Aerial Mineral Survey Method and Apparatus Using Pulsed Laser Beam to Vaporize Surface Material

The present invention pertains to airborne mineral surveying and more particularly to methods and apparatus for remotely vaporizing and spectrally analyzing a mineral sample.

Various types of passive remote sensors have been employed in attempts to deduce the composition of earth materials from an airborne platform. These includes gamma ray spectrometry which yields direct measurement of near-surface abundance of certain elements, but is limited in practice to only a few rare radioactive isotopes. Fluorescence spectrometry, applicable for several minerals, commonly yields data that are representative of surface contamination rather than the underlying mineral. Furthermore, the fluorescence spectra of different minerals tends to be ambiguous and are not indicative of the abundance of the elements or compounds being sought.

Therefore there exists a need for apparatus to rapidly analyze a great number of mineral samples for quantity and makeup by analysis means transported by a platform located a distance from the mineral samples.

An aircraft is equipped with a high-energy laser and a spectrometer. Both the laser beam generated by the laser and the spectrometer are aimed at a common point on the earth's surface along the aircraft track. As the aircraft traverses the survey area the laser is energized at selected intervals to vaporize material at a small spot on the surface below. The vaporized material produces atomic emission radiation which is characteristic of that material, this radiation is collected by the spectrometer and analyzed to determine the type and quantity of chemical elements present in the sample. The aiming of the laser beam and spectrometer is accomplished by a stabilized platform in the aircraft. Multiple samples are taken at each spot on the earth's surface so as to penetrate the outer layers and collect samples more representative of the bulk of material in the sample region.

For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is an elevation view of an aircraft for collecting mineral samples according to the present invention,

FIG. 2 is a perspective illustration of a laser beam generator and spectral analyzer mounted on a stabilized platform, and

FIG. 3 is a schematic diagram of a laser focusing mechanism and spectrometer collection apparatus in accordance with the present invention.

Referring to FIG. 1, there is shown an aircraft 10 carrying a mineral survey package 12 over a survey area 14. The package generates a laser beam 16 which is directed downward from the aircraft and strikes the survey area 14 at a sample point 18. The minerals at the sample point are vaporized and the resulting material produces radiation 20 which is projected outward over a wide range from the sample point 18. A portion of the radiation 20 is returned to the survey package 12 where it is analyzed to determine the makeup of the mineral structure at the sample point 18.

The survey package 12 is illustrated in further detail in FIG. 2. A high-energy laser 22 generates a laser beam which is directed to an optical system 24 that directs the laser beam 16 downward toward the earth. The resulting radiation 20 is returned to the survey package 12 where it is collected by the optical system 24 and transferred to a spectrometer 28 for mineral analysis. A relatively low power laser 29 generates a laser beam which is also directed downward to the sample point on the earth's surface.

The laser 22, optical system 24 and spectrometer 28 are mounted on a stabilized platform 30 for directing the laser beam 16 toward the sample point 18 on the earth. The platform 30 is connected to the aircraft fuselage by a fixed frame 32. Within the fixed frame 32 a roll gimbal 34 provides stabilization about a roll axis 36 and a pitch gimbal 38 provides stabilization about a pitch axis 40. The roll and pitch gimbals 34 and 38 are driven so as to direct the laser beam 16 to the selected point below the aircraft.

The stabilized platform 30 is inertially stabilized about two axes, roll and pitch. It is located so that the laser beam and spectrometer field-of-view are directed downward in approximate coincidence with the yaw axis 42 of the aircraft to minimize the effect of yawing motion on the pointing of the beam. An inertial reference sensor on the platform 30 senses any residual 3-axis motion of the platform with respect to inertial space and provides torquing commands to pitch and roll gimbals. Rate bias commands, derived from ground speed and altitude-above-ground, are provided to compensate for translation of the aircraft during activation of the laser and spectrometer.

The altitude information is also applied to adjust the focus of the laser beam expander and spectrometer optics, described below, to maintain the ground surface within the depth of focus of the optics. Other well known optical stabilization methods, such as the inertia-referenced coelostat, may be substituted for the stabilized platform 30.

The optical system 24 is schematically illustrated in greater detail in FIG. 3. The laser 22 is driven by a pulse control circuit 43 to generate the laser beam 16 which is annular and is directed to a focus mirror 44 moveable in the directions indicated by the arrow 46. An annular laser beam can be generated by using an unstable optical resonator. The mirror 44 reflects the annular laser beam 16 to a Cassegrain expander mirror 48 which in turn directs the beam to an annular flat reflector 50. The reflector 50 directs the laser beam as an annular converging beam toward the sample point 18 on the earth's surface. The focus mirror 44 is moved back and forth in the direction of the arrows 46 in order to focus the laser beam 16 to have a preselected diameter as it strikes the earth at the sample point 18. The radiation 20 generated by the vaporized mineral sample is returned to the optical system 24 to a converging reflector 52 which directs the radiation 20 to a focus reflector 54 which in turn directs the returned radiation into an entrance slit 56 of the spectrometer 28. The focus reflector 54 is moved in the direction of the arrows 58 in order to focus the image of the radiation 20 on the entrance slit 56.

The data produced by the spectrometer 28 is channeled to a recorder 60 which records the data to permit additional correlation and analysis after collection.

Both the downward directed laser beam 16 and the returned radiation 20 pass through a window 61 in the base of the aircraft 10. A suitable window material is zinc selenide, however, the window may alternatively be an opening in the aircraft.

The optical system illustrated in FIG. 3 generates a downward directed laser beam coaxial with the spectrometer line-of-sight to eliminate the problem of parallax which would be present if the coaxial orientation was not employed. The annular laser beam 16 generated by the laser 22 is a conventional coherent laser beam. The focus mirror 44 reflects this beam to the Cassegrain expander mirror 48. This mirror reflects the annular beam to the annular flat reflector 50. By movement of the focus mirror 44 in the direction of the arrows 46 the convergence of the beam is varied as it is provided to the annular flat reflector 50. The variation in this focus causes the beam 16, which is annular and conical, to be concentrated at a focus point. This focus point is selected to be of a predetermined size and the focus mirror 44 is adjusted to produce this focus point on the surface of the earth. The radiation 20 generated by the vaporized minerals is returned through the interior of the annular beam 16 and transferred to the optical elements 52 and 54 which direct the radiation into an entrance slit 56 of the spectrometer 28. The focus mirror 54 is positioned so as to project the image of the vaporized minerals from the sample point 18 into the spectrometer 28. Both the spectrometer optics and the laser optics are mounted on the same stabilized platform 30, and therefore, the spectrometer will at all times be aimed at the point of impact of the laser beam 16.

In operation over land areas the laser beam 16 as shown in FIG. 1 is directed at the sample point 18 on the surface of the earth with a power density sufficient to vaporize a sample of the surface material and then heat the resulting vapor to a temperature sufficient to yield atomic emission radiation, typically in the range from ultraviolet to near infrared. This radiation is returned to the spectrometer which measures the radiation wave lengths and intensities to determine the type of mineral vaporized. The analysis can reveal not only the types of minerals present but also the quantitative ratios of the minerals. A portion of the radiation 20 will be absorbed by various gases in the atmosphere but this absorption is well characterized and can easily be compensated for.

When a high-energy laser is operated in the atmosphere, a fraction of the energy of the laser is absorbed by the atmosphere thereby heating the air along the path of the laser beam. This heated air, which is of a differing density from surrounding air, creates an optical disturbance within the atmosphere to cause the laser beam to be diverged from its desired focus. In order to overcome this problem, the laser of the present invention is operated in a pulsed mode with the pulse width shorter than the thermalization time of the dominant radiation absorbing molecules (e.g., CO.sub.2) of the air. Therefore, for each pulse, there is insufficient time for the air to be heated and create an optical disturbance to interfere with the beam. However, an optical disturbance is still created following the pulse, and if the next pulse is directed through the same air space, the optical distortion problem will again be encountered. But, since the aircraft is moving, the pulse-to-pulse interval is selected such that the aircraft advances by at least one beam aperture diameter between pulses. Hence, the line-of-sight from the aircraft to the surface spot is moved ahead of the disturbance created by the previous pulse.

Due to the short period and limited energy of the laser pulse, only a very thin layer of surface material will be vaporized by each of the pulses. The vapor produced by the pulse and caused to radiate will be representative of only the very thin layer which has been vaporized. Therefore, with a series of pulses directed to the same sample point deeper and deeper layers of material will be vaporized and therefore sampled. The power applied to the sample is of such magnitude that the surface material will virtually explode outward and clear the sample area for the next laser pulse. Therefore each sample will produce radiation substantially associated with only the particular layer which has been vaporized. This procedure is particularly useful in survey sampling of land areas where the rock structure is covered with a thin weathered layer which is chemically different from the bulk of the rock material. A typical type of weathered layer is referred to as "desert varnish" and is typically of a limited thickness which can be easily penetrated by the laser beam. At a sample point, the vapor produced by each laser pulse can be individually examined in real time and correlated with known spectra to provide instant identification of selected materials.

Since a number of laser pulses are directed to each sample point during a time period, the aircraft will move a given distance, therefore the aim of the laser beam must be compensated for this movement so that each of the pulses strikes the same sample point. The sample points themselves are selected at given distances along the track of the aircraft dependent upon the sample rate desired and the speed of the aircraft.

A set of operating parameters suitable for a wide-area mineral survey to be taken from a small aircraft are as follows. The laser is operated to generate pulses at a rate of 200 per second with each pulse having an energy level of 25 joules. Each pulse has a duration of 50 microseconds and a group of 20 pulses are directed to each sample point for each burst. The burst rate or rate of sampling is selected to be 1.333 per second. The output aperture diameter of the laser beam at the aircraft is 0.25 meters and the beam has a width of 17 arc secs.

These parameters are selected to be compatible with an aircraft speed of 85 meters/sec. (190 MPH) operating at an altitude of 100 meters (328 ft.) with a travel distance between bursts of 63.8 meters (209 ft.).

The diameter of the sample point is then 8 millimeters which together with the 25 joule energy level produces an energy density of 50 joules per square centimeter. In a typical rock sample this power level will produce a penetration for each pulse of 0.05 millimeters resulting in a total penetration with 20 pulses of 1.0 millimeter. This power level produces a surface irradiance of about 1 million watts per square centimeter with a peak electric field of about 25 kilovolts per centimeter. This electric field value is less than the dielectric strength of the air thereby preventing electrical breakdown of the air. The time periods of the pulses are so short as to lose only about 1% of the energy due to heat transfer to the surrounding rock.

A laser source suitable for use in the present invention is the model 971 gas transport laser manufactured by GTE Sylvania, modified for pulsed operation. This carbon dioxide laser produces a continuous output power of 5,000 watts with a 12 millimeter output beam. If the aircraft power system is inadequate to drive the laser, an outboard turbo alternator can be provided to generate the necessary electrical power. This laser operates at a wave length of 10.6 micrometers.

The spectrometer for use in the present system can be a model TN-1710 diode array rapid scan spectrometer system manufactured by Tracor Northern of Middleton, Wis. This system has a spectral response ranging from 200 to 1100 nanometers with a dynamic range of 4,096:1.

In a further embodiment of the present invention, a second lower power laser source 29 is employed to illuminate the zone of vaporization of the mineral sample after the high energy laser pulse has terminated. The atomic emission of the vaporized material decays rapidly due to expansion and cooling but the vapor itself remains present for a short period longer, and if this vapor is illuminated by a low power laser 29, for example, a nitrogen laser operating at 337.1 nanometers and emitting 5 millijoules of energy, remote Raman spectrometry of the vaporized material can be obtained before the vapor has diffused into the surrounding atmosphere. After the vapor itself has been diffused the same laser may be used to illuminate the freshly cleared surface and cause that surface to yield a fluorescence spectrum which can likewise be analyzed for mineral content. The second low power laser 29 can be pulsed after each high-energy laser pulse for analyzing each surface sample on a pulse for pulse basis. Thus, the atomic emission, Raman and fluorescence spectra can all be obtained in conjunction with each high-energy laser pulse for comprehensive analysis of the minerals at the sample point.

The analysis techniques described above are suitable for both day and night operation. For daytime surveying, concurrent photography can be used to advantage by synchronizing the camera shutter with the laser pulse. A flash of visible radiation will be produced from the irradiated spot which will be recorded along with the surrounding terrain to facilitate determination of the exact location of the sample point.

Although several embodiments of the invention have been illustrated in the accompanying drawings and described in the foregoing detailed description, it will be understood that the invention is not limited to the emobdiments disclosed but is capable of numerous rearrangements, modifications and substitutions without departing from the scope of the invention.