The infrared reflexion-spectra of minerals and rocks are used for remote sensing of targets. The reflexion-spectra of silicate rocks vary quite significantly from mineral to mineral in the wave length region from 8 to 12 micrometers. The rock forming minerals like quartz, feldspar, mica and the clay minerals show very different spectral shapes and positions of their maximum of the spectral reflexion. The presence of a good atmospherical window in that spectral region makes the method of differential-reflexion measurement feasible, for remote sensing application. A tunable CO2-laser was used as transmitter for infrared radiation. Laboratory tests showed the feasibility of the method under different simulated environmental conditions. Because of the very narrow bandwidth of the laser-emission lines, reflexion-spectra with extremely high spectral resolution were obtained

Steinmann, C. R.

English

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REMOTE INFRARED SPECTROSCOPY OF
THE EARTH
by
C. R. Steinmann
Deutsche Forschungs- und Versuchsanstalt
fUr Luft- und Raumfahrt e.V.
P.O. Box 90 60 58
D-5000 KOLN 90
GERMANY
A B S T R A C T
Increasing interest in the mineralogical application of infrared spectroscopy
has recently been shown by geologists, petrologists, and pedologists to use this
method as a tool for mineral identification. Several laboratories have con-
ducted studies of the infrared absorption characteristics of minerals and rocks.
Mineralogists are still in the preliminary stage of compiling a library of re-
ference curves that are prerequisit for the correlation of spectral character-
istics, molecular lattice-structures and for the identification of unknown
minerals and mixtures.
Very little interest has been shown for the infrared reflexion-spectra which
correlates to the absorption spectra.
In this investigation the infrared reflexion-spectra of minerals and rocks are
used for remote sensing of the targets.
The reflexion-spectra of silicate rocks vary quite significantly from mineral
to mineral in the wave length region from 8 to 12 um. The rock forming
minerals like quartz, feldspar, mica and the clay minerals show very different
spectral shapes and positions of their maximum of the spectral reflexion.
The presence of a good atmospherical window in that spectral region makes the
method of differential-reflexion measurement feasible, for remote sensing
application.
As transmitter for infrared radiation we used a tunable C02~Laser working with
sufficient power and stability in the spectral band between 9-1 fim and 11.6 /am
at more than 100 emission-lines.
Laboratory tests showed the feasibility of the method under different simulated
environmental conditions. Because of the very narrow bandwidth of the Laser-
emission lines we could obtain reflexion-spectra with extremely high spectral
resolution when tuning the Laser stepwise from line to line covering the
spectrum between 9-1 and 11.6 Jim.
Next the method was tested using a light twin-engined aircraft. As IR-trans-
mitter a J> Watts tunable CW-C02~Laser was used.
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The reflected Laser-energy was detected with the heterodyne method using the
natural Doppler-shift which occurs when the Laser-beam looks some degrees
forward in flight direction, as local oscillator.
The signal to noise ratio was still 25 at an altitude of 4.000 ft. The
discrimination of different surface materials was already possible
Development of the airborne Laser-system and more fundamental laboratory in-
vestigations are in progress.
1.) Introduction
There are many techniques for remote sensing of the earth operating in diffe-
rent portions of the electromagnetic spectrum ranging from the ultraviolet to
the microwave region. These techniques have been implemented by systems that
are either passive or active. All these systems can detect surface features suited
to each individual technique. However, it is still difficult to discriminate
different types of rocks and soils. Because of this need an active airborne
remote sensing system is developed based on infrared Laser-spectroscopy in
the spectral region of the crystal-lattice resonant frequencies (residual
rays) of the rock forming minerals.
2.) Laboratory Investigations and Results
To investigate the feasibility of the method and to identify the different
parameters of noise, a Laser-spectroscopy facility has been installed.
The system consists of a tunable C02-Laser, power supply, Laser-gas-supply,
vacuum pump, cooling system, beam forming module, pyroelectric detector,
chopper and lock-in-amplifier, radiometer and stripchart-recorder.
The Laser was developed by the Battelle-Institut in Frankfurt, Germany, and
is tunable between 9-1 Jim and 11.6 .urn on abt. 100 emission lines with an
average power of 10 W.
The Laser-beam was directed onto the rock-sample. The reflected energy, de-
tected and divided by the total output energy. The Laser was then tuned
stepwise from emission-line to emission-line.
The reflected radiation enables to draw curves of the spectral reflectivity
with very high spectral resolution.
Ten representative samples were investigated. The influence of surface-
structure, temperature and moisture on the spectral shape was investigated tod.
It could be shown that the influence of temperature and moisture is very
little. The influence of the surface varied the height of the reflexion peaks,
but not their spectral position.
958
FIGURE 1. SCHEMATIC OF THE LABORATORY MODEL
OF THE C02 LASER REMOTE SPECTROMETER
-| i r-p-i
12 94 95 as no nz ni ffi£ na no
FIGURE 2. RELATIVE REFLEXION VERSUS
WAVELENGTH
959
ORIGINAL PAGE IS
OF POOR QUALITY
10.6 10.6 11.0 [/*•]
FIGURE 3. RELATIVE REFLEXION OF A POLISHED
SAMPLE OF GRANITE VERSUS WAVELENGTH
FIGURE 4. RELATIVE REFLEXION OF A ROUGH NATURAL
SURFACE SAMPLE OF GRANITE VERSUS WAVELENGTH
960
3.) Airborne System development
Commissioned by the West German Federal Ministry for Research and Technology
an aircraft-borne measuring system is being developed which is to be used for
spectroscopic investgations of the earth's surface and quantitative analysis
of air pollutants and natural atmospheric trace gases.
Using tunable CW-IR-molecular Lasers, the method of differential absorption
or reflection measurements is applied. The Laser-radiation scattered back
from the earth's surface is detected in the aircraft by means of the hetero-
dyne technique.
The measuring system has a weight of 285 kg, a volume of 1.7 nr and a power
input of 1.2 kW. It consists of the optomechanical subsystem, which com-
prises the Laser head, all optical components (including telescope), the
detector system and the instruments for inflight calibration and adjustment,
a 19" rack containing the measurement electronics, the data recording system
and the Laser supply equipment.
| CO; lour I
I 1
! COj loitr !
FIGURE 5. SCHEMATIC OF THE AIRBORNE
LASER SPECTROMETER
961
4.) System Installation and Test
The flight tests of the system were carried out in the Dornier 28 Skyservant
of the "Deutsche Forschungs- und Versuchsanstalt fur Luft- und Raumfahrt
(DFVLR)" at altitudes between 500 and 4000 ft. The signal to noise-ratio of
about 25 , that has been achieved proves the feasibility and superiority of
the selected detection method (heterodyne detection using the natural Doppler
effect) .
Differential measurements have not yet been performed because the functional
model for these preliminary flight test was equipped with one Laser only.
U-A
FIGURE 6. CONFIGURATION OF THE AIRBORNE IR LASER
SPECTROMETER IN THE AIRCRAFT
962
Results
The first series of tests of the CO- airborne Laser-spectrometer were con-
ducted in September 1976 totalling 7 hours of flight and tests.
First taxiing tests showed good results. The high acoustic and electro-
magnetic fields of the engines and radio didn't interfere with the Laser
detector.
The program consisted of the following items:
Calibration of telescope focus for altitudes between
500 and 4000 ft.
Calibration of the deflection angle to produce a constant
Doppler-shift.
Demonstration of the independence from topography
to Doppler-shift.
Documentation of the reflectivity of different
surface materials.
Measurement of Signal to Noise Ratio at different
altitudes.
For documentation of the different reflectivity of the surface, photographs
were taken. Fig. 7 shows reflexion signal of a forest with signatures of
a highway and a clearing.
In later missions in June 1977 two Laser frequencies will be used, making
feasible a differential spectroscopic investigation by applying the ratio
of the different spectral reflections.
963
POOR QUALITY
FIGURE 7. DETECTED HETERODYNE SIGNAL
FOREST WITH CROSSING HIGHWAY AND CLEARING
Flight Sept. 13, 1976
Altitude
Speed
y-axis
x-axis
Integration time
Length of profile
1500 ft
50 m/sec
10 mV maximum
3 inch/min
0.25 sec
2 km
964
Conclusions
An experimental airborne Laser-Infrared-Spectrometer-System based on the
interaction of Laser-radiation with the crystal lattice frequencies of the
surface minerals is in development.
Laboratory investigations have shown the feasibility of the method.
First test flights have shown the feasibility of the heterodyne-detection
method of the re-emitted Laser-radiation.
Based on the laboratory and airborne data the following can be concluded:
Detection of most of the rock forming minerals and clay-minerals with
Laser differential measurements is feasible.
With a two frequency system many geological questions e.g. basic-acid
ratio of igneous rocks can be discriminated.
The geometrical resolution is about 10 m at 10.000 ft altitude.
The Laser-Spectroscopic System operated reliable in the light aircraft.
Acknowledgements
This work is being sponsored by the German Federal Ministry for Research
and Technology. It is being performed by the Deutsche Forschungs- und
Versuchsanstalt fur Luft- und Raumfahrt (DFVLR).
The opto-mechanical portion of the experimental airborne Infrared-Laser-
Spectrometer was designed and fabricated by the Battelle-Institut, Frankfurt,
Germany.
References
1.) "Infrared Mineralogical Techniques for Ore Search";
Bruce G. Wachter; R.I.P. Lyon Stanford University,
Dept. of Mineral Engineering, Sept. 1970
2.) "Untersuchungen zur Fernerkundung von Gesteinen aufgrund des
Reflexionsverhaltens fur Laser-Strahlung im Reststrahlenbereich";
C. R. Steinmann, Deutsche Forschungs- und Versuchsanstalt fUr Luft-
und Raumfahrt, Dezember 1973-
3.) "Entwicklung eines flugzeuggetragenen Mefisystems zur Erkennung der
ErdoberfISche und Atmosphare sowie analytische Untersuchungen im Labor";
Battelle-Institut, Frankfurt, Germany,
Report under Contract 01 TL 026 - Ak/RT/WRT 20714.
965
~FIGURE 8. OPTO-MECHANICAL FACILITY TO PERFORM
LASER-SPECTROSCOPIC INVESTIGATIONS OF ROCKS
*
:
FIGURE 9. IN HOUSE LABORATORY TEST FACILITY.
TUNABLE C02-LASER IN FRONT
966
FIGURE 10. MEASURING SYSTEM WITHOUT SHOCK-MOUNTS.
THE VACANT SPACE NEXT TO THE LASER IS RESERVED
FOR THE SECOND C02~LASER
FIGURE 11. A VIEW OF THE CO -LASER-IR-SPECTROMETER
MOUNTED IN THE AIRCRAFT
967


Remote infrared spectroscopy of the earth

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