BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention disclosed herein relates to well logging tools and, in particular, to an improved light source and light detector for well logging tools. 
   2. Description of the Related Art 
   In exploration for hydrocarbons, it is important to male accurate measurements of properties of geologic formations. In particular, it is important to determine the various properties with a high degree of accuracy so that drilling resources are used efficiently. 
   Generally, oil and gas are accessed by drilling a borehole into the subsurface of the earth. The borehole also provides access for talking measurements of the geologic formations. 
   Well logging is a technique used to take measurements of the geologic formations from the boreholes. In one embodiment, a “logging tool” is lowered on the end of a wireline into the borehole. The logging tool sends data via the wireline to the surface for recording. Output from the logging tool comes in various forms and may be referred to as a “log.” Many types of measurements are made to obtain information about the geologic formations. One type of measurement involves determining gravitational acceleration or gravity. 
   Measurements of gravity can be used to determine information related to the mass of a surrounding formation. For example, measurements of gravity can be used to measure depletion of oil in the surrounding formation as water replaces the oil. When water replaces oil in the formation, the mass of the formation and, therefore, a gravitational force exerted by the formation will increase because water is denser than oil. 
   Measurements of gravity can also be used to determine true vertical depth in the borehole. The true vertical depth is important to know because borehole depth is a common factor among various logs. The various logs may be viewed side-by-side to form a composite picture of the geologic formations. Even small errors in determining the borehole depth can corrupt logging data. Horizontal deviations of the borehole, which can corrupt the logging data, can be accounted for by determining the true vertical depth using gravitational measurements. 
   A logging tool used for measuring gravity may employ a gravimeter that relates changes in gravitational acceleration to changes in light. This type of gravimeter requires a light source and a light detector. In order to obtain accurate measurements, it is important for the light source and the light detector to operate in a stable manner in the environment of the borehole. 
   In general, temperature in the borehole increases with increasing depth. In some instances, the temperature can be as high as 260° C. Additionally, the light source and the light detector may be subject to shocks of acceleration while traversing the borehole. To survive the rigors of the borehole environment, the light source and the light detector can be built using solid state technology. For example, the light source may include a laser diode and the light detector may include a photodiode. 
   Accurate measurements usually require that the wavelength of light emitted from the laser diode not shift more than ten ppm. In addition, the intensity of the light emitted from the laser diode should remain constant. With respect to the photodiode, the output should also remain stable for a stable input of light. With increasing temperature, the intensity of the light emitted from a laser diode decreases and the wavelength of the light increases to longer wavelengths. At high enough temperatures, such as temperatures within the borehole, most conventional laser diodes stop working. Providing stable laser light wavelength generally requires that the temperature of the conventional laser diode be maintained to within 0.001° C. Maintaining temperatures with this accuracy can be difficult in the borehole environment. 
   Therefore, what are needed are a light source and a light detector that can operate throughout a range of high temperatures and require less-stringent temperature control. 
   BRIEF SUMMARY OF THE INVENTION 
   Disclosed is an embodiment of an instrument for measuring gravitational acceleration from within a borehole, the instrument including: a light source having a semiconductor that comprises a bandgap greater than about two electron volts (eV); and a gravimeter for receiving light from the light source and providing output light with a characteristic related to the gravitational acceleration, the gravimeter implemented by at least one of a nano electromechanical system (NEMS) and a micro electromechanical system (MEMS); wherein the light source and the gravimeter are disposed in a housing adapted for insertion into the borehole. 
   Also disclosed is one example of a method for measuring gravitational acceleration from within a borehole, the method including: placing a light source and a gravimeter in the borehole, the light source having a semiconductor that has a bandgap greater than about two electron volts (eV), the gravimeter implemented by at least one of a nano electromechanical system (NEMS) and a micro electromechanical system (MEMS); and illuminating the gravimeter with light emitted from the light source wherein the light is used to perform the measuring. 
   Further disclosed is an embodiment of a system for measuring gravitational acceleration from within a borehole, the system including: a logging tool; a light source having a semiconductor that has a bandgap greater than about two electron volts (eV); a gravimeter for diffracting light emitted from the light source, the diffracted light having an intensity related to the gravitational acceleration, the gravimeter implemented by at least one of a nano electromechanical system (NEMS) and a micro electromechanical system (MEMS); a light detector having a semiconductor that has a bandgap greater than about two eV, the light detector for measuring the intensity; and a data collector for providing measurement data to a user. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings, wherein like elements are numbered alike, in which: 
       FIG. 1  illustrates an exemplary embodiment of a logging tool in a borehole penetrating the earth; 
       FIG. 2  illustrates aspects of an exemplary embodiment of a light source, an optical filter and a light detector; 
       FIGS. 3A and 3B , collectively referred to as  FIG. 3 , illustrate an exemplary embodiment of a gravimeter; 
       FIGS. 4A and 4B , collectively referred to as  FIG. 4 , present graphs depicting an effect of the optical filter; 
       FIG. 5  illustrates an exemplary embodiment of a fiber Bragg grating; 
       FIG. 6  illustrates an exemplary embodiment of an optical cavity; 
       FIG. 7  illustrates an exemplary embodiment of a computer coupled to the logging tool; and 
       FIG. 8  presents one example of a method for measuring gravitational acceleration from within the borehole. 
   


   DETAILED DESCRIPTION OF THE INVENTION 
   The teachings provide techniques for measuring gravitational acceleration from within a borehole. In particular, the techniques provide for stable measurements of gravitational acceleration with varying temperatures within the borehole. 
   Gravitational acceleration is measured using a gravitational accelerometer, also referred to as a “gravimeter.” The gravimeter receives light and alters the light in accordance with an amount of gravitational acceleration sensed by the gravimeter. 
   The techniques include a light source for emitting light to the gravimeter. The light has a wavelength and an intensity that are stable over a range of temperatures to which the light source is exposed. In addition, the techniques provide a light detector that generates an output that is stable over a range of temperatures to which the light detector is exposed. Stability of the light source and stability of the light detector are provided by using semiconductors that include wide bandgap materials in conjunction with achievable temperature control. A narrower range of wavelengths, achieved using the wide band gap materials, results in more accurate measurements of gravitational acceleration than would occur if wavelength was allowed to drift by more than 10 ppm. 
   For convenience, certain definitions are provided. The term “gravimeter” relates to a sensor for measuring gravitational acceleration. The sensor receives light from a light source and relates a change in gravitational acceleration to a change in characteristics of light emitted from the sensor. Absolute gravitational acceleration can be measured with the gravimeter by relating the change in gravitational acceleration to a reference calibration point. The term “stable” relates to an output or parameter of a device that does not vary significantly with respect to an application. The term “light source” relates to a device that emits light for use in a sensor. In accordance with the teachings herein, the light source is maintained stable in a downhole environment. The term “light detector” relates to a device that generates an output (referred to as “photocurrent”) in relation to the power of light (referred to as “incident light power”) entering the device. The term “responsivity” refers to the ratio of generated photocurrent to the incident light power. In accordance with the teachings herein, the responsivity of a light detector made with wide bandgap materials is stable over a range of temperatures of interest to a user. The range of temperatures includes those temperatures that may be encountered by the logging tool in the borehole. 
   The term “bandgap” relates to an energy difference between the top of a valence band and the bottom of a conduction band in a semiconductor. Electrons in the conduction band are generally free to move to create an electrical current. Generally, only electrons, which have enough thermal energy to be excited across the bandgap, are available for conduction. 
   The term “housing” relates to a structure of a logging tool. The housing may used to at least one of contain and support a device used with the logging tool. The device may be at least one of the light source, the optical filter, and the light detector. 
   Referring to  FIG. 1 , one embodiment of a well logging tool  10  is shown disposed in a borehole  2 . The logging tool  10  includes a housing  8  adapted for use in the borehole  2 . The borehole  2  is drilled through earth  7  and penetrates formations  4 , which include various formation layers  4 A- 4 E. The logging tool  10  is generally lowered into and withdrawn from the borehole  2  by use of an armored electrical cable  6  or similar conveyance as is known in the art. In the embodiment of  FIG. 1 , an instrument  5  for measuring gravitational acceleration is disposed within the housing  8 . Also depicted in  FIG. 1  is an electronic unit  9 , which receives and processes output data from the instrument  5 . 
   In some embodiments, the borehole  2  includes materials such as would be found in oil exploration, including a mixture of liquids such as water, drilling fluid, mud, oil and formation fluids that are indigenous to the various formations. One skilled in the art will recognize that the various features as may be encountered in a subsurface environment may be referred to as “formations.” Accordingly, it should be considered that while the term “formation” generally refers to geologic formations of interest, that the term “formations,” as used herein, may, in some instances, include any geologic points of interest (such as a survey area). 
   For the purposes of this discussion, it is assumed that the borehole  2  is vertical and that the formations  4  are horizontal. The teachings herein, however, can be applied equally well in deviated or horizontal wells or with the formation layers  4 A- 4 E at any arbitrary angle. The teachings are equally suited for use in logging while drilling (LWD) applications, measurement while drilling (MWD) and in open-borehole and cased-borehole wireline applications. In LWD/MWD applications, the logging tool  10  may be disposed in a drilling collar. When used in LWD/MWD applications, drilling may be halted temporarily to prevent vibrations while the logging tool  10  is used to perform a measurement. 
     FIG. 2  illustrates aspects of an exemplary embodiment of the instrument  5  that includes a light source  20 , an optical filter  22 , a gravimeter  24  and a light detector  26 . Referring to  FIG. 2 , the light source  20  provides emitted light  21  to the optical filter  22 . The optical filter  22  filters the emitted light  21  and provides filtered light  23  to the gravimeter  24 . Generally, the filtered light  23  has a narrower range of wavelengths (and corresponding frequencies) than the emitted light  21 . The gravimeter  24  receives the filtered light  23  and provides output light  25  to the light detector  26 . The light detector  26  is used to measure at least one of intensity, frequency, and angle of the output light  25 . 
   The gravimeter  24  may be built using solid state fabrication techniques to survive the environment of the borehole  2 . Solid state fabrication also results in the gravimeter  24  having dimensions small enough to fit within the housing  8 . In one embodiment, the gravimeter  24  is implemented by at least one of a Nano Electromechanical System (NEMS) and a Micro Electromechanical System (MEMS) as is known to those skilled in the art of NEMS and MEMS. In this embodiment, a proof mass is used to measure gravitational force. The proof mass is coupled to a diffraction grid such that at least one dimension of the diffraction grid changes with displacement of the proof mass. The diffraction grid is used along with the light source  20  and the light detector  26  to act as an interferometric displacement sensor. The filtered light  23  may be diffracted by the diffraction grid to provide the output light  25 . Characteristics of the output light  25  can be measured by the light detector  26  and correlated to the displacement of the proof mass to determine the gravitational force. By knowing the mass of the proof mass and the gravitational force, the gravitational acceleration can be determined. The filtered light  23  with stable characteristics and a narrow range of wavelengths can provide for improved accuracy in gravitational measurements. Similarly, the light detector  26  with stable responsivity can also provide for improved accuracy in the gravitational measurements. 
     FIG. 3  illustrates an exemplary embodiment of the gravimeter  24  that is implemented by at least one of a NEMS and a MEMS. A top view of the gravimeter  24  is depicted in  FIG. 3A . Referring to  FIG. 3A , the gravimeter  24  includes a proof mass  30  coupled to a diffraction grid  31 . The proof mass  30  is suspended by springs  32  coupled to a support substrate  33 . The springs  32  provide a counter-force to the force of gravity while allowing displacement of the proof mass  30  due to the force of gravity. In the embodiment depicted in  FIG. 3A , the proof mass  30 , the diffraction grid  31 , and the springs  32  are implemented by at least one of the NEMS and the MEMS. 
     FIG. 3B  illustrates a side view of the gravimeter  24 .  FIG. 3B  depicts the gravimeter  24  with the light source  20 , the optical filter  22 , and the light detector  26 . The diffraction grid  31 , the light source  20 , the optical filter  22 , and the light detector  26  form an interferometric displacement sensor  34 . Referring to  FIG. 3B , the springs  32  allow movement of the proof mass  30  in substantially vertical direction  35 . As the proof mass  30  moves, at least one dimension defining the diffraction grid  31  changes. In turn, intensity of the output light  25  is related to the at least one dimension. Thus, by measuring the intensity, displacement of the proof mass  30  can be determined. Further, the displacement can be correlated to an amount of gravitational force or gravitational acceleration imposed on the proof mass  30 . 
   Wide band gap materials are used to make the light source  20  and the light detector  26 . The wide bandgap materials provide stability for the light source  20  and the light detector  26  throughout a range of temperatures. Thermally generated electrons and holes can increase noise and change the wavelength (and corresponding frequency) of the emitted light  21 . Similarly, the thermally generated electrons and holes can increase noise and reduce response of the light detector  26 . By using the wide bandgap materials, the number of electrons and holes that are thermally excited to the conduction band is significantly reduced. Reducing the number of thermally excited electrons and holes in the conduction band results in the emitted light  21  having a stable wavelength and the light detector  22  having a stable responsivity. In addition, a reduction of thermally excited electrons and holes in the conduction band results in decreased noise in the light source  20  and the light detector  26 . 
   The color of a light ray or photon corresponds to the wavelength and the associated energy of the photon. For example, blue light has a wavelength of 450 nanometers (nm) and a photon energy of about 2.76 electron volts (eV). The wide bandgap materials are associated with light towards the blue end of the light spectrum. Generally, semiconductors having wide bandgaps emit or respond to photons that have an energy corresponding to the energy of the bandgap. Thus, the light source  20  and the light detector  26  that are associated with light towards the blue end of the light spectrum provide for improved thermal behavior. 
   Many types of the wide bandgap materials may be used to build the light source  20  and the light detector  26 . Examples of the wide bandgap materials include Gallium Phosphide (GaP), Gallium Nitride (GaN), and Silicon Carbide (SiC). With the exception of GaP (550 nm wavelength), these wide bandgap materials emit or respond to light in the ultraviolet range (100 nm-400 nm). 
   Recently developed wide bandgap (e.g. 405 nm) laser diodes can operate at higher temperatures than conventional laser diodes. The wide bandgap laser diodes have less wavelength shift with temperature (0.05 nm/° K) than the conventional laser diodes. A wavelength shift of 0.05 nm/° K corresponds to wavelength stability of 123 ppm/° C. Therefore, the wide band gap laser diodes require temperature maintenance to within about 0.081° C. to achieve 10 ppm wavelength stability, which can be achieved downhole. Intensity of the light emitted from the wide band gap laser diodes can be maintained by adjusting the current through the wide band gap laser diodes. An exemplary embodiment of a wide band gap laser diode for use as the light source  20  is a blue violet laser diode (405 nm) model number DL-3146-151 manufactured by SANYO Electric Company, LTD of Tottori, Japan. 
   The optical filter  22  filters the emitted light  21  to provide the filtered light  23  with a narrow range of wavelengths.  FIG. 4  presents graphs depicting the effect of the optical filter  22  on the emitted light  21 .  FIG. 4A  illustrates an exemplary graph  40  of intensity versus wavelength for the emitted light  21 .  FIG. 4B  illustrates an exemplary graph  41  of intensity versus wavelength for the filtered light  23 . 
   One embodiment of the optical filter  22  is a fiber Bragg grating as shown in  FIG. 5 . The fiber Bragg grating forms an optical waveguide with at least one of periodic and aperiodic perturbations of the effective refractive index of a core of the waveguide. Referring to  FIG. 5 , a fiber Bragg grating  50  includes a cladding  51  and a core  52 . Light is transmitted in the core  52  and reflected from the cladding  51 . The core  52  includes at least one of periodic and aperiodic perturbations  53  (or grating  53 ) of the effective refractive index of the core  52  as depicted in  FIG. 5 . The effect of the grating  53  is that the fiber Bragg grating  50  can reflect a narrow range of wavelengths of light incident on the grating  53 , while passing all other wavelengths of the incident light. The result is that the filtered light  23  depicted in  FIG. 5  has a narrower range of wavelengths than the emitted light  21  incident on the grating  53 . 
   Another embodiment of the optical filter  22  is a Fabry-Perot cavity. For optical channel separation, telecommunications Fabry-Perot cavity filters exist that are stable to within a picometer. Stability to within a picometer for a 405 nm light source corresponds to 2.5 ppm wavelength stability. A fiber Fabry-Perot tunable filter is available from Micron Optics Inc. of Atlanta, Ga. 
     FIG. 6  depicts a Fabry-Perot cavity  60 . Referring to  FIG. 6 , the cavity  60  includes mirrored surfaces  61  and a spacer medium  62  in optical contact with the mirrored surfaces  61 . The emitted light  21  entering the cavity  60  will reflect multiple times from the mirrored surfaces  61 . Only certain wavelengths of the emitted light  21  will be sustained by the cavity  60 . The other wavelengths of the emitted light  21  will be suppressed by destructive interference. The result is that the filtered light  23  depicted in  FIG. 6  has a narrower range of wavelengths than the emitted light  21  incident on the cavity  60 . 
   Generally, the wavelengths of the light sustained by the cavity  60  are determined by a distance, D, between the mirrored surfaces  61  as shown in  FIG. 5 . In some embodiments, the cavity  60  can be built using solid state technology such as that used to fabricate semiconductor devices. Fabricating the Fabry-Perot cavity  60  using solid state technology provides a cavity in which light is reflected many times before dissipating. Thus, a solid state Fabry-Perot cavity  60  is efficient in providing light with a narrow bandwidth. 
   In order to provide the filtered light  23  with little or no variations in the narrow range of wavelengths throughout a range of temperatures, the fiber Bragg grating  50  and the Fabry-Perot cavity  60  may be built using glass having a low coefficient of thermal expansion to achieve 0.01 ppm stability in wavelength. The glass may be referred to as “low expansion glass.” In one embodiment of the fiber Bragg grating  50 , the core  52  is made from low expansion glass. In one embodiment of the Fabry-Perot cavity  60 , the spacer medium  62  is made from low expansion glass. The coefficient of thermal expansion for the low expansion glass used in embodiments of the optical filter  22  can be less than 0.2 ppm/° K over a range of temperatures in the borehole  2 . One example of low expansion glass is ULE® glass manufactured by Coming Specialty Materials of Corning, N.Y. Another example of low expansion glass is ZERODUR® glass manufactured by Schott AG of Mainz, Germany. 
   Generally, the well logging tool  10  includes adaptations as may be necessary to provide for operation during drilling or after a drilling process has been completed. 
   Referring to  FIG. 7 , an apparatus for implementing the teachings herein is depicted. In  FIG. 7 , the apparatus includes a computer  70  coupled to the well logging tool  10 . Typically, the computer  70  includes components as necessary to provide for the real time processing of data from the well logging tool  10 . Exemplary components include, without limitation, at least one processor, storage, memory, input devices, output devices and the like. As these components are known to those skilled in the art, these are not depicted in any detail herein. 
   Generally, some of the teachings herein are reduced to an algorithm that is stored on machine-readable media. The algorithm is implemented by the computer  60  and provides operators with desired output. The output is typically generated on a real-time basis. 
   The logging tool  10  may be used to provide real-time measurements of various parameters such as gravity for example. As used herein, generation of data in “real-time” is taken to mean generation of data at a rate that is useful or adequate for making decisions during or concurrent with processes such as production, experimentation, verification, and other types of surveys or uses as may be opted for by a user or operator. As a non-limiting example, real-time measurements and calculations may provide users with information necessary to make desired adjustments during the drilling process. In one embodiment, adjustments are enabled on a continuous basis (at the rate of drilling), while in another embodiment, adjustments may require periodic cessation of drilling for assessment of data. Accordingly, it should be recognized that “real-time” is to be taken in context, and does not necessarily indicate the instantaneous determination of data, or make any other suggestions about the temporal frequency of data collection and determination. 
   A high degree of quality control over the data may be realized during implementation of the teachings herein. For example, quality control may be achieved through known techniques of iterative processing and data comparison. Accordingly, it is contemplated that additional correction factors and other aspects for real-time processing may be used. Advantageously, the user may apply a desired quality control tolerance to the data, and thus draw a balance between rapidity of determination of the data and a degree of quality in the data. 
     FIG. 8  presents one example of a method  80  for measuring gravitational acceleration from within the borehole  2 . The method  80  calls for placing (step  81 ) the light source  20  and the gravimeter  24  in the borehole  2 . Further, the method  80  calls for illuminating (step  82 ) the gravimeter  24  with light emitted from the light source  20  wherein the light is used to perform the measuring. 
   In certain embodiments, a string of two or more logging tools  10  may be used where each logging tool  10  includes at least one light source  20 , the optical filter  22  and the light detector  26 . In these embodiments, a response from each logging tool  10  may be used separately or combined with other responses to form a composite response. 
   In support of the teachings herein, various analysis components may be used, including digital and/or analog systems. The digital and/or analog systems may be used in the electronic unit  9 . In one embodiment, the electronic unit  9  may be a data collector (data collector  9 ) for providing measurement data to a user. The electronic unit  9  may be disposed at least one of in the logging tool  10  and at the surface of the earth  7 . The system may have components such as a processor, storage media, memory, input, output, communications link (wired, wireless, pulsed mud, optical or other), user interfaces, software programs, signal processors (digital or analog) and other such components (such as resistors, capacitors, inductors and others) to provide for operation and analyses of the apparatus and methods disclosed herein in any of several manners well-appreciated in the art. It is considered that these teachings may be, but need not be, implemented in conjunction with a set of computer executable instructions stored on a computer readable medium, including memory (ROMs, RAMs), optical (CD-ROMs), or magnetic (disks, hard drives), or any other type that when executed causes a computer to implement the method of the present invention. These instructions may provide for equipment operation, control, data collection and analysis and other functions deemed relevant by a system designer, owner, user or other such personnel, in addition to the functions described in this disclosure. 
   Further, various other components may be included and called upon for providing for aspects of the teachings herein. For example, a power supply (e.g., at least one of a generator, a remote supply and a battery), cooling unit, heating unit, sensor, transmitter, receiver, transceiver, antenna, controller, lens, optical unit, electrical unit or electromechanical unit may be included in support of the various aspects discussed herein or in support of other functions beyond this disclosure. 
   It will be recognized that the various components or technologies may provide certain necessary or beneficial functionality or features. Accordingly, these functions and features as may be needed in support of the appended claims and variations thereof, are recognized as being inherently included as a part of the teachings herein and a part of the invention disclosed. 
   While the invention has been described with reference to exemplary embodiments, it will be understood that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications will be appreciated to adapt a particular instrument, situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
An instrument for measuring gravitational acceleration from within a borehole, the instrument including: a light source having a semiconductor that comprises a bandgap greater than about two electron volts (eV); and a gravimeter for receiving light from the light source and providing output light with a characteristic related to the gravitational acceleration, the gravimeter implemented at least one of a nano electro-mechanical system (NEMS) and a micro electro-mechanical system (MEMS); wherein the light source and the gravimeter are disposed in a housing adapted for insertion into the borehole.