to measure gravity in a local area. A pair of gravity sensors can be used cooperatively to detect a differential gravity between two locations. Multiple differential gravity sensors can be used to develop a three-dimensional map of gravity across a field or other region. Such 3-D mapping has been proposed in order to monitor fluid flow in-situ in subterranean reservoirs, such as oil fields. 
     In order to be effective in such applications, a gravity sensor must be extremely sensitive. For example, sensitivity below 1 micro-Galileo is often necessary. Such extreme sensitivity, however, requires very high immunity to noise sources. Error can be introduced into the output signal of a gravity sensor from noise sources such as electromagnetic interference, horizontal components in the acceleration of a free-falling mass, mechanical misalignment of sub-components, mechanical shock, and Coriolis forces that arise due to the rotation of the Earth. 
     Gravity sensors have been developed that are based on the principle of balancing the weight of a fixed mass with forces from a normal or superconducting spring. Gravity sensors such as these, however, can be difficult to setup and calibrate. In addition, such gravity sensors can be sensitive to environmental influences such as temperature or vibration. 
     Small gravity sensors, specifically designed for direct insertion into areas such as a borehole, have been developed. In some instances, these small gravity sensors utilize piezoelectric launchers to vertically launch a pair of test masses upward so that they can subsequently free-fall downward. An interferometer arrangement is used to monitor the acceleration of their falling masses after each reaches its apex. In addition to some of the drawbacks of other prior-art gravity sensors, the sensitivity of these gravity sensors is limited due to shock and vibration associated with their piezoelectric launchers. This mechanical energy manifests itself as noise into the output signal, reducing signal-to-noise ratio. 
     There exists a need, therefore, for a gravity sensor that avoids or mitigates some or all of the problems associated with prior-art gravity sensors. 
     SUMMARY OF THE INVENTION  
     The present invention provides a gravity sensor based on an optical interferometer comprising a test mass that includes a retro-reflector pair. Some embodiments of the present invention are particularly well-suited for use in systems such as inertial guidance systems, density measurement systems, and gravity monitors in boreholes of subterranean oil fields. 
     In some embodiments, a gravity sensor comprises an optical interferometer that is formed by splitting input light into a free-space reference beam and a free-space sample beam. The reference beam is launched into free-space at a reference port and the sample beam is launched into free-space at a sample port. The reference and sample ports are aligned so that the reference beam and sample beam are launched toward each other along an optical axis. A test mass, capable of free-fall along the optical axis, comprises a first retro-reflector and a second retro-reflector. The first retro-reflector reflects the reference beam back toward the reference port. The second retro-reflector reflects the sample beam back toward the sample port. The local gravity at the gravity sensor is determined by monitoring the relative phases of the reflected sample and reference beams as the test mass free-falls along the optical axis. 
     In prior-art gravity sensors, the free-space path length of a sample beam is directly dependent upon the position of a test mass that falls along an optical axis. In such systems, this sample beam is combined with a fixed-path-length reference beam. The change in the relative phase between the sample beam and reference beam provides an indication of the rate at which the test mass falls and, therefore, a measure of the gravity at the position of the gravity sensor. 
     Like prior-art gravity sensors, the present invention also provides a sample beam whose free-space path length is directly dependent upon the position of a test mass that falls along an optical axis. In contrast to the prior art, however, the present invention comprises a reference beam whose free-space path length is also directly dependent upon the position of the test mass on the optical axis. As the test mass falls, the free-space path length of the reference beam increases at the same rate that the free-space path length of the sample beam decreases. This effectively doubles the rate at which the phases of the sample beam and reference beam change. As a result, an optical interferometer in accordance with the present invention has sensitivity that can be as much as twice that of prior-art systems. 
     In an illustrative embodiment, input light is split by a 2×2 splitter/combiner into a reference path and a sample path. Light in the reference path is carried by a first optical fiber to a first gradient-index (GRIN) lens, at which the reference signal is launched into free space. In similar fashion, light in the sample path is carried by a second optical fiber to a second GRIN lens, at which the sample signal is launched into free space. The reference signal is reflected back to the first GRIN lens by a first retro-reflector located on a test mass. The sample signal is reflected back to the second GRIN lens by a second retro-reflector located on the test mass. The reflected reference signal and sample signal are captured at the GRIN lenses and carried to the 2×2 splitter/combiner, where they are combined into a single output light signal. The intensity of this output light signal is based on the relative phases of the reference signal and the sample signal. The phase of each of these signals is based upon the distance between the test mass and each of the two GRIN lenses; therefore, the intensity of the output signal is based upon the position of the test mass. 
     An embodiment of the present invention comprises: a test mass having a first axis; a first retro-reflector, wherein the first retro-reflector reflects light along a first direction that is aligned with a first reflection axis; and a second retro-reflector, wherein the second retro-reflector reflects light along a second direction that is aligned with a second reflection axis, wherein the first retro-reflector, second retro-reflector, and test mass are physically coupled, and wherein the first reflection axis, second reflection axis, and the first axis are substantially collinear, and further wherein the first direction and second direction are opposite directions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  depicts a schematic diagram of details of an oil field fluid flow measurement system in accordance with an illustrative embodiment of the present invention. 
         FIG. 2  depicts a schematic diagram of details of a prior-art gravity sensor. 
         FIG. 3  depicts a gravity sensor in accordance with the illustrative embodiment of the present invention. 
         FIG. 4  depicts a method for measuring localized gravity at a location in accordance with the illustrative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION  
     The present invention is suitable for use in many applications, including oil field mapping, inertial guidance navigation, and homeland security applications wherein a localized material density measurement might be used to identify hidden cargo or the presence of a secret tunnel. An illustrative embodiment is provided, wherein full-field differential gravity monitoring is used to model the fluid distribution in an oil field by mapping the relative gravity across the expanse of an oil field. 
       FIG. 1  depicts a schematic diagram of details of an oil field fluid flow measurement system in accordance with an illustrative embodiment of the present invention. Measurement system  100  comprises gravity sensing system  102 , and oil wells  104 - 1  and  104 - 2 . 
     Gravity sensing system  102  is a system for monitoring fluid movement in the oil field. Gravity sensing system  102  comprises gravity sensors  106 - 1  and  106 - 2  (referred to, collectively, as gravity sensors  106 ), cables  108 - 1  and  108 - 2  (referred to, collectively, as cables  108 ), and processor  110 . To monitor fluid flow in the oil field, multiple gravity sensors are inserted directly into the boreholes of oil wells distributed around the oil field. It should be noted that an oil field fluid flow measurement system will typically comprise more than two gravity sensors; however, the illustrative embodiment is depicted with only two gravity sensors for clarity. Each of the gravity sensors provides a signal based on the sensed gravity at its respective location. Processor  110  develops a map of the gravity gradient based on these signals, which provides an indication of the oil distribution through the oil field. 
     Each of gravity sensors  106  includes a test mass that is enabled to undergo a free fall. The characteristics of the free-fall are based on the gravity at the location of the falling test mass. Each gravity sensor then provides an output signal based on the free-fall of its test mass. These output signals are carried to processor  110  via cables  108 . For example, the output signal of gravity sensor  106 - 2  is carried to processor  110  via cable  108 - 2 . A representative gravity sensor  106  is described in more detail below and with respect to  FIG. 3 . 
     Processor  110  is a conventional data processing system for storing data and performing computation. Processor  110  receives output signals from gravity sensors  106 - 1  and  106 - 2  and compiles a full-field gravity image based on these output signals. In some embodiments, processor  110  comprises a single laser source for providing input light to each of gravity sensors  106 . In some embodiments, processor  110  comprises detectors for receiving output optical signals from one or more of gravity sensors  106 . 
       FIG. 2  depicts a schematic diagram of details of a prior-art gravity sensor. Gravity sensor  200  is representative of a class of gravity sensors that are based on interferometric measurement techniques. Gravity sensor  200  comprises laser  202 , splitter/combiner  208 , mirror  218 , lens  222 , test mass  224 , optical fibers  206 ,  214 ,  216 , and  238 , vacuum chamber  228 , and detector  242 . 
     Vacuum chamber  228  protects test mass  224 , retro-reflector  226 , and lens  222  from its surrounding environment and also enables a vacuum environment that reduces the effect of air resistance on the rate of fall of test mass  224 . Vacuum chamber  228  is typically oriented so that when test mass  224  free falls, it travels along axis  230 . 
     In operation, laser  202  provides input light signal  204 , which is conveyed to splitter/combiner  208  on optical fiber  206 . Input light signal  204  is substantially coherent light that is characterized by a nominal wavelength. Typically, laser  202  is a frequency-stabilized coherent light source, such as a gas laser, semiconductor laser, fiber laser, diode pumped laser, and the like. 
     Splitter/combiner  208  splits input light signal  204  into reference signal  210  and sample signal  212 . Reference signal  210  is conveyed to mirror  218  on optical fiber  214 . Sample signal  212  is conveyed to lens  222  on optical fiber  216 . 
     Mirror  218  reflects reference signal  210  back toward splitter/combiner  208  as reflected reference signal  220 . Reflected reference signal  220  is conveyed to splitter/combiner  208  on optical fiber  214 . 
     At lens  222 , sample signal  212  is launched into free space as free-space sample signal  232 , which propagates to retro-reflector  226 . Retro-reflector  226  reflects free-space sample signal  232  back to lens  222  as reflected free-space sample signal  234 . Retro-reflector  226  receives free-space sample signal  232  along lens optical axis  244  and reflects reflected free-space sample signal  234  back along lens optical axis  244 . Lens  222  collects the light contained in reflected free-space sample signal  234  and couples it into optical fiber  216  as reflected sample signal  236 . Reflected sample signal  236  is conveyed to splitter/combiner  208  on optical fiber  216 . 
     At splitter/combiner  208 , reflected reference signal  220  and reflected sample signal  236  are combined into output optical signal  240 . Output optical signal  240  is conveyed on optical fiber  238  to detector  242 . Detector  242  generates an electrical signal whose instantaneous value is based on the instantaneous intensity of output optical signal  240 . The intensity of output optical signal  240  is based on the interference of reflected reference signal  220  and reflected sample signal  236  when they combine at splitter/combiner  208 . 
     As one skilled in the art will recognize, the interference of reflected reference signal  220  and reflected sample signal  236  occurs as a function of their relative phases at splitter/combiner  208 . The phase of each of reflected reference signal  220  and reflected sample signal  236  is the total distance that that signal has traveled (i.e., its total path length). 
     The total path length traveled by the light contained in reflected reference signal  220  is a fixed distance (i.e., twice the length of optical fiber  214 ). Since this path length is fixed, the phase of reflected reference signal  220  at splitter/combiner  208  remains fixed. 
     The total path length traveled by the light contained in sample signal  236  is twice the length of optical fiber  216  (a fixed distance) and the free-space distance traveled by free-space sample signal  232  and reflected free-space sample signal  234 . The distance between the reflective surfaces of retro-reflector  226  is disregarded since it is substantially negligible (and fixed). The instantaneous phase of reflected sample signal  236  at splitter/combiner  208 , therefore, is based on the instantaneous separation between lens  222  and test mass  224 . 
     During a gravity measurement, test mass  224  is enabled to free-fall along axis  230  toward lens  222 . As test mass  224  falls, the phase of reflected sample signal  236  at splitter/combiner  208  changes commensurately. Specifically, the change in phase of reflected sample signal  236  is proportional to twice the change in the separation between lens  222  and retro-reflector  226 . As a result, the intensity of output optical signal  240  changes at twice the rate that test mass  224  falls. 
     Detector  242  receives output optical signal  240  and provides an electrical output signal whose instantaneous value is based on the instantaneous intensity of output optical signal  240 . As a result, the electrical output signal is based on the localized gravity that acts on test mass  224 . 
       FIG. 3  depicts a gravity sensor in accordance with the illustrative embodiment of the present invention. Gravity sensor  106  comprises laser  202 , splitter/combiner  208 , first lens  308 , second lens  222 , test mass  310 , optical fibers  206 ,  214 ,  216 , and  238 , vacuum chamber  320 , and detector  242 . 
       FIG. 4  depicts a method for measuring localized gravity at a location in accordance with the illustrative embodiment of the present invention. Method  400  is described herein with continuing reference to  FIGS. 1 and 3 . 
     Method  400  begins with operation  401 , wherein interferometer arrangement  308  is provided. Interferometer arrangement  308  comprises source  202 , splitter/combiner  208 , reference arm  310 , sample arm  306 , and detector  242 . 
     It should be noted that arms  310  and  306  are designated as “reference arm” and “sample arm” only to facilitate distinguishing them, and optical signals that they convey, from one another. One skilled in the art will recognize, after reading this specification, that these designations are arbitrary and either of arms  310  and  306  can be designated as a reference arm or sample arm. 
     Reference arm  310  comprises optical fiber  214 , lens  308 , and the free-space distance between lens  308  and retro-reflector  316 . The path length of reference arm  310  comprises twice the length of optical fiber  214 , the thickness of lens  308 , and twice the separation distance between lens  308  and retro-reflector  316  (neglecting the separation between the reflective surfaces of retro-reflector  316 ). 
     In similar fashion, sample arm  306  comprises optical fiber  216 , lens  222 , and the free-space distance between lens  222  and retro-reflector  226 . The path length of sample arm  306  comprises twice the length of optical fiber  216 , the thickness of lens  222 , and twice the separation distance between lens  222  and retro-reflector  226  (neglecting the separation between the reflective surfaces of retro-reflector  226 ). 
     At operation  402 , input light signal  204  is split into reference signal  210  and sample signal  212  at splitter/combiner  208 . Reference signal  210  is conveyed to lens  308  on optical fiber  214 . Sample signal  212  is conveyed to lens  222  on optical fiber  216 . Reference signal  210  and sample signal  212  have the same phase as they leave splitter/combiner  208 . 
     Optical fibers  214  and  216  are optically coupled with lenses  308  and  222 , respectively, within vacuum chamber  306 . Vacuum chamber  320  comprises suitable feed-throughs that enable optical fibers to pass through its outer wall while still enabling vacuum chamber  320  to hold a desired vacuum level (e.g., an internal pressure below 10 −3  Torr). In some embodiments, vacuum chamber  320  comprises one or more getters to further ensure the maintenance of a suitable vacuum environment within the chamber. 
     Axis  230  defines the desired axis along which test mass  310  free-falls. Vacuum chamber  320  is oriented so that axis  230  is aligned with the gravity to be sensed. Vacuum chamber  320  comprises a material suitable for supporting a vacuum environment. Suitable materials for vacuum chamber  320  include, without limitation, metals, plastics, glasses, composite materials, and ceramics. It will be clear to one skilled in the art how to make and use vacuum chamber  320 . 
     At operation  403 , reference signal  210  is launched into free-space at lens  308  as free-space reference signal  312 . Lens  308  substantially collimates free-space reference signal  312 . In some embodiments, lens  308  is a conventional GRIN lens affixed at the end of optical fiber  214 . 
     At operation  404 , sample signal  212  is launched into free-space at lens  222  as free-space sample signal  232 . Lens  222  substantially collimates free-space sample signal  232 . In some embodiments, lens  222  is a conventional GRIN lens affixed at the end of optical fiber  216 . 
     As will be clear to one skilled in the art, the relative phases of free-space reference signal  312  and free-space sample signal  212 , as they are launched into free-space at their respective lenses, are functions of the path lengths from splitter/combiner  208  to lenses  308  and  222 . In some embodiments, these path-lengths are equal so that the phases of free-space reference signal  312  and free-space sample signal  212  are substantially the same as they enter free-space. 
     At operation  405 , a free-fall of test mass  310  along axis  230  is enabled. In some embodiments, the free-fall of test mass  310  is enabled by launching the test mass upward along axis  230  to an apex, at which it begins to free-fall downward along axis  230 . In some embodiments, test mass  310  is raised upward along axis  230  and then dropped. It will be clear to one skilled in the art how to enable the free-fall of test mass  310 . 
     Test mass  310  is an object of known mass and comprises retro-reflectors  314  and  226 . Retro-reflectors  314  and  226  are conventional corner cube mirrors that are reflective at the wavelength of input light signal  204 . Retro-reflector  314  is characterized by optical axis  326 , which is collinear with axis  230 . Retro-reflector  226  is characterized by optical axis  244 , which is also collinear with axis  230 . In some embodiments, Axis  230  and optical axes  244  and  326  are aligned with each other, but not collinear. For the purposes of this specification, including the appended claims, axes are considered aligned with each other of they are collinear, or if they are non-collinear but are substantially parallel. In some embodiments, at least one of retro-reflectors  314  and  226  is a different type of retro-reflector, such as a fish-eye retro-reflector, and the like. 
     At operation  406 , free-space reference signal  312  is received from lens  308  along axis  230 . Retro-reflector  314  reflects free-space reference signal  312  in an upward direction along axis  230  as reflected free-space reference signal  316 . Reflected free-space reference signal  316  is captured by lens  314  and coupled into optical fiber  214  as reflected reference signal  318 . Optical fiber  214  conveys reflected reference signal  318  to splitter/combiner  208 . 
     The phase of reflected reference signal  318  at splitter/combiner  208  depends upon the total path length of reference arm  304 . Since the length of optical fiber  214  is fixed, a change in the phase of reflected reference signal  318  at splitter/combiner  208  depends only on a change in the position of retro-reflector  314  (and, therefore, the position of test mass  310 ) along axis  230 . 
     At operation  407 , free-space sample signal  232  is received from lens  222  along axis  230 . Retro-reflector  226  reflects free-space sample signal  232  in a downward direction along axis  230  as reflected free-space sample signal  234 . Reflected free-space sample signal  234  is captured by lens  222  and coupled into optical fiber  216  as reflected sample signal  236 . Optical fiber  216  conveys reflected sample signal  236  to splitter/combiner  208 . 
     The phase of reflected sample signal  236  at splitter/combiner  208  depends upon the total path length of sample arm  306 . Since the length of optical fiber  216  is fixed, a change in the phase of reflected sample signal  236  at splitter/combiner  208  depends only on a change in the position of retro-reflector  226  (and, therefore, the position of test mass  310 ) along axis  230 . 
     At operation  408 , reflected reference signal  318  and reflected sample signal  236  are combined as output optical signal  322 . Reflected reference signal  318  and reflected sample signal  236  will combine either constructively or destructively depending on their relative phases. The instantaneous intensity of output optical signal  322 , therefore, is based on the relative instantaneous phases of reflected reference signal  318  and reflected sample signal  236 . 
     At operation  409 , detector  242  receives output optical signal  322  and generates output signal  324 . The instantaneous value of output signal  324  corresponds to the instantaneous intensity of output optical signal  322 . Output signal  324  is conveyed to processor  110  on cable  108 . 
     An aspect of the present invention is that the distance between lens  308  and retro-reflector  314  increases at the same rate that the distance between lens  222  and retro-reflector  226  decreases as test mass  310  falls along axis  230 . As a result, the rate at which the intensity of output optical signal  322  changes is twice the rate of the phase change of each of reflected reference signal  310  and reflected sample signal  232 . Since the phase of each of reflected reference signal  310  and reflected sample signal  232  changes at twice the rate at which test mass  310  falls, the intensity of output optical signal  322  changes at four times the rate at which test mass  310  falls. As discussed above, and with respect to  FIG. 2 , the rate of change of the intensity of output signal  240  based on falling test mass  224 . The sensitivity of gravity sensor  106  to gravity at the location of test mass  310 , therefore, is twice that of prior art systems that rely on a fixed phase reference signal. 
     At operation  410 , processor  110  receives output signal  324  and computes a value for gravity at the location of test mass  310 . 
     At operation  411 , processor  110  computes a value for the difference in gravity at the locations of gravity sensors  106 - 1  and  106 - 2  (i.e., test masses  310 - 1  and  310 - 2 ). 
     It is to be understood that the disclosure teaches just one example of the illustrative embodiment and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.