Abstract:
Accelerometers for determining acceleration and methods of fabricating an accelerometer are disclosed. In one embodiment, the accelerometer includes a frame, a mass movably suspended on the frame, a fixed element having a rounded surface that does not move with respect to the frame, a movable element having a rounded surface that moves with the mass, and a sensing coil of optical waveguide wrapped around the rounded surfaces to detect movement of the mass in response to acceleration based on interferometric sensing of a change in length of the sensing coil. A method of fabricating the accelerometer includes suspending the mass in the frame and wrapping the optical waveguide around the rounded surfaces. Sensitivity and low fabrication cost of the accelerometers enables their use for integration within an ocean bottom seismic cable. Further, the accelerometer may be an in-line or a cross-line accelerometer depending on the arrangement within the frame.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional of U.S. patent application Ser. No. 11/018,944 filed Dec. 21, 2004, now U.S. Pat. No. 7,243,543 which is a continuation-in-part of U.S. patent application Ser. No. 10/933,132 filed Sep. 2, 2004, now U.S. Pat. No. 7,013,729, which is a continuation application of Ser. No. 10/366,900, now U.S. Pat. No. 6,789,424 filed Feb. 14, 2003, which is a continuation of application Ser. No. 09/410,634, now U.S. Pat. No. 6,575,033, filed Oct. 1, 1999, all of which are herein incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   Embodiments of the present invention generally relate to highly sensitive accelerometers. More particularly, embodiments of the invention relate to optical accelerometers for applications such as integration into ocean bottom seismic cables. 
   2. Description of the Related Art 
   Marine seismic exploration surveys for the exploration and monitoring of hydrocarbon producing zones and reservoirs utilize seismic cables deployed on the ocean floor. The cable includes an array of accelerometers capable of detecting ground acceleration on the ocean floor produced by acoustic vibrations. 
   One common type of accelerometer includes a mass-spring transducer housed in a sensor case. The sensor case couples to a moving body, the ocean floor, whose motion is inferred from the relative motion between the mass and the sensor case. Such accelerometers relate the relative displacement of the mass with the acceleration of the case, and therefore the ocean floor. Obtaining an ocean bottom seismic (OBS) survey requires placing the seismic cables along the ocean floor, generating seismic waves that travel downward through the earth and reflect off of underground deposits or changes in formation, and recording the reflected seismic waves detected by the accelerometers. Thus, the sensitivity of the accelerometer directly affects the quality of the data acquired by the OBS survey making many prior accelerometers designs unacceptable due to insufficient sensitivity. 
   Several problems exist with using conventional electrical accelerometers in cable arrays in the ocean. In particular, electrical accelerometers require an insulated electrical conductor for transmitting electrical signals, which can short if the electrical conductor becomes damaged and is exposed to sea water. Further, most high performance piezoelectric accelerometers require power at the sensor head which may be difficult to provide due to the substantial cable length. Also, multiplexing of a large number of such sensors is not only cumbersome but tends to occur at a significant increase in weight and volume of an accelerometer array, as well as a decrease in reliability. Additionally, piezoelectric accelerometers tend to operate poorly at the lowest frequencies in the seismic band. 
   Many systems and methods for OBS surveying do not retrieve the cable arrays for redeployment and reuse. During a single OBS survey, cable arrays with several thousand accelerometers may be utilized. The large quantity of accelerometers required along with the practice of abandoning the deployed cable arrays after one use makes the cost of the accelerometers very critical. Prior designs of both optical and electrical accelerometers often require a complicated assembly procedure and a large number of specially made parts, thereby increasing the cost to manufacture the accelerometers. 
   Therefore, there exists a need for an inexpensive optical accelerometer with increased sensitivity for applications such as integration into OBS cable arrays. 
   SUMMARY OF THE INVENTION 
   Embodiments of the invention generally relate to accelerometers for determining acceleration and methods of fabricating an accelerometer. In one embodiment, the accelerometer includes a frame, a mass movably suspended on the frame, a fixed element having a rounded surface that does not move with respect to the frame, a movable element having a rounded surface that moves with the mass, and a sensing coil of optical waveguide wrapped around the rounded surfaces to detect movement of the mass in response to acceleration based on interferometric sensing of a change in length of the sensing coil. A method of fabricating the accelerometer includes suspending the mass in the frame and wrapping the optical waveguide around the rounded surfaces. Sensitivity and low fabrication cost of the accelerometers enables their use for integration within an ocean bottom seismic cable. Further, the accelerometer may be an in-line or a cross-line accelerometer depending on the arrangement within the frame. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
       FIG. 1  is a schematic that depicts a Bragg grating interferometric sensing system as an exemplary sensing system in which embodiments of the invention may be utilized. 
       FIG. 2  is a perspective view of an assembled in-line accelerometer. 
       FIG. 3  is an exploded view of the in-line accelerometer shown in  FIG. 2 . 
       FIG. 4  is a perspective view of the in-line accelerometer shown in  FIG. 2  as it would appear during assembly thereof with a counter mass supported within a frame by two diaphragms. 
       FIG. 5  is a perspective view of the in-line accelerometer shown in  FIG. 2  as it would appear during assembly thereof after the addition of a stationary half cylinder to the frame and a movable half cylinder to the mass. 
       FIG. 6  is a perspective view of an assembled cross-line accelerometer. 
       FIG. 7  is an exploded view of the cross-line accelerometer shown in  FIG. 6 . 
       FIG. 8  is a perspective view of the cross-line accelerometer shown in  FIG. 6  as it would appear during assembly thereof with a counter mass hinged to a frame. 
       FIG. 9  is a perspective view of the cross-line accelerometer shown in  FIG. 6  as it would appear during assembly thereof after the addition of a stationary half cylinder to the frame and a movable half cylinder to the mass. 
       FIG. 10  is a sectional view of an in-line accelerometer having a spring to bias a counter mass and hence a movable half cylinder. 
       FIG. 11  is a perspective view of an in-line accelerometer having four springs to bias a movable half cylinder directly. 
       FIG. 12  is a perspective view of an in-line accelerometer with integral components. 
       FIG. 13  is a perspective view of an in-line accelerometer according to another embodiment. 
       FIG. 14  is a partial sectional view of the in-line accelerometer of  FIG. 13  taken across a top of the in-line accelerometer. 
       FIG. 15  is a partial sectional view of the in-line accelerometer of  FIG. 13  taken across a side of the in-line accelerometer. 
       FIG. 16  is a graph of the measured performance of a tested sample of the accelerometer illustrated in  FIG. 13 . 
   

   DETAILED DESCRIPTION 
   Embodiments of the invention generally relate to an optical accelerometer. The accelerometer may be coupled to any surface or structure subjected to acceleration to be sensed. In one particular application, the highly sensitive accelerometers described herein may be disposed within sensor stations spaced along a seismic cable used to obtain an ocean bottom seismic (OBS) survey. As described in greater detail herein for some embodiments, each accelerometer may include a pair of fiber optic sensors separated by a length of optical fiber, forming an interferometer. Each sensor in the pair may reflect a narrow wavelength band of light having a central wavelength. Each accelerometer may operate at a different wavelength band and central wavelength such that the signals may be easily detected using Wavelength Division Multiplexing (WDM) techniques. Alternatively, the signals may be separated in time using Time Division Multiplexing (TDM). 
     FIG. 1  schematically illustrates a simplified optical waveguide interferometric accelerometer system  100 . The accelerometer system  100  includes a sensing coil  102  comprised of a number of tightly wrapped turns of an optical waveguide  104  (such as an optical fiber) around a sensing assembly  106 . Embodiments of the present invention include configurations where the sensing coil  102  may be disposed on or within an elastic member. The sensing assembly  106  should be understood as generically representing any of the inventive sensing assemblies subsequently described herein. The sensing coil  102  is bounded by a pair of Bragg gratings  110 ,  112  that have the same Bragg wavelength (λB). In some applications, it may not be practical to form the sensing coil  102  and the Bragg gratings  110 ,  112  along a continuous section of optical waveguide. In that case, the individual components, such as input and output optical waveguides  120 ,  130 , the sensing coil  102 , and the Bragg gratings  110 ,  112  can be individually formed and then spliced together.  FIG. 1  illustrates such splices using slash marks  136 . 
   The sensing coil  102  acts as a sensor since the length (L) of the sensing coil  102  depends on the diameter of the sensing assembly  106 , which, in turn, depends on the acceleration experienced by the sensing assembly  106 . Well known interferometric interrogation techniques, such as Fabry-Perot, Michelson, or Mach-Zehnder, can determine the length of the sensing coil  102 . For example, a series of optical pulses from a pulse generator  114  can be applied to the sensing coil  102  through the input optical waveguide  120 . Reflections of optical pulses from the Bragg gratings  110 ,  112 , which are partially transmissive, are detected by a detector  116  and analyzed by an analyzer  118 . By assessing the phase shift in the pulses that are reflected from the two Bragg gratings  110 ,  112 , the length of the sensing coil  102  can be determined. 
   Acceleration causes a change in length ΔL of the length L and a corresponding change in the round trip path of pulses reflected from the second Bragg grating  112 , which causes the phase relationship between the light pulses detected at the detector  116  to vary. The analyzer  118  senses the phase variance and provides an electrical output that corresponds to the acceleration. The output optical waveguide  130  can be connected to other optical components or sensors deployed along with the accelerometer system  100 . Other strain sensing techniques including the use of piezoelectric, electronic or electric strain gauges may be used to measure the variations in strain on the sensing coil  102  such as those described and shown in FIGS. 15-23 of U.S. Pat. No. 6,575,033, entitled “Highly Sensitive Accelerometer,” which is herein incorporated by reference in its entirety. 
   The sensing assembly  106  may include a mass-spring arranged within the sensing coil  102  to provide either an in-line accelerometer or a cross-line accelerometer. Movement of the mass in response to acceleration results in the change in length of the sensing coil  102 . 
     FIG. 2  illustrates an assembled in-line accelerometer  200  that includes a counter mass  202 , a stationary half cylinder  204 , a movable half cylinder  206  movably coupled with the counter mass  202 , a sensing coil  208  wrapped around the half cylinders  204 ,  206 , and a frame formed by first and second frame plates  210 ,  211  held together by four bolts  212 . 
     FIG. 3  shows the in-line accelerometer  200  in an exploded view with first and second diaphragms  300 ,  301  positioned to support the counter mass  202  between the frame plates  210 ,  211 . The sensing coil  208  preferably includes windings of optical fibers that form an elastic member responsive to movements of the movable half cylinder  206  with respect to the stationary half cylinder  204  by elongating or relaxing resulting in detectable changes in length. Thus, the half cylinders  204 ,  206  and the counter mass  202  provide the sensing assembly such that the sensing coil  208  lengthens or shortens to produce a signal corresponding to the acceleration. 
   For example, the counter mass  202  displaces within the frame plates  210 ,  211  in the direction indicated by arrow  216  when the in-line accelerometer  200  accelerates in the opposite direction indicated by arrow  217 . In this particular case, the tension in the sensing coil  208  increases as the movable half cylinder  206  moves away from the stationary half cylinder  204  such that the fiber length of the sensing coil  208  increases. Similarly, the counter mass  202  displaces within the frame plates  210 ,  211  in the direction indicated by arrow  217  when the in-line accelerometer  200  accelerates in the opposite direction indicated by arrow  216  such that the movable half cylinder  206  moves toward the stationary half cylinder  204  and the fiber length of the sensing coil  208  decreases. As previously described, this change in length results in a detectable change in phase angle between the signals reflected from the sensors (e.g., Bragg gratings) separated by the sensing coil  208 . 
     FIG. 4  illustrates the in-line accelerometer  200  as it would appear during assembly thereof with the counter mass  202  supported between the frame plates  210  (shown transparent),  211  by the diaphragms  300 ,  301  (not visible). With reference to  FIG. 3 , ends of the bolts  212  with reduced diameters extend through apertures  302  at the corners of the frame plates  210 ,  211  until a shoulder formed by the reduced diameter abuts the frame plates  210 ,  211 . The first and second diaphragms  300 ,  301  secure to the center of the first and second frame plates  210 ,  211 , respectively, such as by welding. Each of the diaphragms  300 ,  301  couple to opposite ends of the counter mass  202 . A short member such as post  304  may extend from the ends of the counter mass  202  to facilitate attachment thereof with the diaphragms  300 ,  301 . Diaphragms  300 ,  301  flex in the direction of arrows  216 ,  217  to permit movement of the counter mass  202  in the axis along these directions. However, the diaphragms  300 ,  301  substantially prevent movement of the counter mass  202  along other axes since the diaphragms  300 ,  301  are stiff in these axes. 
     FIG. 5  shows the in-line accelerometer  200  as it would appear during assembly thereof after the addition of the stationary half cylinder  204  and the movable half cylinder  206 . In particular, the stationary half cylinder  204  secures to the bolts  212  extending from the second frame plate  211  on the side of the second frame plate  211  opposite from the counter mass  202 . Since the movable half cylinder  206  is positioned adjacent a face of the first frame plate  210  opposite from the counter mass  202 , a center aperture  306  (shown in  FIG. 3 ) through the first frame plate  210  enables coupling of the movable half cylinder  206  with the counter mass  202  using any type of conventional connector. The accelerometer may additionally include blocks  214  (shown transparent) secured to the bolts  212  extending from the first frame plate  210  on the side of the first frame plate  210  opposite from the counter mass  202 . The blocks  214  provide further support to the bolts  212  and protect and guide the movement of the movable half cylinder  206 . Once assembled, the movable half cylinder  206  freely moves between the fixed blocks  214  with the movement of the counter mass  202 , which moves with respect to the frame plates  210 ,  211 , the stationary half cylinder  204  and the blocks  214  that are all locked together by the bolts  212 . The sensing coil  208  increases the effective spring constant of the mechanical resonator made by the counter mass  202  and the sensing coil  208 , thereby improving the frequency response of the in-line accelerometer  200 . 
   As is apparent from  FIG. 5 , winding of the sensing coil  208  around the half cylinders  204 ,  206  to complete the in-line accelerometer  200  can be accomplished easily and performed directly thereon after all other assembly of the in-line accelerometer  200  is complete. Thus, there is no need for a separate manufacturing process to form the sensing coil  208  which may facilitate assembly and reduce cost. During winding of the sensing coil  208 , the diaphragms  300 ,  301  may be used as springs to pre-strain the sensing coil  208  such that the sensing coil  208  is responsive to movement of the movable half cylinder  206  in both directions indicated by arrows  216 ,  217 . In addition, the design of the in-line accelerometer  200  utilizes a relatively small number of parts in order to further simplify the manufacturing process. Furthermore, parts required for the design of the in-line accelerometer  200  such as the half cylinders  204 ,  206 , the counter mass  202 , and/or the blocks  214  may be made using polymers along with efficient molding techniques to further reduce manufacturing costs. 
     FIG. 6  illustrates an assembled cross-line accelerometer  600  that includes a hinged counter mass  602 , a stationary half cylinder  604  (shown transparent), a movable half cylinder  606  movably coupled with the hinged counter mass  602 , a sensing coil  608  disposed around the half cylinders  604 ,  606 , and a frame formed by a first frame plate  610 . 
     FIG. 7  shows the cross-line accelerometer  600  in an exploded view. Similar to the in-line accelerometer  200  shown in  FIGS. 2-5 , the sensing coil  608  preferably includes windings of optical fibers that form an elastic member responsive to movements of the movable half cylinder  606  with respect to the stationary half cylinder  604  by elongating or relaxing. Again, the half cylinders  604 ,  606  and the hinged counter mass  602  provide the sensing assembly. However, the cross-line accelerometer  600  detects cross-line acceleration instead of in-line acceleration as detected by the in-line accelerometer  200  previously discussed. Thus, the action of the sensing coil  608  lengthens or shortens the optical fibers and produces a signal corresponding to the acceleration as the counter mass  602  displaces in the direction indicated by arrows  616 ,  617  depending on the direction of acceleration along the axis identified by the arrows  616 ,  617 . 
     FIG. 8  shows the cross-line accelerometer  600  as it would appear during assembly thereof with the counter mass  602  hinged to the first frame plate  610 . In particular, the first frame plate  610  includes a mounting clamp  612  secured at one end thereto. Two blades  614  located in-line with one another and made of a material such as steel extend from the top of the first frame plate  610  in a direction facing the opposite end of the first frame plate  610  from where the mounting clamp  612  is located. The blades  614  connect to approximately the center of the hinged counter mass  602  to permit pivotal movement of the hinged counter mass  602  with respect to the first frame plate  610 . Thus, the blades  614  flex in one plane identified by arrows  616 ,  617  while the blades  614  substantially prevent movement of the counter mass  602  along other axes since the blades are stiff in these axes. Furthermore, the blades  614  represent a spring pulling the hinged counter mass  602  back to its center position during operation. 
     FIG. 9  shows the cross-line accelerometer  600  as it would appear during assembly thereof after the addition of the stationary half cylinder  604  and the movable half cylinder  606 . Specifically, the stationary half cylinder  604  rigidly secures by any conventional connection to the end of the first mounting plate  610  opposite from the mounting clamp  612 . The movable half cylinder  606  mounts directly to the hinged counter mass  602  using any conventional connection. For some embodiments, the location of the movable half cylinder  606  and the stationary half cylinder  604  may be transposed such that the stationary half cylinder  604  is adjacent the hinge point of the mass  602 . Appropriate tolerances remain between parts (e.g., the movable half cylinder  606  and the mounting clamp  612 ) of the cross-line accelerometer  600  after assembly thereof to not inhibit the required travel of the hinged counter mass  602  with respect to the frame plate  610  and the stationary half cylinder  604 . Thus, pivoting of the hinged counter mass  602  caused by acceleration of the cross-line accelerometer  600  in the direction of arrows  616 ,  617  effectively increases or decreases the separation between the half cylinders  604 ,  606  upon the rotational movement of the movable half cylinder  606  coupled to the mass  602 . 
   Referring back to  FIGS. 6 and 7 , a second frame plate  611  may be secured to the top of the stationary half cylinder  604 . Additionally, the cross-line accelerometer  600  may further include a biasing member such as a spring  700  located on the opposite side of the hinged counter mass  602  from the blades  614 . The spring  700  rests within a spring retainer  704  on the first frame plate  610  and acts against the first frame plate  610  and an extension  702  extending from the hinged counter mass  602 . In this position, the spring  700  biases the end of the hinged counter mass  602  against the force in the direction indicated by the arrow  617  generated by pre-tension of the sensing coil  608  that tends to pull the hinged counter mass  602  out of its center aligned position. The spring  700  increases the effective spring constant of the mechanical resonator made by the hinged counter mass  602  and the sensing coil  608 , thereby improving the frequency response of the cross-line accelerometer  600 . 
   The cross-line accelerometer  600  shares many of the benefits of the in-line accelerometer  200 . For example, winding of the sensing coil  608  around the half cylinders  604 ,  606  to complete the cross-line accelerometer  600  can be accomplished easily and performed directly thereon after all other assembly of the cross-line accelerometer  600  is complete. In addition, the design of the cross-line accelerometer  600  utilizes a relatively small number of parts that may be made using polymers along with efficient molding techniques to further simplify the manufacturing process and further reduce manufacturing costs. 
     FIG. 10  illustrates a cross section view of an in-line accelerometer  1000  substantially similar to the in-line accelerometer  200  shown in  FIGS. 2-5  and explained above. However, the in-line accelerometer  1000  illustrated in  FIG. 10  includes a spring  1050  disposed about the outside of a counter mass  1002  to bias the counter mass  1002  and hence a movable half cylinder  1006 . One end of the spring  1050  is supported by a frame plate  1011  of the in-line accelerometer  1000  such that the other end of the spring  1050  that is in contact with a shoulder  1052  of the counter mass  1002  acts to push the counter mass  1002  away from a stationary half cylinder  1004 . Thus, the bias of the counter mass  1002  and the movable half cylinder  1006  away from the stationary half cylinder  1004  by the spring  1050  can be used to aid in applying a pre-strain to a sensing coil  1008  disposed around the half cylinders  1004 ,  1006 . The spring  1050  can be relatively soft with a long stroke to obtain the required force to pre-strain the sensing coil  1008 . The long stroke and softness of the spring  1050  increases the efficiency and scale factor compared to use of a short and stiff spring, such as a diaphragm used to pre-strain the sensing coil  1008 . Since the spring  1050  is used to pre-strain the sensing coil  1008 , a diaphragm  1300  that only has to effectively guide movement of the counter mass  1002  can be made softer. 
     FIG. 11  shows an in-line accelerometer  1100  that includes four springs  1150  (only three are visible) to directly bias a movable half cylinder  1106  away from a stationary half cylinder  1104 . In this embodiment, the four springs  1150  located away from an area where a counter mass  1102  is disposed enable pre-straining of a sensing coil  1108  in a manner similar to the spring  1050  shown in  FIG. 10  and described above. The counter mass  1102  mounts within a central housing  1110  by use of diaphragms (not visible). As with other embodiments described herein, the stationary half cylinder  1104  rigidly couples to the central housing  1110  while the movable half cylinder  1106  moves with the counter mass  1102 . Four pins  1151  (only three are visible) couple to a perimeter of the central housing  1110  and extend toward an inside face of the movable half cylinder  1106  without coming into contact with the movable half cylinder  1106 . The pins  1151  serve as supports for the springs  1150  that are concentrically disposed about the pins  1151  in order to prevent buckling of the springs  1150 . One end of each of the springs  1150  is supported relative to the central housing  1110  such that the other end of each of the springs  1150  that is in contact with the movable half cylinder  1106  acts to push the movable half cylinder  1106  away from the stationary half cylinder  1104 . Thus, the bias of the movable half cylinder  1106  away from the stationary half cylinder  1104  by the springs  1150  can be used to aid in applying a pre-strain to the sensing coil  1108  disposed around the half cylinders  1104 ,  1106 . 
     FIG. 12  illustrates an in-line accelerometer  1200  with integral components. The in-line accelerometer  1200  includes a counter mass  1202 , a stationary half cylinder  1204 , a movable half cylinder  1206  and a central frame  1210  that are all formed from a single piece of steel by wire cutting or laser cutting to make the required splitting of the components. The cutting is through the whole body of the in-line accelerometer  1200 . Internal cuts  1260  define the counter mass  1202  within the central frame  1210  and form one side of a diaphragm region. An outer cut  1262  defines the stationary half cylinder  1204  that is rigid with respect to the central frame  1210 . Slots  1263  define the movable half cylinder  1206  that moves with the counter mass  1202 . The half cylinders  1204 ,  1206  can be formed by milling. Alternatively, the half cylinders  1204 ,  1206  can be separate components added to the body such as partial tubular components or components made separately in a lath. A sensing coil  1208  is shown invisible around the half cylinders  1204 ,  1206 . 
     FIG. 13  shows an in-line accelerometer  1300  according to another embodiment. Similar to the other embodiments described herein, the in-line accelerometer  1300  includes a counter mass  1302  (visible in  FIGS. 14 and 15 ), a stationary half cylinder  1304 , a movable half cylinder  1306 , a central frame  1310  and a sensing coil  1308  around the half cylinders  1304 ,  1306 . In addition to the in-line accelerometer utilizing a relatively small number of parts, the two half cylinders  1304 ,  1306  may be substantially identical to further reduce manufacturing costs. Two bolts  1312  secure the stationary half cylinder  1304  to the central frame  1310 . 
     FIGS. 14 and 15  illustrate partial sectional views of the in-line accelerometer  1300 . An assembly bolt  1314  extends through a longitudinal central bore of the counter mass  1302  and a first diaphragm  1319  where an end of the assembly bolt  1314  couples to a face of the movable half cylinder  1306  facing the counter mass  1302 . On the other side of the counter mass  1302  from the movable half cylinder  1306 , a nut  1316  attaches to the assembly bolt  1314  to engage a diaphragm clamp  1315  on an opposite side of a second diaphragm  1318  from the counter mass  1302 . Accordingly, this arrangement of the assembly bolt  1314  sandwiches the counter mass  1302  between the two diaphragms  1318 ,  1319  such that the movable half cylinder  1306  moves with the counter mass  1302  suspended by the diaphragms  1318 ,  1319 . Additionally, an o-ring  1320  may be disposed between the central housing  1310  and the stationary half cylinder  1304 . 
     FIG. 16  is a graph showing measured performance of a tested design of the accelerometer  1300  by plotting a relative response of the accelerometer to an excitation force on a test shaker. The results shown in the graph are obtained by monitoring the accelerometer across a range of frequencies when the accelerometer is installed in an oil-filled housing to reduce mechanical resonance. As evidenced by the graph, the specific accelerometer provides a response with a flat curve within a desired range of operation and a peak corresponding to the mechanical resonance that is damped by the oil. The damping can be made even more efficient by using oil with a higher viscosity. Additionally, the frequency of the mechanical resonance can be changed based on the mass and spring constant selected for the accelerometer. 
   For any geometry of the wraps described herein, more than one layer of fiber may be used depending on the overall fiber length and sensitivity desired. It is further within the scope of the present invention that the sensing coil may comprise the optical fiber disposed in a helical pattern (not shown) about the half cylinders. 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 acceleration sensitivity and other parameters desired to be measured, for example, the magnitude of the acceleration. Furthermore, the half cylinders generally provide rounded surfaces for wrapping the sensing coil thereon to prevent straining and sharp bending of the sensing coil. However, the surface supporting the sensing coil may be any other shape than rounded such as flat, angled or undulated. In addition, various elements of the accelerometers  200 ,  600  may be integrated into a single element for some embodiments. For example, the stationary half cylinder  204  may be integral with the second frame plate  211 . 
   While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.