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Interpretation of thermal prospection on bare soils

Authors:
Archaeometry
23,
2
(19812169-187.
Printed
in Great Britain
INTERPRETATION
OF
THERMAL PROSPECTION ON BARE SOILS
M.
C.
PERISSET and A. TABBAGH
Centre de Recherches Gkophysiques, Centre National de la Recherche Scientifique, Garchy,
581
50
Pouilly -sur-Loire, France
1.
INTRODUCTION
Thermal prospection, by which we measure the surface temperature
of
the ground, has interest-
ing characteristics for archaeological research, especially in surveying large areas, since a thermo-
graphy corresponds to several square kilometers. Like other surveying techniques, it has its
limitations and a more complex interpretation of the results than electrical or magnetic pros-
pecting. In addition, with organisational data processing and cost problems, the difficulties
encountered in interpretation explain why this method has been of little use until now. Tem-
perature can be measured by direct contact as well as from the radiation transmitted by the
ground surface: but only a radiometer can be operational in the field, because in order to com-
pare the points of the surface it is necessary that the temperature should not vary during the
time of the survey, which is limited to a few minutes. In the presence of vegetation one does
not measure the ground surface temperature but that of the plants, which is controlled by plant
transpiration. Archaeological features may be very well detected by plant temperature, as in the
case of Villeneuve la Guyard on the
4th
May 1976 (Tabbagh 1979), but it is a non-direct detec-
tion and the interpretation needs knowledge about plant metabolism and soil conditions on
which it depends. This knowledge is necessary in order to determine at which stage of plant
development it will be interesting to make a survey. The interpretation of results for bare soils
is a direct application of the physical laws of thermal exchanges.
In
previous papers several rules
have been established (Tabbagh 1973 and 1977a):
(1) soil temperature depends on solar radiation and exchanges with the atmosphere,
(2)
the diurnal variation of the flux of heat in the ground has a too low penetration depth to
give a measurable anomaly linked to features lying below the cultivated superficial layer,
(3)
slow transient variations of the flux may induce anomalies revealing archaeological features.
Besides the calculation of a two layer analytical model, a programme computing a three dimen-
sional (time and two space dimensions) numerical model (Tabbagh 1977a) has been written to
obtain the anomaly of a cylindrical feature covered by a superficial layer for any variation of
flux against time. Using these tools it is now possible (if we know the flux variations before the
survey) to improve the interpretation of the results obtained from the last four years experi-
ments. This will be done hereafter, but it is still very useful to define some general interpreta-
tion rules, which allow us to forsee the amplitudes of anomalies, to decide to fly or not and to
understand and quickly judge the results.
A
brief summary of the instrumentation and processing techniques is also appropriate. The
photoconducting detector (Hg, Cd, Te) measures the luminance of a small area of the soil sur-
face in a narrow frequency band (10.5-12.5pm for example) defined by a filter. The size of the
area is determined by the value of the instantaneous field
of
view (IFOV), the value
of
which
169
170
M.
C
Perisset
and
A.
Tabbagh
is
2.8
mrad in the case of the
ARIES
radiometer used (Monge et Sirou 1975). The scan of each
line is made by the rotation of a mirror
in
a direction perpendicular to the
flight
path. The
motion
of
the aircraft ensures progression from one line to the next: the distance,
d,
between
two adjacent lines is equal to the ratio of the aircraft velocity to the rotation speed of the
mirror,
N.
The distance between points along the line of scan is determined by the sampling
angle:
e
=
1.74mrad. To obtain optimum resolution, the lowest aircraft velocity and hghest
rotation speed are selected, The flying altitude,
h,
is then deduced to allow a regular measure-
ment grid:
ha
=
d.
For instance, if
V
=
40m/s (90Mph) and
N
=
36
Hz,
the interval between
lines
of
scan
d
=
1.1
1
m, and since
h
=
d/a,
the altitude should be 632 m (2050 feet). In the air-
craft, the data are recorded
in
P.C.M.
(one byte per point,
900
points per line) on a
high
density
analogic tape, and are then transferred in the laboratory on a computer-compatible tape
(9
tracks,
1600
bpi). Subsequent processing requires the use
of
a computer such as that of the
Rheinisches Landemuseurn in
Bonn,
and fa&
into
four stages.
(1)
Geometrical corrections:
for
roll
from the data recorded for each line from a gyroscope
linked to the radiometer, for pitch using data from the same gyroscope and for the panoramic
effect by resampling the line at the
hor
step.
(2)
Temperature coding translates the flux values into temperature values (always on
8
bits)
by using calibration data.
(3)
Image processing to obtain a more readable picture, using the programm library of the
image processing center at Bonn (Scollar 1980)
for
the following operations: (a) spikes elimina-
tion by median fdtering (b) contrast enhancement by mean and variance equalisation on a
moving 100
x
100 point window (c) destriping by equalisation of the mean of each line to ?hat
of
a
200
line moving band
(d)
low pass fitering.
(4)
Creation
of
a negative film using
a
scale of SOpm/point.
2.
GENERAL PROBLEMS IN INTERPRETATION
2.1
Anomaly
sign
and
contrast coefficient
If we assume that the flow of heat by convection is negligible. two independant thermal proper-
ties are necessary to describe the behaviour of temperature in the soil: the conductivity,
k,
defined by Fourier's law
Cp
=
--grad0 where
q5
is the flux and
0
the temperature, the volu-
metric specific heat,
0,
defined by dQ
=
CvdB/dt where
dQ
is the amount of heat stored by
unit volume during the time dt. The difhsivity,
I?
=
k/Cv,
is a more convenient parameter in
the calculations since it appears in the heat conduction equations,
The thermal inertia,
k
P=-=@%
fl
is also useful because the response of an homogeneous ground is inversely proportional to
P.
If
we consider the simple case of a three media model as in figure 1, seven parameters may
influence the sign of the anomaly for a positive single pulse flux: the thickness of the superficial
layer and
two
properties for each medium. Numerical calculation does
not
show directly the
role of each parameter as is the case in analytical calculation (for example the amplitude of a
magnetic anomaly is proportional to the difference of magnetic susceptibility between the
Interpretation
of
thermal prospection on bare soils
171
1
I
I
1
I
I
I
Figure
1
~lindrical three media model used
in
the calculations
feature and the surrounding soil, and this is directly readable in the writing of the analytical
expression of the anomaly). The only solution is to execute the programme for the several cases
which are considered as representative. In this way it has been established (Tabbagh 1977a) that
the superficial layer (1) attenuates the anomaly if its thickness or its conductivity increases and
if its diffusivity decreases, but the layer has no effect on the sign of the anomaly which only
depends on the properties of the features
(2)
and of the surrounding soil (3).
To
determine the
influence of these four parameters, their position in the formulae for analytical calculations
have to be considered. For a two-layered ground for example the properties of the second layer
appear in the expression of the surface temperature through the ratio
pz
-
p1
pz
+p1
-~
(Tabbagh 1973) (1 for the first layer, 2 for the second). If we use the same idea for the three
medium model, and a contrast coefficient
p3
-
p2
c
=
--
P3+Pz'
it is systematically verified that the anomaly increases with
c
and has the same sign as
c
when
the flux is positive (soil gaining heat) and the opposite sign when the flux is negative. This result
is illustrated by figure 2, which shows the anomalies for
t
=
3 days after a 15
W
flux during two
days for the three following cases:
a
feature 0.6 m large and
0.3
m depth with,
in
the first case:
rz=0.5O
x
10"mz/s,
k2=
1.20w{m/",
r3=
0.60
x
IOdm2/s and
k3=
1.80w/m/",
so
that
c
=
0.156, in the second case
rz
=
0.6
x
lO"mTs,
kz
=
1.60w/m/O,
r3
=
0.80
x
10"mz/s and
k3
=
1.35 w/m/"C
so
that
c
=
-
0.156 and
in
the third case
r2
=
0.6
x
m2/s,
k2
=
1.60w/m/",
r3
=
0.80
x
m2/s and
k3
=
2.53 w/m/"
so
that
c
=
0.156. In all the three
cases the superficial layer is the same: thickness
h
=
0.2 m, diffusivity m2/s
and
conductivity
kl
=
1 w/m/". The amplitudes of the corresponding anomalies are
+
0.25",
-
0.23"C and
+
0.22"C; the sign of the anomaly is that of
c
and its magnitude is not strictly
proportional to
c.
When the values of
r3
and
k3
are higher, third case, there is a better heat
distribution and the anomalies are lower. In practice a higher value of the conductivity of the
superficial layer corroborates this effect and the anomalies
will
be in general more visible on
low conductivity and diffusivity subsoil, for example silt or chalk, than on soils developed on
gravel, sandstone or granite. The three cases chosen in figure 2 correspond to experimental
data:
the first one was a ditch filled with topsoil in hard limestone, the second one was building
materials
in
low conductivity gravel, the third one building materials in high conductivity sand.
=
0.50
x
172
M.
C
Perisset
and
A.
Tabbagh
7,
0,so
r2
z0,60
lo-'
k
,
z
2,53
tz
z
1,60
<
=
0,80
lo-'
t,
=
1,JS
rz
0,60
kz
=
1/60
Figure
2
OSOX
10"mz/s,k,= lW/m/"Candh=0.20m
Response obtained at
t
=
3
days for several contrast coefficients, after a two days
pulse,
for
r,
=
Coefficients similar to
c
can be used in other physical methods, for example in resistivity sur-
veying the anomaly created by a subsurface body depends on a coefficient
P3--PZ
P3
+
Pz
where
p
is the electrical resistivity (Spahos
1979).
2.2
Reversal
of
heat
flux
When the
flux
keeps the same direction. the same sign, the interpretation rules are rather
simple; the sign
of
the anomaly is directly given by that of the contrast coefficient, and, taking
into account the geological or pedological context, an interpretation can be proposed for the
anomalies. The only difficult case is that of alluvial soils where rapid change from low conduc-
tivity to high conductivity gravels may be encountered and where
c
may be positive as well as
negative for
a
given feature.
When, a few days before the measurements, a reversal
of
flux direction occurs, the interpre-
tation is more complex. The sign of the anomaly not only depends
on
the contrast coefficient
but
also
on
the delay
to
reverse the anomaly. This delay is a function
of
the thickness and the
diffusivity
of
the superficial layer. In figure
3
the time variation
of
the anomaly magnitude is
shown
for
a flux of
-
20w/mz during two days, followed by
+
20w/mZ during two days and
then a nu1 flux, with several values for the superficial layer parameters
and
with
rz
=
0.45
x
lo-'
mz/s,
kz
=
0.80
w/m/"C,
r3
=
0.70
x
I
O-6mz/s
and
k3
=
1.80
w/m/'. One can see that if
Interpretation
of
thermal prospection on bare soils
W/mZ
20
10
0
-10
-
20
173
-
-
i
i
i
1
i
i
1
2
3
4
5
6
7
8
days
-
A0
02
0
-
0,r
-
0,4
A0
or2
0
-
0,z
-
0.4
A0
94
0
-
0'4
-0P
A0
0#4
0
-
0,4
-
Qa
-1,z
kl
=
1,30
W/m/'
-6
2
OC
/i
=0,70
10
m
1s
-
h,
=
0,30
m
'c
l',
=
0,45
10-'mZ/s
h
1
=
0,30
m
t
1
"
6
=
0,45
10-6mZ/s
hl
=0,15
m
1
1
1
Figure
3
ent types of superficial layer
Variation
of
magnitude
of
the anomaly
in
the case of an inversion of the
flux
of
heat, for differ-
174
M.
C
Perisset
and
A.
Tabbagh
the prospection is undertaken at
t
=
4
days the anomaly
will
be hot or cold according
to
the
diffusivity and the thickness of layer
1.
The two'instants when it would be easier to understand
the results are
t
=
2.5 days and
t
=
6
or
7 days. However it is better to avoid making measure-
ments
in
such periods, because the anomalies are low due to the opposite effects of the succes-
sive variations
of
the flux.
(In
figure 3, a contrast coefficient
of
0.29 was taken to enhance the
anomaly magnitude).
3.
DETERMINATION
OF
SLOW
TRANSIENT VARIATIONS
OF
THE FLUX
OF
HEAT
INTO THE GROUND
The knowledge of flux variations prior to the measurement time is of great importance in inter-
pretation. It is necessary therefore to determine these variations with precision. However the
method of measurement has to be practicable, that is to say heavy equipment in the field
should be avoided and enough information available each day about the flux
to
be able to
decide whether to fly
or
not.
Comparisons have been made between different measurements and calculation methods
(Carson and Moses 1963, Pirisset 1980). The most reliable consists in measuring the tempera-
ture variation with time at several points
on
a vertical profile, and calculating the flux,
Q(t),
by
decomposing it in a series of step functions beginning at regular intervals
At.
At time
ti
=
iAt,
the temperature,
O(z,
t),
in
an homogeneous soil may be writtm by the expression
where
z
is the depth;S is defined by
Z
S(Z,
m)
=
ierfc
ierfc being the integral of the complementary error function
e-x
*
ierfc(x)
=
~
-x
erfc(x)
i
dn
and
m
erfc(x)
==
2
i
e-uzdu)
.
Q,
represents the successive values of the flux and the series of
Q,
values is calculated step by
step from the temperature differences between
tl
and This calculation can be done with
one measuring point
only,
but if several depths are available the use
of
the least squares method
reduces the influence
of
measurement errors. For example
Q1
is
calculated by minimising the
expression
A,
2
j=1
(j
referring to depth and
Aej
being the observed temperature difference between
t
=
At
arid
t
=
0);
dA
from
-
=
0
dQ
1
Interpretation
of
thermal prospection
on
bare
soils
175
P
ZjABjS(zj,
1)
Q
=-
can be deduced,
2
ZjS(Zj,
1)2
1
Q,
is then calculated taking
Q,
into account and
so
on. Temperature measurements are made
with a simple device of
4
or
5
thermistors (platinum thermistors for example) and the associated
electronics, the data being recorded on
a
cassette tape once or twice an hour. It is also possible
to use the measurements made by the meteorological stations network. Each departmental
station in France records soil temperatutes at four depths, (0.1 m, 0.2 m,
0.5
m
and
1
m) one or
three times a day.
As
it has been established (Perisset and Tabbagh 1980) that slow variations
of
the flux are well correlated for neighbouring stations and fairly uniform for a region, the net-
work is sufficiently dense to avoid the necessity of setting up any other point of measurement
in the surveyed area itself.
The flux variations preceding the four series
of
$:xperiments already made are represented in
figures 4 to 7: from 24th Apnl 1976 to 8th May (figure 4), from 23rd November to 22nd
December 1977 (figure
S),
froni 26th February
1979
to 14th March and from 7th March to
21st March
1980.
For the flight
of
the 4th May 1976, the
flux
was
positive since the 29th
April and the variation can be approximated by a constant (10W/m2) flux during the four days
before the flight. Consequently the interpretation is relatively simple, but
it
would have been
better to wait two or three days more to profit from the higher flux values between the 4th and
the 8th May. For the flight of the 20th December 1977 over the area around the
town
of
Lion en Beauce and over the Bassie (Seine valley upstream
of
Montereau) the flux was negative;
but, while the conditions were relatively good for the Basge, since the flux approached
-
15
W/m2 during four days, they were not
so
favourable in Beauce since the flux was around
-5
W/rn2.
It was then necessary to correct the values of flux estimated from air temperature
variations (Tabbagh 1978) and to reconsider the interpretation of results. The flights of March
1979 took place in a positive flux period; the conditions on the 8th March (Bassee) were less
favourable
than
on the 14th March (Aisne Valley and Champagne ardenaise) because the
inflow of the heat begun
on
10th March was at
its
maximum effect on the
14th,
while on the
8th the positive pulse (2nd-5th March) had already passed three days before. The flight of
March 1980 took place in a bad period owing to low amplitude of the
flux
and a reversal two
days before the measurements. According to subsequent discussions the observed anomalies
were due
to
superficial layer heterogeneities.
All these flux values are used hereafter in the interpretations and discussion of results.
4.
INTERPRETATION OF ANOMALIES OBTAINED
FOR
KNOWN FEATURES
4.1.
Neolithic camps
of
Noyen s/Seine and Grisy s/Seine (Seine et Mame, France)
The interrupted ditches of the great middle neolithc enceintes of Noyen s/Seine and Grisy
s/Seine (Mordant
C.
and
D.
1972) have a sufficient volume to be detected even with a bad
ground resolution. During excavations it was possible to measure thermal properties of the pit
fillings;
those
of the surrounding gravel could not be determined due to the inability of the
instrument, but the compactness of the gravel implies high values of conductivity and
diffusivity. Measurements of the properties of the superficial layer were made at different
periods of the year. These sites were flown over in May 1976, December 1977 and March 1979.
On the May 1976 thermographs the pits generate anomalies where they are dug into gravel
(Tabbagh 1977b), but they do not appear in silt and this is well explained by the low coefficient
176
k
=
1.80
W/m/'
-
M.
C
Perisset
and
A.
Tabbagh
FLUX
OF
HEAT
IN
THE
GROUND
(in
W/rn*)
so
20
10
0
-
10
-
20
20
10
0
-10
20
10
0
-10
TROYES
r
=
0,60
10-6m2/s
k
=
1,60 W/m/'
t
&
tn4
t
U
ORLEANS-BRICY
f
=
0,SO
m*/s
24252627262330
12
3
4
5
6
7
8
April
1976
May
1976
Figure
4
and the
8th
May
Flux
of heat calculated by decomposition in a step function series between the 24th
April
1976
contrast between the
fill
and the silt.
The
detailed map of temperature shows anomalies of
0.5"C
magnitude on each site (Tabbagh 1977b).
Let
us
consider the following model:
(a) superficial layer: thickness
h
=
0.24m,
kl
=
1.35 W/m/"C,
rl
=
0.72
x
mz/s
(assuming
that it is well packed)
Interpretation
of
thermal prospection on bare soils
177
N
c
\
3
e
.-
-
a
a
z
2
0
0
w
I
I-
z
a
l-
W
I
lL
0
X
3
-I
LL
i
1
!5
?
1
I
7
1
I
178
30-
20
10
M.
C
Perisset
and
A.
Tabbagh
FLUX
OF
HEAT
IN
THE
GROUND
(
in
W/m*)
-
-
1
February
1979
March
1979
Figure
6
1979
and the 14th March
Flux
of
heat calculated by decomposition
in
a step functions series between the 26th February
(b)
feature: top width2m (alittleless than theactualvalue), depth0.60m,kz= 1.25 W/mrC,
(c)
Surrounding gravel: k3
=
2.40
W/m/"C,
r3
=
0.90
x
10%
mz/s
so
P3
=
2.53
x
1
O3
US1
and
c
=
0.177.
For a constant
flux
of
I0
W/m2 beginning at
12
h on the
30
April, we obtain at
t
=
92
h
an
anomaly
of
+
0.46"
magnitude, which agrees well with the experimental data if we consider the
uncertainty which exists in the estimation
of
gravel properties and the values
of
superficial layer
parameters at the exact time
of
the flight.
rz
=
0.50
x
10-6m2/s
so
P,
=
1.77
x
lo3
USI.
Interpretation
of
them1 prospection
on
bare
soils
FLUX
OF
HEAT
IN
THE
GROUND
(
in
W/m'
10
TROYES
20
1
-
-
0-
f
~0.70
lo-'
n'/r
k
=
1,60 W/m/*
10
c
n
U
-20
-I0
t
AUXERRE
7
=
0.75
lo-'
m'/s
20
k
=1,90 W/m/*
10
-
0
--
-10
-
-20
L
NEVERS
rs
O,SO
10-'
mZ/*
k
=
1,15
W/m/'
20
t-
n
-
-lo
20
t
BOURGES
P
=
0.70
1OSmZ/s
20
t-
k
=
1,60W/m/*
7
- -
':
20
10
1
StGERMAIN
MS
BOlS
F=
0.76
10"
m*/s
k
=
1.82
W/m/*
0
I
I
-
-lo
t
Lr"
-
20
t
tl~l~llll'lllli
7
b
9
(0
11
12
15
14 15 16 17
lb
19
20
Ll
Morrh
1980
179
Figure
I
March 1979
Flux
of
heat calculated
by
decomposition in
a
step functions series between the 7th and the 21st
180
M.
C
Perisset and
A.
Tabbagh
The 20th December 1977 results do not show pits (Pkrisset 1978). This
is
well explained
for
the Grisy site where, according to near infra-red results, the soil is not bare but covered by
stubble and aftercrop; vegetation acts here as a screen. At the Noyen site, where the soil was
bare, the strip of silt does not appear either but traces
of
excavation are very strong. This means
that the
soil
response is mostly that of the superficial layer, and can be explained for several
reasons. First, time of flight
(11
h 30GMT) was too late and the anomalies were due to the
effect
of
the flux diurnal variation on the heterogeneities of the superficial layer. Secondly, the
recent ploughing may have considerably reduced the diffusivity of
this
layer, with
k,
=
0.90
W/m/"C and
rl
=
0.30
x
10*m2/s and keeping the other model parameters constant one
obtains, for a
-
15 W/m2 flux beginning at 12 h on 16th December, an anomaly of
-
0.38'
and
for a
-
10W/mZ flux a 0.25' anomaly. But
this
explanation is not entirely satisfactory since the
L.S.B.
corresponds to
0.08'
and an anomaly of 0.25' can be distinguished. Thirdly the flux
magnitude may be lower than the values at Troyes and Auxerre stations: the datum for Bricy is
about
-
5 W/m2 but the data in Seine et Marne are not available in the vicinity of the sites and
the cold wave may have been smaller in Bassie
or
locally on the sites than at the stations due to
the influence of the river. Having a
-
5
W/mZ flux the anomaly is only of 0.12' and may be hid-
den by noise in the imagery.
The survey we carried out in March 1979 at Noyen is difficult to interpret due to big
disturbances generated by surface stripping and excavations. At Grisy the results show, as for
May 1976, the part
of
the enceinte which
is
dug in gravel.
On
the detailed chart (figure
8)
the
anomaly is around 0.4'C. Keeping the same model and the following values
for
the flux:
t
=
0
on 2nd March at 6h, 12 W/m2 during 24 hours,
18
W/mZ between 24 and
48
h, 12 W/m2 from
48
h to 72 h,
-
10
W/mZ between 72 and 96 h, 12 W/m2 from 96 h to 120 h, then
0
W/m2 and
3 W/mZ on the morning of 8th March, the calculated anomaly is
0.45"
and in good agreement
with the experimental results.
4.2
Geological variations
on
the south western part
of
the area centered
on
the
town
of
Lion en
Beauce (Loiret, France)
The thermal prosp$ction in December 1977 of the whole area gives us a large amount of inter-
esting archaeological information (Tabbagh
1978,
Fourteau and Tabbagh 1979) through micro-
relief anomalies
and
subsurface heterogeneity detection. It
is
important
to
discuss the interpre-
tation of
this
second type of anomaly, in particular about the depth of the heterogeneities
which generate anomalies. The south western part of the township had been flown over in May
1976 and a comparison is possible in spite of the bad quality of the imagery for the first fly.
Therefore the study is limited to this part
of
the township.
Plate 1 shows the thermography
of
May 1976, plate 2 that of December 1977 and figure 9
topography and geological limits as they arz defined on the 1/50.000 map. In plate 1,
roll
and
pitch are not corrected, neither is the effect of projection of a plane on a cylinder; the fields
with vegetable canopy are
8'
colder and show black on the picture.
A
good correlation appears between geological formations and surface temperature on the
20th of December 1977. For a negative flux the Beauce limestone (mla2 in figure 9) is hot, the
marl
of
Blamont and the plateau silt are cold except when they are mixed with sand. On the
4th
of
May 1976 the situation is reversed, this corresponds to the fact that flux was positive.
These effects are not linked to the relief but to soil or subsoil constitution: the limestone is hot
and
it
corresponds to a topographic hollow. We have to determine if the anomalies are direct
subsoil effects
or
if
a
correlative horizontal variation
of
the constitution of the superficial layer
Interpretation
of
thermal prospection
on
bare soils
GRISY
LES
ROUQUEUX
181
0
10
m
Figure
8
camp
of
Les
Rouqueux,
Grisy,
France (wave length
11.5
fim)
Apparent temperature
on
the 8th March
1979
at
9
h
58
G.M.T.
on
the ditch
of
the neolithic
is needed to explain the temperature differences, near
lo,
obtained between hot and cold areas.
So
we compare the calculation results (table
1)
between a model with uniform superficial layer
(first case) and a non uniform one (second case).
In
May 1976, with a flux beginning
on
30th April at 12h and having the successive daily
values:
2.5,
13,6 and
10W/m2,
we obtain a temperature difference of +0.15"C. In December
1977 with a flux beginning
on
16th December at 12 h and having the successive daily values of
-4,
-2,
-6 and -7 W/m2, we obtain
-O.lO°C.
It is necessary to consider that the superficial
layer plays an important role in the difference. In the second case we obtain 035°C for the 4th
May 1976 and
-0SO"C
for the 20th December 1977; these values are in better agreement
with observed differences. In this example the geological formations are well detected thanks to
the variations
of
the superficial layer constitution. For the December 1977 flight, when the flux
intensity was low, archaeological features could not be detected directly but only in the case
when the superficial layer was contaminated by them,
this
remark partially explains the diffi-
culties met with in recognizing the form or the type of the features.
4.3. Pro
to historic enclosures
4.3.1
Necropolis
of
La Tombe (Seine et Mame, France)
The necropolis of
La
Tombe is local-
ised in a reshaped clayed formation
on
chalky flowed materials.
A
square enclosure was known
by air photography but no excavation had been made
on
this site
so
that
no
direct measure-
ment of thermal properties was available but the type
of
subsoil implies that the contrast co-
efficient
is
positive for a ditch. This is confirmed by the anomalies obtained (plate 3 and figure
10)
in
December 1977 since the enclosures are cold.
182
M.
C.
Pirisset and
A.
Tabbagh
LION
EN
BEAUCE
Figure
9
Altitudes in meters and geological contours in the
S.
W.
part
of
the Lion de Beauce township. ml
b:
burdigalian
[clay).
mla3: upper aquitanian, mark
of
Blamont. mla2: upper aquitanian, Beauce limestone.
LP
Silt
of
Plateau.
This result confirms also that plateau
soils
are better for thermal detection than alluvial de-
posits;
this site
is
not far from Noyen but the features are less important in volume,
4.3.2
Coulommes
Necropolis
(Ardennes, France)
The results of the March
1979
flight have
not
been checked yet
so
that the interpretation of the anomalies obtained in the
so
called place
'le ban perdu' as protohistoric enclosures is still hypothetic. The anomaly is hot,
0.4"C
higher
than surrounding points and this agrees with the positive contrast which exists between chalk
and earthen ditches. On the neighbouring site of Saulce champenoise, the filling
of
a
ditch has
the following properties:
k2
=
0.84
W/m/" and
rz
=
0.46
x
10-6m2/s and the chalk
k3
=
1.46
W/m/" and
r3
=
0.5
1
x
10"
m2/s,
so
that
c
=
0.24.
Plate
1
35
G.M.
T.
(wave length
11.5
pm)
Thermography
of
the
S.W.
part
of
the
Lion
en
Beauce township
on
the 4th May 1976 at 9h
Plate
2
10h40
G.M.T.
(wavelength
II.Sfim)
Thermography
of
the
S.
W.
part
of
the
Lion
en Beauce township
on
the 20th December
1977
at
Plate
3
G.M.
T.
(wave length
11.5
fim)
Thermography
of
the necropolis
of
La
Tombe
(Seine et Mmne), 20th December
1977
at
11
h
52
Plate
4
Thermography
of
Sennecay
and
Levet (Cher), 19th March
1980
at
11
h
18
G.M.
T.,
(wave length
11.5
pm)
First
case:
uniform
superfichl
layer
layered earth
1
:
layered earth
2:
h
=id.
k,
=
idem
h
=
0.30
m
k,
=
1.3
W/m/"
k,
=
1.4 W/m/'
k,
=
2.0
W/m/"
Second case:
non
unvorm
superjkhl
layer
layered earth
1:
h
=
0.30
m
k,
=
1
.lo
W/mp
k,
=
1.40 W/mp
layered earth
2:
h
=
idem
k,
=
1.40 W/m/"
k,
=
2.0
W/m/'
Table
1
rl
=
0.50
X
10"
mz/s
rl
=
0.60
X
10" m2/s (marl
or
silt)
r,
=
idem
r,
=
0.70
X
10-6mz/s (Beauce limestone)
rl
=
0.50
X
10-6mz/s
r,
=
0.60
X
r,
=
idem
r,
=
0.70
X
10L6mys (Beauce limestone)
mys (marl or
silt)
B
differences
3
z
May 1976 (positive flux)
-t
0.15'C
%
ff
Dec. 1977 (negative flux)
-0.10'C
3
P
R,
a
%
c,
May 1976 (positive flux)
+
0.85'C
Dec. 1977 (negative flux)
-0.5O"c
8
3
184
M.
C.
Perisset
and
A.
Tabbagh
0
500
m
P
Figure
10
Interpretation diagram
of
plate
3;
in dotted
lines
are represented the ancient roads and field
limits
0
SO
n
P
Figure
11
limits
Interpretation diagram
of
plate
4;
in dotted lines are represented the ancient ways and field
4.4
Traces
of
ancient
roads
and field
limits
The 19th March 1980 flight over the path
of
the planned motorway
A71
occurred during an
unfavourable period, because the
flux
was
negative
in
this
region (figure
7,
Bourges and
St
Germain) until the 15th March and changed on the 16th. However, interesting conclusions can
Interpretation
of
thermal prospection
on
bare
soils
185
be drawn about the depth of anomalies caused. The flux had the following values:
-
15 W/m2
from the 13th at
6
h to the 14th then -5, -8,O, 2.5,6 and finally 17 W/m2 on the 19th March
morning.
Let
us consider
a
conducting feature, stony way or wall, with a
-
0.22 contrast coef-
ficient with the surrounding silt covered by a uniform superficial layer
(h
=
0.22m,
kl
=
13OW/m/",
rl
=
0.5
x
m2/s): it
will
give a hot anomaly of 0.10"C of magnitude. If the
superficial layer above .the features has a greater conductivity (due to the presence of stones)
the anomaly
will
be colder and for a relatively low contrast coefficient of
-
0.125 between the
superficial layer above the feature and the undisturbed superficial layer, will have a magnitude
of
-
0
3°C.
Plate 4 shows a part of the thermography on the outskirts of the townships of Levet and
Sennecay (Cher, France); the network of ancient lanes corresponds to cold tracks. These anom-
alies are due to the effect of the positive flux, which occurred, between the 16th March and
the time of the flight, on the superficial layer itself as is confirmed by the fact that modern
field limits are well marked. These results lead to two conclusions: (a) in such flux conditions
the anomalies
of
deep origin will be completely masked by those of superficial layer origin,
(b)
as was the case for Lion en Beauce in December 1977, these last anomalies are sufficiently
interesting to justify the flight.
5.
PROBLEMS
OF
EMISSIVITY
VARIATIONS
Spectral emissivity is defined by the ratio
of
the emittance of a given body to that of the black
body at the same temperature and frequency. It is characteristic
of
chemical composition and
crystalline structure of the body observed by the radiometer. At first sight emissivity contrast
may exist between the material of an archaeological feature and the surrounding soil, but in
order to be detected by the radiometer, the feature has to out crop and in such a case it could
be directly observed very easily even without a thermal prospection. Consequently it is likely
that direct use of emissivity contrast will not be of great use
in
the future. Nevertheless it is
interesting to know the emissivity variations which could be detected at the ground surface,
so
as to be able to identify them and to determine their origin. Several attempts have been made
to plot emissivity using multichannel radiometers (Vieillefosse 1978, Patoureaux 1979). Four
channels are necessary, the 3-5~m band and three in the 8-14pm window, while the radio-
meter
ARIES
we used (Monge and Sirou 1975) has only two numerical channels. Since this
radiometer has an internal calibration, it is however possible to calculate apparent tempera-
tures. Using two channels, we may observe emissivity variations of the ground surface (because
it is unlikely that between two different frequencies they are exactly similar) and determine
approximately the differences between emissivities. Two infra-red channels were used during
March 1980 experiments, one (number 1) centered at 11.5pm where the emissivity is high
-
between 0.95 and 1
.OO
according to moisture (Buettner
et al.
1965) for soils and vegetation,
the other (number
2)
centeredat 8.7pm corresponds to silica and silicates low absorption bands
(Vincent 1975). The results of the comparison between the two channels are as follows:
(a) The average apparent temperatures in channel
2
are generally
1°K
lower than those of
channel 1.
(b) This difference is smaller for vegetation, meadow or wood,
so
that they appear relatively
hotter in channel 2 thermography.
(c) Over very specific targets such as roads or house roofs, channel 2 may give a lower tem-
perature, reaching
a
difference of
6°K.
(d) the set of anomalies appearing on bare soils are similar on the two channels.
186
M.
C.
Pirisset
and
A.
Tabbagh
Taking
as
an
example the flight undertaken in the N.E. of La Charitti s/Loire (NiBvre,
France) along the RN151 Road, the average apparent temperatures are 12.3"C on channel
1
1
1 A"C on channel 2. If we neglect the effect
of
the transmission through the atmosphere, and
we suppose that the emissivity
is
1
for clmnel
1,
the true temperature
is
285.3"K and the
apparent temperature corresponds to an emissivity of
0.985.
If channel
1
emissivity
is
0.96,
that of channel
2
will be 0.945.
So
the difference is around
1.5%.
For
a
petroleum pitch
covered road
(RN151)
the apparent temperature is 19°C on channel
1
and 15.5"C on channel 2
and the deviation in emissivity is
5%.
For a granite gravel covered road (D179) there is 17°C for
channel
1
and 11°C for channel 2, which is exactly the apparent temperature of the surround-
ing meadows, the emissivity deviation is here
9.5%
but
this
case is the most extreme one ever
met with. Two conclusions are to be drawn from this comparison:
(a)
Only one channel is sufficient to detect temperature anomalies and it is better to use the
second one in the visible and near infrared red band in order
to
make easier localisations and
give information on the
soil
and vegetation status.
(b)
The channel centred at
11.5
pm
wdl be used because, the emissivity being more uniform,
the interpretation is simplified, and because a road or
a
house which can be useful for localisa-
tion
will
not disappear.
6.
CONCLUSIONS
As
for other prospecting methods, what is important to know about thermal prospection is its
limits.
lks
method depends very much on climatic conditions, which cannot be controlled and
it is necessary, when taking a decision about a flight, to be able
to
define the characteristics
of
the features which will be detected and of those which will not.
This
study
presents the conclusions of four experiments. The conclusions are encouraging
and they lead
us
to plan a more systematic use of this method. First because we know, through
the study of slow transient flux variations, the favourable moment, secondly because we know
how to interpret the results, and finally because even in unfavourable flux conditions two types
of anomalies can give fruitful information
viz
the microreliefs and superficial layer heterogenei-
ties where contaminated by underlying archaeological features.
A
last remark must be made about the geographical area where our conclusions are valuable.
All
these studies were made
in
the bassin Parisien and as flux variations are more important in
continental climate and less important in oceanic climate, experiments should be undertaken in
the western part of France
or
in Great Britain to determine the exact limits of the method.
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... There are four significant thermodynamic properties that affect the thermal behavior of material: conductivity, diffusivity, inertia, and volumetric heat capacity. Excellent explanations in archaeological literature exist [1,9] (pp. 311-313), [6] (pp. ...
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... The problems that led to the scarce use of this technique, outside of satellite imagery, were mainly related to the high cost and the poor spatial and radiometric resolution offered. Between the 1970-80s, a large and cooled scanning radiometer was used with liquid nitrogen that recorded thermal images on long rolls of film [11][12][13]. Despite the difficulty, the methodological foundation for the use of thermography in archaeology was developed in those early years. ...
... Despite the difficulty, the methodological foundation for the use of thermography in archaeology was developed in those early years. In particular, Périsset and Tabbagh [13] and Périsset [15] conducted a series of controlled tests to quantify the thermal behavior of some archaeological elements with respect to the surrounding terrain. The work in [15] presents a summary and tests for soil thermodynamics, including abundant mathematical notation and physical properties used to understand the results, and how analysis can be applied to different archaeological contexts. ...
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Kamarina, located in southern Sicily (Italy), was an important Greek colony since its foundation in the sixth century BC. Archaeological excavations, carried out since the twentieth century, uncovered only limited portions of the site so far. Despite the importance of the Greek colony, the presence of remarkable buildings that archaeologists expected to bring to light has not found fully correspondence in the archaeological excavations. Consequently, the integrated geophysical prospection carried out in the study area is aimed to support and address the future archaeological investigations. After the photographic and thermographic survey obtained by an unmanned aerial vehicle, we performed a systematic survey through ground magnetic and GPR methods over an area of 6200 m². The acquisition procedures have been optimized in order to get the best results combining high resolution and elevated speed of acquisition. The results derived from the three geophysical techniques have been conveniently combined by means of a cluster analysis, allowing us to clearly identify a series of buried archaeological features. Because of their geometrical characteristics, often in good agreement with the spatial arrangement of the archaeological remains at the surface, these buried archaeological features can be interpreted as roads, walls, or buildings foundations in which the various construction phases of the city can be clearly recognized. The integrated approach has proven to be essential for a robust interpretation of the archaeogeophysical investigation.
... A thermal camera can assess the amount of infrared radiation being emitted and translate that into a thermogram, more commonly known as a thermal image. The principle of archaeological thermography is that as the sun rises and sets during the course of the diurnal cycle, subsurface remains will absorb and emit infrared radiation, with the amount of infrared transference dependent on variables such as moisture, material and density (Casana, Kantner, Wiewel, & Cothren, 2014;Fourteau & Tabbagh, 1979;Haley, Johnson, & Stallings, 2002;Perisset & Tabbagh, 1981;Scollar, Tabbagh, Hesse, & Herzog, 1990;Tabbagh, 1977). If the emitted thermal radiation differs from that of soil surrounding the feature, it may be detected by a thermal camera (Kvamme, 2008a). ...
... If the emitted thermal radiation differs from that of soil surrounding the feature, it may be detected by a thermal camera (Kvamme, 2008a). This technique has been successfully employed on a number of archaeological projects since the 1970s, utilizing a variety of thermal cameras and aerial photography platforms such as balloons (Haley et al., 2002), helicopters (Ben-Dor, Kochavi, Vinizki, Shionim, & Portugali, 2001;Lunden, 1985), planes (Berlin, Ambler, Hevly, & Schaber, 1977;Perisset & Tabbagh, 1981;Sever & Wagner, 1991), and powered parachute (Kvamme, 2006(Kvamme, , 2008b. ...
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Infrared thermography, or thermal imaging, has been used as a remote sensing technique to determine whether subsurface features such as walls or pits generate a heat differentiation from the surrounding earth. To date, this form of remote sensing has been notoriously difficult to perform due to cost, low-resolution thermal cameras and an inability to provide a stable aerial photographic platform. Furthermore, thermal fluctuations produced by archaeological remains are highly volatile, and are dependent on a multitude of variables such as soil moisture, particle size, and the construction materials of features. These issues have restricted the use of infrared thermography within archaeology. Yet, with the rapid development and adoption of unmanned aerial vehicles (UAVs) over the last decade as well as developments in thermographic technology, thermal imaging is now affordable and can be attached with relative ease to a multitude of UAVs. This paper reviews a new, low-cost, FLIR Systems thermal camera made specifically for UAVs, which were successfully employed at the Ancient Methone Archaeological Project, Pieria, Greece. By utilizing the FLIR Vue Pro, along with a DJI Phantom 2, aerial thermography was performed at the site at a cost far below that proposed by previous studies, with results also equalling or exceeding those methodologies. In combination with high resolution aerial photogrammetry, this methodology has helped to clarify previous archaeological investigations at the site, as well as revealing significant rectilinear subsurface remains.
... Thus, a subsurface material of different thermal inertia from its surrounding soil matrix will retain or lose heat at a different rate. If this contrast is sufficient, it can affect the temperature of the surrounding soils, producing differences in surface radiance detectable by the sensor [2]. Harnessing these thermal properties is critical to successful thermographic prospection of archaeological contexts. ...
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Over the last decade, we have witnessed momentous technological developments in unmanned aircraft systems (UAS) and in lightweight sensors operating at various wavelengths, at and beyond the visible spectrum, which can be integrated with unmanned aerial platforms. These innovations have made feasible close-range and high-resolution remote sensing for numerous archaeological applications, including documentation, prospection, and monitoring bridging the gap between satellite, high-altitude airborne, and terrestrial sensing of historical sites and landscapes. In this article, we track the progress made so far, by systematically reviewing the literature relevant to the combined use of UAS platforms with visible, infrared, multi-spectral, hyper-spectral, laser, and radar sensors to reveal archaeological features otherwise invisible to archaeologists with applied non-destructive techniques. We review, specific applications and their global distribution, as well as commonly used platforms, sensors, and data-processing workflows. Furthermore, we identify the contemporary state-of-the-art and discuss the challenges that have already been overcome, and those that have not, to propose suggestions for future research.
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While archaeologists have long understood that thermal and multi-spectral imagery can potentially reveal a wide range of ancient cultural landscape features, only recently have advances in drone and sensor technology enabled us to collect these data at sufficiently high spatial and temporal resolution for archaeological field settings. This paper presents results of a study at the Enfield Shaker Village, New Hampshire (USA), in which we collect a time-series of multi-spectral visible light, near-infrared (NIR), and thermal imagery in order to better understand the optimal contexts and environmental conditions for various sensors. We present new methods to remove noise from imagery and to combine multiple raster datasets in order to improve archaeological feature visibility. Analysis compares results of aerial imaging with ground-penetrating radar and magnetic gradiometry surveys, illustrating the complementary nature of these distinct remote sensing methods. Results demonstrate the value of high-resolution thermal and NIR imagery, as well as of multi-temporal image analysis, for the detection of archaeological features on and below the ground surface, offering an improved set of methods for the integration of these emerging technologies into archaeological field investigations.
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Thermal infrared imaging, or thermography, is the remote sensing technique of detecting variations in ground temperature caused by exposed or subsurface archaeological remains either absorbing or radiating heat. Despite its conception in the 1970s, the practice has to date been rarely utilized, as a result of the high cost of the technology and the complex interplay of environmental variables. However, recent studies have demonstrated the effectiveness of the technique, especially when combining modern thermal cameras with unmanned aerial vehicles (UAV). Yet these papers often focus on mid‐ to high‐altitude flights, where the technique is only effective at detecting larger thermal anomalies. This article presents a new method for terrestrial thermography, developed for the Zagora Infrared Photogrammetry Project (The University of Sydney and the Australian Archaeological Institute at Athens). The project undertook a six week thermal investigation of the Early Iron Age site of Zagora and the surrounding hinterland utilizing the newest commercial thermal cameras and UAVs. The method of terrestrial thermography involves using photographic poles and photogrammetry to create high‐resolution thermal orthophotographs, which allow the detection of smaller thermal anomalies, providing significantly more detail than aerial thermography. Several features were discovered using this method, including a possible kiln, which would be the first ever identified at the site.
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Locating the subtle and uneven deposition of human activities across the landscape continues to challenge archaeologists. Existing tools (e.g. excavation, shovel testing, pedestrian survey, and terrestrial geophysics) have proven effective at locating many types of archaeological features but remain time-consuming and difficult to undertake on densely vegetated or topographically complex terrain. As a result of these limitations, key aspects of past communities remain largely outside of archaeological detection and interpretation. This flattening of past lifeways not only affects broader understandings of these communities, but can also negatively impact the preservation of archaeological sites. This paper presents the detection of archaeological features through an analysis of drone-acquired thermal, multispectral, and visible light imagery, alongside historical aerial photography, in the area surrounding Middle Grant Creek (11WI2739), a late prehistoric village located at Midewin National Tallgrass Prairie in Will County, IL. Our investigations discovered a probable housing area and a ritual enclosure, increasing the area of the site from 3.4 ha to 20 ha. The proposed housing and ritual areas of this village also help contextualize finds from the ongoing archaeological excavations at Middle Grant Creek. More broadly, results demonstrate the valuable contributions that these relatively new archaeological survey methods have in shaping our understandings of the archaeological landscape and highlight the importance of integrating them into the archaeological toolkit.
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While a long history of experimental data shows that aerial thermal images can reveal a wide range of both surface and subsurface archaeological features, technological hurdles have largely prevented more widespread use of this promising prospecting method. However, recent advances in the sophistication of thermal cameras, the reliability of commercial drones, and the growing power of photogrammetric software packages are revolutionizing archaeologists' ability to collect, process, and analyze aerial thermal imagery. This paper provides an overview of the theory behind aerial thermography in archaeology, as well as a discussion of an emerging set of methods developed by the authors for undertaking successful surveys. Summarizing investigations at archaeological sites in North America, the Mediterranean, and the Near East, our results illustrate some contexts in which aerial thermography is very effective, as well as cases in which ground cover, soil composition, or the depth and character of archaeological features present challenges. In addition, we highlight novel approaches for filtering out noise caused by vegetation, as well as methods for improving feature visibility using radiometric thermal imagery.
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For more than half a century, air photographs made with hand held cameras have made major contributions to the discovery of sites in northern Europe. Most appear as discolorations in crops or soils. Photos taken over many years show different details which only a proper drawing on a large scale plan can reproduce. Two useful algorithms for the improvement of picture quality and for geometric correction, creating pseudo-vertical imagery are described. They can be used in standard mini computers with reasonable processing times and storage.
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The study of historical field boundaries is based at first on the documentary evidence, but in the case of Lion-en-3eauce township there is no thing older thorn cje 1 808 land- register. But thermal prospection brings up useful information showing the headland ridges caracterised by the association of a warm strip and of cold strip (those ridges were controled by altmetric measurment on the field) ; other old limits appear which correspond to ancient ditches. The date of these boundaries can be shown by excavation of certain ones and of the sites located by surface remains, which allow a relative chronology to be obtain.
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Infrared signals received in the 8- to 12- window by weather satellites and air- craft are dependent on surface temperature (T,), surface emissivity (e), and atmospheric interference. It seems that nowhere can variations of e be neglected in order to evaluate T, data correctly. Methods of determining e of typical surface materials are presented. They are: (1) determinations from reflection data obtained from polished rock samples run on a spectro- photometer; (2) a fieldworthy device constructed by the authors, called the emissivity box; and (3) e as inferred from Tiros data. Over the Sahara, emissivity of SiO2-containing surfaces is frequently below 0.95. Emissivity of water and oil on water are shown to differ.
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Micrometeorologists have long recognized the role of the heat-storage capacity of the soil in such problems as the occurrence of frost and dew, forecasting of maximum and minimum temperatures, partition of solar energy between the soil and atmosphere, etc. Meteorologists are becoming increasingly aware of the importance of thermal properties of the soil on larger-scale processes such as the modification of air masses, formation of tornadoes, development of weather systems, and the general circulation.The heat energy stored per unit area by the soil can be calculated if the temperature and heat capacity of the soil as functions of time and depth are known. In this investigation, the daily and annual cycles of soil temperature have been calculated from the soil temperature data collected by the Meteorology Group at the Argonne National Laboratory. Since data on soil moisture were not available, it was necessary to estimate the heat capacity of the soil from several spot measurements of the heat-capacity profiles and the rainfall record.
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Thermal infrared scanners with multiple channels in the 8-14-µm wavelength region are important for geological remote sensing because this spectral region contains important compositional information about silicate rocks and minerals that cannot be duplicated by remote sensors operating elsewhere in the electromagnetic spectrum. Emittance minima in this spectral region, caused by interatomic oscillations, occur at different wavelengths depending on silicate rock type. It has been demonstrated that an image constructed from a signal that is proportional to a ratio of radiances in two thermal scanner channels can be used to map compositional (chemical and mineral) variations in silicates, while suppressing temperature variations across a scanned scene. Theoretical studies indicate that future infrared scanners with eight to twelve channels in the 8-14-µm region might be used to produce an image that could be simply level-sliced (divided into discrete gray levels) to map silicate rocks according to traditional rock classification charts. This is a field in which sensor technology is still the limiting factor. However, improvements in extrinsic sensor properties, especially an increase in the number of spatially coregistered detector elements in a single dewar, are more important than improvements in intrinsic properties, such as detectivity.
Computer restitution and enhancement of extreme oblique archaeologird air photos for archaeological cartography. Proceeding o f the XXth Symposium for Archaeometry, revue d'ArchiomPtrie no 5
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