2-d velocity structure of the buried ancient canal of xerxes

8/11/2019 2-D Velocity Structure of the Buried Ancient Canal of Xerxes

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.Journal of Applied Geophysics 47 2001 2943

www.elsevier.nlrlocaterjappgeo

2-D velocity structure of the buried ancient canal of Xerxes:an application of seismic methods in archaeology

V.K. Karastathis a,), S. Papamarinopoulos b, R.E. Jones c

aInstitute of Geodynamics, National Obseratory of Athens, P.O. Box. 20048, Athens 118-10, Greece

bDepartment of Geology, Uniersity of Patras, Rio 261-10, Greece

cDepartment of Archaeology, Uniersity of Glasgow, Glasgow G12 8QQ, UK

Received 15 September 2000; accepted 23 March 2001

Abstract

An ancient buried canal whose existence had been disputed even in antiquity has been detected and described by the

modern seismic methods of geophysics. Its dimensions concur with those described by the ancient historian, Herodotus. The

2-km long canal is located in the Chalkidiki peninsula in northern Greece, and was constructed some 2500 years ago by the

Persian King Xerxes.

Beyond the classical processing of the seismic data, inverse seismic modeling was also implemented, giving an improved

and more complete picture. The inverse modeling tested the validity of the results of the seismic refraction and reflection

seismics and provided 2-D velocity structure profiles. Over much of the isthmus, it was possible to trace the route of the

ancient canal by connecting the deepest points of all the sections. q 2001 Elsevier Science B.V. All rights reserved.

Keywords:Ray tracing; Refraction; Inversion; Seismic methods; Archaeology; Canals

1. Introduction

During the last decade, a group of researchers

from Britain and Greece, working under the auspices

of the British School of Archaeology at Athens,

carried out various surveys to detect, map and gener-

ally study the ancient buried Canal of Xerxes in the .Chalkidiki peninsula, Northern Greece Fig. 1 . The

course of the supposed canal is close to two modern

villages, Nea Roda and Tripiti. In accordance withthe great historians, Herodotus Histories VII, 2224,

. .37, 122 and Thucydides History 4, 109 , the an-

cient canal was constructed on the orders of the

)

Corresponding author. Fax: q30-1-3490180. .E-mail address: [email protected] V.K. Karastathis .

Persian King Xerxes at the start of the Persian

invasion of Greece in 480 BC in order for his fleet to

avoid rounding the treacherous Cape Athos. Twelve

years previously, the Persian fleet of Mardonius had

been destroyed in a storm when rounding that cape.

According to Herodotus, the canal had a length of

about 2 km and width of about 30 m; its construction

must have been a considerable feat of civil engineer- .

ing. Isserlin 1991 has made a detailed analysis ofthe expected dimensions of the canal based on previ-

ous research, including that of Choiseul-Gouffier .1809 who not only presented a sectional drawing

of the canal but also estimated the volume of earth 3.required in the canals construction c. 250,000 m .

The existence of the canal was disputed in ancient

times as well as in the recent past, despite the

0926-9851r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. .P I I : S 0 9 2 6 - 9 8 5 1 0 1 0 0 0 4 5 - 3


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( )V.K. Karastathis et al.rJournal of Applied Geophysics 47 2001 29 4330

Fig. 1. The Chalkidiki peninsula in northern Greece. The Canal lies at the narrowest point of the Athos peninsula, from the village of NeaRoda to Tripiti.

general validity of Herodotus and Thucydides ac- .counts Paparrigopoulos, 1955 . For instance, the

.ancient geographer Strabo Geography VII, fr 35

refers to Demetrius of Skepsis who doubted

Herodotus description, claiming that the existenceof a rocky plateau, with a length of one stadium 185

.m and very close to the coast of Tripiti, would have

made any attempt to build a canal impossible; in-stead, he proposed there was a short slipway at the

Tripiti end, linking the sea with the canal, along

which the ships were dragged.

Today, the visible indications of any kind of

structure are very limited. As a result, this putative,

deeply buried structure has become the target of a

large multi-disciplinary study drawing together ar-chaeologists, geophysicists, geologists and others Is-

serlin 1991; Karastathis and Papamarinopoulos, 1994,.1997; Isserlin et al., 1996; Jones et al., 2000 . Of the

geophysical datasets magnetics, ground-penetrating.radar, resistivity and seismics , only seismics gave

adequate results and furthermore they supported the

hypothesis of a canal across much of the isthmus.

These results have in turn been confirmed by analy-sis of sediments from boreholes Isserlin et al., 1996;

.Jones et al., 2000 . The overview by Jones et al. .2000 of the archaeological, topographic, geologi-

cal, drilling and geophysical investigations was pre-

pared before the travel time inversion of the seismic

data was carried out.

2. The geological setting

The geological survey in the area of the canal .Syrides, 1990; see also Jones et al., 2000 mapped

alluvium, sand, silt and outcrops of granite and .gneiss Fig. 2 . Lacustrine limestone is also present

in places. However, our interest focuses mainly on

the materials associated with the canal. Thus, Syrides .Isserlin et al., 1996; Jones et al., 2000, Table 4

distinguished two categories of sediments from cores

obtained from boreholes in the area. The first cate-

gory, which is the filling material of the canal,

consists of grey-brown to dark brown-blackish, mas-

sive silty coarse sands with scattered small pebbles.

The blackish colour due to organic matter at some

horizons indicates deposition in a non-oxidizing,

aquatic environment. The second category is more

compact, brown-reddish coarse silty sand with vary-

ing proportions of clay and scattered small pebbles.

Although the interface between these categories of


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( )V.K. Karastathis et al.rJournal of Applied Geophysics 47 2001 29 43 31

Fig. 2. Geological map of the isthmus area. The white color indicates alluvium, the half circle filling shows the landslips and the brick

filling shows the Lacustine. The sands are presented in grey, and the granite and gneiss by symmetric and asymmetric cross hatching

.respectively from Syrides, 1990 .

sediment was clear enough in the cores, its absolute

depth was not entirely constant for reasons that are

not yet fully understood.

3. The specifications of the seismic survey

Evaluation of the historical and archaeological

information about the existence of the canal and itsprobable location raised three questions.

Is there a buried channel that can be related to

the ancient canal?

Was it possible for a canal to have been dug in .the area of Tripiti Fig. 1 where the bedrock is

close to the surface?

What were the dimensions and shape of the

canal when it was in operation?

Eight seismic refraction and four seismic reflec-

tion profiles were positioned and carried out in such

a way as to answer these questions. The positions ofthe profiles are shown in Figs. 3 and 4 southern and

.northern parts of the isthmus, respectively . The

seismic survey was conducted in two phases. The

first phase, in 19931994, aimed to detect the canaland describe its section at one location one reflec-

.tion and one refraction profileD D in Fig. 3 . Its1 2results, published in detail by Karastathis and Papa-

.marinopoulos 1997 and not discussed further here,

were sufficiently encouraging to warrant a second

phase, carried out in 19961997, which would hope-

fully shed light on the three main questions posed.

The equipment used for the acquisition of the data

was a 24 channel EG&G Geometrics 2401 seismo-

graph with 24 Mark Products geophones of 8 Hzresonant frequency for the refraction profiles and 24

geophones of 100 Hz for the reflection ones. In both

the refraction and reflection surveys, the source was .a buffalo gun Pullan and MacAulay, 1987 . Verti-

cal stacking of three to five repeats was used to

improve the signal.

The refraction seismic profiles 96-2, 96-8, 96-11,

96-12, 96-16, 96-17 had geophone intervals of 2 m .and 96-4 had 2.5 m Figs. 3 and 4 . The positions of

the reflection profiles A A , B B and C C , are1 2 1 2 1 2also shown in Fig. 3. The geophone interval at the

B B and C C reflection profiles was 1 m and at1 2 1 2 .A A which aimed to sense to greater depths was1 2

2 m. The fold of the reflection survey was 1200%.

The geophones had an end-on setting.

In most of the measurements, the greatest prob-

lem was attributable to bad weather conditions, par-

ticularly strong wind and rain. The noise from the


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Fig. 3. The positions of the seismic profiles placed along thelikely southern course of the canal the photograph was taken in

.1992; Source: Ministry of Environment of Greece .

wind could not be reduced adequately by vertical

stacking even after 10 repeats. The problem was

tackled by burying the geophones and choosing the

proper explosion time from the noise monitoring

system of the seismograph. Owing to the moisture

from the rain, it was necessary to protect the geo-

phone take-outs by positioning them on small forked

pieces of wood.

4. Data processing by classical methods

The data processing of the seismic refraction data

was based on the Generalized Reciprocal Method . .GRM of Palmer 1980, 1981 and the software

.GREMIX 2 Interpex, 1990 . This method was able

to describe complicated structures with quite irregu-

lar interfaces. The method is also capable of han-

dling horizontal velocity inhomogeneities as well as

velocity reversals and hidden layers in some cases.

Examples of the data are presented in Fig. 5. Al-

though there were 11 shots per spread, the software

used could process only nine, as shown in the figure.

Three of the seven resulting profiles are presented in

Fig. 6.

The seismic reflection data were processed by

means of the Seismic UNIX v. 30 of the Center for

Wave Phenomena of Colorado School of Mines .Cohen and Stockwell, 1997 and Eavesdropper 3 of

Steeples and Miller of the Kansas Geological Survey .1992 . The processing sequence in all profiles was

more or less similar. At the start of the processing,

the traces were examined one by one in order to

reject those with very poor signal-to-noise ratio.

The ground-roll had a dominant frequency of

about 60 Hz and a velocity of only 130 mrs, whilethe airwave from the buffalo gun had a dominant

frequency of 150 Hz. Therefore, velocity filtering

was not adequate to suppress the coherent noise.

Thus, muting of the coherent noise was attempted

with satisfactory results. The refraction first arrivals

were also muted. To ensure the separation of the

refraction from the reflection arrivals many ray-trac-

ing experiments were done, and statics and bandpass

filtering were also applied. For the rejection of the

Fig. 4. The positions of the seismic profiles at the northern end ofthe canal close to the modern village of Nea Roda the photograph

.was taken in 1992; Source: Ministry of Environment of Greece .


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Fig. 5. Four examples of the refraction data used for the GRM processing. Only nine shots per spread are presented since the package used

had this limitation in processing.

short period reverberations a deconvolution proce-

dure was effectively implemented. Residual statics

were also applied. Migration made the slopes of the

canal slightly steeper but also created strong artifactson the seismic sections. The migration implemented

in the processing was of the Gazdag type and used a

velocity model compatible with the seismic inverse

modeling. Examples of respectively data processing,

an unmigrated reflection profile and migrated reflec-

tion profiles are shown in Figs. 7 and 8a,b.

5. Data processing with inversion method

The data from the seismic refraction profiles were

enriched with more shot records, and a ray-tracing

inverse modeling was attempted with the use of the .program Rayinvr Zelt and Smith, 1992 to achieve

two goals. The first was to test that the validity of

the models had been achieved by classical seismics,

and the second was to acquire a velocity model that

could satisfy the seismic measurements and the bore- .hole data Jones et al., 2000 .

The inversion of seismic traveltimes achieved the

simultaneous determination of the 2-D velocity andinterface structure, using any type of seismic wave.

The method and its advantages over the classical

forward modeling are presented in detail in Zelt and .Smith 1992 , and some additional features on the

.program are presented in Zelt and Forsyth 1994 .

Besides improving the time required for data pro-

cessing, the method estimates model parameter reso-

lution, uncertainty and non-uniqueness. The algo-

rithm is usually used in crustal studies such as those .reported by Zelt and Ellis 1989 , OLeary et al.

. .1995 , Zelt and White 1995 and Clowes et al. . .1995 . Zelt 2000 has also applied the method on

an engineering scale to find the possible models that

would agree with his first arrival dataset. The models

he used gave an overall RMS misfit between ob-

served and predicted travel times equal to 2.5 ms. As

regards archaeological applications, seismic inver-

sion techniques have not yet been extensively used.


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Fig. 6. Three examples of the refraction profiles processed by the GRM method.


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. . . .Fig. 8. a An example of an unmigrated reflection profile at C C . b Two migrated seismic reflection sections at C C left and B B1 2 1 2 1 2 ..right .

.However, Merlanti and Musante 1994 have worked

in Italy with straight ray algorithms, and Witten et al. .1995 with diffraction tomography at a prehistoric

site in Israel.

In the present study, the velocity models that

resulted after the inversion method on refraction

profiles 96-12, 96-11, 96-8, 96-4, 96-2, 96-16 and

96-17 are shown in Fig. 9ag. Examples of process-

ing of the inversion method showing the ray-tracing

coverage and the comparison between the observed

and the predicted travel times are shown in Fig.

10ac.

Fig. 11 shows two examples of plots presenting

the value of the diagonal of the resolution matrix.

The values for the velocity and depth nodes are

presented by a contour plot and the size of the

circles, respectively. Although ideally the diagonals

of the resolution matrix are one, values of greater

than 0.50.7 are considered to indicate reasonably .well resolved model parameters Zelt, 1999 . The

general view is that all the models are well resolved.

The areas with no ray coverage have normally ze-

roed resolution. Poor resolution also has the deepest

point of the canal do to the also poor ray coverage.


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. . . Fig. 9. a Velocity structure of the refraction profile 96-12. b Velocity structure of the refraction profile 96-11. c Velocity structu . . structure of the refraction profile 96-4. e Velocity structure of the refraction profile 96-2. f Velocity structure of the refractio

. .refraction profile 96-17. Triangles denote geophone positions in a g .


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To test the stability of the models and to get a

measure of the spatial resolution we carried out a test . .Fig. 12 proposed by Zelt and Smith 1992 . We

perturbed the value of a node in the central area of

the model in a such way as to produce a significant

travel time anomaly with respect to the picking

uncertainties, but not too high to produce an altering

of the path distribution. For this perturbed model, we

produced a new model travel time dataset. The per-

turbed data were then inverted with the real final

model as a starting model and involving all model

parameters that were determined at the same time as

the selected parameter during the inversion for the

final model. If the final model was well resolved

about the selected parameter, the model produced

after the inversion of the perturbed data would have

all the other parameters, beyond the selected one,

equal to the corresponding parameters in the final

model. By contrast, if the model was poorly resolvedabout the selected parameter, then the perturbation of

the parameter would be smeared into adjacent .boundary or velocity or both parameters. The ex-

tent of this smearing can be considered as a measure

of the spatial resolution.

6. Results

The refraction and reflection profiles indicated the

existence of the ancient canal and its main features. .The C C reflection Fig. 8 and 96-12 refraction1 2

.profiles Fig. 6 , which were conducted at the same

place, gave very similar results regarding the width

of the canal: 3034 m depending on the point of

measurement. They also arrived at the same depth

for the bottom of the canal: absolute elevation of

about y4 m at maximum. The depth conversion in

the reflection profile was based on the stackingvelocity estimated from the velocity analysis. In the

. Fig. 10. a Top: The first arrival raypaths through a two-layer possible model for the line 96-12. Bottom: Comparison of observed bars. .represent"1.5 ms uncertainty and predicted travel times shown by curves. Overall RMS misfit is 1.485 ms for the 278 arrivals. b Top:

The first arrival raypaths through a two-layer possible model for the line 96-11. Bottom: Comparison of observed bars represent"1.5 ms. .uncertainty and predicted travel times shown by curves. Overall RMS misfit is 1.492 ms for the 298 arrivals. c Top: The first arrival

.raypaths through a two-layer possible model for the line 96-16. Bottom: Comparison of observed bars represent "1.5 ms uncertainty and

predicted travel times shown by curves. Overall RMS misfit is 0.968 ms for the 434 arrivals.


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.Fig. 10 continued .

area over the canal, the stacking velocity had values

about 680 mrs with the datum set to the 13.2 m.

The inverse modeling of the 96-12 data confirmed

the results of the classical processing and gave a

possible model of the velocity structure of the filling

of the canal. The two-layer model with a fill material

having a gently increasing velocity gradient from 0.3

to 1.5 kmrs gave a solution with an overall RMS


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Fig. 11. Two examples of the resolution plots of the estimated parameters. The contour plot presents the resolution of the velocity and the

size of the black dots the resolution of the boundary depth.

.misfit of 1.485 ms Figs. 9a and 10a . The velocityof 1.5 kmrs for the lower depth of the first layer is

in agreement with the expected water saturation at

these depths found from nearby wells. A slope of 308

for the sides of the canal can be clearly estimated

from the tomogram. Without the inversion results

this estimate would have been problematic since the

refraction and reflection profiles could not give a

clear image of the sides. The model gives the origi-

nal top of the canal to be 5 6 m below present

ground surface. As far as the maximum depth is

concerned, we cannot be sure about the exact shape

of the canal at depths greater than present sea level

because the coverage of the rays is very poor at these

depths. Thus both a V-shaped model at y4 m

elevation or a trapezoid one at for example y2 m

elevation are acceptable.

The 96-11 refraction profile has been arranged in .combination with 96-12 Fig. 6 in order to check the

direction of the canal at this point. The two linescross each other at a 308 angle. Taking the equal

depth position connecting the deepest midpoints, the

result is the possible centerline of the canal shown in

Fig. 3. The inversion improved the results of 96-11,

altering slightly the estimation of its deepest point

and also changing the direction of the canal. With

the inversion results, the canal seems to be parallel to .the direction of the road nearby Fig. 3 . The overall

RMS misfit between the calculated times and ob-

served for this model was 1.492 ms. The model

estimates the width of the canal to be about the 30 m .see Figs. 9b and 10b compared with about 2930

m when the two refraction profiles are combined.

The velocity structure of 96-11 is very similar to that

of 96-12. .The B B reflection profile Fig. 8 , which was1 2

.processed in a similar way to C C Fig. 8 , gave the1 2deepest point at absolute elevation ofy4 m, which


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.Fig. 12. A single parameter resolution test. The final real model C was perturbed by raising the 40 m node by 3 m. The resulting model

. .A was used to produce a new model travel time dataset. The inversion of the real model C with the perturbed travel time dataset gave . .model B. The difference between B and C is shown in the panel D. Contour lines show the difference between the velocity values. The

line graph with black squares indicates the difference in the boundary nodes. Note that the boundary node at 40 m is raised by 3 m as it

should be. The difference in the velocity values is very small, indicating that the extent of smearing of the perturbation paramater is very

limited. Velocity is given in kmrs.

is in good agreement with the result from the refrac- .tion profile 96-8 Fig. 6 . Estimation of the width of

the canal can also be derived from the reflection

profile: 3035 m. With an overall misfit between

observed and calculated travel times of about 1.448

ms the inversion gave the model shown in Fig. 9c,

confirming what the reflection and refraction meth-

ods had indicated. The modeling confirmed the esti-

mates of the other two methods. The velocity struc-

ture is also similar to the 96-11 and 96-12 profiles.

The reflection profile A A attempted to give1 2some information about the depth of the bedrock in

the area west of Tripiti. The bedrock was shown as a

reflector at 6080 ms. The datum is at 6.4 m, and

the velocity analysis for the bedrock reflector gave a

stacking velocity of about 1400 mrs. Since the

depth conversion gave a depth to bedrock of more

than 40 m, the scenario proposed by Demetrius of

Skepsis that the canal could not be dug at this point

because of shallow bedrock and therefore did not

meet the sea seems most unlikely. The refractionprofile 96-4 and its inversion profile RMS misfit

.1.772 ms showed that a possible correlation of this

channel structure with the canal could not be ex-


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.cluded Fig. 9d . The velocity values of the seismic

waves of the lower layer indicate a saturated material

since the position of the line is very close to the sea.

This may obscure the expected structure, and there-

fore the only indication for the existence of the canal

on that point is a smaller than the expected depres-

sion which can be correlated with only the central

part of the canal. A further examination of Demetrius

of Skepsis hypothesis was made at the refraction .line 96-2 close to the beach Tripiti Fig. 3 . Although

a small pond lying just inland from the beach pre-

vented a full investigation by land seismics, it was at

least possible to ascertain the depth of bedrock to the

side of the pond, bearing in mind that a limestone

outcrop is visible to the east of the pond and a gneiss

one just to the west of it. In the event, the results

seemed clear enough, the refraction profile showing

that a canal of the expected depth could have been

opened. A layer of very low velocity could accountfor the first 3 m at the west end of the profile.

Beneath this layer, the velocity was higher, about 1.5

kmrs, a value that would be expected from a well-

saturated sediment but not bedrock. The inversion .profile RMS misfit 1.528 ms agreed with these

results and also gave the contact between the lime- .stone and the softer sediments Fig. 9e .

At the northern end of the canal, near the village

of Nea Roda, the whole area now consists of flat,

slightly marshy land up to 180 m in width. Two

. .early travellers, Spratt 1847 and Leake 1835 ,reported that there was a small lagoon in this area

that probably correlated with the route of the canal,

but this lagoon has since been drained and is no

longer visible. No clear indication of the presence of

the canal can be observed in the sections of refrac-

tion profile 96-16. Combining the results of the

GRM and inversion, a horizon corresponding to the

sea intrusion and two small rises is observed which .can scarcely relate to the canal Fig. 9f . Essentially,

the marshy environment has obscured any informa-

tion about the existence of the canal. The RMS .misfit of the data Fig. 10c in this inversion profile

was only 0.968 ms.

The refraction profile 96-17 and its inversion

results indicate the existence of a small depression

which, although potentially similar to the expected

dimensions of the canal, is probably a modern con- .struction Fig. 9g .

7. Conclusions

The seismic survey has made a substantial contri-

bution to the geoarchaeological project at Xerxes

Canal. Although the results of the classical reflection

and refraction seismics precisely described the main

features of the canal, that is its maximum depth andwidth, they left aside questions about the slopes of

its sides as well as the interior structure of its fill.

The slopes were not systematically similar in their

shape, and the information about the grade of homo-

geneity of the filling was very poor. The classical

refraction seismics could neither describe a possible

vertical gradient in the velocity of the fill nor give

any estimate about the velocity of its lower part.

Shallow reflection seismics provided an average esti-

mate of the velocity of the fill but could not give any

realistic model of the velocity structure. By contrast,the inversion of the refraction data has helped con-

siderably in deriving a realistic model for the canal

that is in accordance with the other geophysical data

as well as with the geological, sediment analysis and

historical information.

The structure of the canal as derived from the

seismic data and seismic inversion can be described

as follows.

The upper width of the canal is about 30 m.

The vertical distance between the top and thebottom of the canal is 5.58 m.

The slopes of the sides of the canal are about 308.

The bottom of the canal is at an absolute depth of

up to y4 m.

The first of these results is in excellent agreement

with what Herodotus described. The estimated depth

is also in a good agreement with the depth given by .Herodotus 4.5 m . There is also agreement with the

suggested dimensions put forward by Choiseul- .Gouffier 1809 except with respect to the canal

sides which he estimated to be at a greater angle .458 .

The successful implementation of 2-D seismic

inversion modeling has shown that a future 3-D

seismic tomography survey could give us an even

better image of the ancient canal.


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Acknowledgements

The authors would like to thank Dr. B.S.J. Isser-

lin, former head of the Department of Semitic Stud-

ies of Leeds University, for his direction as well as

comments on the present paper, Dr. J. Uren for the

topographic measurements used in the static correc-

tions and the location of the profiles, Dr. C. De

Wispelaere, Director of the NATO Science for Sta-

bility Programme Office, and the members of its

steering group for encouragement and financial sup-

port, the Greek Ministry of Culture for issuing the

permit for the fieldwork, Dr. B. Tsigarida of the

Archaeological Service at Thessaloniki, the British

School at Athens, and finally the Greek General

Secretariat of Research and Technology of Ministry

of Development for funding the project.

They are also indebted to C.A. Zelt for providing

the program RAYINVR.Special thanks to Dr. J. Stockwell, researcher at

the Center of Wave Phenomena of Colorado School

of Mines, for his advice on some special topics of

seismic processing and providing the software Seis-

mic Unix. We also thank Professor D.K. Smythe and

Dr. J. Schmidt for discussion.

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