Abstract:
An oil and gas exploration system and method for land and airborne operations, the system and method used for locating subsurface hydrocarbon deposits based upon a remote detection of trace amounts of gases in the atmosphere. The detection of one or more target gases in the atmosphere is used to indicate a possible subsurface oil and gas deposit. By mapping a plurality of gas targets over a selected survey area, the survey area can be analyzed for measurable concentration anomalies. The anomalies are interpreted along with other exploration data to evaluate the value of an underground deposit. The system includes a differential absorption lidar (DIAL) system with a spectroscopic grade laser light and a light detector. The laser light is continuously tunable in a mid-infrared range, 2 to 5 micrometers, for choosing appropriate wavelengths to measure different gases and avoid absorption bands of interference gases. The laser light has sufficient optical energy to measure atmospheric concentrations of a gas over a path as long as a mile and greater. The detection of the gas is based on optical absorption measurements at specific wavelengths in the open atmosphere. Light that is detected using the light detector contains an absorption signature acquired as the light travels through the atmosphere from the laser source and back to the light detector. The absorption signature of each gas is processed and then analyzed to determine if a potential anomaly exists.

Description:
The U.S. Department of Energy has certain rights in this present invention under DOE contract No. DE-FGO3-92ER81318 and The U.S. Air Force has certain rights in this present invention under AF contract No. F29601-94-C-0065. 
    
    
     BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     This invention relates to an oil and gas exploration system and method of detecting gases in the atmosphere and more particularly, but not by way of limitation, to detecting hydrocarbon gases using field-scale, differential absorption lidar (DIAL) sensing techniques operating in a mid-infrared, 2 to 5 micrometers, spectral range. 
     (b) Discussion of Prior Art 
     Gases, that are trapped or generated within the earth, can escape and travel through the earth to the earth&#39;s surface and then into the atmosphere. While in the earth, the gases come in contact with or are created by deposits of hydrocarbons and thereby acquire some signature of the deposit. The atmosphere can be monitored for gases that are associated with the deposits of hydrocarbons. The subject invention described herein addresses the measurement of gases associated with a potential oil and gas deposit. The gas concentrations are then mapped over a survey area and the maps are analyzed for concentration anomalies as described herein. The gas anomalies are interpreted along with other exploration data to evaluate the value of the underground deposit. 
     An association of gases detected in the atmosphere with a hydrocarbon deposit may be direct or indirect. An example of a direct association is the release of hydrocarbon gases to the atmosphere from subsurface oil and gas deposits. The association is direct in that the gas itself is emitted into the atmosphere, albeit with a potentially modified composition. 
     Methane is produced from the thermal or biological breakdown of coal. The gas detected (methane) is not the same as the natural resource (coal), so the term “indirect” is used to describe this association. The term indirect association does not imply that the scientific basis for the association is weak. The process of converting coal to methane is well characterized in the scientific literature. Also, coalbed methane is a new and expanding area of production for the natural gas industry. 
     In the discussion of the subject invention, the term “target gases” is used to indicate gases that are associated either directly or indirectly with deposits of hydrocarbons. The measured atmospheric concentrations of target gases form the basis of the new exploration tool as described herein. Target gases must have some uniqueness to their association with the hydrocarbon deposit. For example, methane is produced in a number of ways. It may occur in the atmosphere as a result of emission from a hydrocarbon deposit, emission from a coal deposit, emission from wetlands with active populations of methane producing bacteria, emission from a leaking natural gas pipeline, etc. Sources of methane other than the hydrocarbon deposit are said to be environmental interferences. Environmental interferences complicate the association between a target gas and the hydrocarbon deposit and will vary in magnitude and type with standard geological factors such as soil type, hydrology, subsurface structure and composition as well as atmospheric conditions, weather and land use. 
     A non-unique gas such a methane is a useful target gas for fossil fuels if combined with additional measurements to create a unique association. Examples of additional measurements are: concentrations of other gases such as ethane, propane, etc., isotopic composition of the methane, type of vegetation present, soil moisture, proximity of pollution sources and wind direction. Individual gases or gas combinations that have very unique associations with the hydrocarbon deposit provide the most valuable exploration signatures. 
     Laser absorption spectroscopy (LAS) is a sensitive means of quantifying molecular concentrations in a variety of situations not amenable to other techniques. A main advantage of LAS is that the measurements is done “in situ” which enables rapid measurements with good spatial resolution in harsh environments such as plasma, high vacuum and chemical reaction chambers. For an absorption experiment, the ratio of the transmitted beam intensity to the initial beam intensity, I(∀,x)/Io(∀,x=O), is related to an absorber concentration, n, by Beer&#39;s Law.            I        (     v   ,   x     )           I   o          (     v   ,     x   =   0       )                     -     σ        (   v   )            nx                              
     The molecular cross-section at frequency, ∀, is denoted σ(∀) and the path length over which the laser travels by x. For any given signal to a noise ratio (SNR) for the measurement of I/Io, the measurement sensitivity can be increased by increasing the path length. This patent application includes a differential absorption lidar (DIAL) which samples long paths through the atmosphere. 
     A wide range of instruments have been developed which successfully detect most trace gases in the atmosphere. These instruments can be loosely categorized into point techniques which sample air at a specific point in space and remote sensing systems such as the numerous satellite-based systems which provide large-scale measurements of gas concentrations. There are numerous types of gas sources which, because of their unique spatial and temporal properties, cannot be accurately characterized by these techniques. For example, an underground reservoir which might contain methane, carbon dioxide or gases from polluted ground waters, can leak intermittently from many points along a surface fracture. Monitoring emissions from such sources requires a system which can measure minute concentrations quickly and over long paths. Long path differential absorption lidars (DIALs) meet these requirements. 
     There are a number of prior art patents that describe oil and gas exploration systems that include means for detecting trace gases in the atmosphere. Some of these systems operate in the microwave or the ultraviolet wavelength region. These systems are unlike the subject invention which operates in the mid-infrared wavelength range. 
     The following patents are mentioned since the systems described therein operate in the mid-infrared wavelength region for detecting hydrocarbon gases. In U.S. Pat. No. 4,450,356 to Murray et al., a frequency-mixed CO 2  laser beam is used for remote detection of gases in the atmosphere. The laser beam system uses frequency doubling and frequency summing in crystals to produce wavelengths near 3 micrometers. This type of CO 2  laser system&#39;s wavelength is not continuously tunable. 
     In U.S. Pat. No. 4,489,239 to Grant et al., a portable remote laser sensor is described for detecting methane gas. The system requires the use of two lasers. The two lasers operate at two different wavelengths, each of which is fixed. The detector must be kept at liquid nitrogen temperatures and does not operate at room temperatures. Further, the two lasers are not tunable and are used for detecting methane only. In U.S. Pat. No. 4,871,916 to Scott, a laser system, using glass lasers, is described for detecting explosive amounts, 40,000 parts per million (ppm), of methane only. In this system, the wavelength region is near 1.3 micrometers and the lasers are not tunable. 
     In U.S. Pat. Nos. 5,157,257 and 5,250,810 to Geiger, a mid-infrared light hydrocarbon DIAL lidar is described. The system uses six distinct coherent beams formed by six different lasers. The beams are combined into a single transmitted beam. While the six lasers are tunable, they include crystals which are easily damaged by high energy laser pulses. The complexity of this type of DIAL is not conducive to use in the field. Also, the spectral width is too broad to resolve the absorption bands of many key gases. 
     None of the above mentioned patents disclose or describe the unique features, structure, function and method steps of the subject oil and gas exploration system and method of detecting trace amounts of hydrocarbon gases in the field and in the mid-infrared region using a DIAL system with a single spectroscopic grade laser source. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is a primary object of the subject invention to provide a unique oil and gas exploration tool which is able to detect sub parts per million (ppm) amounts of hydrocarbon gases in the atmosphere. 
     Another object of the invention is the subject DIAL system can be used for mapping a plurality of target gases over a selected survey area. The maps are analyzed for concentration anomalies that indicate the presence of a potential hydrocarbon reservoir. 
     Yet another object of the DIAL system is its ability to achieve high energy laser pulses, in the mid-infrared wavelength range, 2 to 5 micrometers, with excellent spatial and spectral quality. The term “spectroscopic grade laser” used herein refers to lasers that produce light with a spectral width at least a factor of ten less than the spectral width of the target gas absorption bands used to make the DIAL measurements. Good spectral control enables the system to detect trace amounts of specific gases, such as ethane, methane and propane. 
     Still another object and key feature is the DIAL system is tunable for choosing appropriate wavelengths to measure different gases. Because the system is tunable, absorption bands of interference gases, such as water vapor and CO 2 , can be avoided and the wavelengths can be chosen resulting in the most sensitive detection of a particular target gas. 
     A further object of the invention is the system allows trace gases to be accurately detected over one mile (two mile round trip) paths and greater and in a time span of under one minute. 
     Another object in the invention is the system is housed in a mobile trailer for transporting to various field locations. The system is operable and reliable for field applications and under harsh weather conditions. Also, the mid-infrared laser beam emitted is eye safe at all energy levels and at all wavelengths. 
     The system includes a differential absorption lidar (DIAL) system with a spectroscopic grade laser, a laser light and a light detector. The laser light is broadly tunable in the mid-infrared wavelength range, 2 to 5 micrometers, of the electromagnetic spectrum from which the appropriate wavelengths to measure different hydrocarbon gases and avoid absorption bands of interference gases can be chosen. The DIAL system with the laser light source is housed in a mobile platform for field operation. Also, the DIAL system can be used on an airborne platform for airborne survey applications. The laser light has sufficient optical energy to measure atmospheric concentrations of a selected gas over a path as long as a mile and greater. The detection of the target gas is based on optical absorption measurements at specific wavelengths in the open atmosphere. Light that is detected, using the light detector, contains an absorption signature acquired as the light travels through the atmosphere from the laser source and back to the light detector. The absorption signature of each target gas is processed and then analyzed to determine the gas concentration in the atmosphere. The target gas concentrations are mapped and analyzed to determine presence of concentration anomalies. 
     These and other objects of the present invention will become apparent to those familiar with the different types of hydrocarbon gas detection systems and methods when reviewing the following detailed description, showing novel construction, combination, and elements as herein described; and more particularly defined by the claims, it being understood that changes in the embodiments to the herein disclosed invention are meant to be included as coming within the scope of the claims, except insofar as they may be precluded by the prior art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the components making up the subject DIAL system for detecting trace amounts of hydrocarbon gases. 
     FIG. 2 illustrates a normalized lidar signal vs. wavenumber, near 3.37 micrometers, for ambient urban air over a 480 meter path. This graph shows data from the subject DIAL system indicating the presence of water vapor and methane in the ambient atmosphere. 
     FIG. 3 illustrates a theoretical absorption methane spectrum in the 3.365 to 3.373 micrometer range. 
     FIG. 4 illustrates a theoretical absorption water vapor spectrum in the 3.365 to 3.373 micrometer range. 
     FIG. 5 illustrates data showing that the system described herein demonstrates the generation of two mid-infrared laser pulses separated by less than 1 millisecond. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The accompanying drawings illustrate complete preferred embodiments of the present invention according to the best modes presently devised for the practical application of the principles thereof, and in which: 
     In FIG. 1, a differential absorption lidar DIAL system operating in a 2-5 micrometer wavelength band is illustrated and having general reference numeral  10 . The DIAL system  10  is based on a Raman-shifted, Cr:LiSAF laser. A similar DIAL system, but not described for use in detecting trace amounts of hydrocarbon gases for oil and gas exploration, is illustrated in U.S. Pat. No. 5,583,877 issued to MacPherson et al. and assigned to Ophir Corporation, Littleton, Colo. The subject matter found in this patent is incorporated herein by reference. 
     A brief description of Raman shifting follows as an aid in clearly describing various embodiments of the invention. The use of a Raman cell to shift the wavelength of light is well-described in the prior art. In basic terms of energy conservation, the process can be described as the energy of photons encountering gas in the Raman cell is shifted by an amount equal to the change in vibrational energy of the gas in the Raman cell. Expressed mathematically, ΔEphoton=ΔEvibgas where ΔEphoton is the change in energy of the photon and Evibgas is the change in the vibrational energy of the gas in the Raman cell. This process also shifts the wavelength of the photons because the energy and the wavelength of a photon are related by the formula, E=hc/λ, where E designates the energy of the photon, h is Planck&#39;s constant, c is the speed of light and λ is the wavelength of the photon. The energy of incident photons may be shifted to higher energy (anti-Stokes Shift) or to lower energy (Stokes shift) and multiple shifts can occur. Light that undergoes a single Stokes shift is referred to as S 1  light and light that under two Stokes shifts is referred to as S 2 . Similarly, light that undergoes a single anti-Stokes shift is referred to as AS 1  and two anti-Stokes shifts results in AS 2  light. The incident light is designated S 0 . In a typical operation, S 0  light is incident on the Raman cell and a filter is used after the Raman cell to eliminate AS 2 , AS 1 , S 0  and S 1  light components leaving only S 2  to comprise the mid-infrared light. 
     The DIAL system  10  is unique in that it achieves continuously wavelength-tunable, high energy laser pulses with excellent spatial and spectral quality. The system allows individual gases to be accurately quantified over one mile (two mile round trip) paths and greater in under one minute. While not shown in the drawings, the system is housed in a mobile trailer allowing it to be used to survey different types of exploration sites. Also, the system  10  has been developed to operate reliably even under harsh weather conditions. 
     The system  10  broadly includes the following. A laser oscillator  12  optically connected to a laser amplifier  14 . A recirculating water chiller  16 , using buffered distilled water, is connected to both the oscillator  12  and amplifier  14 . The chiller  16  is used for cooling Cr:LiSAF laser rods in the oscillator  12  and amplifier  14 . 
     A 750-950 nm pulsed near infrared laser light  18  is directed outwardly through a first partial reflective mirror  20  and a polarization filter  21  to a hydrogen filled Raman cell  22 . A portion of the light  18  is reflected to a spectrometer  24 , which in turn is electrically connected to detection electronics  26 . From the Raman cell  22 , the light pulses pass through a wavelength filter  23  to remove residual near infrared, anti-Stokes and first Stokes components. From the wavelength filter  23 , 2-5 micrometer pulsed mid-infrared light  28  is directed through a second partial reflective mirror  30 , through a third partial reflective mirror  32  and through transmitting optics  29  onto a transmission mirror  36 . The transmission mirror  36  is mounted in a two-axis rotational stage to allow spatial scanning of the laser light  28 . 
     The transmission mirror  36  is operated under a mirror scan control  38 . The second partial reflector  30  directs a portion of the light  28  to a reference gas cell  44  and onto a reference detector  46 . The reference detector  46  is electrically connected to the detection electronics  26  for monitoring transmission through the reference gas cell  44 . 
     The third partial reflector  32  directs a portion of the light  28  to a normalization detector  40 , which in turn is electrically connected to detection electronics  26  for measuring the laser pulse energy. 
     From the transmission mirror  36 , the 2-5 micrometer light  28  is transmitted to a retroreflector  48  over a transmission path through the atmosphere, as indicated by arrow  50 . Redirected light  52 , also shown as an arrow, is reflected back to the transmission mirror  36  to receiving optics  54  and received by a sample detector  56 . The sample detector  56  is electrically connected to the detection electronics  26 . 
     The DIAL system  10  also includes a diode seed laser  58  optically connected to the laser oscillator  12  and also electrically connected to, a computer  60  and optically connected to a wavelength meter  62 . The wavelength meter  62  is electrically connected to the computer  60 . A Q switch controller  64  and a flashlamp controller  66  are both electrically connected between the computer  60  and the laser oscillator  12 . The flashlamp controller  66  is also electrically connected to the laser amplifier  14 . 
     The laser amplifier  14  is a Cr:LiSAF, single pass amplifier. Together the oscillator  12  and amplifier  14  generate near infrared Q-switched pulses with pulse energies of 40 millijoules and pulse widths of 80 nanoseconds at a pulse repetition frequency of 2 Hz. Also, the pulses are in the 780 to 980 nanoseconds spectral band. 
     The configuration of the diode seed laser  58 , the laser oscillator  12 , the laser amplifier  14  and the Raman cell  22  are chosen in order to improve the reliability of the laser light source under harsh field conditions. For example, the pulses extracted from the oscillator  12  are amplified by a factor of two in a single pass through the amplifier  14 . For equivalent output pulse energies, this optical configuration reduces the thermal load on the Cr:LiSAF crystal rods incorporated into the oscillator  12  and thereby reduces the energy density in the oscillator cavity relative to a similar configuration without an amplifier stage. Though not shown in FIG. 1, the coupling between the diode seed laser  58  and the laser oscillator  12  can be actively controlled using standard techniques. Active control increases the seeding efficiency, but adds complexity to the system. 
     The Cr:LiSAF is a uniaxial crystal, which rotates the polarization of light as the light passes through the crystal according to the standard principals governing polarization rotation in birefringent crystals. The magnitude of the birefringent effect is very sensitive to the temperature of the Cr:LiSAF crystals. The temperature of the cooling water circulated by the chiller  16  is controlled with fluctuations in a range of plus or minus 1 degree C. Small misalignments of the optical axis of the Cr:LiSAF amplifier rod in the amplifier  14  result in large temperature driven polarization changes in the pulses that pass through the amplifier. Polarization effects throughout the remainder of the optical path are overcome with the use of the polarization filter  21  between the amplifier  14  and the Raman cell  22 . 
     Referring now to the Raman cell  22 , stimulated Raman scattering of the light  18  in hydrogen is used to shift the near infrared Cr:LiSAF laser pulses into the mid-infrared light  28 , where many of the hydrocarbon gases have strong absorption bands. Stokes shifts of approximately 4155 wavenumbers arise from the Q(1) vibrational Raman transition in the hydrogen. The hydrogen is held at room temperature and at a pressure of 10 atmospheres. Second Stokes conversion of the Cr:LiSAF pulses to the mid-infrared is enhanced by use of a multiple pass design for the Raman cell  22 . 
     The stimulated Raman scattering has several important advantages over other nonlinear wavelength shifting techniques. The subject technique is extremely efficient with up to an 80 percent quantum efficiency in conversion of the near infrared light  18  to the mid-infrared light  28  when the laser energy is well above the stimulated Raman scattering threshold for shifting the wavelength of the light. This technique is inherently “phase-matched”. Also, the mid-infrared light  28 , can be tuned anywhere over a 2.2 to 5.2 micrometer wavelength band simply by tuning the near-infrared laser light  18  from 780 to 980 nanometers. This key feature is in contrast to other prior art nonlinear methods for producing mid-infrared light, as mentioned above, where optical crystals need to be rotated or heated in a controlled fashion to achieve tuning. Further, another advantage of the use of the Raman cell  22  is optical damage does not occur to the nonlinear medium which is hydrogen. 
     Referring to the transmission of the 2-5 micrometer light  28  to the retroreflector  48 , anti-Stokes (AS 1  and AS 2 ), first Stokes (S 1 ) and residual near infrared components (S 0 ) are removed from the second Stokes output of the Raman cell  22  with the wavelength filter  23 . The wavelength filter  23  is a 3.3 micrometer, anti-reflection coated germanium filter. This ensures that only an eyesafe laser is transmitted into the atmosphere. The beam of the light  28  is expanded to a 4 inch diameter, collimated and directed through the atmosphere to the retroreflector  48  as shown in FIG.  1 . The retroreflector  48  is a 40 centimeter diameter array of corner-cube reflectors. While the retroreflector  48  is mentioned and is ideal for this type of system, it should be kept in mind various naturally occurring and man made physical objects can be used for reflecting the light beam. The redirected light  52  from the retroreflector  48  is collected by the receiving optics  54 . The receiving optics  54 , for example, can include a collection telescope such as a 12.5 inch custom Newtonian telescope. As indicated by arrow  50 , the optical path may be up to or greater than one mile with the system  10  having sufficient pulse energy to extend well beyond this range. 
     The three detectors  40 ,  46  and  56  are used in the system  10  to measure the laser pulse energies at various points in the system  10 . The detectors are uncoated pyroelectric crystals with an active diameter of 2 millimeters and include.integrated battery powered pre-amplifiers. Other types of detectors may also be used. 
     The computer  60  is used to the control the system  10 , to perform health checks to verify proper functionality, log the numerous data channels, process the data to give a real time or time-averaged gas concentration value and display data. The software used by the computer  60  was developed for controlling various subsystems and peripheral equipment interfaced with the computer. Serial ports of the computer are used to collect auxiliary data such as wind speed and direction, temperature, barometric pressure and humidity from a weather station and Global Positioning System (GPS) coordinates of the mobile unit housing the system  10  and the retroreflector  48 . Also, the computer  60  receives inputs from the detectors  40  and  46  and in addition to the signal from the main redirected light  52  which are automatically logged for each laser shot to monitor the quality of the data and verify that the system is operating properly. 
     A small portion of the mid-infrared pulse light  28  is detected for outgoing pulse normalization. Another small portion of the mid-infrared light  28  is sent through the reference gas cell  44 . The cell  44  is filled with a known composition of gas and provides a turbulence-free differential absorption measurement which can be used to verify that the outgoing light  28  has a proper wavelength and spectral width. 
     Coarse, near infrared wavelength measurements are calculated from the output of a diode array detector mounted on the output port of the spectrometer  24 . Properly seeded pulses are much narrower (spectrally) than the spectrometer resolution while unseeded pulses are not. This feature provides feedback to an operator during the initial optimization of the seeding alignment. 
     Also shown in FIG. 1 are auxiliary sensors  68  connected to the computer  60 . The auxiliary sensors  68  include a weather station and GPS receivers. The temperature, humidity, barometric pressure, wind speed and wind direction from the weather station are recorded with each lidar pulse of the system  10 . The latitude and longitude of the mobile trailer and the retroreflector  48  are determined for each atmospheric path. The communication with the GPS receiver that is located at the retroreflector  48  is achieved with a battery powered radio frequency modem. Differential GPS readings with a standard deviation of less than plus or minus 2 meters at distances to 1500 meters are used to accurately measure the path length in order to extract calibrated wavelength concentration measurements that are described herein. 
     While not shown in FIG. 1, a first alternate embodiment of the DIAL system  10  is now described and referred to as a “seeded Raman” embodiment. In the seeded Raman embodiment, the seed laser  58  operates at mid-infrared wavelengths and is optically coupled directly to the Raman cell  22  rather than connected to the near-infrared oscillator  12 . There are several benefits of the seeded Raman embodiment. The first benefit is that lower pulse energy densities are required to achieve equivalent Raman shifting efficiency. Lower optical energy densities lead to less power consumption, less chance for optical damage, and thereby increased lifetime of the system. The second benefit is that the wavelength meter  62  in the DIAL system  10  can be replaced with a gas cell, similar to the reference gas cell channel described in the preferred embodiment and associated detector and detection electronics. Gas cells are simple, rugged devices with no moving parts and require no periodic recalibration. Finally, the variation in pulse energy for light  18  produced by the near-infrared oscillator  12  and the amplifier  14  is reduced in the absence of seeding by the seed laser  58 . 
     A second alternate embodiment of the DIAL system  10  is now described and called a “double pulse” embodiment. The double pulse embodiment takes advantage of two laser pulses generated from the Cr:LiSAF laser rods in the near-infrared oscillator  12 . The two laser pulses are called a “double pulse” because the two pulses can be generated with less than a millisecond delay between the two pulses. The pulse pairs can be generated at a 2 Hz rate already described for the DIAL system  10 . Further, one pulse of the pulse pair can be emitted with a wavelength corresponding to an “on line”, absorbed, wavelength and the other pulse of the pulse pair can be emitted with.a wavelength corresponding to an “off line”, not absorbed, wavelength in order to perform a differential absorption measurement. 
     In FIG. 5, data is shown to demonstrate the ability of the subject invention to support the double-pulse embodiment. The plot shown in FIG. 5 shows the intensity of the S 0 (pump) beam as a function of time. Without Raman shifting, the pulses are expected to exhibit a Gaussian shape. The high intensity portion of each pulse is missing because at these intensities a significant portion of the pump beam is shifted from the near-infrared (S 0 ) to the mid-infrared (S 2 ). The two pulses occur 43 microseconds apart and both show conversion to S 2 . 
     The benefit of using pulse pairs for the differential absorption measurement is that the atmosphere appears frozen on this time scale, because atmospheric motions that affect open path optical measurements occur on longer time scales. Therefore, the on line and off line measurements occur essentially simultaneously, which is the preferred technique for DIAL measurements. Further, the double pulse embodiment is possible only if the Raman cell  22  is capable of shifting both the pulses of a pulse pair from the near infrared to the mid-infrared spectral range. A second benefit is that the data acquisition rate is essentially doubled even though the main repetition rate of the system remains at 2 Hz. In the preferred embodiment of the DIAL system  10  described above, the reference detector  46 , the normalization detector  40  and the sample detector  56  were all described more specifically to be pyroelectric detectors. Obviously to those skilled in the art, the relatively inexpensive pyroelectric detectors have to be replaced with higher speed detectors in order to detect the individual pulses of each pulse pair. 
     A third alternate embodiment of the DIAL system  10  is now described and called a “combined seeded Raman and double pulse” embodiment. This combined embodiment provides the benefits and advantages of the seeded Raman embodiment and the double pulse embodiment discussed above. 
     A fourth alternate embodiment of the DIAL system  10  is now described and called a “multiple wavelength” embodiment. The multiple wavelength embodiment is an alternative to the double-pulse embodiment. A common thread for the embodiments described heretofore is that each pulse produced by a given embodiment of the DIAL system  10  contains light with a single wavelength and is denoted as the mid-infrared light  28 . Accordingly, differential absorption measurements are made by scanning the wavelength of the mid-infrared light  28 . In the multiple wavelength embodiment, the filter  23  is designed to transmit S 2  light and one or more of the AS 2 , AS 1 , S 0  and S 1  components produced or transmitted by the Raman cell  22 . Light, comprised of the multiple wavelengths is transmitted through the atmosphere, reflected and received in the same manner as a single wavelength mid-infrared light  28 . 
     The functions of the reference detector  46 , the normalization detector  40  and the sample detector  56  are fulfilled with wavelength selective detection in order to carry out differential optical absorption measurements with a single laser pulse. Wavelength selective detection can be achieved with the use of composite detectors that quantify the amount of light received at different wavelengths or through the use of wavelength dependent beam splitters and additional non-wavelength specific detectors. 
     The benefit of the multiple wavelength embodiment is that the multiple wavelengths are transmitted, reflected and received simultaneously. Therefore, atmospheric turbulence has little impact. A second benefit is that the non-mid-infrared beams can be used to collect other data about the atmosphere. The other data includes windspeed, aerosol concentrations, aerosol composition and water vapor concentration. Appropriate precautions must be taken to ensure eye safety when using this embodiment, since the multiple wavelength laser beam is not necessarily eye safe when the wavelength filter  23  is removed. 
     In FIG. 2, a normalized lidar signal vs. wavenumber spectrum, near 3.37 micrometers, is illustrated for ambient urban air over an open path. The open path is 480 meters and data is for a 0.008 nanometer scan. The data show optical absorption features for methane and water with a total of seven features in this spectral region. The laser source in the system  10  is sufficiently narrow to resolve two water absorption features that are separated by just 0.0005 nanometers. The near infrared tuning range of the injected seeded laser oscillator  12  and amplifier  14  is greater than 0.5 nanometers without any operator intervention. 
     In FIG. 3, a theoretical absorption methane spectrum based upon the 1996 HITRAN molecular database is illustrated. This spectrum is in a 3.365 to 3.373 micrometer range. 
     In FIG. 4, a theoretical absorption water vapor spectrum based upon the 1996 HITRAN molecular absorption database is illustrated. This spectrum is also in the 3.365 to 3.373 micrometer range. 
     While the invention has been shown, described and illustrated in detail with reference to the preferred embodiments and modifications thereof, it should be understood by those skilled in the art that equivalent changes in form and detail may be made therein without departing from the true spirit and scope of the invention as claimed, except as precluded by the prior art.