Patent Publication Number: US-2022229198-A1

Title: Sensor assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND 
     Seismic surveying, or reflection seismology, is used to map the Earth&#39;s subsurface. During a seismic survey, a controlled seismic source emits low frequency seismic waves that travel through the subsurface of the Earth. At interfaces between dissimilar rock layers, the seismic waves are partially reflected. The reflected waves return to the surface where they are detected by one or more seismic sensors. In particular, the seismic sensors detect and measure vibrations induced by the waves. Ground vibrations detected by the seismic sensors at the earth surface can have a very wide dynamic range, with displacement distances ranging from centimeters to angstroms. Data recorded by the seismic sensors is analyzed to reveal the structure and composition of the subsurface. 
     BRIEF SUMMARY 
     Some embodiments disclosed herein are directed to a sensor assembly for a seismic sensor. In an embodiment, the sensor assembly includes an electrically conductive outer housing, and an electrically insulating holder disposed within the outer housing. The holder comprises a recess. In addition, the sensor assembly includes a sensor element disposed within the recess of the holder. The sensor comprises a sensing element, and is electrically insulated from outer housing by the holder. 
     Other embodiments disclosed herein are directed to a seismic sensor. In an embodiment, the seismic sensor includes an outer housing having a central axis, an upper end, a lower end, and an inner cavity. In addition, the seismic sensor includes a proof mass moveably disposed in the inner cavity, wherein the outer housing is configured to move axially relative to the proof mass, and a plurality of biasing members disposed within the inner cavity and configured to flex in response to axial movement of the outer housing relative to the proof mass. Further, the seismic sensor includes a sensor assembly disposed in the inner cavity and axially positioned between the proof mass and the lower end of the outer housing. The sensor assembly includes an electrically conductive sensor housing, and an electrically insulating holder disposed within the sensor housing. The holder includes a recess. In addition, the sensor assembly includes a sensor element disposed within the recess of the holder. The sensor comprises a piezoelectric element, and wherein the sensor element is electrically insulated from the sensor housing by the holder. 
     Still other embodiments disclosed herein are directed to a method of manufacturing a seismic sensor. In an embodiment, the method includes (a) inserting a sensor element within a recess of an electrically insulating holder, (b) enclosing the holder and the sensor element within an electrically conductive sensor housing after (a), and (c) engaging the sensor housing with an end of a carrier after (b). In addition, the method includes (d) suspending a proof mass within the carrier via a plurality of biasing members. Further, the method includes (f) inserting the carrier, the sensor housing, and the proof mass within an outer housing after (c) and (d) such that the sensor element is deflected when the carrier moves relative to the proof mass. 
     Embodiments described herein comprise a combination of features and characteristics intended to address various shortcomings associated with certain prior devices, systems, and methods. The foregoing has outlined rather broadly the features and technical characteristics of the disclosed embodiments in order that the detailed description that follows may be better understood. The various characteristics and features described above, as well as others, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings. It should be appreciated that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes as the disclosed embodiments. It should also be realized that such equivalent constructions do not depart from the spirit and scope of the principles disclosed herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a detailed description of various exemplary embodiments, reference will now be made to the accompanying drawings in which: 
         FIG. 1  is a schematic representation of a seismic surveying system for surveying a subsurface earthen formation according to some embodiments; 
         FIG. 2  is a perspective view of an embodiment of a seismic sensor which may be used within the system of  FIG. 1  according to some embodiments; 
         FIG. 3  is a longitudinal cross-sectional view of the seismic sensor of  FIG. 2 ; 
         FIG. 4  is a perspective view of the battery and tabs of the seismic sensor of  FIG. 2 ; 
         FIG. 5  is an exploded perspective view of an embodiment of a sensor assembly which may be used within the seismic sensor of  FIG. 2  according to some embodiments; 
         FIG. 6  is a perspective view of the sensor assembly of  FIG. 5  with the cover plate removed; 
         FIG. 7  is a perspective view of the sensor assembly of  FIG. 5  with the cover plate installed; 
         FIG. 8  is another perspective view of the sensor assembly of  FIG. 5 ; 
         FIG. 9  is a perspective view of the sensor assembly of  FIG. 5  installed onto the carrier of the seismic sensor of  FIG. 2 ; 
         FIG. 10  is an enlarged perspective view of the sensor assembly of  FIG. 5  installed onto the carrier of the seismic sensor of  FIG. 2 ; and 
         FIG. 11  is a longitudinal, cross-sectional view of the seismic sensor of  FIG. 2  detailing the position and arrangement of the sensor assembly of  FIG. 5  therein. 
     
    
    
     DETAILED DESCRIPTION 
     The following discussion is directed to various exemplary embodiments. However, one of ordinary skill in the art will understand that the examples disclosed herein have broad application, and that the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to suggest that the scope of the disclosure, including the claims, is limited to that embodiment. 
     The drawing figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in interest of clarity and conciseness. 
     In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . .” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection of the two devices, or through an indirect connection that is established via other devices, components, nodes, and connections. In addition, as used herein, the terms “axial” and “axially” generally mean along or parallel to a given axis (e.g., central axis of a body or a port), while the terms “radial” and “radially” generally mean perpendicular to the given axis. For instance, an axial distance refers to a distance measured along or parallel to the axis, and a radial distance means a distance measured perpendicular to the axis. Further, when used herein (including in the claims), the word “about,” “generally,” “substantially,” “approximately,” and the like mean within a range of plus or minus 10%. 
     As previously described, during a seismic survey, seismic sensors are used to detect reflected seismic waves to reveal the structure and composition of the subsurface. One type seismic sensor relies on capacitance to generate the electrical signal. With one example, these sensors can be constructed as Microelectromechanical systems (MEMS) using micro machined silicon with metal plating applied to facing components on opposite sides of a small plated and spring loaded mass. These MEMS sensors often have the advantage of small size and weight compared to a moving coil geophone. The movement of the MEMS proof mass relative to the outer fixed plates creates variable capacitance that is detected as a signal proportional to the acceleration of the sensor displacement. During operations, electromagnetic interference from other electromagnetic components both within and around the seismic sensor may interfere with the MEMS sensor&#39;s ability to produce quality data. 
     Accordingly, embodiments disclosed herein include seismic sensors, particular MEMS type seismic sensors that include enclosed sensor assemblies for providing electromagnetic shielding for the sensor element(s) housed therein. In at least some embodiments, the enclosed and shielded sensor assemblies are insulated (partially or totally) from any surrounding electromagnetic interference, so that the data quality may be improved (e.g., from that captured by a non-shielded sensor assembly). In addition, at least some embodiments of the sensor assemblies disclosed herein include an electrically insulating holder, which may allow the sensor element(s) within the sensor assembly to be separated (and thus insulated) from an electrically conductive outer housing (or shielding) of the sensor assembly. Further, without being limited to this or any other theory, embodiments of the sensor assemblies disclosed herein may be easier to manufacture and assemble than other sensor assembly designs. 
     Referring now to  FIG. 1 , a schematic representation of a seismic surveying system  50  for surveying a subsurface earthen formation  51  is shown. As shown in  FIG. 1 , the subsurface  51  has a relatively uniform composition with the exception of layer  52 , which may be, for example, a different type of rock as compared to the remainder of subsurface  51 . As a result, layer  52  may have a different density, elastic velocity, etc. as compared to the remainder of subsurface  51 . 
     Surveying system  50  includes a seismic source  54  disposed on the surface  56  of the earth and a plurality of seismic sensors  64 ,  66 ,  68  firmly coupled to the surface  56 . 
     The seismic source  54  generates and outputs controlled seismic waves  58 ,  60 ,  62  that are directed downward into the subsurface  51  and propagate through the subsurface  51 . In general, seismic source  54  can be any suitable seismic source including, without limitation, explosive seismic sources, vibroseis trucks and accelerated weight drop systems also known as thumper trucks. For example, a thumper truck may strike the surface  56  of the earth with a weight or “hammer” creating a shock which propagates through the subsurface  51  as seismic waves. 
     Due to the differences in the density and/or elastic velocity of layer  52  as compared to the remainder of subsurface  51 , the seismic waves  58 ,  60 ,  62  are reflected, at least partially, from the surface of the layer  52 . The reflected seismic waves  58 ′,  60 ′,  62 ′ propagate upwards from layer  52  to the surface  56  where they are detected by seismic sensors  64 ,  66 ,  68 . 
     The seismic source  54  may also induce surface interface waves  57  that generally travel along the surface  56  with relatively slow velocities, and are detected concurrently with the deeper reflected seismic waves  58 ′,  60 ′,  62 ′. The surface interface waves  57  generally have a greater amplitude than the reflected seismic waves  58 ′,  60 ′,  62 ′ due to cumulative effects of energy loss during propagation of the reflected seismic waves  58 ′,  60 ′,  62 ′ such as geometrical spreading of the wave front, interface transmission loss, weak reflection coefficient and travel path absorption. The cumulative effect of these losses may amount to a 75 dB, and in cases more than 100 dB, in amplitude difference between various waveforms recorded by sensors  64 ,  66 ,  68 . 
     The sensors  64 ,  66 ,  68  detect the various waves  57 ,  58 ′,  60 ′,  62 ′, and then store and/or transmit data indicative of the detected waves  57 ,  58 ′,  60 ′,  62 ′. This data can be analyzed to determine information about the composition of the subsurface  51  such as the location of layer  52 . 
     Although seismic surveying system  50  is shown and described as a surface or land-based system, embodiments described herein can also be used in connection with seismic surveys in transition zones (e.g., marsh or bog lands, areas of shallow water such as between land and sea) and marine seismic survey systems in which the subsurface of the earthen formation (e.g., subsurface  51 ) is covered by a layer of water. In marine-based systems, the seismic sensors (e.g., seismic sensors  64 ,  66 ,  68 ) may be positioned in or on the seabed, or alternatively on or within the water. In addition, in such marine-based systems, alternative types of seismic sources (e.g., seismic sources  54 ) may be used including, without limitation, air guns and plasma sound sources. 
     Referring now to  FIGS. 2 and 3 , an embodiment of a seismic sensor  100  is shown. In general, seismic sensor  100  can be used in any seismic survey system. For example, sensor  100  can be used for any one or more of sensors  64 ,  66 ,  68  of seismic surveying system  50  shown in  FIG. 1  and described above. Although sensor  100  can be used in land or marine seismic survey systems, it is particularly suited to land-based seismic surveys. Generally speaking, seismic sensor  100  may include many similar components to those discussed in U.S. Pat. No. 10,139,506, filed Mar. 12, 2015, which is hereby incorporated by reference in its entirety for all purposes. 
     In this embodiment seismic sensor  100  includes an outer housing  101 , an inductive spool assembly  130  disposed within housing  101 , a carrier  140  disposed in housing  101 , and a sensor assembly  300  disposed within housing  101  and coupled to carrier  140 . Housing  101  has a central or longitudinal axis  105 , a first or upper end  101   a , a second or lower end  101   b , and an inner chamber or cavity  102 . Ends  101   a ,  101   b  are closed and inner cavity  102  is sealed and isolated from the environment surrounding sensor  100 , thereby protecting the sensitive components disposed within housing  101  from the environment (e.g., water, dirt, etc.). In addition, housing  101  includes a generally cup-shaped body  110  and an inverted cup-shaped cap  120  fixably attached to body  110 . 
     Body  110  has a central or longitudinal axis  115  that is coaxially aligned with axis  105 , a first or upper end  110   a , and a second or lower end  110   b  defining lower end  101   b  of housing  101 . In addition, body  110  includes a base  111  at lower end  110   b  and a tubular sleeve  112  extending axially upward from base  111  to upper end  110   a . Base  111  closes sleeve  112  at lower end  110   b ; however, sleeve  112  and body  110  are open at upper end  110   a . As a result, body  110  includes a receptacle  113  extending axially from upper end  110   a  to base  111 . Receptacle  113  forms part of inner cavity  102  of housing  101 . 
     In this embodiment, body  110  of outer housing  101  includes a pair of connectors  118   a ,  118   b . Connector  118   a  is provided on base  111  and connector  118   b  is provided along sleeve  112 . Connector  118   a  includes rectangular throughbore  119   a  extending radially therethrough and a hole  119   b  extending axially from lower end  110   b  to throughbore  119   a . Hole  119   b  is internally threaded and threadably receives the externally threaded end of a spike (not shown) used to secure sensor  100  to the ground during seismic survey operations. Throughbore  119   a  enables a rope or the like (not shown) to be attached to sensor  100  for storage or deployment. In particular, the rope may be folded double and inserted throughbore  119   a . Thus, bore  119   a  has a width of at least twice the diameter of the rope. The loop formed by the portion of folded rope extending through bore  119   a  is then placed around the sensor  100 . In this manner, a plurality of sensors  100  can be coupled to a single rope without side ropes, hooks or other mechanisms that can complicate the handling of multiple sensors  100 . 
     The connector  118   b  is disposed along the outside of sleeve  112  proximal upper end  101   a . In general, connector  118   b  provides an alternative connection point for handling of sensor  100  during deployment and retrieval. In this embodiment, connector  118   b  is an eye connector or throughbore to which a rope, lanyard, hook, carabiner or the like can be releasably attached. Connector  118   b  can also be used in a manner similar to throughbore  119   a , thereby allowing a rope to be folded double and inserted through the hole of connector  118   b . Thus, the bore of connector  118   a  has a width of at least twice the diameter of the rope. The loop formed by the portion of folded rope extending through the bore of connector  118   b  is then placed around the sensor  100 . In this manner, a plurality of sensors  100  can be coupled to a single rope without side ropes, hooks or other mechanisms that can complicate the handling of multiple sensors. In this embodiment, the entire body  110  (including base  111  and sleeve  112 ) is made via injection molding. 
     Referring still to  FIGS. 2 and 3 , cap  120  has a central or longitudinal axis  125  that is coaxially aligned with axis  105 , a first or upper end  120   a  defining upper end  101   a  of housing  101 , and a second or lower end  120   b . In this embodiment, cap  120  has the general shape of an inverted cup. In particular, cap  120  includes a planar cylindrical top  121  at upper end  120   a  and a tubular sleeve  122  extending axially downward from top  121  to lower end  120   b . Top  121  closes sleeve  122  at upper end  120   a ; however, sleeve  122  and cap  120  are open at lower end  120   b . As a result, cap  120  includes an inner chamber or cavity  123  extending axially from lower end  120   b  to top  121 . An annular flange  126  extends radially outward from sleeve  122  proximal lower end  120   b.    
     Cap  120  is fixably attached to body  110  such that cap  120  is coaxially aligned with body  110  and such that lower end  120   b  of cap  120  seated within upper end  110   a  of body  110  and upper end  110   a  of body  110  coupled to flange  126 . Body  110  and cap  120  are sized such that an interference fit is provided between lower end  120   b  of cap  120  and upper end  110   a  of body  110 . In this embodiment, body  110  and cap  120  are made of the same material (polycarbonate), and thus, are can be ultrasonically welded together to fixably secure cap  120  to body  110 . More specifically, as shown in  FIG. 3  an annular ultrasonic weld W 110-120  is formed between the opposed radially outer surface and radially inner surface of sleeves  122 ,  112 , respectively, at ends  120   b ,  110   a . Weld W 110-120  defines an annular seal between cap  120  and body  110  that prevents (or at least restricts) fluid communication between cavities  113 ,  123  and the environment surrounding sensor  100 . 
     Referring still to  FIGS. 2 and 3 , a power source or supply  190  and electronic circuitry  195  are removably mounted to carrier  140  within housing  101 , particularly within cavity  113  of body  110 . In this embodiment, power supply  190  is a battery and electronic circuitry  195  is in the form of a circuit board (e.g., PCB). Thus, power supply  190  may also be referred to as battery  190  and electronic circuitry  195  may also be referred to as circuit board  195 . Electronic circuitry  195  is fixably mounted to carrier  140  within housing  101 . In addition, a battery  190  is movably disposed within housing  101  such that battery  190  is configured to move axially relative to housing  101  (with respect to axis  105  described below), carrier  140 , and circuitry  195  during operations. Generally speaking, battery  190  includes a first or upper end  190   a  and a second or lower end  190   b , opposite upper end  190   a . When battery  190  is inserted within cavity  102  of housing  101 , upper end  190   a  of battery  190  is more proximate upper end  101   a  than lower end  101   b  and lower end  190   b  of battery  190  is more proximate lower end  101   b  than upper end  101   a.    
     Inductive spool assembly  130  is used to inductively charge the battery  190  from the outside of sensor  100  (e.g., wirelessly). In this embodiment, spool assembly  130  is mounted within cavity  123  of cap  120  and includes a cylindrical sleeve-shaped body  131  and a coil  136  wound around body  131 . Coil  136  is electrically coupled to circuit board  195  with wires or other suitable conductive paths (not shown) that enable the transfer of current to circuit board  195 , which in turn charges battery  190  during charging operations. 
     Referring still to  FIGS. 2 and 3 , in this embodiment, carrier  140  supports circuit board  195  and a light guide  128  within cavity  102  of outer housing  110 . In this embodiment, carrier  140 , circuit board  195 , and light guide  128  are fixably coupled to outer housing  101  and do not move relative to outer housing  110 , however, battery  190  is movably coupled to carrier  140 , and thus, battery  190  (which may be referred to herein as a “proof mass” for seismic sensor  100 ) can move axially relative to carrier  140 , circuit board  195 , light guide  128 , and outer housing  101 . 
     Carrier  140  has a central or longitudinal axis  145  coaxially aligned with axis  105 , a first or upper end  140   a  extending through inductive spool assembly  130 , and a second or lower end  140   b  axially adjacent base  111 . Carrier  140  has an axial length that is substantially the same as the axial length of cavity  102 . Thus, upper end  140   a  engages top  121  of cap  120  and lower end  140   b  is coupled to sensor assembly  300  which in turn is supported by base  111  of body  110 . More specifically, carrier  140  is axially compressed between cap  120  and body  110 . As a result, movement of carrier  140  relative to outer housing  101  is generally restricted (or prevented entirely) during operations, so that carrier  140  is fixably secured or mounted within housing  101 . 
     Referring still to  FIGS. 2 and 3 , carrier  140  includes an axially extending internal recess or pocket  144 . Pocket  144  is defined by an upper end surface  149 , a lower end surface  147 , and a cylindrical surface  148  extending axially between end surfaces  149 ,  147 . Battery  190  is disposed within pocket  144  but does not contact carrier  140 . In particular, the dimensions of pocket  144  are greater than the dimensions of battery  190  (e.g., the radius of surface  148  is greater than the outer radius of battery  190 , and the axial distance between end surfaces  149 ,  147  is greater than the length of battery  190  between ends  190   a ,  190   b ). In this embodiment, battery  190  is oriented parallel to but is slightly radially offset from aligned axes  105 ,  145 . In particular, the central axis (not shown) of battery  190  is radially offset from axes  105 ,  145  by about 1.0 to 1.5 mm. 
     Referring specifically now to  FIG. 3 , carrier  140  also includes a projection  146  that extends generally radially within pocket  144 , and that is axially positioned between upper end  190   a  of battery  190  and upper surface  149 . In addition, carrier  140  includes a first or upper annular recess  150 , and second or lower annular recess  151 . Upper annular recess  150  extends radially outward from cylindrical surface  148  of pocket  144  within carrier  140  proximate upper end  110   a  of body  110  but axially below projection  146 , and lower annular recess  151  extends radially outward from cylindrical surface  148  of pocket  144  proximate base  111 . Further, carrier  140  includes a throughbore  142  extending through lower surface  147  of pocket  144  in a direction that is generally parallel to aligned axes  105 ,  145 . 
     Referring still to  FIG. 3 , elongate curved L-shaped light guide  128  is fixably secured to carrier  140  generally axially above pocket  144  within cavity  123  of cap  120 . In this embodiment, light guide  128  is integral with and monolithically formed with carrier  140 . Light guide  128  is generally “L” shaped, and thus includes a first end  128   a , a second end  128   b  and a 90° curve or corner  129  between ends  128   a ,  128   b . As will be described in more detail below, light guide  128  wirelessly communicates data to/from circuit board  195  through top  121 . To facilitate the transmission of light, light guide  128  and top  121  are made of a clear material. In this embodiment, the entire cap  120  (including top  121  and sleeve  122 ) and guide  128  are made of a clear polycarbonate. 
     Referring now to  FIGS. 3 and 4 , battery  190  has a cylindrical shape and is coupled to circuit board  195  with a pair of tabs  200 . In particular, tabs  200  are disposed at the ends  190   a ,  190   b  of battery  190  and are spring loaded to axially compress battery  190  therebetween (e.g., with respect to aligned axes  105 ,  145 ). In this embodiment, tabs  200  are made of metal (e.g., steel, such as spring steel), and provide both a physical and electrical connection between battery  190  and circuit board  195 . Thus, tabs  200  enable battery  190  to provide power to circuit board  195  and the various functions performed by the components of board  195  during seismic survey operations, and enable board  195  to provide power to battery  190  during inductive charging operations. 
     In this embodiment, each tab  200  is a resilient, semi-rigid element through which battery  190  is supported within pocket  144  of carrier  140 . As best shown in  FIG. 4 , each tab  200  comprises a resilient disc  201 , a plurality of prongs  202  extending radially from disc  201 , and a connector  203  extending radially from disc  201  (e.g., with respect to axis  105  previously described). Connector  203  includes an axially extending raised bump or projection  203   a  (e.g., axially with respect to axis  105  previously described). As best shown in  FIG. 4 , disc  201  has a semi-cylindrical shape including a straight edge  201   a  and a semi-circular edge  201   b  extending from straight edge  201   a . Prongs  202  extend from straight edge  201   a  and connector  203  extends from semi-circular edge  201   b  opposite prongs  202 . 
     For purposes of clarity and further explanation, the tab  200  coupled to upper end  190   a  of battery  190  may be referred to as the upper tab  200   a  and the tab  200  coupled to lower end  190   b  of battery  190  may be referred to as the lower tab  200   b . Generic references herein to “tabs  200 ” refer to both the upper tab  200   a  and lower tab  200   b . The semi-circular edge  201   b  of upper tab  200   a  is seated in upper recess  150  of carrier  140 , and the semi-circular edge  201   b  of lower tab  200   b  is seated in lower recess  151  of carrier  140 . As best shown in  FIG. 3 , projection  203   a  of connector  203  in upper tab  200   a  is seated within upper recess  150 , and projection  203   a  of connector  203  of lower tab  200   b  is seated in lower recess  151 . The positioning of edges  201   b  and connectors  203  in recesses  250 ,  251  maintains the outer periphery of tabs  200  generally static or fixed relative to carrier  140  and outer housing  101 . In this embodiment, prongs  202  of tabs  200  extend through circuit board  195  and are soldered thereto. 
     Referring still to  FIGS. 3 and 4 , upper tab  200   a  includes a central projection  208  and a plurality of uniformly circumferentially-spaced through cuts or slots  207  radially positioned between projection  208  and edges  201   a ,  201   b . Upper tab  200   a  is oriented such that central projection  208  faces and extends toward upper end  190   a  of battery  190  in an axial direction (e.g., axially with respect to aligned axes  105 ,  145 ). In addition, projection  208  forms or defines a receptacle or recess  206  on an opposing side of upper tab  200   a  (e.g., a side of upper tab  200   a  that faces axially away from upper end  190   a  of battery  190 ). Projection  208  is fixably coupled to the upper end  190   a  of battery  190 . In particular, in this embodiment terminal wall  206   b  of projection  208  is spot welded to the upper end  190   a  of battery  190 . 
     Lower tab  200   b  does not include a projection  208  and recess  206  as described above for upper tab  200   a  and instead includes a cylindrical post  163  extending axially therefrom (see  FIG. 3 ). As best shown in  FIG. 3 , cylindrical post  163  extends axially away from lower end  190   b  of battery  190  and through throughbore  142  when lower tab  200   b  is installed within cavity  102  as described above. As will be described in more detail below, post  163  can freely move axially within throughbore  142  as outer housing  101  and carrier  140  axially reciprocate relative to battery  190  during operations. 
     Referring still to  FIGS. 3 and 4 , each slot  207  within tabs  200  extends axially through the corresponding tab  200 . In addition, each slot  207  spirals radially outward moving from a radially inner end proximal central projection to edges  201   a ,  201   b . In this embodiment, four slots  207  are provided, each pair of circumferentially adjacent inner ends of slots  207  are angularly spaced 90° apart about axis  145 , each pair of circumferentially adjacent outer ends of slots  207  are angularly spaced 90° apart about axis  145 , and each slot  207  extends along a spiral angle measured about axis  145  between its ends of about 360°. The radially inner ends of slots  207  on upper tab  200   a  are radially adjacent projection  208 , and the radially inner ends of slots  207  on lower tab  200   b  are radially adjacent post  163 . 
     As previously described, tabs  200  provide electrical couplings between battery  190  and circuit board  195 . In addition, tabs  200  function like flexures or biasing members for suspending battery  190  within pocket  144 . Accordingly, tabs  200  may also be referred to as flexures or biasing members. In particular, tabs  200  are resilient flexible elements that flex and elastically deform in response to relative axial movement of outer housing  101  and carrier  140  relative to battery  190 . In addition, tabs  200  radially bias battery  190  to a central or concentric position within pocket  144  radially spaced from carrier  140 . In particular, the presence of spiral slots  207  enhances the flexibility of tab  200  in the region along which slots  207  are disposed, thereby allowing that region to flex in the axial direction (up and down) with relative ease. Spiral slots  207  also enhance the flexibility of each tab  200  in the radial direction. However, spiral slots  207  may generally resist some flexing of tabs  200  in the radial direction. Due to the relatively high degree of flexibility of tabs  200  in the axial direction, when an axial load is applied to tabs  200  by carrier  140  or battery  190 , slots  207  generally allow free relative axial movement between central projection  208  and edges  201   a ,  201   b  on upper tab  200   a  and free relative axial movement between post  163  and edges  201   a ,  201   b  on lower tab  200   b . However, due to the more limited flexibility in the radial direction, when a radial load is applied to tabs  200  by carrier  140  or battery  190 , slots  207  may generally resist relative some (but not necessarily all) radial movement between the central projection  208  and edges  291   a ,  291   b  of upper tab  200   a  and between post  163  and edges  201   a ,  201   b  of lower tab  200   b . Thus, tabs  200  bias battery  190  and carrier  140  back into substantial coaxial alignment with axes  105 ,  145  (but with the radial offset of battery  190  as previously described above). 
     Referring still to  FIGS. 3 and 4 , a biasing member  250  is installed within pocket  144  of carrier  140  and is engaged with upper tab  200   a . Generally speaking, biasing member  250  is a flat spring that includes a first end  250   a , a second end  250   b , and a body  252  extending between ends  250   a ,  250   b . Body  252  includes a first or fixed portion  253  and a second or free portion  254 . Fixed portion  253  extends from first end  250   a , and free portion  254  extends from fixed portion  253  to second end  250   b . A projection  260  is mounted to free portion  254  of biasing member  250 , proximate second end  250   b.    
     As shown in  FIG. 3 , fixed portion  253  is disposed about projection  146  in receptacle  144  such that free portion  254  and projection  260  extend generally axially from projection  146  toward upper tab  200   a  and battery  190 . In particular, projection  260  is biased into recess  206  by body  252  to further axially compress battery  190  between tabs  200  and biasing member  250  and to promote alignment between projection  260 , upper tab  200   a , and battery  190  in a direction that is parallel to and radially offset from aligned axes  105 ,  145 . Thus, the engagement between projection  260  and recess  206  may further bias battery  190  toward a central position within pocket  144  in the radial direction with respect to aligned axes  105 ,  145 . 
     Referring now to  FIGS. 5-8 , sensor assembly  300  has a central or longitudinal axis  305  that is coaxially aligned with axis  105  (see e.g.,  FIG. 3 ). In addition, sensor assembly  300  generally includes a cup  320 , a holder  340 , a sensor element  360 , and a cover plate  380  all arranged or stacked along axis  305 . As will be described in more detail below, cup  320 , holder  340 , and cover plate  380  function to insulate sensor element  360  from electromagnetic interference that may be present within cavity  102  of sensor  100 . Each of the specific components of sensor assembly  300  will now be described in more detail below. 
     Referring still to  FIGS. 5 and 6 , cup  320  includes a first end  320   a , and a second end  320   b  opposite first end  320   a . In addition, cup  320  includes a planar base plate  322  disposed at second end  320   b  that is oriented radially relative to axis  305 , and a plurality of wall segments  324  that extend axially from an outer periphery  323  of base plate  322  to first end  320   a . Together, base plate  322  and wall segments  324  form a recess  325  that extends axially from first end  320   a  to plate  322 . As will be described in more detail below, recess  325  receives the other components of sensor assembly  300  (e.g., holder  340 , sensor element  360 , cover plate  380 , etc.) during operations. 
     In addition, a plurality of holes or apertures extend into recess  325  through the base plate  322  and wall segments  324 . In particular, cup  320  includes a plurality of first apertures  326 , a plurality of second apertures  327 , and a plurality of third apertures  328 . 
     The plurality of first apertures  326  each extend through corresponding ones of the wall segments  322  and through a portion of base plate  322  at the outer periphery  323 . In this embodiment, the first apertures  326  are uniformly angularly spaced about axis  305 . More specifically, there are a total of three first apertures  326  spaced approximately 120° apart from one another about axis  305 . The plurality of second apertures  327  each extend through corresponding ones of the wall segments  324  at the intersection of the corresponding wall segments  324  and the periphery  323  of base plate  322 . In this embodiment, second apertures  328  are uniformly angularly spaced about axis  305 . More specifically, there are a total of two second apertures  327  disposed radially opposite one another (i.e., disposed approximately 180° from one another) about axis  305 . The plurality of third apertures  328  each extend through corresponding ones of the wall segments  324  at points that are axially spaced from periphery  323 . In this embodiment, third apertures  328  are uniformly angularly spaced about axis  305 . More specifically, there are a total of three third apertures  328  spaced approximately 120° apart from one another about axis  305 . 
     Cup  320  comprises a conductive material (e.g., a metal). In some specific embodiments, cup  320  may comprise, for example, steel, aluminum, copper, etc. As will be described in more detail below, cup  320  is configured to conduct electrical current and/or interference away from sensor element  360  during operations so as to improve operations thereof. In addition, in this embodiment, wall segments  324  are monolithically formed with base plate  322 , and thus each comprises the same material. However, it should be appreciated that in other embodiments, wall segments  324  and base plate  322  may be formed as separate bodies or members that are connected or coupled together. 
     Referring still to  FIGS. 5-8 , holder  340  includes a first end  340   a , and a second end  340   b  opposite first end  340   a . In addition, holder  340  includes a planar base  342  at second end  340   b  that is oriented radially relative to axis  305 , and an annular wall  344  extending axially from base  342  to first end  340   a . In particular, wall  344  extends axially from an outer periphery  343  of base  322  and includes an upper planar annular shoulder or surface  341  at first end  340   a . Together, base  342  and wall  344  form a recess  345  that extends axially from first end  340   a  to base  342 . As will be described in more detail below, recess  345  receives sensor element  360  therein during operations. 
     A pair of projections  350  extend radially outward from wall  344 , and a pair of axially extending recesses  352  extend axially inward to wall  344  from annular surface  341  to projections  350 . In this embodiment, each of the projections  350  and corresponding recesses  352  are radially opposite (i.e., disposed approximately 180° from) one another about axis  305 . 
     In addition, holder  340  includes a receptacle  348  disposed along wall  344 . In particular, receptacle  348  extends axially upward from annular wall  341  and includes a pair of slots  347 . As will be described in more detail below, slots  347  are configured to receive corresponding electrical connectors therein for electrically connecting sensor element  360  to circuit board  195  (see  FIG. 3 ) during operations. Further, as is best shown in  FIG. 6 , holder  340  also includes a retention recess  349  that extends radially outward and into wall  344  at receptacle  348 . 
     As is best shown in  FIG. 5 , a plurality of apertures  354  each extend into recess  345  through wall  344  and through a portion of base plate  342  at the outer periphery  343 . In this embodiment, the apertures  354  are uniformly angularly spaced about axis  305 . More specifically, there are a total of three apertures  354  spaced approximately 120° apart from one another about axis  305 . 
     Holder  340  comprises an electrically insulating material (e.g., a polymer, a composite (e.g., fiberglass), etc.). As will be described in more detail below, holder  340  is configured to electrically insulate sensor element  360  from other conductive portions of sensor assembly  300  during operations (e.g., cup  320 , cover plate  380 , etc.). 
     Referring still to  FIGS. 5-8 , sensor element  360  comprises a disc or substrate  362  that includes an outer periphery  363 . In addition, sensor element  360  includes a piezoelectric element  364  comprising one or more layers of a rigid piezoelectric ceramic material disposed on a substrate which forms the outer periphery  363 . In some embodiments, the piezoelectric ceramic material comprises lead zirconate titanate (PZT) which is regarded as low cost and relatively strong. Disc  362  may be electrically conductive and may comprise beryllium copper or brass in some embodiments. The one or more layers of piezoelectric ceramic material of element  364  may be bonded to (and potentially disposed between) one or more layers of the substrate to provide a substantially flat member. During operations, the sensor element  360  may have a sufficient elastic compliance so as to support the proof mass (e.g., battery  190 ) of sensor  100  without fracturing. In addition, the sensor element  360  (including the one or more layers of piezoelectric ceramic material and substrate) may have a bending stiffness which is greater than the piezoelectric ceramic material alone. In some embodiments, the sensitivity and resonance peak frequency of the sensor element  360  may be set based on various factors (e.g., the diameter and thickness of sensor element  360 —particularly of the piezoelectric ceramic material, the ratio of Titanium to zirconium in the piezoelectric ceramic material, etc.). 
     Cover plate  380  includes a planar base plate  382  that is oriented radially relative to axis  305  and that includes an outer periphery  383 . A recess or notch  385  extends radially inward from periphery  383 . In addition, an aperture  384  extends through plate  382  at a position that is spaced from periphery  383  but is radially shifted slightly from axis  305 . 
     A plurality of tabs or projections extend generally axially from periphery  383  of base plate  382 . In particular, base plate  382  includes a plurality of first tabs  387 , a plurality of second tabs  388 , and a plurality of third tabs  389 . The plurality of first tabs  387  each extending from periphery  383  of base plate  382 . In this embodiment, the first tabs  387  are uniformly angularly spaced about axis  305 . More specifically, there are a total of three first tabs  387  spaced approximately 120° apart from one another about axis  305 . The plurality of second tabs  387  each also extend from the periphery  383  of base plate  382 . In this embodiment, there are two second tabs  388  disposed on opposing angular sides of recess  385 . In particular, each second tab  388  is disposed angularly between recess  385  and corresponding ones of the first tabs  387 . The plurality of third tabs  389  each also extend from the periphery  383  of base plate  382 . In this embodiment, there are two third tabs  389  disposed along periphery  383  generally on a side of cover plate  380  that is radially opposite recess  385 . In addition, a first connector tab  386  extends axially upward from a wall or border of notch  385 . 
     Cover plate  380  comprises a conductive material, such as, for example, a metal. In some specific embodiments, cover plate  380  may comprise, for example, steel, aluminum, copper, etc. In addition, in some embodiments, cover plate  380  may comprise the same conductive material as cup  320 ; however, this may not be the case in other embodiments. As will be described in more detail below, cover plate  380  is configured to conduct electrical current and/or interference away from sensor element  360  and into cup  320  during operations so as to improve operations thereof. 
     Referring now to  FIGS. 5 and 6 , to construct sensor assembly  300 , holder  340  is inserted axially within recess  325  of cup  320  until second end  340   b  engages with base plate  322 . In addition, annular wall  344  slidingly engages with wall segments  324  of cup  320  as holder  340  is inserted axially within recess  325 . Further, when seated within recess  325 , holder  340  is angularly aligned with cup  320  about axis  305  such that the plurality of apertures  354  on holder  340  are angularly aligned with the plurality of first apertures  326  in cup  320 . Referring briefly again to  FIG. 8 , the aligned apertures  354 ,  326  provide open path ways into recess  345  of holder  340  from second end  320   b  of cup  320 . 
     Referring again to  FIGS. 5 and 6 , when holder  340  is inserted axially within recess  325  of cup  320 , projections are aligned with and are inserted within the second apertures  327  in cup  320 . In this embodiment, the insertion of holder  340  within recess  325  of cup  320  causes sliding engagement between projections  350  and the wall segments  324  carrying second apertures  327 , such that these wall segments  324  are deformed radially outward from axis  305 . Once projections  350  axially align with apertures  327 , the corresponding wall segments  324  rotate radially inward so as to fully engage projections  350  through apertures  327 . As a result, once holder  340  is installed within recess  325  of cup  320  in this manner, the engagement between projections  350  and second apertures  327  prevents accidental withdrawal or rotation of holder  340  from or within, respectively, recess  320 . 
     Either before or after holder  340  is inserted within recess  325  of cup  320 , sensor element  360  is installed within recess  325 . In particular, sensor element  360  is inserted within recess  325  such that periphery  363  is inserted radially into retention recess  349  of holder  340 . Thereafter, a pair of electrical leads or connectors  370 ,  371  are installed within slots  347  of receptacle  348  to provide electrical connection with sensor element  360  and other components within sensor  100  (e.g., circuit board  195 ). 
     In particular, the pair of electrical connectors  370 ,  371  comprises a first connector  370  and a second connector  371 . First connector  370  includes a central body  372 , a first conductive lead  374  extending from body  372 , and a second conductive lead  376  extending from body  372 . Similarly, second connector  371  includes a central body  373 , a first conductive lead  375  extending from body  373 , and a second conductive lead  377  extending from body  373 . In this embodiment, the body  372  and leads  374 ,  376  of first connector  370  are all made of electrically conductive materials (e.g., a metal), and may, in some embodiments, be formed of the same material (e.g., such that body  372  and leads  374 ,  376  of first connector  370  are monolithically formed). Similarly, in this embodiment, body  373  and leads  375 ,  377  of second connector  371  are also all made of electrically conductive materials, and may, in some embodiments, be formed of the same material (e.g., such that body  373  and leads  375 ,  377  of second connector  371  are monolithically formed). As best shown in  FIG. 6 , body  372  of first connector  370  is installed within one of the slots  374  of receptacle  348  such that second lead  376  engages with metallic disc  362  of sensor element  360  and first lead  374  extends axially (generally) from first end  340   a  of holder  340  via the corresponding slot  347 . In addition, body  373  of second connector  371  is installed within the other of the slots  374  of receptacle  348  such that second lead  377  engages with piezoelectric element  364  of sensor element  360  and first lead  375  extends axially (generally) from first end  340   a  of holder  340  via the corresponding slot  347 . In some embodiments, second leads  376 ,  377  (or some portion thereof) may be soldered or otherwise conductively secured to disc  362  and element  364 , respectively, of sensor element  360 ; however, such connection is not required. In addition, in this embodiment, each of the leads  376 ,  377  of connectors  370 ,  371 , respectively are biased into engagement with sensor element  360  (specifically disc  362  and piezoelectric element  364 , respectively). In one or more embodiments, first connector  370  is a separate piece from second connector  371 . First connector  370  is connected to disc  362 . Second connector  371  is connected to element  364  via second lead  377 . By configuring first connector  370  to the outer edge of disc  362 , the quality-of-response of piezoelectric element  364  of sensor element  360  can be improved. Second connector  371  (as shown in  FIG. 6 ) can include a bend in order to increase the length of second connector  371 . This increase in length can increase the flexibility of second connector  371 , in order to generate less counter-force when the disc  362  is deformed, and thus increasing the flexibility of second connector  371  can exhibit improved sensor performance. 
     Referring now to  FIGS. 5-7 , once sensor element  360  and connectors  370 ,  371  are installed within holder  340  as described above, cover plate  380  is also inserted within recess  325  of cup  320  until base plate  382  engages with annular surface  341  on holder  340 . In particular, cover plate  380  is angularly aligned with holder  340  and cup  320  so that notch  385  is angularly aligned with receptacle  348  and first tabs  387  are angularly aligned with the wall segments  324  of cup  320  that carry third apertures  328 . Still more specifically, as cover plate  380  is axially advanced within recess  325 , first tabs  387  slidingly engage with the radially inner surface of the wall segments  324  carrying third apertures  328 . The engagement between first tabs  387  and these corresponding wall segments  324  causes the wall segments  324  to deflect radially outward or away from axis  305  until first tabs  387  are aligned with third apertures  387 . Thereafter, tabs  387  are received through apertures  328  so that the corresponding wall segments  324  rotate radially inward toward axis  305  thereby securing cover plate  380  to cup  320 . In addition, when cover plate  380  is installed within recess  325  of cup  320  as described above, second tabs  388  and third tabs  389  and periphery  383  are engaged with corresponding ones of the wall segments  324 . Thus, once installed within assembly  300 , cover plate  380  is electrically coupled to cup  320  via contact between tabs  387 ,  388 ,  389  and periphery  383  of cover plate  380  and wall segments  324  of cup  320 . 
     Accordingly, when sensor assembly  300  is fully constructed as described above, cover plate  380  and cup  320  form an outer housing  310  (or sensor housing  310 ) that receives sensor element  360  and holder  340  therein. In addition, sensor element  360  is installed within housing  310  such that no contact is formed between sensor element  360  (or connectors  370 ,  371 ) and either cover plate  380  and cup  320 . Rather, sensor element  360  and connectors  370 ,  371  are in contact with holder  340  within housing  310 . Because cover plate  380  and cup  320  are constructed from electrically conductive materials, the housing  310  forms an electrically conductive shell around sensor element  360  that protects or shields sensor element  360  from electromagnetic interference generated outside of housing  310 . In other words, housing  310  forms a so-called “Faraday cage” about sensor element  360 . 
     Referring now to  FIGS. 3 and 9 , once constructed, sensor assembly  300  is secured to the lower end  140   b  of carrier  140  so that together, inductive spool assembly  130 , carrier  140 , circuit board  195 , sensor assembly  300 , battery  190 , and biasing members  200 ,  180  may be installed within outer housing  101 . In particular, as shown in  FIG. 10 , the third tabs  389  on cover plate  380  engage with corresponding recesses  143  formed on an outer surface of carrier  140  so that sensor assembly  300  is secured to lower end  140   b  of carrier  140  as described above. In particular lower end  140   b  of carrier  140  is engaged with base plate  382  of cover plate  380 . 
     Referring again to  FIG. 9 , when sensor assembly  300  is mounted to lower end  140   b  of carrier  140  as described above, leads  374 ,  375  of connectors  370 ,  371 , respectively (see e.g.,  FIG. 6 ) are engaged with corresponding portions or components on circuit board  195  (e.g., such as solder pads on circuit board  195 ). In this embodiment, the leads  374 ,  375  may also be soldered to circuit board  195 ; however, in other embodiments, leads  374 ,  375  simply contact circuit board  195  during operations. Thus, once leads  374 ,  375  are engaged (and soldered in this embodiment) to circuit board  195 , sensor element  360  is electrically coupled to circuit board  195 . In addition, when sensor assembly  300  is engaged with lower end  140   b  of carrier  140 , connector tab  386  of cover plate  380  is also engaged with (and possibly soldered to) a corresponding portion or component on circuit board  195  (e.g., such as a soldering pad as described above for leads  374 ,  375 ). In this embodiment, the connector tab  386  provides a grounding contact for housing  310  that is electrically coupled to the ground of circuit board  195 . Moreover, in some embodiments, all (or some) of the leads  374 ,  375 ,  386  are biased into engagement with circuit board  195  (or solder pads disposed on circuit board  195 ). 
     Thus, the sensor assembly  300  may be assembled and electrically coupled to other components within seismic sensor  100  (e.g., circuit board  195 ) with relative ease (e.g., a technician does not need to route additional wiring between sensor assembly  300  and circuit board  195  after sensor assembly  300  is attached to lower end  140   b  of carrier  140 ). Rather, the arrangement and design of connectors  370 ,  371 , and connector tab  386  may provide a predetermined alignment to the appropriate locations or contacts on circuit board  195 , so that the mechanical attachment of sensor assembly  300  to lower end  140   b  of carrier  140  as described above also facilitates the above described electrical connections. 
     Referring to  FIG. 11 , when sensor assembly  300  is mounted to lower end  140   b  of carrier  140  as described above, post  163  is inserted through aperture  384  in cover plate  380  so that distal end  163   b  of post  163  engages with piezoelectric element  386  of sensor element  360 . Thus, as post  163  moves axially within throughbore  142  and aperture  384 , distal end  163   b  transfers forces and pressure to sensor element  360  so that element  360  (particularly piezoelectric element  364 ) begins to generate electrical signals that are indicative of the vibrations transferred to sensor  100  during operations as described in more detail below. 
     Referring now to  FIGS. 3 and 11 , after sensor assembly  300  is secured to lower end  140   b  of carrier  140 , both are inserted within cavity  102  of housing  101  such that lower end  320   b  of cup  320  (particularly base plate  322 ) is engaged with base  111 . In addition, as best shown in  FIG. 11 , when lower end  320   b  of cup  320  in sensor assembly  300  is seated against base  111 , a plurality of internal projections  114  formed within body  110  of housing  101  are inserted through the aligned apertures  326 ,  354  and engage directly with sensor element  360  (particularly with metallic disc  362 ). Thus, once carrier  140  and sensor assembly  300  are installed within cavity  102  of housing  101 , sensor element  360  is axially and radially suspended within recess  345  and does not directly contact holder  340 . Rather, sensor element  360  is in direct contact with housing  101  via projections  114 . While not specifically shown in  FIG. 11 , in this embodiment, there are a total of three projections  114  that are generally aligned with the aligned apertures  326 ,  354  of sensor assembly  300 . 
     Referring now to  FIGS. 2, 3, and 11 , during seismic surveys, a plurality of sensors  100  are coupled to the surface of the earth (e.g., in place of sensors  64 ,  66 ,  68  in system  50  shown in  FIG. 1 ). Each sensor  100  may, for example, be attached to a spike which is pushed into the earth. Alternatively, the entire sensor  100  may be buried, or placed at depth in a borehole. Regardless of how sensors  100  are coupled to the earth, each sensor  100  is preferably positioned with axis  105  oriented in a generally vertical direction (e.g., aligned with the force of gravity). 
     The arrival of a compressional seismic wave causes outer housing  101  and the components fixably coupled thereto (e.g., spool assembly  130 , carrier  140 , circuit board  195 , light guide  129 , etc.) to move in a generally vertical direction. The inertia of the proof mass (which in this embodiment comprises battery  190  as previously described above) within outer housing  101  causes the proof mass to resist moving with the displacement of the outer housing  101  and carrier  140 , and consequently the outer housing  101  and carrier  140  reciprocate axially relative to the proof mass, as permitted by tabs  200  and biasing member  250 . This movement causes tabs  200  and free portion  254  (including engagement member  260 ) of biasing member  250  to flex or be deflected and the load of the proof mass to be taken up by the sensor element  360 , via post  163 . The axial reciprocation of the outer housing  101  and carrier  140  relative to the proof mass generally continues as the compressional seismic wave passes across sensor  100 . 
     During the axial reciprocations of the outer housing  101  and carrier  140  relative to the proof mass, the sensor element  360  is cyclically deflected by post  163 . As previously described, when mechanical stress is applied to sensor element  360  due to deformation or deflection by post  163 , the piezoelectric ceramic material of piezoelectric element  364  generates an electrical potential (piezoelectric effect). The electrical potential is conducted to circuit board  195  via leads  376 ,  374  of first connector  370  and/or leads  373 ,  373  of second connector  371  (see  FIG. 6 ). The circuit board  195  (or components thereof) may sample and store the conducted electrical potential in memory as a measure of the amplitude of the seismic vibration. Thus, during operations, the sensor element  360  generates a signal that is indicative of the vertical movement of the outer housing  101  relative to the proof mass (e.g., battery  190 ) as induced by the seismic vibration. The data stored in memory on the circuit board  195  can be communicated to an external device for further consideration and analysis (e.g., via light guide  228 , and top  221  as previously described). 
     During these operations, sensor element  360  is shielded from electromagnetic interference by the conductive housing  310  as previously described above. Such electromagnetic interference may be generated by other electronic components within sensor  100  (e.g., battery  190 , inductive spool assembly  130 , circuit board  195 , etc.) or by sources disposed outside of sensor  100  (e.g., other electronic components disposed adjacent to sensor  100  during a seismic survey). Thus, by enclosing sensor element  360  within a conductive housing  310  as described above, the amount of signal noise caused by such electromagnetic interference may be reduced (or eliminated entirely). Accordingly, the quality of seismic signals collected by the seismic sensors disclosed herein (e.g., sensor  100 ) may be improved. 
     While exemplary embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teachings herein. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the systems, apparatus, and processes described herein are possible and are within the scope of the disclosure. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims. Unless expressly stated otherwise, the steps in a method claim may be performed in any order. The recitation of identifiers such as (a), (b), (c) or (1), (2), (3) before steps in a method claim are not intended to and do not specify a particular order to the steps, but rather are used to simplify subsequent reference to such steps.