Method and apparatus for detecting a tracer gas using a single laser beam

A method and apparatus for airborne prospecting for base and precious metal deposits, petroleum and natural gas deposits, geothermal steam deposits, and leaks in natural gas pipelines. A trace gas associated with the deposits in the near-surface atmosphere is detected with a differential absorption optical technique utilizing only a single laser beam to perform remote differential optical measurement. Anomalies in the tracer gas, which are indicative of underground deposits, are detected by transmiting a laser beam having narrow linewidth laser pulses at a high repetition rate with a center wavelength approximately equal to the atomic absorption line of the tracer gas to the area under investigation. The pulses are directed toward the investigated area from an airborne platform, reflected off the ground, and are collected by a detector on the airborne platform. Each pulse received is then broken down into a portion containing energy which is coincident with the absorption line of the tracer gas and a portion which is non-coincident with the absorption line of the tracer gas using a special optical filter. A tracer gas cell removes all the energy from the pulse which is coincident with the tracer gas absorpotion line, and the energy detected from the tracer gas cell corresponds to the amount of energy in the pulse which is off-resonance. By subtracting the off-resonance energy from the total energy received, it is possible to calculate the energy in the pulse which is received in the on-resonance spectral interval.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to detecting a tracer gas in the near-surface 
atmosphere. More specifically, the present invention relates to a system 
for remotely locating mineral deposits, petroleum and natural gas 
deposits, geothermal steam deposits and leaks in natural gas pipelines by 
detecting a tracer gas using a single laser beam. 
2. Description of the Prior Art 
In the early stages of man's quest for natural gas and petroleum, almost 
all drilling for underground deposits followed a pattern of selecting 
drill sites by the proximity to oil and gas seeps which could be perceived 
without the aid of any equipment. Soon, however, deposits causing these 
visible seeps became exhausted, and those who wished to prospect for 
petroleum had to begin to rely on other methods. Over the past century, 
other techniques have been discovered and developed employing, for 
example, the refraction seismograph, the reflection seismograph, the 
magnetometer, and the gravameter. Together with geological analysis, the 
use of these geophysical tools has helped the prospector in his seemingly 
impossible task of locating with some degree of accuracy an underground 
deposit. 
Without any direct surface indication, however, such indirect geophysical 
methods of prospecting are necessarily time consuming, laborious, 
complicated and expensive. In addition, and perhaps most importantly, 
these techniques have only marginally improved the exploratory drilling 
success ratio, i.e., the number of successful wells drilled versus the 
total number of wells drilled, since the period after the exhaustion of 
deposits associated with visible seeps and before such geophysical methods 
were available. 
It would therefore be an enormous improvement if means were found to detect 
the most tenuous of seeps which have so far remained undetected in order 
to prospect for underground deposits. In most cases, these seeps will be 
located above "fault trap" types of deposits since the fault allows for a 
relatively easy communication of light hydrocarbon gases from the deeply 
buried deposit to the surface. However, undetected seeps may also be 
located above stratographic, anticline, reef, upward unconformity, salt 
dome and pinch out types of reservoirs. Obviously, they may be more easily 
distinguished above shallow reservoirs and over reservoirs which have a 
relatively porous caprock. 
Numerous ground-based methods exist for measuring the concentration of 
hydrocarbons entrained in the soil and those which exist in the 
near-surface atmosphere. These methods necessitate that measurements be 
performed on the ground, however, in the vicinity of the seep. Surveying 
large amounts of land through the use of such ground-based methods is 
expensive and time consuming. 
To overcome this problem, prospectors have developed a technique for 
remotely detecting light hydrocarbon molecules as they may exist emanating 
from the ground, indicating the presence of a deposit. Thus, for example, 
U.S. Pat. No. 3,651,395 to Owen et al teaches that light hydrocarbon 
molecules may be emanating from an underground hydrocarbon deposit. Owen 
et al teach the detection of the light hydrocarbon molecules through the 
use of their microwave re-radiation characteristics. In this scheme, 
target molecules are bombarded with microwave radiation that comes from a 
microwave transmitter on board an airborne platform. The light hydrocarbon 
molecules, after being struck by the microwave energy, rebroadcast at a 
different frequency. The particular frequency shift is characteristic of 
the target molecule in question. The rebroadcast radiation may then be 
monitored at the airborne platform. 
Problems exist with this method in that it is necessary to bombard the 
earth's surface with microwave energy. In order for the light hydrocarbon 
molecules to radiate sufficient energy, a great deal of energy must be 
transmitted from the airborne platform. Even then, the signal received 
from the molecules is very weak, introducing sensitivity problems. 
U.S. Pat. Nos. 4,100,481 to Gournay and 4,132,943 to Gournay et al both 
teach the use of microwave energy to excite gas. U.S. Pat. No. 3,351,936 
to Feder also teaches the use of electromagnetic waves (either radio 
frequency or microwave) to explore subterranean structures. Two different 
radar wavelengths which have different penetration characteristics are 
transmitted and reflections are detected. 
U.S. Pat. No. 3,741,653 to Svetlinchny teaches the use of an aircraft as a 
base for a laser measurement system. This system is intended only to 
monitor ground contours and not to detect the presence or location of 
particular materials. However, the remote detection of gases employing 
lasers is taught in Murray, "Remote Measurement of Gases Using Discretely 
Tuneable Infrared Lasers" in Proceedings of the Society of Photo-Optical 
Instrumentation Engineers, Vol. 95, pp. 96-104 (1976). This article 
teaches the projection of two laser beams having different frequencies 
through a remotely located sample chamber. Radiation reflected from a 
topical feature is detected, and the differential amount of absorption 
between the two laser beams is employed as an indication of the 
concentration of the sample gas in the remotely located chamber. 
U.S. Pat. Nos. 3,861,809 to Hall and 3,807,876 to Nakahara both teach the 
measurement of the amount of absorption of light of a particular frequency 
to determine the concentrations of particular gases. 
It is known to those skilled in the art that trace gases having spectral 
absorption characteristics in the ultraviolet range can be detected 
remotely, during prospecting, through the use of differential absorption 
laser radar (DIAL) techniques. Ahmed et al, "Remote Monitoring of Gaseous 
Pollutants by Differential Absorption Laser Techniques", Environmental 
Protection Agency, EPA-600/2-80-049, for example, teaches the remote 
detection of sulphur dioxide and nitrogen dioxide in the ultraviolet 
spectral region. In addition, Alden et al, "Remote Measurement of 
Atmospheric Mercury Using Differential Absorption Lidar", Optics Letters, 
Vol. 7, No. 5, May 1982 teaches the remote detection of sulphur dioxide 
and nitrogen dioxide in the ultraviolet spectral region. Also, Alden et al 
teach the remote detection of atmospheric mercury using differential 
absorption laser radar. Browell et al, in "Airborne Differential 
Absorption Lidar System For Water Vapor Investigations", Optical 
Engineering, January/February 1981, Vol. 20, No. 1, pp. 84-90, teach the 
use of DIAL to detect water vapor. However, the disadvantages of these 
techniques stem from the fact that two separate laser beams must be used 
to perform the differential absorption measurement. 
Using two separate laser beams to perform the differential absorption 
measurement severely limits the sensitivity of the measurement. It is also 
known to those skilled in the art that during each measurement, the 
turbulent effects of the atmosphere play a role in degrading the 
measurement sensitivity. As taught by Killinger and Menyuk, "Remote 
Probing of the Atmosphere Using a CO.sub.2 DIAL System", IEEE J. Quant. 
Elect. Vol. QE-17, No. 9, September 1981, introducing a distinct time 
delay between the time of travel of one laser pulse with respect to the 
time of travel of another laser pulse will increase the statistical noise 
of the measurement. Thus, the sensitivity achievable using this technique 
is limited, and such a time delay between the two laser pulses is 
necessary when operating a single laser differential absorption lidar 
(DIAL Lidar) in the "sequential" mode due to the recovery time of (the 
laser the laser can not fire fast enough). 
As disclosed in U.S. patent application Ser. No. 531,729, now abandoned, a 
DIAL Lidar can also operate in the "simultaneous" mode. In this mode, two 
physically separate laser beams are sent out simultaneously. Thus, there 
is no time delay between them. However, this mode is generally not 
feasible for gases having their absorption spectra in the ultraviolet 
because of the difficulty of manufacturing spectral bandpass filters which 
will allow one but not both of the beams to pass through. Furthermore, 
even if the problem of manufacturing sufficiently workable bandpass 
filters could be solved, thus providing a means to discriminate between 
the two beams when they return to the measurement platform, there is still 
a disadvantage in operating with two physically separate laser beams in 
the ultraviolet in the "simultaneous" mode. When operating in either the 
sequential or the simultaneous mode, for example, beam pointing errors 
degrade the measurement. This effect is caused by the fact that the two 
pulses may propagate through slightly different beam paths and reflect off 
slightly different sections of the ground. Thus, the difference in their 
measured intensities may be due to other factors rather than due to the 
presence of the tracer gas in the atmosphere. 
The use of two laser beams also constitutes a serious practical 
disadvantage because the apparatus necessary to perform the measurement 
has to consist of two physically separate laser systems. The cost of two 
laser systems is a serious disadvantage. In the case of operation in the 
simultaneous mode, the apparatus has to consist of two physically separate 
laser systems in order to be able to generate the two different 
wavelengths simultaneously. In the case of operation in the sequential 
mode, on the other hand, the apparatus may also consist of two separate 
laser systems because of the need to fire over time intervals very much 
smaller than existing laser repetition rates will allow one to fire over 
because of the need to minimize the measurement error associated with 
changes in intensity due to a turbulent atmosphere. Thus, it is desirable 
for the time interval between pulses to be very small, which heretofore 
has been impossible with a single laser because of the inability to fire a 
single laser fast enough (on time frames less than 100 microseconds). 
Canadian Pat. No. 808,760 to Bradley et al teaches a DIAL technique 
employing a single laser simultaneously producing two narrow-band 
frequencies. However it is difficult to achieve flexibility in selecting 
the frequencies produced and to control the relative power levels at the 
two frequencies. 
It is also known to those skilled in the art that hydrocarbon deposits, 
mineral deposits, and geothermal steam deposits can be located through the 
use of mercury vapor anomalies. These anomalies occur in the soil gas and 
in the near-surface atmosphere. As taught in the article to Kartsev, the 
mercury vapor anomalies may indicate the presence of a buried petroleum or 
natural gas deposit because mercury as an atom in the environment often 
becomes absorbed into organic materials in sedimentary basins, and the 
element often becomes associated with petroleum and natural gas in buried 
pools. Furthermore, as taught by D'Itri and D'Itri, Mercury Contamination: 
A Human Tragedy, Wiley & Sons, New York (1977), up to 1.4 million pounds 
per year of mercury are released into the atmosphere from the burning of 
fossil fuels such as coal, oil, and natural gas. Accordingly, discoveries 
of high concentrations of mercury vapor are often indicative of the 
presence of an oil or gas deposit, since mercury is trapped in the 
supergene enrichment process by recycling into carboniferous precursor 
beds. 
Also, it is known to those skilled in the art that deposits of precious and 
base metals may be detected through the use of mercury vapor anomalies in 
the soil gas and in the near-surface atmosphere. Hawkes and Williston, 
"Mercury Vapor as a Guide to Lead-Zinc-Silver Deposits", Mining Cong. J., 
December 1962, for example, teaches that patterns of mercury vapor may 
exist above concealed mineral deposits which can be used as exploration 
targets in the search for ore bodies. It is also known to those skilled in 
the art that geothermal steam deposits can be located with the aid of 
mercury vapor soil gas surveys. Matlick and Buseck, "Exploration for 
Geothermal Areas Using Mercury: A New Geochemical Technique", Proc. Second 
U.N. Geotherm, Symp., Govt. Printing Office (1976), for example, teaches 
the advantages of this mercury vapor technique in prospecting for 
geothermal deposits. 
Thus, there is a need for a method and apparatus to overcome the 
limitations of having to use two physically separate laser beams to make a 
DIAL lidar measurement in order to locate a tracer gas such as mercury 
vapor in the ultraviolet range such that petroleum and natural gas, 
geothermal steam, and precious and base metals deposits can be located. A 
better method and apparatus is also needed for locating the presence of 
leaks in natural gas pipelines from a remote location, for while the 
presence of leaks somewhere in the line can be ascertained by a measured 
pressure drop, locating the leak exactly is much more problematical. 
SUMMARY OF THE INVENTION 
The purpose of the present invention is to overcome the disadvantages noted 
above in prior art atmogeochemical prospecting devices by eliminating the 
need for the use of two lasers or the sequencing of laser pulses in a 
differential absorption technique. 
This purpose is accomplished in the present invention by using a laser 
transmitter which provides high-power, laser pulses at a high repetition 
rate with a bandwidth including the resonant frequency of the tracer gas 
but being broader than the resonance of the gas. The pulses are then 
directed toward the area under investigation for the tracer gas. In most 
cases, the apparatus will be carried aboard an airborne platform, and the 
area of investigation will be the area beneath the aircraft as it moves 
along the flight path. The pulses travel down through the atmosphere, 
reflect off the ground, and then are collected back onboard the aircraft 
by means of a large aperture telescope. 
Each pulse has a portion of its energy coincident with the absorption line 
of the tracer gas and a portion which is non-coincident. As each pulse 
returns to the aircraft, it is broken down into these component portions 
by means of a special optical filter or "analyzer". The analyzer consists 
of a beam splitter, two photodetectors, and a filter which may consist of 
an optical cell containing a saturated volume of the tracer gas. The beam 
splitter splits the received pulse into two physical parts. The first part 
goes directly to a photodetector which measures the total power or energy 
contained in the pulse. The second part passes through the tracer gas cell 
and then impinges on the other photodetector which measures the energy or 
power falling on it. Since the tracer gas cell will remove all the energy 
from the pulse which is coincident with the tracer gas absorption line, 
the only energy the second photodetector will measure is that amount of 
energy in the pulse which is "off-resonance". By subtracting the 
off-resonance energy from the total energy received, it is possible to 
calculate the energy in the pulse which was received in the "on-resonance" 
spectral interval. 
The present invention then takes these detected on-resonance and 
off-resonance values and compares them with the known values of the 
on-resonance and off-resonance energies which were first sent out by the 
transmitter. The concentration of the tracer gas in the atmospheric path 
of the light pulse is then calculated in a processing unit continuously as 
the airborne platform upon which the present invention is placed flies 
over various terrains. In this manner, it can be determined whether or not 
any anomalous concentrations of the tracer gas exist in the near-surface 
atmosphere. If any such anomalous concentrations are found, they are 
correlated with the existence of underground mineral or hydrocarbon 
deposits or with the presence of a leak in a natural gas pipeline. 
The present invention may be used as a first-stage exploration tool with 
discovered anomalies verified later by geophysical or other means, or it 
may be used as a corroborating tool itself, verifying what electromagnetic 
pulse or other exploration means may have identified. Accordingly, the 
method and apparatus of the present invention represents an improvement 
over the previous techniques for atmogeochemical prospecting for 
underground deposits in that it uses only a single ultraviolet laser beam 
to effect a remote quantitative measurement of a tracer gas such as 
mercury vapor in the exterior atmosphere.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENT OF THE INVENTION 
As noted in the background of the invention, mercury vapor has been used as 
a pathfinder gas for locating underground deposits of petroleum, natural 
gas, geothermal steam, and precious and base metals. Mercury vapor is a 
particularly good pathfinder gas because it has a differential absorption 
spectra with a very sharp transition as shown in FIG. 5a. This is so 
because the differential absorption of the mercury gas is caused by an 
atomic transition and not a molecular transition in response to the 
incident light. As also shown in FIG. 5a, the incident light is more 
readily absorbed at the frequency corresponding to the atomic absorption 
line of mercury, which occurs at 2536.50 Angstroms. Accordingly, the 
present invention will be described below with reference to mercury vapor 
as a tracer gas. However, as will be apparent to one skilled in the art, 
detection of other tracer gases using the disclosed method and apparatus 
is within the scope of the present invention. 
It is not generally known that leaks in pipelines can be located by 
detecting mercury vapor anomalies in the soil gas and near-surface 
atmosphere above buried natural gas transmission and distribution lines. 
There is no known use of this technique in the industry. However, the 
present inventor has discovered that mercury is present in the natural gas 
being transmitted through the transmission lines because the contamination 
is originally present in the gas when it gets pumped out of the ground, 
and chemical "scrubbing" does little to remove it. The mercury vapor thus 
travels along with the commercial gas and escapes along with it if there 
is a leak in the pipeline. Generally, mercury vapor is highly absorbed by 
the iron in the pipeline walls; however, there are measurable quantities 
of mercury vapor in the pipeline hundreds of miles from the original 
field, thus rendering the mercury vapor soil gas or near-surface 
atmospheric technique a highly viable one for detecting leaks in 
pipelines. Such a technique is generally shown in FIG. 1, in which the 
apparatus of the present invention is mounted on an airborne platform on a 
helicopter which flies over the buried natural gas pipeline being checked 
for leaks. 
Turning now to FIG. 2, the optical schematic of the means for generating 
the laser beam and detecting the reflected laser radiation for such 
purposes are shown. The apparatus of the present invention consists of a 
laser transmitter 10 which generates a specific bandwidth, high-powered 
pulse of radiation 12 centered at the atomic absorption line of the tracer 
gas. As noted above, by way of example, it will be assumed for purposes of 
the present description that the tracer gas is mercury vapor having an 
absorption line centered at 2536.5 Angstroms. Accordingly, in the 
preferred embodiment, laser transmitter 10 transmits pulses of high 
powered laser radiation 12 centered at 2536.5 Angstroms. 
As shown in FIG. 3, in the preferred embodiment, the laser transmitter 
consists of an excimer laser 14 which pumps a dye laser 16. Such lasers 
are readily available. For example, the excimer laser may be a Lambda 
Physik Model EMG-53MSC excimer laser, and the dye laser may be a Lambda 
Physik Model FL2002e dye laser. The excimer laser outputs light 18 into 
dye laser 16 which is then pumped up so as to output light 20. Dye laser 
output 20 then pumps a potassium pentaborate optical crystal 22 which 
doubles the frequency of the dye laser light output to 2536.5 Angstroms; 
however, a Raman cell or other similar device may also be used to double 
the frequency of the dye laser output. The laser transmitter is capable of 
pulsing at a rate of 200 Hz with approximately 0.5 millijoule of output 
energy in a 0.05 cm-1 cm spectral width. For purposes of the present 
invention, it is desirable for the spectral width of the laser transmitter 
output to be centered at the atomic absorption line of the tracer gas and 
to have a spectral bandwidth greater than that of the tracer gas. Such 
limitations can be met using the laser transmitter apparatus shown in FIG. 
3 for a tracer gas such as mercury vapor. 
Referring back to FIG. 2, the outputted laser light 12 is split into two 
portions before it is projected toward the target. Beam splitter 24 taps 
off a small portion 26 of the beam after it emerges from laser transmitter 
10. This small portion, representing a sampling of approximately 5%, of 
the total pulse energy, is used for reference purposes. It is then split 
into two parts by beam splitter 28, each part of which represents 
approximately 50% of the total sampled portion. Sampled portion 30 is 
directed toward photodetector 32 which measures the total sampled pulse 
energy. Sampled portion 34, on the other hand, is directed through a 
calibration cell 36 which contains a known concentration of the tracer gas 
(mercury vapor) maintained at a fixed temperature. The temperature of the 
mercury vapor is fixed by use of a temperature stabilization mechanism 
(not shown). 
Calibration cell 36 is of a precise length and is used to determine the 
effective molecular absorption coefficient for each laser pulse so as to 
determine the effective absorption cross-section for the mercury vapor. 
Calibration cell 36 removes part of the energy in the reference pulse 
which is coincident with the absorption line of mercury. This provides a 
means of determining the linewidth of each outgoing pulse 12 as well as 
determining the center frequency of each outgoing pulse 12. Pulse energy 
38 outputted by calibration cell 36 is then measured by photodetector 40. 
Photodetectors 32 and 40 may be of any commercially available type, such as 
pyroelectric, photoconductive, or photovoltaic detectors, or they may be 
photomultiplying tubes. These detectors may be cryogenically cooled, 
thermoelectrically cooled or they may be at room temperature. 
Beamsteering mirrors 42 are used to direct the main portion of the beam 
emerging from transmitter 10 to beam expanding telescope 44 which is 
mounted collinearly with the receiving telescope 46. The beam is then 
expanded to increase the beam size to a practical diameter for efficient 
prospecting in pipeline leakage control or atmogeochemical prospecting. 
The width of the beam is governed by the amount of interference which the 
system may tolerate. For example, if the beam is spread out too far, too 
much outside interference may be detected; however, if the beam is too 
narrow, it may be extremely difficult to cover a very large prospecting 
area with the beam in a given time interval. Once the desired beam width 
is chosen, beam 48 is projected down to the surface of the earth. On its 
way to the surface, and after it has reflected off topographical target 
50, the beam may encounter mercury vapor 52 in the atmosphere. An amount 
of the light radiation is absorbed by mercury vapor 52, the amount 
depending upon the amount of mercury vapor present. 
After the beam has reflected off topographical target 50 and propagated 
back to the airborne platform, return beam 54 collected by large aperture 
Dobsonian Newtonian reflecting telescope 46. Focusing optics 56 ensure 
that the beam is properly focused. Field stop 62 ensures that the 
radiation which passes further into the receiver is spatially coincident 
with the projected cone of radiation from the laser transmitter. After 
passing through field stop 62, the detected radiation impinges on bandpass 
filter 66 which removes all stray radiation outside the preferred spectral 
region. In the preferred embodiment, bandpass filter may be, for example, 
an Oriel Model 56400 Mercury Line Interference Filter which has a 110 
Angstrom FWHM bandwidth and a transmission of 20% at the center wavelength 
of 2536.5 angstroms. 
Outputted beam 68 then enters analyzer 70, which can be seen in more detail 
with reference to FIG. 4. As shown in FIG. 4, analyzer 70 contains a beam 
splitter 72 which splits the detected radiation into two parts. The first 
part 74 is detected by photodetector (photomultiplying tube) 76. The 
second part 78 passes through an optical cell 80 containing a saturated 
volume of the tracer gas (mercury vapor) at a regulated temperature. The 
temperature of the cell is regulated by a temperature regulation mechanism 
82. The temperature of the near-surface atmosphere being targeted may be 
determined by a remote temperature sensor (not shown) which relays the 
temperature to the temperature regulating mechanism for adjustment. In the 
absence of such a temperature adjustment mechanism, the temperature of the 
cell may be kept at a temperature which is judged to approximate the 
average temperature of the area under investigation. It is important that 
the temperature of the mercury vapor in the optical cell be regulated and 
kept at the temperature of the area under investigation because of the 
wide fluctuation its absorption cross-section can have with temperature. 
The effect of optical cell 80 is to absorb all of the optical energy in the 
central spectral portion of the beam coinciding with the optical 
absorption line of mercury. Thus, the only energy which passes through 
optical cell 80 is that which is contained in the "wings", or regions of 
the beam's spectrum which are on either side of the mercury absorption 
line. The intensity of beam 84 after it passes through optical cell 80 is 
detected by photodetector (photomultiplying tube) 86. As shown in FIG. 5B, 
the portion of the detected energy centered at the mercury absorption line 
is removed by optical filter 80. In the preferred embodiment of the 
present invention, photodetectors 76 and 86 may be Thorn EMI Model 6818QB 
photomultiplying tubes which have a very high responsivity in this 
wavelength region. 
Referring now to FIG. 6, a schematic of the receiving and control 
electronics of a single beam mercury vapor LIDAR prospecting system in 
accordance with the preferred embodiment of the present invention is 
shown. 
As shown, the signals outputted by photodetectors 32 and 40 associated with 
calibration cell 36 are preamplified by preamplifiers 90 and 92, and the 
signals outputted by the photomultiplying tubes 76 and 86 are preamplified 
by preamplifiers 94 and 96, respectively. The output of preamplifiers 
90-96 are then digitized by analog to digital converter 98. In the 
preferred embodiment, analog-to-digital converter 98 may be a LeCroy 
Research 2249 SG Separately Gated ADC with a charge integrating feature. 
The ADC channel gates for analog-to-digital converter 98 are opened by 
trigger generator 100 which functions as a discriminator and a gate 
generator. A control signal from trigger generator 100 causes the 
appropriate channel of converter 98 to accept data from preamplifiers 
90-96 and convert it to digital form. 
A particular type of discriminator used in the preferred embodiment of 
trigger generator 100 is a threshold level discriminator, but in general, 
any type of discriminator may be used to establish the detection of the 
light pulse by detectors 76 or 86 including zero-crossing discriminator 
and signal rise time discriminators. In addition, a peak sensing 
analog-to-digital converter may be employed. In fact, the discriminator 
may be replaced with an external altimeter to open the appropriate channel 
of converter 98 after a preselected delay from the laser firing 
corresponding to the detected altitude. 
The preferred gate generator for trigger generator 100 is the LeCroy Model 
2323 Dual Gate and Delay Generator which, upon command from the 
discriminator, supplies a 100 nanosecond gating pulse on line 102 to 
analog-to-digital converter 98. Trigger generator 100 is informed of the 
fact that the light pulses have returned to the measurement platform by 
the presence of signal current in preamplifier circuit 94 (preamplifier 
circuit 96 may also be used), the presence of which is detected by trigger 
generator 100. 
As well as providing gate signals for the analog-to-digital converter 98, 
trigger generator 100 supplies a signal to a time-to-digital converter 
(TDC) 104 to tell it to stop counting. In the preferred embodiment, 
time-to-digital converter 104 may be a LeCroy Model 4201 Time to Digital 
Converter. Time-to-digital converter 104 is employed to measure the time 
of flight of the transmitted laser pulse in order to measure the distance 
to a topographical target. This distance measurement is supplied to 
microcomputer 60. 
Synchronization of the present invention is controlled by clock 112. In 
response to a signal from microcomputer 60, switch relay unit 114 causes 
clock 112 to begin producing signals. In response to the signal from clock 
112, laser transmitter 10 fires. Included within laser transmitter 10 is 
another clock which generates sync pulses which are applied to 
analog-to-digital converter 98 to cause appropriate channels to open to 
cause data from preamplifiers 90 and 92 to be converted into digital form. 
Also, as is clear to those skilled in the art, laser transmitter 10 need 
not have an internal clock. Instead, an external clock or any other timing 
mechanism may be employed to control the firing sequence of the laser and 
the opening of channels in analog-to-digital converter 98. 
Signals indicating the firing of laser transmitter 10 are also applied to 
time-to-digital converter 104. As noted above, this unit is employed to 
generate an indication of the distance the laser beam travels between 
generation and detection. Upon receipt of the firing signal from laser 
transmitter 10, converter 104 begins counting. When trigger generator 100 
receives an indication that laser radiation has been received by detector 
76, trigger generator 100 generates a control signal on line 102 to cause 
the counter in time-to-digital converter 104 to stop counting. The count 
stored in converter 104 is a measure of the round-trip travel time of the 
laser radiation and is immediately transformable into the distance that 
the beam travels upon multiplication by the speed of light. 
Data converted by analog-to-digital converter 98 is applied to 
microcomputer 60 along with the distance data generated by time-to-digital 
converter 104. The intensities of the two different spectral portions of 
the received beam digitized by the analog-to-digital converter 98 are then 
analyzed by microcomputer 60 together with the measured intensities of the 
transmitted laser pulse 12. Microcomputer 60 calculates the concentration 
of the mercury vapor in the target area by using the following equation: 
##EQU1## 
where: N.sub.atmos =Concentration of mercury vapor in atmosphere in parts 
per billion, 
N.sub.cell =Concentration of mercury vapor in calibration cell 36 in parts 
per billion, 
L=Length of calibration cell 36 in meters, 
P=Intensity of received radiation, on-resonance, in watts, 
P'=Intensity of received radiation, off-resonance, in watts, 
R=Distance to target in meters, 
P.sub.c I =intensity of transmitted radiation, on-resonance, in watts and, 
P.sub.c '=Intensity of transmitted radiation off-resonance in watts. 
It should also be noted that the total power transmitted and received (as 
detected by detectors 32 and 76, respectively) minus the power "in the 
wings" (as detected by detectors 40 and 86, respectively) is equal to the 
power at the on-resonance frequency. Detectors 40 and 86, of course, 
indicate off-resonance power transmitted and received respectively, 
directly. All of this raw measurement data (including position data to be 
described below) is stored in digital form in backup memory 116. 
Microcomputer 60 calculates the concentration of the target gas for each 
laser pulse and then shows the measured concentration in real-time on 
display 118, which may be, in the preferred embodiment, a Princeton 
Graphics Model SR 12 high-resolution CRT monitor. Also, microcomputer 60 
receives position information from position indicator 120, which feeds 
coordinates to microcomputer 60 for determining the position of the 
differential absorption measurement, either with respect to fixed ground 
stations, or known satellite positions. At present, satellite positioning 
systems are limited by the fact that they do not give measurements in 
real-time 100 percent of the time. However, further advances in the state 
of the art are expected to change this soon so that precise, real time 
position information will be available all of the time. 
There are many readily commercially available radio positioning systems 
which are applicable for giving a position of an aircraft. These are, in 
general, much more accurate than simple navigational aids such as LORAN-C; 
therefore, the measurement is fixed much more precisely over the ground. 
The readily commercially available radio positioning systems applicable to 
the present invention operate in the microwave or radio portion of the 
spectrum. Some are limited to line of sight measurement (approximately 80 
kilometers at 1.0 kilometer altitude), but others are not. In addition to 
using the radio positioning system on board which reads out the position 
of the plane in real-time, it is also possible, in accordance with the 
general principles of the invention, to calculate the aircraft's position 
by the principle of "dead reckoning", i.e., using the knowledge of the 
speed of the aircraft and the time between each laser measurement. In a 
preferred embodiment of the present invention, however, position indicator 
120 may be a Texas Instruments Model TI4100 Navstar Global Positioning 
System. The output of this navigation receiver is then stored along with 
the mercury vapor concentration data calculated for that point in memory 
device 116 for later processing. In a preferred embodiment, memory device 
116 is a Kennedy Model 6455 Cartridge Tape Drive. 
Instead of measuring the time of travel of the laser beam with 
time-to-digital converter 104, alternate approaches may be taken in order 
to determine an indication of the distance traveled to determine the 
target gas concentration. Thus, if an airborne platform is used for the 
laser and detectors, an external altimeter (either of the barometric or 
microwave variety) may be used. In fact, a fixed distance may be employed 
representing the average diameter of a cloud of target gas. This approach 
recognizes the fact that target gas from a gas seep will remain close to 
the surface of the earth and not be distributed evenly over the path 
between the laser and the point of reflection. 
The manner in which the preferred embodiment operates will now be described 
with reference to FIG. 7, which shows a flow chart of the operations 
performed by microcomputer 60. At the beginning of the measurement cycle, 
after initialization step 200, microcomputer 60, at step 202, generates a 
command for relay switch 114 to begin a laser firing sequence. As a 
result, switch 114 instructs laser transmitter 10, with its built-in 
clock, to begin a laser firing sequence. As a result, laser transmitter 10 
fires and immediately sends a signal to analog-to-digital converter 98 to 
cause it to sequentially convert data from detectors 32 and 34 via 
preamplifiers 90 and 92, respectively. At steps 204 and 206, microcomputer 
60 sequentially reads this data from converter 98. At the same time, a 
signal from laser transmitter 10 also causes time-to-digital converter 104 
to begin counting. 
Radiation 12 from laser transmitter 10 is transmitted toward the area under 
investigation. Eventually, that beam is reflected and detected by detector 
76. This data is received by trigger generator 100, which then produces a 
signal 102 transmitted to both analog-to-digital converter 98 and 
time-to-digital converter 104. The output signal to converter 104 from 
trigger generator 100 causes converter 98 to process data from detectors 
76 and 86. The output signal 102 to time-to-digital converter 104 causes 
the counter within converter 104 to stop counting. Then, at steps 208 and 
210, microcomputer 60 reads the data from detectors 76 and 86 which has 
been converted by analog-to-digital converter 98. Finally, at step 212, 
microcomputer 60 reads the count stored in time-to-digital converter 104 
as an indication of the distance the laser beam traveled. 
At step 220, microcomputer 60 reads the position at which the measurement 
was taken from position indicator 120. At step 222, all of the raw data 
that has been collected from detectors 32, 40, 76 and 86, time-to-digital 
converter 104, and position indicator 120 are stored in backup memory 116 
as a single record. Then, at step 224 the concentration of the target gas 
is calculated using Equation 1 and all of the data that has been 
collected, the concentration then being displayed on real-time display 
118. 
This concludes the basic measurement cycle. However, in the preferred 
embodiment, it is typical that the cycle should be continuously repeated. 
At some point, however, the test will be stopped. At step 226, 
microcomputer 60 determines whether a command has been received to 
indicate that the test should be stopped. If it is to be stopped, at step 
228, a signal is generated to deactivate relay switch 114. 
If the test is to be repeated, control passes to step 230, at which it is 
determined whether the tenth test has just been performed. If it has not, 
processing returns to step 204, at which time clock 112 causes the firing 
sequence to be repeated. Obviously, microcomputer 60 must be synchronized 
roughly with clock 112 and the clock within laser transmitter 10. 
If the test which has just been completed is the tenth test as determined 
by step 230, microcomputer 60 instructs analog-to-digital converter 98 to 
sequentially process data from detectors 32, 40, 76 and 86 with no laser 
radiation being received at step 232. This procedure allows for 
measurement of preamplifier drift. This drift is stored continuously with 
every tenth cycle, and the results are stored in backup memory 116 step 
234 and is useful in later error analysis. Control then returns to step 
204. 
It is advantageous to employ as high a test repetition rate as possible. 
When several laser measurements are performed over the same spot on the 
ground, the specially-resolved concentration measurements can be increased 
in accuracy. This increase, which is brought about by averaging the 
back-scattered laser shots, is found to be proportional to the square root 
of the number of times that the measurement is repeated. The increase in 
accuracy which results from averaging over several laser shots is due to 
the effect of averaging out large scale atmospheric fluctuations which 
affect statistically the propagation of the laser beam. 
The present invention, utilizing the above-described apparatus, is able to 
resolve target concentrations down to 1.0 parts per billion in a one meter 
optical path from a range of about 400 feet. The concentrations of 
atmogeochemical anomalies vary, but many are in the range of 1 to 1000 
parts per billion in the first meter or so of atmosphere. Thus, the 
present invention represents an important tool for atmogeochemical 
prospecting. 
The foregoing description of the presently preferred exemplary embodiment 
has been directed to petroleum and natural gas prospecting in which 
mercury vapors are detected. However, as noted at the outset, it should be 
understood that the general principles of the invention may be used in 
prospecting for other minerals as well, since many mineral deposits are 
associated with significant quantities of a known target gas at the 
surface immediately above their underground location or with 
characteristic contaminants in the surface soil. These gases or 
contaminants can originate from chemical reactions, for instance, of the 
mineral ore with water, or the gases or contaminants can result from 
radioactive reactions. 
Utilizing the absorption spectrum of the target molecule, the present 
invention can be employed to detect the location of the associated mineral 
deposits. Detections of such gases are possible as long as the on/off 
resonance wavelengths are close enough together to fit within the spectral 
region defined by a laser beam with a spectral region centered about the 
frequency coinciding with the absorption line of the target gas and as 
long as the laser light includes the on/off resonance wavelengths of the 
target gas without being too weak to be used for prospecting. Accordingly, 
detection of other such target gases is within the scope of the present 
invention. 
Although only a few exemplary embodiments of this invention have been 
described in detail above, those skilled in the art will readily 
appreciate the many modifications that are possible in the exemplary 
embodiment without materially departing from the novel teachings and the 
advantages of the invention. Accordingly, all such modifications are 
intended to be included within the scope of the invention as defined in 
the following claims.