Abstract:
A spatial heterodyne spectrometer may employ an integrated computational element (ICE) to obtain a measure of one or more fluid properties without requiring any moving parts, making it particularly suitable for use in a downhole environment. One illustrative method embodiment includes: directing light from a light source to illuminate a sample; transforming light from the sample into spatial fringe patterns using a dispersive two-beam interferometer; adjusting a spectral weighting of the spatial fringe patterns using an integrated computation element (ICE); focusing spectral-weight-adjusted spatial fringe patterns into combined fringe intensities; detecting the combined fringe intensities; and deriving at least one property of the sample.

Description:
BACKGROUND 
     Modern oil field operators demand access to a great quantity of information regarding the parameters and conditions encountered downhole. Such information typically includes characteristics of the earth formations traversed by the borehole and data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” can be performed by several methods including wireline logging, “logging while drilling” (LWD), and tubing-conveyed logging. 
     In wireline logging, a probe or “sonde” is lowered into the borehole after some or all of the well has been drilled. The sonde hangs at the end of a long cable or “wireline” that provides mechanical support to the sonde and also provides an electrical connection between the sonde and electrical equipment located at the surface of the well. In accordance with existing logging techniques, various parameters of the earth&#39;s formations are measured and correlated with the position of the sonde in the borehole as the sonde is pulled uphole. 
     In LWD, the drilling assembly includes sensing instruments that measure various parameters as the formation is being penetrated, thereby enabling measurements of the formation while it is less affected by fluid invasion. While LWD measurements are desirable, drilling operations create an environment that is generally hostile to electronic instrumentation, telemetry, and sensor operations. 
     Tubing-conveyed logging, like wireline logging, is performed in an existing borehole. Unlike wireline logging, tubing-conveyed logging enables a logging tool to travel where a wireline-suspended tool cannot, e.g., in a horizontal or ascending borehole. Tubing-conveyed logging tools typically suffer from restricted communications bandwidths, meaning that acquired data is generally stored in memory and downloaded from the tool when the tool returns to the surface. 
     In these and other logging environments, measured parameters are usually recorded and displayed in the form of a log, i.e., a two-dimensional graph showing the measured parameter as a function of tool position or depth. In addition to making parameter measurements as a function of depth, some logging tools also provide parameter measurements as a function of azimuth. Such tool measurements have often been displayed as two-dimensional images of the borehole wall, with one dimension representing tool position or depth, the other dimension representing azimuthal orientation, and the pixel intensity or color representing the parameter value. 
     Once a borehole has been drilled, operators often wish to perform downhole formation testing before finalizing a completion and production strategy. Fluid sampling tools enable operators to draw fluid (i.e., gas or liquid) samples directly from the borehole wall and measure contamination levels, compositions, and phases, usually based on the properties (e.g., optical properties, electrical properties, density, NMR, and PVT properties) of the materials drawn into the sample chamber. Existing downhole fluid analysis tools may have a limited reliability due to, e.g., insufficient instrumentation to perform in-situ analysis, or conversely, too many moving parts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Accordingly, there are disclosed in the drawings and detailed description specific embodiments of methods, systems, and downhole tools that employ spatial heterodyne integrated computational element (“SH-ICE”) spectrometers. In the drawings: 
         FIG. 1  shows an illustrative environment for logging while drilling (“LWD”). 
         FIG. 2  shows an illustrative environment for wireline logging. 
         FIG. 3  shows an illustrative environment for tubing-conveyed logging. 
         FIG. 4  shows an illustrative formation fluid sampling tool. 
         FIGS. 5A-5C  show illustrative embodiments of a SH-ICE spectrometer based fluid analyzer. 
         FIGS. 6A-6D  illustrate a wavelength-to-spatial fringe relationship. 
         FIG. 6E  shows an illustrative combined spatial fringe intensity. 
         FIG. 6F  shows an illustrative spatial fringe image. 
         FIG. 7A  shows an illustrative multiplex integrated computational element (“ICE” 
         FIG. 7B  shows an illustrative spatially-dependent ICE. 
         FIG. 7C  shows an illustrative multiplex spatially-dependent ICE. 
         FIG. 8  is a flowchart of an illustrative downhole fluid analysis method. 
     
    
    
     It should be understood, however, that the specific embodiments given in the drawings and detailed description do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed in the scope of the appended claims. 
     DETAILED DESCRIPTION 
     Various systems and methods for performing optical analysis with combined spatial-heterodyne (“SH”) integrated computational element (“ICE”), or “SH-ICE” spectrometer, Light from a light source encounters a material to be analyzed, such as a formation fluid sample, a borehole fluid sample, a core sample, or a portion of the borehole wall. The encounter can take various forms, including transmission (attenuation) through the sample, reflection from the sample, attenuated total reflectance (evanescent wave), scattering from the sample, and fluorescence excitation. In any event, the spectral characteristics of the material are imprinted on the light beam and can be readily analyzed with the spectrometer to obtain a measure of characteristics of the substance such as concentrations of selected components. The disclosed spectrometer is believed to be capable of laboratory-quality measurements in a wide range of contexts including a hostile downhole environment, 
     Context 
     The disclosed systems and methods are best understood in the context of the larger systems in which they might be employed.  FIG. 1  shows an illustrative logging while drilling (LWD) environment. A drilling platform  2  supports a derrick  4  having a traveling block  6  for raising and lowering a drill string  8 . A kelly  10  supports the drill string  8  as it is lowered through a rotary table  12 . A drill bit  14  is driven by a downhole motor and/or rotation of the drill string  8 . As bit  14  rotates, it creates a borehole  16  that passes through various formations  18 . A pump  20  circulates drilling fluid through a feed pipe  22  to kelly  10 , downhole through the interior of drill string  8 , through orifices in drill bit  14 , back to the surface via the annulus around drill string  8 , and into a retention pit  24 . The drilling fluid transports cuttings from the borehole  16  into the pit  24  and aids in maintaining the integrity of the borehole. 
     A LWD tool  26  is integrated into the bottom-hole assembly near the bit  14 . As the bit extends the borehole  16  through the formations  18 , logging tool  26  collects measurements relating to various formation properties as well as the tool orientation and various other drilling conditions. The logging tool  26  may take the form of a drill collar, i.e., a thick-walled tubular that provides weight and rigidity to aid the drilling process. As explained further below, tool assembly  26  includes a optical fluid analysis tool that monitors wellbore fluid properties. A telemetry sub  28  may be included to transfer measurement data to a surface receiver  30  and to receive commands from the surface. In some embodiments, the telemetry sub  28  does not communicate with the surface, but rather stores logging data for later retrieval at the surface when the logging assembly is recovered. 
     At various times during the drilling process, the drill string  8  may be removed from the borehole as shown in  FIG. 2 , Once the drill string has been removed, logging operations can be conducted using a wireline logging tool  34 , i.e., a sensing instrument sonde suspended by a cable  42  having conductors for transporting power to the tool and telemetry from the tool to the surface, A wireline logging tool  34  may have pads and/or centralizing springs to maintain the tool near the axis of the borehole as the tool  34  is pulled uphole. As explained further below, tool  34  can include a formation fluid sampler that extends a probe against a borehole wall to draw fluids into a sample analysis chamber. A surface logging facility  44  collects measurements from the logging tool  34 , and includes a computer system  45  for processing and storing the measurements gathered by the logging tool. 
     An alternative logging technique is logging with coil tubing.  FIG. 3  shows an illustrative coil tubing-conveyed logging system in which coil tubing  54  is pulled from a spool  52  by a tubing injector  56  and injected into a well through a packer  58  and a blowout preventer  60  into the well  62 . (It is also possible to perform drilling in this manner by driving a drill bit with a downhole motor.) in the well, a supervisory sub  64  and one or more logging tools  65  are coupled to the coil tubing  54  and optionally configured to communicate to a surface computer system  66  via information conduits or other telemetry channels. An uphole interface  67  may be provided to exchange communications with the supervisory sub and receive data to be conveyed to the surface computer system  66 . 
     Surface computer system  66  is configured to communicate with supervisory sub  64  during the logging process or alternatively configured to download data from the supervisory sub after the tool assembly is retrieved. Surface computer system  66  is preferably configured by software (shown in  FIG. 3  in the form of removable information storage media  72 ) to process the logging tool measurements (including the fluid component measurements described further below). System  66  includes a display device  68  and a user-input device  70  to enable a human operator to interact with the system software  72 . 
     In each of the foregoing logging environments, the logging tool assemblies preferably include a navigational sensor package that includes directional sensors for determining the inclination angle, the horizontal angle, and the rotational angle (a.k.a. “tool face angle”) of the bottom hole assembly. As is commonly defined in the art, the inclination angle is the deviation from vertically downward, the horizontal angle is the angle in a horizontal plane from true North, and the tool face angle is the orientation (rotational about the tool axis) angle from the high side of the wellbore. In accordance with known techniques, wellbore directional measurements can be made as follows: a three axis accelerometer measures the earths gravitational field vector relative to the tool axis and a point on the circumference of the tool called the “tool face scribe line”. (The tool face scribe line is typically drawn on the tool surface as a line parallel to the tool axis.) From this measurement, the inclination and tool face angle of the logging assembly can be determined, Additionally, a three axis magnetometer measures the earth&#39;s magnetic field vector in a similar manner. From the combined magnetometer and accelerometer data, the horizontal angle of the logging assembly can be determined. 
       FIG. 4  shows an illustrative formation fluid sampler tool  80 . Tool  80  can be a drill collar, a coil tubing joint, or a drilling tubular, but most commonly it is expected to be part of a wireline sonde. Tool  80  extends a probe  82  and a foot  84  to contact the borehole wall  17 , typically driving them outward from the tool body using hydraulic pressure. The probe  82  and foot  84  cooperate to seat the probe  82  firmly against the borehole wall  17  and establish a seal that keeps borehole fluids from being drawn into the tool  80 . To improve the seal, the wall-contacting face of the probe  82  includes an elastomeric material  85  that conforms to the borehole wall  17 . A pump  86  draws down the pressure, prompting fluid to flow from the formation through a probe channel  88 , a sample chamber  90  in fluid analyzer  92 , and a sample collection chamber  94 . The pump  86  exhausts fluid into the borehole  16  through a port  96  and continues pumping until a sampling process is completed. Typically, the sampling process continues until the tool  80  determines that the sample collection chamber  94  is full and any contaminants have been exhausted. Thereafter the sample collection chamber  94  is sealed and the probe  82  and foot  84  are retracted. If desired, the tool  80  can repeat the process at different positions within the borehole  16 . Sample collection chamber  94  may be one of many such sample collection chambers in a cassette mechanism  98 , enabling the tool  80  to return many fluid samples to the surface. 
     Spatial-Heterodyne Integrated Computational Element Spectrometer 
       FIGS. 5A-5C  show illustrative embodiments of a SH-ICE spectrometer based fluid analyzer. In  FIG. 5A , a light source  502  shines light through an inlet window  504  into a sample (shown here as fluid flow stream  506 ). The light source  502  can he either broadband or narrowband. For the purposes of this disclosure, the term “broadband” is used to distinguish from narrowband sources that provide only isolated peaks in their spectrum. The broadband sources contemplated for use downhole have continuous spectrums in the range of 200-400 nm (for UV absorption and fluorescence spectroscopy), 1500-2300 nm (for special purpose spectroscopy, e.g. GOR (gas to oil ratio) determination), and 400-6000 nm (for general purpose VIS-IR spectroscopy), These examples are merely illustrative and not limiting. One readily available source suitable for this purpose is a tungsten-halogen incandescent source with a quartz envelope, generating light across the 300-3000 nm range. Arc lamps, broadband fluorescent sources, broadband quantum light sources, or a combination of a number of relatively narrowband light sources (such as LEDs) may also be suitable light sources. Suitable narrowband light sources are lasers and single wavelength LEDs, Such narrowband light sources may be used for single wavelength excitation spectroscopy (e.g. Raman and Fluorescence). 
     The illustrated sample is a fluid flow stream  506  sandwiched between the inlet window  504  and an outlet window  508 . Windows  504  and  508  are made from a transparent material (e.g., quartz, diamond, sapphire, zinc selenide) so that the main effect on the spectrum of the light is produced by attenuation as the light passes through the fluid flow stream  506  (i.e., transmission spectroscopy). Other spectrometer configurations may cause the light to interact with the sample (which, in some tool configurations, may be a surface of a solid) via reflection, diffuse reflection, attenuated total reflectance, scattering, or fluorescence. Conversely, some spectrometer embodiments cause the light to pass multiple times through the sample to increase the transmission-induced attenuation. 
     The light from the sample chamber may captured by a collimation element such as a mirror or lens  510 . Spectrometer embodiments employing a narrowband source would typically include a notch filter  511  to block the central frequency emitted by the light source  502  to prevent the intensity at this wavelength from overwhelming the measurements at nearby frequencies, The notch filter  511  can be positioned anywhere on the optical path after the sample (e.g., fluid flow stream  506 ). 
     One or more apertures  512  may be positioned at various points along the optical path to define the light into a beam and limit the effects of the beam periphery. A dispersive two-beam interferometer  514  employs a beam splitter  516  to split the incoming light beam into two beams that travel along first and second optical paths before being recombined by the beam splitter  516  into an outgoing beam. (A 50/50 splitter is preferred, but not required.) 
     Light traveling along the first path interacts with a diffraction grating  518  or other dispersive element that reflects the light at an angle that is dependent on its wavelength, in other words, the beam that returns to the splitter has the spectral components propagating with wavelength-dependent wavefront angles. Similarly, the light traveling along the second path interacts with a second diffraction grating  520  or other dispersive element that produces a return beam with spectral components propagating with wavelength-dependent wavefronts angles. The dispersive elements  518 ,  520  are positioned to provide the opposite wavefront angles. As the outgoing beam reaches a detector  530 , the difference in propagation angles produces a set of interference fringes. As explained below with reference to  FIGS. 6A-6E  (taken from Roesler, U.S. Pat. No. 5,059,027, “Spatial Heterodyne Spectrometer and Method”), the fringes vary based on the wavefront angle. 
     For a baseline or reference wavelength λ 0 , the wavefront angles in both beams are aligned, producing no fringes as indicated in  FIG. 6A , Graph  602  shows that the intensity as a function of position on a detector (e.g., detector  530  in  FIG. 5A ) is constant at this wavelength. As the wavelength increases, the wavefront angles of the two beams become increasingly different.  FIG. 6B  shows the wavefronts at an angle that produces one fringe on the detector (the intensity variation in graph  604  results when the path difference between the wavefronts varies from −λ/2 on one edge of the detector to +λ//2 on the other edge).  FIG. 6C  shows the wavefronts at an angle that produces two fringes on the detector (the intensity variation in graph  606  results when the phase difference between the wavefronts varies from −λ to +λ. As indicated in  FIG. 6D , each increment of the wavelength by a value δλ adds one fringe across the width of the detector. (Graph  608  shows n fringes across the width of the detector.) 
       FIGS. 6A-6D  illustrate examples of what occurs when only a single wavelength is present. When multiple wavelengths are present, the intensity vs. position relationship becomes more complex, as indicated by graph  610  in  FIG. 6E . Nevertheless, a spatial Fourier transform can separate out the contributions from the individual wavelengths. 
     The actual image cast by the outgoing beam on the detector is two dimensional.  FIG. 6F  (excerpted from a figure in N. Gromer et al., “Raman spectroscopy using a spatial heterodyne spectrometer: proof of concept”, Appl. Spectroscopy v65, n8, 2011) shows an illustrative two dimensional image  612 . Along the width of the detector (i.e., in the x-dimension), the image demonstrates a complex fringe dependence, whereas along the height of the detector (i.e., in the y-dimension) the intensity is relatively constant. The signal-to-noise ratio may be improved by summing or averaging the columns of the image together before analyzing the fringe structure. 
     Returning to  FIG. 5A , the foregoing discussion neglects the presence of element  522 , which as explained in greater detail below, is an integrated computation element (ICE) that modifies the outgoing beam image before it strikes detector  530 . The ICE  522  is included to exploit the observation that, in addition to spatial intensity variation, the image also contains wavelength-dependent intensity (“color”) variation, enabling further processing to be done on the image before it is captured by the detector  530 . 
     The ICE  522  operates to weight the various spectral components of the outgoing light beam by corresponding amounts, the weighting template being chosen based on what fluid properties are being measured. Many ICE implementations are known and potentially suitable, including a transparent substrate carrying a multilayered stack of materials having contrasting refractive indices, e.g., silicon and silica, niobium and niobia, germanium and germania, MgF and SiO. Suitable substrates may include BK-7 optical glass, quartz, sapphire, silicon, germanium, zinc selenide, zinc sulfide, various polymers (e.g., polycarbonates, polymethylmethacrylate, polyvinylchloride), diamond, ceramics, and the like. A transparent protective layer may further be provided over the layers with contrasting refractive indices. The relative weightings of different wavelengths are achieved through a judicious selection of the number, arrangement, and thicknesses of the layers to provide various degrees of optical interference at selected transmitted (or reflected) wavelengths. Other illustrative ICE implementations achieve the wavelength-dependent weightings by suitably varying their transmissivity, reflectivity, absorptivity, dispersivity, and/or scattering properties. Such implementations may employ engineered materials, holographic optical elements, gratings, acousto-optic elements, magneto-optic elements, electro-optic elements, light pipes, and digital light processors (DLPs) or other types of micro-electronic mechanical (MEMs) based light manipulation devices. 
       FIG. 7A  shows an illustrative ICE  702  of the multi-layered contrasting-refractive index variety. The illustrated ICE  702  is a multiplex device having different multi-layered structures over different image regions  704 , but it is also contemplated that there may be only a single region  704  over the entire substrate surface. The regions  704  are continuous across the width of the device. Hereafter this type of region is described as “row-oriented”. There is no horizontal spatial dependence to the ICE, meaning that, when employed as ICE  522  in  FIG. 5A , each of the image fringes is processed based solely on their wavelengths. Nevertheless, the fidelity of the fringe measurement is increased by the suppression of irrelevant wavelength intensities (and hence the spatial fringes irrelevant to the fluid property measurement). Note that this ICE embodiment can be positioned nearly any here in the optical path. 
     The use of different ICE structures in corresponding row-oriented regions enables different ICE templates to be applied simultaneously. As the number of regions increases, however, the size of each region decreases correspondingly, reducing the total tight intensity associated with each measurement. In some embodiments, this loss may be compensated by lengthening the measurement time. 
     When the image is captured by a detector  530  ( FIG. 5A ) such as a ICCD (intensified charge coupled device), it is digitized and suitable for digital signal processing. (Other image capture detectors would also be suitable.) As previously mentioned, the processing may include combining measurements from different rows (albeit, different rows within the same region  704 ) to increase the signal to noise ratio, and may further include a spatial Fourier transform to derive the spectral content of the ICE-filtered outgoing beam. Such processing can be done using a software or firmware programmed general purpose processor, or an application specific integrated circuit. Fourier transform processing of the weighed or unweighted spatial pattern would allow for a system that uses SH-ICE to gather spectral data in-situ for calibration or re-calibration. 
     In most cases, however, it is expected that a Fourier transform would not be required, but rather the information in each row could be combined (averaged or summed) together to obtain a single value representative of the ICE-specific measurement (e.g., an analyte concentration). Such a measurement can be performed using software or hardware (e.g., an appropriately wired detector) or, as indicated in  FIG. 5B , a minor or lens  524  that focuses the information from each row onto a row-associated point, yielding a one dimensional line. An array of photodetectors  540  may be provided along the line to enable each photodetector  540  acquire a row-associated measurement. Because the imaging array is now only one-dimensional, it can be further simplified to, a single photodetector  540  and a scanning mirror. The photodetector  540  can take the form of a photodiode, a thermal detector (including thermopiles and pyroelectric detectors), a Golay cell, or a photoconductive element. Cooling can be employed to improve the signal-to-noise ratio of the photodetector  540 . 
     Whether the recombining of spatial fringe information is done optically ( FIG. 5B ) or electronically ( FIG. 5A , after image capture by detector  530 ), signal to noise ratio may be improved by combining the measurements associated with all of the rows in a given region  704 . 
     In  FIGS. 5A and 5B , ICE  522  operates on the transmitted light. The systems can be readily modified to employ the reflected light, as indicated in  FIG. 5C . ICE  526  has a wavelength-dependent reflectivity to provide the desired spectral weighting on the fringes that reach detector  530 . Still other system embodiments measure both the transmitted and reflected light to achieve even higher performance. However, a similar performance is achievable with a multiplex ICE  702  having a regions  704  with complementary ICE templates, or by employing at least one ‘reference’ region that is weighted to a constant value (e.g. neutral density) or left as an unweighted (clear) region. 
       FIG. 7B  shows an alternative ICE  712  that employs a spatial dependence to provide the desired spectral weighting. It employs regions that are continuous across the height of the device, i.e., along the y-axis, Hereafter, this type of region is described as “column-oriented”. Because the spatial dependence corresponds to selected fringes, the wavelength selectivity of the regions can be relaxed. Indeed, some contemplated embodiments employ a mask that equally attenuates all wavelengths in that region of the beam. However, it is believed that the best efficiency will be achieved when at least some wavelength selectivity is combined with at least some spatial dependence, and the highest degree of performance should be achievable when the both the geometry and wavelength selectivity are carefully tailored to the desired measurement. 
       FIG. 7C  shows an illustrative multiplex ICE  722  in which each row-oriented region  724  employs a spatially-dependent ICE structure. As before, the use of multiplexing enables multiple simultaneous measurements, though it does so by corresponding reducing the light intensity available for each measurement. 
     An alternative to a multiplex ICE is the use of multiple ICES that can be sequentially positioned in the light path, e.g., with the use of a rotating filter wheel. As yet another alternative, the ICE can be dynamically changed, e.g., with a programmable acousto-optic ICE (for changeable wavelength dependence) or a programmable electro-optic ICE (for changeable spatial dependence). Dynamically changeable ICEs may use individually controllable pixels of a liquid crystal tunable filter or an acousto-optical tunable filter. Other programmable ICEs include but are not limited to DLP or other types of MEMS based devices. 
       FIG. 8  is a flowchart of an illustrative downhole fluid analysis method. It includes operations represented by blocks shown and described in sequential order, but this sequence is solely for explanatory purposes. In practice, the operations may be performed concurrently or, if sequential, may be performed in a different order or asynchronously. 
     In block  802 , the driller positions the fluid analysis tool downhole, e.g., in a wireline sonde or a LWD collar. In block  804 , fluid (e.g., from the formation) is drawn into a sample cell. In block  806 , the light source is energized and calibrated, in some embodiments, the calibration is performed by measuring light received from the source via a path that bypasses the fluid sample, A measurement correction may be derived from this measurement. In addition, or alternatively, a feedback signal may be derived from a measurement based on the output from the light source and used to adjust the light intensity applied to the fluid sample. 
     In block  808 , the tool illuminates the fluid sample with light from the source and analyzes the transmitted, reflected, or scattered light using a SH-ICE spectrometer, As discussed previously, the spectrometer obtains measurements indicative of fluid properties such as analyte concentrations. In block  810 , these measurements are processed, either by the tool itself or by a surface facility, to derive the fluid properties. Illustrative properties include amount and type of hydrocarbons (e.g., fractions of saturated, aromatics, resins, and asphaltenes), amount and type of gas phase (e.g., CO 2 , H 2 S, etc.), amount and type of liquid phase (e.g., water cut), PVT properties (including bubble point, gas-to-oil ratio, density variation with temperature), concentrations of compounds such as concentration of treatment fluid, and amount of contamination (e.g., drilling fluid) in formation fluid sample. 
     In block  812 , the tool and/or the surface facility communicates and stores the derived information. Contemporaneously, or later, the information is displayed to a user, preferably in the form of a log. In block  814 , the operation of the tool is optionally adjusted in response to the measurement, e.g., by terminating a pumping operation when the contamination level falls below a predetermined threshold. 
     The SH-ICE embodiments shown in  FIGS. 5A-5C  employ a series of discrete optical elements arranged along an optical path, which may further include additional mirrors, lenses, apertures, switches, filters, sources, and detectors. Some contemplated embodiments employ an integrated (“monolithic”) light path component. The integrated component provides reduced sensitivity to temperature changes, pressure changes, vibrations, and shock. A solid block of transparent material (e.g., quartz, sapphire, zinc selenide) is used as the body of the integrated component. Mirror gratings  518 ,  520 , beam splitter  516 , also made of the same material, ICE  522 . (or ICE  526 ), and focusing element  524 , are fused or otherwise attached to this body without any air gaps to maintain the alignment and spacing of the components over a wide range of temperature, pressure, vibration, and shock conditions. 
     Some tool embodiments, rather than being fluid analyzers, analyze a solid that is visible through a window or aperture, such as a core sample or a portion of the borehole wall adjacent to the tool. In such embodiments, the tool tracks the motion of the tool relative to the solid, associating the measurements with time and/or position to construct an image of the sample&#39;s surface. 
     Various techniques to maximize the quality of the measurements would be known to one of ordinary skill in the oil field industry and can be employed. For example, the tool may be outfitted with a reservoir of a reference fluid for downhole calibration of the system and for compensating for contamination on the windows of the flow cell. Detector cooling or temperature compensation can be used Co minimize the effects of temperature drift in the electronics. 
     Various other features can be incorporated into the tool. For example, scattered light can be analyzed to determine the size distribution of particles entrained in a fluid flow. An ultraviolet light source can be included to induce fluorescence in the material, which fluorescence can be analyzed to aid in determining composition of the sample. 
     The spectrometer designs and methods disclosed herein may be used in technologies beyond the oil field including, for example, the food and drug industry, industrial processing applications, mining industries, or any field where it may be advantageous to quickly determine a spectrally-related characteristic of a material. These and other variations, modifications, and equivalents will be apparent to one of ordinary skill upon reviewing this disclosure. It is intended that the following claims be interpreted to embrace all such variations and modifications where applicable.