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Geophysical characterisation of Carlo's V Castle (Crotone, Italy)
M. Bavusi , A. Giocoli, E. Rizzo, V. Lapenna
IMAA-CNR - Hydrogeosite Laboratory, C.da. Fontanelle, 85052, Marsiconuovo (PZ), Italy
a b s t r a c ta r t i c l e i n f o
Article history:
Received 1 November 2007
Accepted 4 September 2008
Keywords:
Archaeology
Castle
Electrical resistivity tomography
Ground penetrating radar
Magnetic method
Time-slice
The Carlo's V Castle, located in Crotone Town, on the Ionian coast of the Calabria Region (Italy), date back to
the 13th century d.C. (Fig. 1). During its long life, the building changed several owners and sustained the
damages and the consequent reconstructions due to the innumerable naval battles. Moreover, the castle
suffered the action of the earthquakes which always afict the region.With the principal aim of detecting the location, depth and geometry of the rests of destroyed structures, a
systematic Ground Penetrating Radar (GPR) survey was carried out in the area inside the boundary walls.
The results are sixty-two one-meter-spaced, ltered and migrated radargrams arranged in four 3D data-sets.
From each data-set, the most signicant time-slice was extracted.
To reduce the ambiguity in the GPR data interpretation, additional geophysical techniques, such as Magnetic
(M), and Electrical Resistivity Tomography (ERT), were carried out with a partial superimposition with the
GPR data. A comparison and a joint interpretation amongst different geophysical data pointed out some very
remarkable features associated to buried remains and possible buried cannonballs.
With the secondary aim to check the presence of an old military walkway linking two bastions a GPR prole
was carried out on the sea side boundary wall. The GPR results are in agreement with an ERT survey carried
out on the same prole and consistent with the presence of an underground passage.
2008 Elsevier B.V. All rights reserved.
1. Introduction
The Carlo's V Castle, sited in the Crotone Town, on the Ionian coast
of the Calabria Region (Italy), suffered the damages due to the naval
battles and changes due to the evolution of the war art under several
owners: Carlo d'Angi in the 13th century, Ruffo family and Alfonso
d'Aragona in the 15th, Carlo IV di Borbone in the 18th. In the 19th
century the castle lost its military signicance and it was partially
dismantled owing to often earthquakes. In the 1960 some restoration
works were carried out. The boundary walls of the castle form a struc-
ture which shows a roughly quadrangularshape in plan view (Fig.1).At
four corners there are two bastions (S. CaterinaandS. Giacomo), placed
on the NW sea side, and two towers (Torre Comendante and Torre
Aiutante) situated on land. The inner area shows an irregular morphol-
ogy for the presence of topographical terraces connected by several
steep slopes (Fig. 1).
In this framework, a geophysical survey was planned in two steps:
1) check some features associated to buried remains of ancient
structures in the area inside the walls; 2) to check a military walkway
linking the two bastions in the sea side wall. Theeld work included a
3D Ground Penetrating Radar (GPR) survey supported by Electrical
Resistivity Tomography (ERT) and magnetic surveys in order to reduce
the ambiguity in the interpretation of data.
2. Geophysical methods
In the recent years the geophysical methods have been rapidly
transformed; new sensors and technologies have made the instru-
ments able to acquire with high sensitivity and acquisition rate.
Moreover, new algorithm for data inversion is developed for all
geophysical parameters. There is currently a wide class of methods
and techniques that allow to obtain extremely detailed images of the
physical properties of the subsoil. Particularly, the magnetic, electro-
magnetic and electrical methods give fast, non-destructive and low
cost tools to obtain quick information for archeological research.
GPR method is appreciated for its non-destructivity and for its
ability to give a real-time, high resolution information (Basile et al.,
2000). In fact, the technique is based on the impulsive emission of
electromagnetic (e.m.) energy and on the reception of the reected
echoes which occurs at the buried discontinuity in the dielectric
constant and electric conductivity (Davis and Annan, 1989). The
acquisition consists in dragging an antenna along a free surface
(generally of the soil) while it emits and receives the e.m. signals. The
result of a GPR survey is a vertical section of the ground in terms ofX
(distance covered by the antenna) and two way time (double time
spent by the e.m. pulse to cover the path antenna/target/antenna.).
The timedepth conversion is possible estimating the electrical
permittivity (r) of the ground. The spatial resolution depends on
the used frequency, constructive features of the used antenna, and the
sampling rate. The use of adequate antennas allows to detect buried
remains but to carry out non-destructive checks on the structures
Journal of Applied Geophysics 67 (2009) 386401
Corresponding author.
E-mail address:[email protected](M. Bavusi).
0926-9851/$ see front matter 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jappgeo.2008.09.002
Contents lists available at ScienceDirect
Journal of Applied Geophysics
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j a p p g e o
mailto:[email protected]://dx.doi.org/10.1016/j.jappgeo.2008.09.002http://www.sciencedirect.com/science/journal/09269851http://www.sciencedirect.com/science/journal/09269851http://dx.doi.org/10.1016/j.jappgeo.2008.09.002mailto:[email protected]8/11/2019 Carlo's v Castle
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be used, but in most cases a line separation of 1m is used to reducethe
time-consumption of the survey (Nuzzo et al., 2002).
Magnetic method is a passive technique appreciated for its
quickness and non-destructivity. Recently, the gradiometric cong-
uration and the use of very sensitive vapour caesium sensors allow
detection of buried remains up to 6m depth (Bavusi et al., 2004).
The technique detects the variations in the geomagnetic eld due
to the presence, in the subsoil, of the buried objects. In fact, all bodies
Fig. 3.Detailed plan view of area AofFig. 2with the traces of GPR, ERT and magnetic surveys.
Fig. 4.Detailed plan view of area
B
ofFig. 2with the traces of GPR and magnetic surveys.
388 M. Bavusi et al. / Journal of Applied Geophysics 67 (2009) 386401
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modify locally the geomagnetic eld proportional to their magnetic
permittivity (r).
Metals provide a greater modication, denedmagnetic anomaly,
than terracotta and stones (Chianese, 2003). The acquisition consists
in walking with the sensor (or the sensors) along a zigzag pathdened by aline spacingand amark spacing. In this way, a large area
can be covered in short time. The result of a magnetic survey is a
magnetic (or gradiometric) map which shows the magnetic anomaly
on the horizontalXYplane. The spatial resolution along the cross-line
direction is dened by theline spacing, while the spatial resolution in
the in-line direction depends on the sampling frequency. Supposing
that onewalks with a velocity of 1 m persecond, a sampling frequency
of 10 Hz provides a spatial resolution of 0.1 m in the sampling
direction. The introduction of a constant mark spacing allows to
compensate possible velocity variations of the operator.
When the magnetometer is equipped with two sensors located at
different heights on the same vertical, it is possible to gain the vertical
gradient of the geomagnetic eld. The gradient allows to avoid the
corrections (for the altitude, latitude, and longitude of diurnalvariation) required for a non-gradiometric magnetic survey.
The ERT method is an active technique more time expensive but
more robust also than the previous ones, thanks to the availability of
inversion routine based on the nite difference.
The most popular inversion routine is the Res2DInv, based on the
smoothness-constrained least-squares method (Loke and Barker,
1996).
The technique is based on the injection of current (I) into the
ground by using a couple of electrode (dened A and B) and the
simultaneous measure of the potential (V) by a second couple of
electrodes (dened M and N). Thesearched parameter is the apparent
electrical resistivity. The inversion process allows to obtain a model in
terms of the real electrical resistivity consistent with the apparent
measured resistivities.
The usefulness of the ERT was demonstrated in the structural
geological geometries such as fault planes in tectonically active areas
(Caputo et al., 2003; Colella et al., 2004), and landslide bodies
(Lapenna et al., 2003). Recently, the ERT was used to improve the
constraint for the detection of archaeological buried features (Rizzoet al., 2005).
3. Survey design
To optimize the geophysical survey, the area inside the boundary
wallswas divided into three areas: A, B and C. Moreover, a fourth
zone, indicated with D, was selected on the sea side boundary wall
linking two bastions (Fig. 2). In each area the superimposition
amongst several techniques was not perfect for logistic problems. In
Fig. 5.Detailed plan view of area CofFig. 2with the traces of GPR and magnetic surveys.
Fig. 6.Picture of area
D
ofFig. 2with the traces of GPR (#66) and ERT (T4) surveys.
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Fig. 7.Comparison between raw (a) and processed (b) radargrams obtained onto line #7 of area A.
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Fig. 9.Time-slices obtained in area Astarting from radargram #28#44. a) time-slice from 0 to 0.4 m; b) time-slice from 0.4 to 0.8 m; c) time-slice from 0.8 to 1.2 m; d) time-
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Fig. 10.Time-slices obtained in area Bstarting from radargram #45#51. a) time-slice from 0 to 0.4 m; b) time-slice from 0.4 to 0.8 m; c) time-slice from 0.8 to 1.2 m; d) time
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Fig. 11.Time-slices obtained in area Cstarting from radargram #52#65. a) time-slice from 0 to 0.4 m; b) time-slice from 0.4 to 0.8 m; c) time-slice from 0.8 to 1.2 m; d) time
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fact, magnetic surveys were not extended where metallic objects, like
railings and fencing, were present. Instead, GPR surveys did not cover
all magnetic investigation areas because of the roughness of the
ground and the presence of several obstacles like walls and
depressions. The ERT surveys were carried out in a small zone of
area A and inarea D to reduce the time-consumption of the survey.
All GPR surveys were carried out by using the GSSI SIR 3000
georadar system equipped with a 400 MHz central frequency, single
fold antenna placed on a trolley equipped with survey wheels. TheGPR acquisitions consisted of several survey proles one-meter
spaced. The system was set with a range of 60ns, the automatic
control in rst reector position, a gain control on four points, low-
pass and high-pass lters of 800 and 30 MHz respectively. Moreover a
scan rate of 32 scans per second and a sampling of 512 samples per
scan were used.
Magnetic measurements were acquired by means of the magnet-
ometer Geometrics G-858 with gradiometric conguration. The
system was set to acquire in bi-directional mode with a sampling
frequency of 5 Hz (corresponding to a spatial resolution of about 0.5m
walking with a velocity of 1m/s) along 1m spaced parallel surveylines. The sensors werexed at a distance equal to 1.0m, the lower one
0.30m from the ground.
Fig. 12.Radargram carried out onto the head of the boundary wall linking S. Caterina and S. Giacomo bastions.
Fig.13.Map of the magnetic gradient of the area inside the boundary wall of the Carlo's V Castle. a) gradiometric map of area A; b) gradiometric map of area B; c) gradiometric
map of area
C
.
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The ERT surveys were carried out by using the IRIS Syscal R2
georesistivimeter with a 32-multielectrode system, with an electrode
spacing of 1.0m.
3.1. Area A
Forty-four radargrams, grouped in two families (from #1 to #27
and #28 to #44 respectively), were acquired in this area. Moreover a
gradiometric map was carried out in the entire area (about4231 m2). Finally three parallel 2.0m spaced WS ERT proles (T1,
T2 and T3 corresponding to the GPR proles #25, #27 and #23) were
carried out to obtain a 2.5D representation (Fig. 3).
3.2. Area B
In area B a group of seven radargrams (from #45to #51) covering
an area of 614 m2 was partially superimposed to a gradiometric
survey which covered an irregular, L-shaped area. In fact, the high
slopesof this area impeded thefullcoverage by theGPR survey,while a
metallic fence limited the magnetic survey area (Fig. 4).
3.3. Area C
In this at area the superimposition between GPR and magnetic
surveywasgood.Fourteenradargrams(from#52 to #65) withdecreasing
lengths were acquired in areaCof about 1414 m2. The gradiometric
survey interested a smaller superimposed area of 10 13 m2 (Fig. 5).
3.4. Area D
Although named area, this zone is indeed a narrow corridor
located onto the head of the boundary wall linking S. Caterina and S.
Giacomo bastions. For this reason a single radargram (#66) was
superimposed to an ERT WS survey along the same prole (Fig. 6).
4. Data processing and results
A GPR data, as well as the ERT data, are generally provided likevertical sections of the ground in the planesXt2or YZrespectively. In
this way it is very hard to compare them with a magnetic or
gradiometric map, which is, on the contrary, a representation of local
geomagneticeld in thehorizontalXYplane. Moreover, since the large
amount of GPR data a compact manner to show them was needed. To
quickly compare all data and provide a compact form to show them,
each GPR data-set was processed to obtain a data volume whose
extract the so-calledtime-slices, is a representation of reectivity of
the ground in the XYplane at a xed time. For the same reason, the
ERTsurvey, whencarried out along several parallel lines, was arrangedto provide several maps at different depths. However, comparison
between GPR and ERT data in the vertical plane was sometimes
necessary.
4.1. GPR data processing and results
All radargrams were subjected to a processing including: static
correction, for the correct positioning of time-zero; remove header
gain, to depurate the data from the eld gains; energy decay, to restore
a correct gain along each wavelet and compensate the energy
spreading;time cut, to show the data up to 50ns; fk-lter, for cutting
high dipping and back scattered noise; bandpass frequency lter,
between 100 and 600 MHz to remove the low and high frequencies
noise; migration with a velocity of 0.15m/ns; trace resampling at
0.05m; andbackground removalto cut the horizontal noise.
Fig. 7 shows a raw data (a) compared with processed data (b).
Although a migration velocity of 0.13m was selected, the radargrams
are showed in terms of time. In fact, the migration velocity represents
an arrangement of several velocities which allow collapse of a great
many hyperbolas. Not necessarily this velocity corresponds to the true
velocity in all radargrams.
So-processed radargrams were interpolated to build a data volume
for each family of parallel radargrams. In this way two data volumes
were obtained for area Aand one each for areas Band C.
Fig. 8 showsve time-slices obtained each 10nsin area A starting
from radargram #1#27 (Fig. 8ae). The greyscale used shows
qualitatively the absolute amplitude of reections: white colour
represents high-amplitude reections; dark-grey, absorption. Some
reective areas start to be delineated between 10 and 20ns (Fig. 8c).Other reective areas are visible between 30 and 40ns (Fig. 8d).The
Fig. 14.Test results between DD and WS devices of tomography T1 carried out in area
A
.
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radargrams #28#44 of area Awere used to build the time-slices of
Fig. 9.In this case also, very remarkable features are visible between
10 and 20ns and between 30 and 40ns respectively (Fig. 9c,d).
In area Bthe time-slices built starting from radargram #45#51
show main reections between 10 and 20ns (Fig. 10c).
The radargrams #52#65 were used to build ve time-slices in
area C (Fig. 11). The highest amplitude is shown between 20 and
30 ns (Fig. 10c), although some remarkable features are visible
between 30 and 40 ns (Fig. 11d). Finally, a radargram (#66) was
carried out onto the head of the boundary wall of area D(Fig. 12).
The radargram shows a chaotic zone upto 30ns. Between 18 and 29 m
along x a riche reection zone is highlighted in the rectangle r.Below 30 ns, horizontal, parallel reectors can be noted (q).
4.2. Magnetic data processing and results
Magnetic data processing included the cleaning of spikes and the
cutting of data between 110 and + 110nT/m. In this way it was
possible to highlight both metallic and non-metallic anomalies with
the same colour scale. The so-ltered data were interpolated by using
the Kriging method with a grid of half meter mesh in both Xand Y
directions. The in-line direction is lightly undersampled, while the
cross-line direction is not much oversampled. In this manner the
spatial resolution in two directions is the same and no stretching
occurs in the pattern of magnetic anomalies. Fig. 13 shows the
magnetic gradient of all areas. The pattern of area A is formed by
several strong ( 110nT/m) dipolar anomalies aligned along different
directions which can be related to buried metal objects (Fig. 13a). In
area B, gradient values are high also in all investigated area, but in
the eastern zone several dipoles seem to form a horseshoe shaped
structure (Fig. 13b). Finally, in area C a strong positive anomaly is
present in the southern zone of the map. Moreover, several dipoles, as
well as the complex structure in the northern zone appear in otherzonesFig. 13c).
4.3. ERT data processing
Acquired ERT data form the so-called pseudosection, a vertical
section of the ground in terms of apparent electrical resistivity.
Obtaining the distribution of true resistivity is made possible by
inverting the data using several algorithms. The used inversion
routine is Res2DInv, based on the smoothness-constrained least-
Fig. 16.ERT T4 carried out on the head of the boundary wall in area
D
.
Fig. 15.2.5 D representation of electrical resistivity obtained by tomographies T1, T2 and T3 of area A.
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squares method (Loke and Barker, 1996) which yields a resistivity
model able to generate a synthetic pseudosection very much alike
with the acquired one. A eld ERT test was applied to check the best
array between DipoleDipole (DD) andWennerSchlumberger (WS)
in area A. The WS technique was chosen because it has shown a
better S/N ratio and larger investigation depth (Fig 14). Before the
inversion process, data areltered to improve the convergence of the
model. For the ERT survey carried out in area A it was possible to
build a data volume interpolating three WennerSchlumberger
tomograpies. The result is shown inFig. 15where the distribution of
resistivities is provided at seven depths starting from 0.25 m. The area
covered by three tomograpies is 4 31 m2, but the interpolation was
performed onto an area of 531 m2 expanding half meter outside for
tomographies T2 and T3. A strongly resistive nucleus is visible
Fig. 17.Combination of data in area A. a) GPR time-slices between 0.8 and 1.2 m for the radargrams #1 #27 and between 1.2 and 1.6 m for the radargrams #28#44; b) map of
magnetic gradient; c) ERT maps at o.75 and 1.27 m respectively.
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between 9 and 13m alongxat depths of 0.75 and 1.27 m. Finally, a WS
ERT was carried out onto the boundary wall linking S. Caterina and S.
Giacomo bastions in area D(Fig. 16). From 1.5 m to 15 m, a resistive
(500,m) and at zone is present up to a depth of 1.8 m where the
base is indicated with q. In it, a very high resistive nucleus is present
between 8 and 13 m along x (p). Below 2.5 m along z, a very low
resistive zone (s) can be related to the presence of water inltration.
The high resistivities (t) located from 16 m to the end along x are
probably due to a strong edge effect consequent the ramp ofFig. 6.Then, they were considered not signicant.
5. Joint interpretation
The last step of this workwas thejoint interpretation ofdata for each
area. A compact form to show the data is required to select more
signicant GPR time-slicesas wellas theERT maps. The most signicant
GPRdata arelocated between 20 and30 ns forall areas.The selected ERT
maps are at 0.75 and 1.27m respectively. Then, GPR data, and ERT data
when present, are compared for areas A, Band Cin form of maps.
For area Dthe comparison was possible in section view.
5.1. Data interpretation of area A
InFig. 17a 3D view of all data is provided. The most signicant
reections are indicated by p, qand t, while an absorption zone
is indicated by rand s(Fig. 17a). Reectionpcorresponds to the
highresistivebodyin the ERT maps (Fig.17c).Sincethe time-slices are
included between 20 and 30 ns and the ERT are at 0.751.27m, we can
calculate a velocity of 0.08 m/ns and a consequent relative electrical
permittivity of about 13. Magnetic anomaly related to object p is
very low (10nT/m) indicating a non-magnetized body. Conductive
zones around zone pcorrespond to the absorption zone in the time-
slices (Fig. 17a,b). Reection qis related to a transition zone with a
value of 50nT/m in the magnetic map (Fig. 17b). On the contrary, the
absorptionzone r is related with a very high magnetic gradient value
indicating the presence of conductive metal objects. The transitionzone between q and r is characterized by a gradient both in the
GPR and in magnetic data. The absorption zone sis located onto a
strong dipolar anomaly in the magnetic map (Fig. 17b). Finally the
reection t is located onto transition zone between a very high-
magnetized zone and area with low values of magnetic anomaly.
Summarizing area A shows: 1) reections associated to non-
magnetized areas or to transition zones between high and low values
of magnetic gradient. Moreover these reections are associated to
high resistive bodies; 2) absorption zones associated to high-
magnetized and very low resistive zones. First evidence can be
explained with the presenceof buriedremainslike drystone walls; the
second one with the presence of conductive, metallic objects.
5.2. Data interpretation of area B
The superimposition between GPR and magnetic data for area B
was very poor because of logistic problems. Reections indicated bypcan be related to a transition zone in the magnetic gradient, while
Fig. 18.Combination of data in area
B
. a) GPR time-slices between 0.8 and 1.2 m; b) map of magnetic gradient.
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the absorption zones q and r correspond to a very high dipolar
anomaly (Fig. 18a,b).
5.3. Data interpretation of area C
Superimposition between GPR and magnetic data in the area C
was very good anda lotof correspondences canbe found between two
kinds of patterns. Absorption zones p and r correspond perfectly to
anomalous zones in the magnetic
eld with high values (Fig. 19a,b).The reection named qin the time-slice corresponds, instead, to a
transition zone in the magnetic gradient with values centered around
zero. Finally, the absorption zone in the time-slice indicated by s
shows very high values of magnetic gradient and the shape seems the
same in two maps. In areaCalso, main reections correspond to the
transition zones into the magnetic eld, while the absorption zones
show a good correspondence with high dipolar magnetic anomalies
related to the presence of magnetic objects.
5.4. Data interpretation of area D
Fig. 20shows the comparison between GPR and ERT data in areaD. Some correspondence can be found between two kinds of data;
reections indicated by pcan be related to the resistive body in the
ERT. The reector q at about 30 ns can be associated to a level placed
at about 1.80m. The GPR data shows that q crosses all the radargram
indicating that the resistive zone t is an artifact due to the edge
effect. Below this level, the high conductive zone scan be related to
an absorption zone in the radargram. Finally, the feature named ris
well represented in both GPR and ERT data. These features are
compatible with a buriedpassageway; in fact the reection r and q
can be related respectively to the top and bottom of a tunnel. The
resistivities included between two features are compatibles with acavity partially lled.
6. Conclusions
Combined geophysical measurements carried out in the Castle of
Crotone were performed to check primarily buried remains (in the
areas A, B and C) and secondly to detect a possible buried
walkway in a portion of the boundary walls (in area D).
A systematic GPReld survey interested the areas A, Band C
with four data-sets of one-meter-spaced radargrams. A complex
sequence processing was performed to enhance the reections and
build a data volume for each data-set. In this way ve time-slices were
built for each area. Then magnetic and ERT (only for areas Aand D)
were carriedout trying to obtain thebestsuperimposition.Unfortunately,
Fig. 19.Comparison between GPR and ERT data for area
C
. a) GPR time-slices between 0.8 and 1.2 m; b) map of magnetic gradient.
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logistic problems impeded a good superimposition for areas A and B.
In area D only a prole was useful to carry out both GPR and ERT
surveys. Finally, the joint interpretation of data for each area allows to
sketch thefollowing conclusions:1) reective zone of theGPR survey can
be related with resistive and nonmagnetic zones. Objects like drystone
walls are compatible with the presence of these signals; 2) absorptive
zones in the time-slices can be related with high dipolar or complex
magnetic anomalies. In these zones, the ERT survey showed very high
conductivities. The intense presence of isolated dipolar magneticanomalies suggests the presence of small metallic objects. Since in the
past, some cannons andcannonball were found in thearea, they couldbe
a candidateto explaintheseevidences. 3) GPR data showed that themost
signicant reections are included between 20 and 30 ns for areas A,B and C; ERT surveys showed signicant resistive objects between
about 0.75 and 1.27m for area A. Matching these data we can infer a
velocity of the medium in area A equaling to 0.08 m/ns and
consequently a relative electrical permittivity of about 13; 4) nally, the
GPR and ERT datain areaD are consistent with thepresence of a tunnel.
Some horizontal reectors could be related to the top and bottom of a
possible buried passageway and the ERT data showed an associated at
resistive body. Moreover, the GPR survey carried out in areaDallowed
to overcome an interpretative problem lied to a strong edge effect in the
ERT data.
Acknowledgment
This work has been supported by the Municipality of Crotone and
the Archaeological Superintendency of Calabria Region.
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Fig. 20.Comparison between GPR and ERT data for area D. a) radargram #66; b) tomography T4.
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