Jökull - 01.12.1972, Síða 34

Prior to the aerial survey reported in the
present paper, oblique aerial photographs of
the Kverkfjöll area were obtained by the
British Royal Air Force on 12 March 1944
(Thorarinsson 1950, Fig. 2). The U. S. Air Force
took vertical cartographic-quality aerial photo-
graphs of part of the Kverkfjöll area on 24
August 1960 (Fig. 3) (Friedman et al. 1969).
The present study was conducted coopera-
tively by the Terrestrial Sciences Laboratory
of the U. S. Air Force Cambridge Researcli
Laboratories; the Iceland National Energy
Authority; the Division of Geosciences of the
Science Institute, University of Iceland; the
Infrared Physics Laboratory of the University
of Michigan; and the U. S. Geological Survey,
as one of a series of aerial infrared thermo-
graphic and photographic surveys of tliermal
manifestations of Iceland’s neovolcanic zone.
INSTRUMENTATION
AND SURVEY DATA
Tlie MlAl airborne infrared optical-mech-
anical line-scanning system, the major instru-
ment in the surveys, utilizes a photovoltaic
indium antimonide solid-state detector sensi-
tive to emitted radiation in the l.Oq to 5.5p,
wavelength region when unfiltered as it was
operated during night missions in Iceland
(Friedman et al. 1969, p. C91—C92). The op-
tical elements of the infrared scanner focus
infrared radiation emitted from the earth’s
surface onto the detector, which transduces the
radiation into wideband electrical signals and
provides an input to an image recorder. The
recording unit electronically records on film a
thermal image of the terrain over which it
passes to produce a continuous strip infrared
image or thermogram. A modification of the
recording unit in 1968 permitted simultaneous
recording of the electronic signal on film and
magnetic tape, thus retaining a semiquantita-
tive record of the infrared signal suitable for
processing by signal amplitude-level slicing
techniques by use of a threshold circuit in the
laboratory.
On 22 August (2357 IMT) continuous gray-
scale infrared images were directly recorded
on 70 mm film (Fig. 4). The resulting tonal
density of the film emulsion is a nonlinear
32 JÖKULL 22. ÁR
function of radiation temperature of the sur-
face features depicted (polarity: black = cool,
white = warm).
According to a modification of the Stefan-
Boltzmann law
W = &r (T4 - T04)
where
W = differential radiant flux per unit area
g = emissivity factor (black body = 1)
cr = Stefan-Boltzmann constant
(1.354 x 10~12 cal • cm~2 deg-1 sec'1
or 5.67 X 10-8 W- m-1 deg-4)
T = absolute temperature of radiating body
<°K)
T0 = absolute temperature of surroundings
(°K)
Emissivity (e) of the surface material (g = 0.95
for basalt surfaces) and actual surface tempera-
ture thus control infrared back radiation and
tlie resulting tonal density of the film, but the
gray scale of the 1966 imagery cannot be
quantitatively calibrated because of electronic
limitations of the detecting system and scale
distortion introduced during direct recording
of the signal on film. Nevertheless, the energy
transfer function is approximately known
(Williams et al. 1968, Fig. 4). The range in
radiation temperature of the surface features
depicted in 1966 probably exceeds 100°C, rang-
ing frorn glacial ice surfaces to open steam
vents (Fig. 5). A sketch map (Fig. 5) derived
from the 1966 infrared image (Fig. 4) shows
the relationship between glacier surfaces
(white), thermal manifestations (black), some
of the larger crevasses, and direction of glacier
melt-water drainage.
The infrared images obtained on 26 August
1968 (2125 through 2303 IMT) were recorded
simultaneously on 70 mm film and magnetic
tape (Fig. 6). The taped data do not have the
introduced distortion inherent in the fihn
record, and were subsequently processed by an
electronic signal-amplitude slicing technique
in which the taped signal was divided into a
number of equal-intensity map units roughly
equivalent to undefined radiation tempera-
tures on the surface (Fig. 7). These amplitude
slices best show the relative temperature and
distribution of hydrothermal features.