Abstract:
A gravimeter that utilizes an electromagnetic launcher for enabling a free-fall of a test mass is disclosed. The electromagnetic launcher propels the test mass upward such that the test mass begins to free-fall once it has reached the apex of its flight. In addition, in some embodiments, the test mass comprises only non-ferromagnetic materials so that the free-fall of the test mass is unperturbed by magnetically induced influences.

Description:
FIELD OF THE INVENTION 
     The present invention relates to gravimeters. 
     BACKGROUND OF THE INVENTION 
     An individual gravimeter can be used to measure gravity in a local area. A pair of gravimeters can be used cooperatively to detect a differential gravity between two locations. Multiple differential gravimeters 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. 
     A gravimeter must be extremely sensitive. For example, sensitivity below 1 micro-Galileo is necessary in many applications. Such extreme sensitivity, however, requires very high immunity to noise sources. Error can be introduced into the output signal of a gravimeter 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. 
     Gravimeters 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. Gravimeters such as these, however, can be difficult to setup and calibrate. In addition, such gravimeters can be sensitive to environmental influences such as temperature or vibration. 
     Gravimeters based on the measurement of the motion of a falling mass have also been developed. A sensor system is used to monitor the acceleration of the falling mass during its free-fall. In some instances, such a gravimeter utilizes a mechanical carriage to lift a test mass to a position from which it can subsequently free-fall downward. Typically, these gravimeters utilize a vacuum chamber to eliminate effects of air resistance on the acceleration of the test mass. There are several disadvantages associated with these gravimeter systems, however. The size and complexity of the mechanical carriage used to position the test mass typically limits the compactness of the system. In turn, the use of a mechanical carriage commonly requires that the vacuum chamber be quite large. As a result, the cost and expense associated with using such free-fall gravimeters precludes their use in many applications. 
     To overcome some of the drawbacks associated with the use of a mechanical carriage, gravimeters that utilize a piezoelectric launcher to vertically launch a test mass upward have been developed. The test mass is launched so that it begins a free-fall downward once it reaches the apex of its flight. In addition to some of the drawbacks of other prior-art gravimeters, however, the sensitivity of these gravimeters is limited due to shock and vibration associated with the piezoelectric launcher itself. This mechanical energy manifests itself as noise into the output signal, thereby reducing signal-to-noise ratio and sensitivity of the gravimeter. 
     There exists a need, therefore, for a gravimeter that avoids or mitigates some or all of the problems associated with prior-art gravimeters. 
     SUMMARY OF THE INVENTION 
     The present invention enables a high sensitivity measurement of gravity without some of the costs and disadvantages for doing so in the prior art. In particular, the illustrative embodiment of the present invention uses an electromagnetic launcher to propel a test mass upward along an axis to an apex. The launcher provides a force impulse that imparts sufficient momentum on the test mass so that it begins to free-fall downward along the axis once it has reached the apex. In some embodiments, the test mass is substantially ferromagnetic material free so that its free-fall is not substantially influenced by perturbations due to the Earth&#39;s magnetic fields, spurious electromagnetic energy, residual magnetic fields associated with the electromagnetic launcher, and the like. 
     In some embodiments, the test mass comprises an electrically conductive element that is supportive of the development of an eddy current in response to a current that flows in a nearby propulsion coil. In some embodiments, this electrically conductive element is a continuous loop of conductive material that surrounds the test mass. In some embodiments, this electrically conductive element is a cylinder of electrically conductive material that surrounds the body of the test mass. In some embodiments, this electrically conductive element is a circular disk of electrically conductive material that is attached to the test mass. In some embodiments, the body of the test mass itself is a cylindrical electrically conductive element. 
     In some embodiments, the test mass is propelled along the axis toward the apex by an armature that is accelerated by the flow of electric current in the propulsion coil. In some embodiments, the armature comprises ferromagnetic materials and the armature is propelled by a magnetic field that develops when the propulsion coil is energized. In some embodiments, the armature comprises an electrically conductive element that is supportive of the development of an eddy current in response to a current that flows in the propulsion coil. Inductive coupling between the electrically conductive element and the energized propulsion coil generates a force that propels the armature, and the test mass, along the axis. 
     An embodiment of the present invention comprises: a test mass, wherein the test mass is substantially ferromagnetic material-free; a housing, wherein the housing encloses the test mass in an environment that has a pressure of less than one atmosphere; a sensor, wherein the sensor provides a signal that is based on a free-fall of the test mass from an apex; and a launcher, wherein the launcher propels the test mass to the apex. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a schematic diagram of details of a gravimeter in accordance with an illustrative embodiment of the present invention. 
         FIG. 2  depicts a schematic diagram of details of a sensor in accordance with the illustrative embodiment of the present invention. 
         FIG. 3  depicts a schematic diagram of details of a test mass system in accordance with the illustrative embodiment of the present invention. 
         FIG. 4A  depicts a method that comprises operations suitable for making a gravity measurement in accordance with the illustrative embodiment of the present invention. 
         FIG. 4B  depicts sub-operations suitable for performing operation  402 .  FIG. 4B  is described with continuing reference to  FIGS. 1-3  and  4 A. 
         FIG. 5  depicts a schematic diagram of details of a test mass system in accordance with an alternative embodiment of the present invention. 
         FIG. 6  depicts a method that comprises operations suitable for propelling a test mass to an apex in accordance with the alternative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts a schematic diagram of details of a gravimeter in accordance with an illustrative embodiment of the present invention. Gravimeter  100  is a system for sensing the gravity at the location of test mass  104 . Gravimeter  100  comprises test mass system  102 , sensor  108 , and processor  112 . 
     Test mass system  102  is a system for enabling the free-fall of test mass  104  along axis  106 . Test mass system  102  is described in more detail below and with respect to  FIG. 3 . 
     Sensor  108  is a sensor for monitoring the motion of test mass  104  along axis  106 . Sensor  108  provides signal  110  to processor  112 . 
     Processor  112  is a conventional processing system for executing software instructions, storing data, and performing computation. Processor  112  computes a value for acceleration of test mass  104  as it free-falls along axis  106 . Processor  112  computes this value based on signal  110 . Processor  112  also provides control signals to test mass system  102  on control line  114 . These control signals include, among other things, a signal for initiating a free-fall of test mass  104 . It will be clear to one skilled in the art how to specify, make, and use processor  112 . 
       FIG. 2  depicts a schematic diagram of details of a sensor in accordance with the illustrative embodiment of the present invention. Sensor  106  comprises source  202 , splitter combiner  204 , mirror  206 , retro-reflector  208  and detector  210 , optical fiber  232 , and lens  234 . Source  202 , splitter/combiner  204 , retro-reflector  210 , mirror  206 , and detector  208  collectively define a Michelson interferometer, wherein the instantaneous length of sense arm  216  is based on the instantaneous position of retro-reflector  208 . 
     Source  202  is a source of coherent light. Source  202  emits light signal  220 , which is conveyed to splitter/combiner  204  on source arm  212 . It will be clear to one skilled in the art how to specify, make, and use source  202 . 
     Splitter/combiner  204  is an optical element that receives light signal  220  and splits it into substantially equal light signals  222  and  224 . Splitter combiner  204  also receives reference signal  226  and sample signal  228 , which are reflected from mirror  206  and retro-reflector  208 , respectively. Splitter combiner  204  combines reference signal  226  and sample signal  228  into output signal  230 . Output signal  230  is conveyed to detector  210  on output arm  218 . It will be clear to one skilled in the art how to specify, make, and use splitter/combiner  204 . 
     Mirror  206  is a conventional mirror that is fixed in position such that the length of reference arm  214  is substantially fixed. It will be clear to one skilled in the art how to specify, make, and use mirror  206 . 
     Retro-reflector  208  is a device that reflects light back toward its source without inducing substantial scattering of the light. As will be described below, and with respect to  FIG. 3 , retro-reflector  208  and test mass  104  are physically coupled so that the instantaneous position of retro-reflector  208  and therefore, the length of sample arm  216 , is based on the instantaneous position of test mass  104  along axis  106 . In the illustrative embodiment, retro-reflector  208  is a corner-cube mirror that is formed in a surface of test mass  104 . In some embodiments, retro-reflector is a layer of material that is substantially reflective for light signal  220 . In some embodiments, retro-reflector  208  is disposed in a recess formed in mass body  302 . In some embodiments, retro-reflector  208  is formed by electro-plating, vapor deposition, evaporation, and the like. In some embodiments, retro-reflector  208  is an element that is encased in mass body  302 . Suitable devices for use in retro-reflector  208  include, without limitation, first surface mirrors, corner-cube mirrors, corner-reflector mirrors, cat&#39;s eye reflectors, and the like. It will be clear to one skilled in the art, after reading this specification, how to specify, make, and use retro-reflector  208 . 
     Lens  234  is a conventional plano-convex GRIN lens that substantially collimates light signal  318  as it emerges from optical fiber  232 . Lens  234  also couples light signal  320  back into optical fiber  232 . In some embodiments, lens  234  is a bulk optic element, such as a refractive lens or a diffractive lens. It will be clear to one skilled in the art how to make and use lens  234 . 
     In operation, splitter/combiner  204  splits light signal  220  into light signals  222  and  224 . Light signals  222  and  224  are substantially in-phase with one another. Light signal  222  is propagates to mirror  206 , which reflects it back toward splitter combiner  204  as reference signal  226 . In similar fashion light signal  224  is propagates to retro-reflector  208 , which reflects it back toward splitter combiner  204  as sample signal  228 . It will be clear to one skilled in the art, after reading this specification, that any or all of light signals  220 ,  222 ,  224 ,  226 ,  228 , and  230  can be conveyed by any convenient means, such as optical fiber, surface waveguides, or, alternatively, propagate through free-space. 
     The phase of reference signal  226 , as received by splitter/combiner  204 , is based on the optical path length of reference arm  214 , which is twice the distance between splitter/combiner  204  and mirror  206 . Since the position of mirror  206  is substantially fixed, the length of reference arm  214  is substantially fixed. As a result, the phase of reference signal  226 , as received by splitter/combiner  204 , is also substantially fixed. 
     The phase of sample signal  228 , as received by splitter/combiner  204 , is based on the optical path length of sample arm  216 . The optical path length of sample arm  216  is substantially equal to a combination of twice the length of optical fiber  232 , twice the thickness of lens  234 , and twice free-space distance  236 . At lens  234 , light signal  224  is launched into free-space and propagates to retro-reflector  208 , for a distance equal to free-space distance  236 . Retro-reflector  208  reflects the free-space light, which then propagates back to lens  234  (i.e., again, free-space distance  236 ). At lens  234 , the free-space light is coupled back into optical fiber  232  as sample signal  228 . The length of optical fiber  232  is fixed, as is the thickness of lens  234 . Free-space distance  236 , however, is dependent upon the position of test mass  104 . As a result, the instantaneous phase of sample signal  228 , as received by splitter/combiner  204 , is based solely on the instantaneous position of test mass  104  along axis  106 . 
     Upon receiving them, splitter/combiner  204  combines reference signal  226  and sample signal  228  into output signal  230 . Reference signal  226  and sample signal  228  combine either constructively or destructively, as a function of their relative phases. The instantaneous amplitude of output signal  230 , therefore, is based on the relative phases of reference signal  226  and sample signal  228  when they are combined at splitter/combiner  204 . When test mass  104  is not moving, the relative phases of these two signals remains constant and, therefore, the amplitude of output signal  230  also remains constant. As retro-reflector  208  moves, however, the amplitude of output signal  230  varies based on that motion. Splitter/combiner  204  provides output signal  230  to detector  210 . 
     Detector  210  is a device for converting light signal  230  into electrical signal  110 . The magnitude of electrical signal  110  is based on the intensity of light signal  230 . Detector  210  provides electrical signal  110  to processor  112 . It will be clear to one skilled in the art how to specify, make, and use detector  210 . 
     Although the illustrative embodiment comprises a sensor that is based on a Michelson interferometer arrangement, it will be clear to one skilled in the art, after reading this specification, how to specify, make, and use alternative embodiments of the present invention that sense the motion of test mass  104  using other technologies including, without limitation, wireless sensors, mechanical position sensors, piezoelectric sensors, optical position sensors, and capacitive position sensors. 
       FIG. 3  depicts a schematic diagram of details of a test mass system in accordance with the illustrative embodiment of the present invention. Test mass system  102  comprises test mass  104 , launcher  306 , and housing  316 . 
     Housing  316  is an enclosure that encloses test mass  104 , launcher  306 , and lens  234  and protects them from their surrounding environment. Housing  316  is evacuated to mitigate the effect of air resistance on the acceleration of test mass  104  while it free-falls. Housing  316  is oriented such that test mass  104  is enabled to free-fall along axis  106 . In some embodiments, housing  316  comprises a ballast that facilitates the alignment of axis  110  with the direction of local gravity. In some embodiments, housing  316  comprises a getter for improving the quality and longevity of a vacuum environment within housing  316 . 
     At lens  234 , light signal  224  is launched into free-space as light signal  318 . As discussed above, and with respect to  FIG. 2 , light signal  318  propagates toward retro-reflector  216  for a distance equal to free-space distance  236 , which is a function of the position of test mass  104 . At retro-reflector  216 , light signal  318  is reflected as light signal  320 , which propagates back through free-space to lens  234  for a distance equal to free-space distance  236 . At lens  234 , light signal  320  is coupled back into optical fiber  232  as sample signal  228 . The phase of light signal  318  at retro-reflector  216  is a function of the phase of light signal  224  at lens  234  and instantaneous distance between lens  234  and retro-reflector  216 . In similar fashion, the phase of light signal  320  at lens  216  is a function of the phase of light signal  318  at retro-reflector  216  and the instantaneous distance between lens  234  and retro-reflector  216 . 
     Launcher  306  is an electromagnetic propulsion system that comprises test mass  104 , power system  312 , and propulsion coil  310 . Launcher  306  generates a propulsive force on test mass  104  when electric current flows in propulsion coil  310 . 
     Power system  312  comprises circuitry that generates, conditions, and manages the delivery of electric current to propulsion coil  310  in response to a signal from processor  112 . It will be clear to those skilled in the art, after reading this specification, how to specify, make, and use power system  312 . In some embodiments, power system  312  is part of processor  112  rather than part of launcher  306 . 
     Propulsion coil  310  is a coil of electrically conductive material suitable for carrying sufficient electric current to generate the magnitude of electromagnetic force required to propel test mass  104  to apex  322 . Propulsion coil  310  is wound on coil body  308 . Propulsion coil  310  and coil body  308  are substantially immovable with respect to housing  316 . In some embodiments, propulsion coil  310  comprises a plurality of propulsion coils, wherein the flow of electric current in the plurality of propulsion coils can be sequenced by power system  312  to enhance the development of propulsive force on test mass  104 . In some embodiments, housing  316  interposes propulsion coil  310  and mass coil  304 . 
       FIG. 4A  depicts a method that comprises operations suitable for making a gravity measurement in accordance with the illustrative embodiment of the present invention.  FIG. 4A  is described with continuing reference to  FIGS. 1-3 . Method  400  begins with operation  401 , wherein test mass  104  is provided such that it is substantially ferromagnetic material-free. Test mass  104  comprises retroreflector  216 , mass body  302 , and mass coil  304 . 
     Mass body  302  comprises a mechanically stable, non-ferromagnetic material. Materials suitable for inclusion in mass body  302  include, without limitation: non-ferromagnetic metals and alloys, such as aluminum, copper, etc.; ceramics; polymers; glasses; composites; and the like. In some embodiments, retro-reflector  208  is disposed in a recess formed in mass body  302 . In some embodiments, retro-reflector  208  is formed by electro-plating, vapor deposition, evaporation, and the like. In some embodiments, retro-reflector  208  is an element that is encased in mass body  302 . 
     Mass coil  304  is a ring of non-ferromagnetic, electrically conductive material suitable for supporting the formation of an eddy current in response to an electric current that flows in propulsion coil  310 . In some embodiments, mass coil  304  is a cylinder disposed on the outer surface of mass body  302 . In some embodiments, mass body  302  comprises a non-ferromagnetic, electrically conductive material and the need for a separate mass coil  304  is obviated. 
     It was recognized by the inventors that the exclusion of ferromagnetic material from test mass  104  affords the present invention significant advantages over gravimeters of the prior art. First, it mitigates or eliminates the influence of the Earth&#39;s magnetic field on the free-fall of test mass  104 . As a result, the accuracy and sensitivity of a gravity measurement made with gravimeter  100  are improved. 
     Second, the lack of ferromagnetic materials enables the use of an electromagnetic launcher to propel test mass  104  into its free-fall position at apex  322 . Inclusion of ferromagnetic materials in test mass  104  would make its free-fall sensitive to residual magnetic fields associated with the decay of electric current flow in propulsion coil  310 . The use of an electromagnetic launcher also enables the generation of a force to slow test mass  104  as it nears the end of its downward travel, thereby enabling a soft landing and increasing the lifetime of test mass  104 . Also, the use of an electromagnetic launcher obviates the need for complicated and expensive mechanical drop mechanisms, such as typically used in the prior art. Further, the lack of a mechanical drop mechanism enables gravimeter  100  to have a reduced form factor as compared to the prior art. As a result, gravimeter  100  can be used in applications that have commonly been unsuitable for prior art gravimeters. 
     Third, it is well-known that some material properties (in particular, magnetic permeability) of ferromagnetic materials are temperature sensitive. Gravimeter  100 , therefore, is suitable for use over a much wider temperature range than gravimeters in the prior art. 
     Finally, known issues with hysteresis and hysteresis drift, vis-à-vis ferromagnetic materials, are mitigated or eliminated. 
     At operation  402 , free-fall of test mass  104  along axis  106  from apex  322  is enabled. The free-fall of test mass  104  is enabled by launching it upward along axis  106  to apex  322 , in response to a launch command provided by processor  112  on control line  114 . 
     At operation  403 , sensor  108  provides signal  110  to processor  112 . Signal  110  is based on the motion of test mass  104  during its free-fall along axis  106 . 
     At operation  404 , processor  108  computes a value for gravity at the location of test mass  104 . 
     In some embodiments, a plurality of gravimeters is used to provide a measurement of differential gravity between two locations. In some embodiments, a plurality of gravimeters is used to provide an average measurement of gravity at a single location. 
       FIG. 4B  depicts sub-operations suitable for performing operation  402 .  FIG. 4B  is described with continuing reference to  FIGS. 1-3  and  4 A. At operation  405 , in response to a command from processor  112 , power system  112  energizes propulsion coil  310  with electric current. 
     Due to inductive coupling between the coils, the flow of current in propulsion coil  310  induces an eddy current in mass coil  304 . The flow of current in the two coils generates a propulsive force on test mass  104  that is directed upward along axis  106 . 
     At operation  406 , the current flow to propulsion coil  310  is terminated. As a result, the generated force is removed from test mass  104 . The magnitude and duration of the current flow in propulsion coil  310  is such that the impulse of force on test mass  104  provides it with sufficient momentum to enable it to reach apex  322  and begin to free-fall back along axis  106 . During its free-fall, test mass  104  accelerates due to the effects of local gravity. 
       FIG. 5  depicts a schematic diagram of details of a test mass system in accordance with an alternative embodiment of the present invention. Test mass system  500  comprises test mass  504 , launcher  502 , and housing  316 . 
     Launcher  502  is an electromagnetic launch system; however, in contrast to launcher  306 , launcher  502  throws test mass  504  by means of propelling an armature that is physically coupled, but not attached, to test mass  504 . 
     Test mass  504  comprises only non-ferromagnetic materials. Materials suitable for use in test mass  504  include, without limitation, non-ferromagnetic metals and alloys, such as aluminum, copper, etc.; ceramics; polymers; glasses; composites; and the like. 
     Armature  506  comprises a ferromagnetic material to facilitate its coupling with the electromagnetic field generated by the flow of electric current in propulsion coil  310 . Suitable materials for armature  506  include, without limitation, steel, iron, Permalloy, nickel and nickel alloys, cobalt and cobalt alloys, and the like. 
       FIG. 6  depicts a method that comprises operations suitable for propelling a test mass to an apex in accordance with the alternative embodiment of the present invention.  FIG. 6  is described with continuing reference to  FIG. 5 . Method  600  begins with operation  601 , wherein propulsion coil  310  is energized with a flow of electric current such that a magnetic field is generated upward along axis  106 . 
     At operation  602 , the magnetic field couples with armature  506 . This induces motion of armature  506  upward along axis  106 . 
     At operation  603 , the upward motion of armature  506  propels test mass  504  upward along axis  106 . 
     At operation  604 , the flow of electric current to coil  310  is stopped and/or reversed to stop the motion of armature  506 . By virtue of its attained momentum, test mass  504  separates from armature  506  and continues to the apex of its flight along axis  106  (i.e., apex  322 ). At apex  322 , test mass  504  begins to free-fall downward along axis  106  in similar fashion to the free-fall of test mass  104 . A value for the gravity at the location of test mass system  500  is then computed as described above and with respect to  FIGS. 1-4A . 
     In some embodiments, armature  506  comprises a circular, electrically conductive element, such as mass coil  304 . In such embodiments, energized propulsion coil  310  and armature  506  inductively couple such that an eddy current develops in the conductive element, which results in the propulsion of armature  504 . In some embodiments, armature  506  comprises only non-ferromagnetic material. 
     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.