US 4,853,543 A · Method and apparatus for detecting a tracer gas using a single laser beam

Abstract

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.

Claims35

Dependent claims

A method of detecting a target gas having a predetermined atomic absorption line, comprising the steps of:

generating a beam of laser radiation having a spectral bandwidth including, while being broader than, said predetermined atomic absorption line of said target gas;
transmitting said beam of laser radiation toward an area to be investigated for said target gas;
splitting the beam reflected from said area into first and second portions, said first portion including the radiation in said reflected beam in the spectral region coinciding with said predetermined atomic absorption line as an on-resonance beam, and said second portion including only the radiation in said reflected beam in the spectral region outside of said spectral region coinciding with said predetermined atomic absorption line as an off-resonance beam;
measuring the intensities of said on-resonance beam and said off-resonance beam and producing an on-resonance energy signal and an off-resonance energy signal, respectively related thereto;
and
determining the absolute concentration of the target gas in said area from the measured intensities.

3 more elements

A method of locating an underground deposit by detecting the presence of mercury gas, comprising the steps of:

(a) generating a beam of ultraviolet laser radiation having a spectral region about a center frequency which coincides with an atomic absorption line of the mercury (Hg) atom;
(b) transmitting said beam of laser radiation toward an area being investigated for the presence of mercury gas;
(c) measuring the intensity of the transmitted laser beam;
(d) detecting said beam of laser radiation after it has been reflected from said area;
(e) splitting the detected beam into first and second portions;
(f) measuring the pulse energy of said first portion of the detected beam;
(g) passing said second portion of the detected beam through a mercury vapor-filled optical cell, said optical cell being kept at a regulated temperature, and said mercury vapor in said optical cell absorbing the optical energy in said second portion of the detected beam which is coincident with the atomic absorption line of mercury;
(h) measuring the pulse energy remaining in said second portion of the detected beam after it has passed through said optical cell as an off-resonance energy signal;
(i) subtracting said off-resonance energy signal from the measured pulse energy of said first portion of the detected beam to get an on-resonance energy signal;
(j) measuring the intensities of said on-resonance energy signal and said off-resonance energy signal;
(k) generating an indication of the distance said transmitted laser beam travels to said area;
(l) determining the concentration of said mercury gas in said area employing the results of steps (c), (j) and (k);
(m) relating said mercury gas concentration to the existence of said underground deposit;
(n) displaying said mercury gas concentration in real time;
(o) determining the position of said area being investigated;
and
(p) storing said mercury gas concentration with the position at which said mercury gas concentration was measured.

14 more elements

An apparatus for locating an underground deposit by detecting the presence of mercury gas, comprising:

means for generating a beam of ultraviolet laser radiation having a spectral region about a center frequency which coincides with an atomic absorption line of the mercury (Hg) atom;
means, responsive to said generating means, for transmitting said beam of laser radiation toward an area being investigated for the presence of mercury gas;
means for measuring the intensity of the transmitted laser beam;
means for detecting said beam of laser radiation after it has been reflected from said area;
means for splitting the detected beam into first and second portions;
means for measuring the pulse energy of said first portion of said detected beam;
a mercury vapor-filled optical cell kept at a regulated temperature for absorbing the optical energy in said second portion of said detected beam which is coincident with the atomic absorption line of mercury;
means for measuring the pulse energy remaining in said second portion of said detected beam after it has passed through said optical cell as an off-resonance energy signal;
processing means, responsive to said first and second portions of said detected beam, for subtracting said off-resonance energy signal from the measured pulse energy of said first portion of said detected beam to get an on-resonance energy signal;
for measuring the intensities of said on-resonance energy signal and said off-resonance energy signal;
for generating an indication of the distance said transmitted laser beam travels to said area;
for determining the concentration of said mercury gas in said area in response to the outputs of said means for measuring the intensity of the transmitted laser beam, said means for measuring the intensities of said on-resonance energy signal and said off-resonance energy signal, and said means for generating an indication of the distance said transmitted laser beam travels to said area;
and for relating said mercury gas concentration to the existence of said underground deposit;
means for displaying said mercury gas concentration in real time;
means for determining the position of said area being investigated;
and
means for storing said mercury gas concentration with the position at which said mercury gas concentration was measured.

14 more elements

Dependencies are inferred from the claim text.

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

Life of this patent

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

Life of this patent

Abstract

Claims35