Estuarine, Coastal and Shelf Science 75 (2007) 205e213www.elsevier.com/locate/ecss

Palaeoenvironment reconstruction in the Lagoon of Venice throughwide-area acoustic surveys and core sampling

Fantina Madricardo a,*, Sandra Donnici a, Alberto Lezziero b, Federica De Carli b,Silvano Buogo c, Paola Calicchia c, Emiliano Boccardi d

a Istituto di Scienze Marine, CNR, Riva Sette Martiri e Castello 1364/a, 30122 Venice, Italyb PHAROS sas, Via della Liberta 12, 30175 Venezia Marghera, Italy

c Istituto di Acustica ‘‘O.M. Corbino’’, CNR, Via del Fosso del Cavaliere 100, 00133 Rome, Italyd c/o Istituto di Scienze Marine, CNR, Riva Sette Martiri e Castello 1364/a, 30122 Venice, Italy

Received 15 October 2006; accepted 13 February 2007

Available online 20 August 2007

Abstract

Since 1999, a joint project called ECHOS has started to explore extremely shallow zones of the lagoon of Venice with the help of acoustictechniques. The main aim of ECHOS is to reconstruct the ancient lagoon environment and investigate its evolution and anthropogenic changes.Although this topic has been the subject of many studies, the use of acoustic methods can solve the problem of correlating the levels intersectedby precision drilling, and it can reveal buried geological and archaeological structures. After the first in situ trials, during the last three years, anextensive acoustic survey was carried out on geo-archaeologically interesting areas in the central and northern Venice Lagoon. For this purpose,a traditional echosounder was modified for use in very shallow waters. A direct comparison of the acoustic and geological data shows the po-tential of acoustic methods for archaeological and geological research aimed at the reconstruction of the ancient morphology of the Venice La-goon. As an example of the encouraging preliminary results that were obtained, we present the discovery of a buried palaeocanal.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Italy; Lagoon of Venice; acoustic survey; very shallow water; core sampling; palaeoenvironment

1. Introduction

In the last few decades, systematic studies carried out onthe formation and evolution process of the Venice Lagoonhave shown that it represents a unique result of natural and an-thropogenic action. These studies investigated and tried to re-construct its origin and its development over time and space upto its current morphological and hydrographic state. The la-goon began to form about 6000 years ago during the late Ho-locene marine transgression (Gatto and Carbognin, 1981) andthe northern part, in particular, originated 5090 � 100 yearsbefore present (BP) (Serandrei-Barbero et al., 2004). Sincethen, a sediment succession of lagoon environments with

* Corresponding author.

E-mail address: fantina.madricardo@ismar.cnr.it (F. Madricardo).

0272-7714/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ecss.2007.02.031

various morphologies and hydrological regimes overlaid theearliest marine deposits (Serandrei-Barbero et al., 2002, 2005).

Over the centuries, natural causes and anthropic activitieschanged radically the path of canals, the form and extensionof salt marshes, the freshwater inflow from the main landand the marine water input. In particular, the position of an-cient coastal strips and the presence during the Roman ageof large emerged areas were discovered (Alberotanza et al.,1977; Favero and Serandrei-Barbero, 1981, 1983; Faveroet al., 1995; Serandrei-Barbero et al., 1997; Lezziero et al.,2005).

On one hand, within the geological research, the samplingof cores and their sedimentary analysis allowed, in some cases,a very detailed reconstruction of the morphology of the lagoonand of its extremely variable environment (Serandrei-Barberoet al., 2006). However, the extreme spatial and temporal

206 F. Madricardo et al. / Estuarine, Coastal and Shelf Science 75 (2007) 205e213

variability of these lagoon morphologies, like channels,marshes and tidal flats, makes it very difficult to correlate hor-izontal facies. Respective palaeoenvironments identified bythe cores give only some information on discrete points.

On the other hand, after the historical studies of Dorigo(1983) about the possible Roman origin of Venice, many Ro-man remains were discovered by the archaeologists (Canal,1998; Canal et al., 2001a,b) in the first 2e3 m depth fromthe mean sea level in the bottom sublayer. Unfortunately, inmost cases, the layers that contain such artefacts are either in-terrupted by the existing canals or appear in extremely shallowwaters (less than 1 m depth) where conventional geophysicalsurvey methods cannot be easily implemented.

In this context, since 1999, a joint research programme be-tween the CNR Institute of Marine Science (Venice) and theCNR Institute of Acoustics (Rome) together with thePHAROS sas has experimented innovative techniques to ex-plore extremely shallow zones in the lagoon, concentratingon well known archaeological sites (Canal and Cavazzoni,2000; Buogo et al., 2005; Madricardo et al., 2006). After thefirst in situ trials, during the last three years, within theECHOS research project, an extensive acoustic survey wascarried out on archaeologically interesting areas in the centraland northern Venice Lagoon, which have been shown to beparticularly valuable from a geological as well as from an ar-chaeological point of view (Canal et al., 1989; Ammermanet al., 1995; Canal, 1998).

Such an extensive acoustic investigation of the lagoon sub-bottom has been systematically carried out for the first time inextremely shallow water, with water levels ranging from about0.5 m to about 5 m. More than 50 km2 in the central and north-ern part of the lagoon were explored. Due to the oscillatingtide level (ranging about 1 m), a large part of this area was ac-cessible only at high tide.

The few previous field trials with acoustic methods in thelagoon are spatially very limited or restricted to the present ca-nals (McClennen et al., 1997; McClennen and Housley, 2006),

whose bottom normally exceeds the depth of the depositionallayer rich in archaeological remains.

The gathered data allowed mapping all the acoustic anom-alies encountered, thus obtaining fundamental information forthe palaeoenvironmental reconstruction of the entire area. Theacoustic method used, although somehow qualitative and stillrequiring ground truth sediment samples, gave indications ofthe sub-bottom layer content over wide areas and providedan extremely useful guide for selection of sample sites forcoring.

As an example, we present in this paper the reconstructionof the path and the structure of a palaeocanal, which is nowcompletely filled up, by correlating the acoustic and geologi-cal data.

2. Study area and experimental method

Out of the about 52 km2 surveyed area, we focus our atten-tion on the northern lagoon sub-basin, for this paper, choosinga study area of about 1.15 km2 close to the islands of Buranoand Mazzorbo (Figs. 1 and 2). The study site is called PaludeS. Caterina and is delimited by the channel south of Burano onthe east and by the Canale di S. Francesco on the west side, bysome natural and artificial salt marshes on the south and bya channel south of Mazzorbo on the north. The site is rela-tively protected from the currents and waves coming fromthe open sea into the lagoon through the Lido inlet by the S.Erasmo Island. It is a submerged mudflat with water depth be-low 1 m. In this area we conducted a very detailed acousticsurvey through parallel lines and we sampled 15 sedimentcores (60 were extracted in the whole investigated area ofFig. 1) to verify the nature of the morphologies identifiedthrough the acoustic investigation.

The acoustic survey was conducted using a modified ELACLAZ 72 single beam echosounder equipped with a 30 kHzLSE 131 transducer. The echosounder emits sound waves invery short pulses (t ¼ 0.15 ms) with a frequency of 30 kHz,

Fig. 1. Map of the Venice Lagoon (left) and an enlargement of the survey area (right): A is the considered zone, Palude S. Caterina.

207F. Madricardo et al. / Estuarine, Coastal and Shelf Science 75 (2007) 205e213

Fig. 2. Effective routes in the survey area.

which propagate in water down to the sea-bottom interface.Here they are partly reflected and partly penetrate into the bot-tom sediments, only to be once again reflected, attenuated, orpropagated. The echosounder measures the intensity of thesound reflected from the sea-bottom and the reverberated con-tributions from the different bottom layer interfaces. The rela-tive parts of these contributions mainly depend on physicalcharacteristics of the sediments: interface roughness, acousticimpedance contrast (between sea-water and bottom, and be-tween different sediment layers under the bottom), in-sediment attenuation, inhomogeneities and the emitted angle(Hampton, 1974; Kinsler et al., 2000; Brekhovskikh andLysanov, 2005).

The LSE 131 is a multi-component transducer that consistsof seven piezoelectric barium titanate elements arranged ina hexagonal array with an effective diameter D ¼ 18.2 cm.In the single element idealization one can consider this config-uration as a plane circular piston, whose Rayleigh distance isR0 � D2/4l ¼ 16.6 cm. This entails that under this idealiza-tion, in spite of the extremely shallow waters, we were operat-ing beyond the Fresnel zone of the beam. This wascorroborated also by the overall good quality of the data. Aswe sometimes operated with 50 cm of water (less than 4R0), though, we cannot always assume the far field approxima-tion to be accurate. However, as in this phase of the study onlya qualitative analysis of the acoustic data was needed, a de-tailed determination of the beam pattern close to the near fieldfalls outside the scope of this paper. This is object of ongoinginvestigation.

In our case, the acoustic (far field) beam aperture of thetransducer is about 16� at �3 dB, giving an ensonified areawith diameters ranging from about 0.14 m at 0.5 m depth toabout 1.4 m at 5 m depth. The echosounder was mounted ona small boat along with a co-located DGPS system TrimbleDSM12, which allows a positioning accuracy within 1 m.

The boat was just 7 m long with a flat bottom drawing30 cm, which allows operating under extreme shallow waterconditions (down to 50 cm depth).

To achieve an optimal performance under our operatingconditions, the echosounder was modified from its originalset-up to improve its resolution: the minimum pulse lengthwas reduced from 0.3 ms to 0.15 ms, and repetition rate wasincreased from 20 pulses/s to 50 pulses/s, which was the max-imum limit that could be achieved by the hardware.

We expect the vertical resolution of the instrument (onehalf of the spatial pulse length) to be of the order of a deci-metre, given the frequency at which the echosounder operates,and assuming an approximately constant compressional wavevelocity v ¼ 1530 m/s. This value is the compressional wavevelocity in sea water at 23 �C, 1 atm and salinity of sedimentpore water.1

The timing of acquisition was provided by a trigger pulsefrom the transmit module of the echosounder, synchronizedwith the electric pulse to the transducer. The received signalswere passed through an analogue processing board for demod-ulation and enhancement of depth resolution. The output of theanalogue board was then acquired using a PCMCIA 12-bitA/D board with sampling rate of 50 kHz. Each acquired tracewas then stored on disk in binary format for further processingand visualization.

1 In the sediments we are dealing with, this value can increase by about 16%

(in case of medium-fine sand), depending on the sediment properties (mean

grain size, porosity, bulk density, attenuation) (Hamilton, 1980; Hamilton

and Bachman, 1982; Bachman, 1985, 1989). Moreover, the compressional

wave velocity is dependent on pore water temperature, salinity and pressure.

Our survey was carried out in spring and summer, and given the variation of

seasonal average values for salinity, temperature and pressure in the Venice la-

goon over the years 2001 to 2003 (see Guerzoni and Tagliapietra, 2006), the

water compressional wave velocity can vary between 1506 and 1537 m/s

over the range of seasonal conditions.

208 F. Madricardo et al. / Estuarine, Coastal and Shelf Science 75 (2007) 205e213

To obtain the highest detail within the capability of the instru-mentation employed, the survey field was organized in parallellines using a dedicated software (Hypack�Max). Line spacingwas set to 2.5 m, a reasonable value considering the error duethe positioning of the DGPS and the difficulty of keeping theboat on course. The actual routes have been plotted in Fig. 2.

A dataset of 730 survey lines, with lengths ranging from 50to 1000 m, has been acquired over an area of 1.15 km2 for a to-tal of about 450 km of acoustic profiles. This high spatial sur-vey accuracy is required to provide a seamless interpretationof the stratigraphic geometry of the studied area.

In order to acquire palaeoenvironmental information aboutthe morphological structures identified by the acoustics, and tobuild a geological database for the Venice Lagoon, groundtruth sediment cores were extracted. The data collected canalso be used for the application of the quantitative methodsfor identification of sediment properties from acoustic reflec-tions (see Tegowski, 2005 and references therein).

The geological data presented here for comparison with theresults of the acoustic survey were obtained from 4 of the 60boreholes drilled, called SA4, SA5, SA8 and SA9. Their geo-graphic positions were determined through a DGPS system,while their lengths varied between 8 and 9.1 m, for all thestructures identified by the acoustic data to be intersected.They were extracted in September 2004 and October 2005with the help of a rotation method with water circulation.The core diameter was 101 mm.

The lithology and sedimentary structures of all extractedmaterials were described, using as altitude reference themean sea level (m.s.l.) of 1897, 23.5 cm below the meanpresent-day level (Cavazzoni, 1977). In particular, the bottomdepth with respect to the m.s.l. was 0.54 m for SA4, 0.58 mfor SA5, 0.83 m for SA8 and 0.7 m for SA9, respectively.The sediment cores were sampled for micromorphological,micropalaeontological and radiometric analysis.

For the micromorphological analysis, four resin impreg-nated thin sections were prepared. They can provide palaeo-climatologic and genetic data through the study of themicrostructure and of the general aspect of the deposits. Thedescriptions of the sections followed the standard rule ofBullock et al. (1985).

Air exposed soil features were found. The verticalehorizontal movement of the iron and calcareous fluid createsnew features that demonstrate the presence of an eroded cal-careous soil at the top of the alluvial succession. In the de-scribed sequence only the C and locally B horizons of thisantique soil are visible.

Thirty sediment samples were taken for micropalaeontolog-ical analysis. The organic content was composed of crushedmollusc shells mixed with abundant tests of benthic foraminif-era (Order Foraminiferida Eichwald, 1830). From each sampleat least 150 foraminiferal specimens were classified accordingto the taxonomic order of Loeblich and Tappan (1987), andtheir percentage abundance was used for statistical dataprocessing.

Radiocarbon ages were calculated on three samplesof shells at the Beta Analytic Inc. and at the CEDAD

laboratories. The samples were analysed using the AcceleratorMass Spectrometry (AMS) technique to determine the amountof 14C content. The conventional 14C age BP is the result afterapplying 13C/12C corrections to the measured age and ‘‘pres-ent’’ is defined as AD 1950.

3. Results

3.1. Acoustic data

The reflection profiles are two-dimensional images, builtthrough n successive acoustic one-dimensional traces. Eachtrace has a definite number of elements (typically 500 sam-ples/trace) and the echo intensity is represented by a colourscale or a 256-level grey scale. Therefore, the image is a two-dimensional matrix of elements, or pixel, whose intensity isrelated to the echo intensity.

Instead of adopting the classical time domain approach,which applies to one-dimensional signals, we chose an imageprocessing approach. Working with images allows us to ana-lyse efficiently two-dimensional data sets. Then, the imagesare treated in order to reduce the noise and improve the visi-bility of interesting structures and details. The aim is to high-light particular structures thanks to shape, dimension andposition parameters, as for example the seabed profile or sub-merged structures, the first reflecting sediment layers and iso-lated targets.

From a first screening of the raw data, it was evident thatthe image signal/noise rate, in this case the rate of the echo in-tensity of the structures and the bottom reverberation noise,could be increased thanks to their spatial distribution in theimage.

The useful signals generally present an extended horizontaldistribution, while noise has a random distribution throughoutthe image. Once the noise was reduced it was possible,through manual texture classification, to identify, classifyand highlight the interesting structures.

This work was done with the help of the LabView softwareplatform and its library for image acquisition and analysisImaq View. This library contains several functions for imageelaboration such as for example intensity level equalisation,specific bidimensional structure filtering, and operators formerging or erasing structures.

The sequence of operators in the filtering procedure is re-ported in Table 1. This sequence was determined with theaim of both minimizing random (speckle) noise and enhancingstructures whose size in pixels along the horizontal axis waslarger than some definite threshold.

In Fig. 3, we show the result of an optimised filtering pro-cedure on an image, which presents the bottom reflection andthe first sediment layers.

The comparison shows that the revealed structures, even ifweakly reflecting, are emphasized with an improved contrastwith respect to the bottom intensity. Moreover, the strong re-duction of the isolated points shows an improved signal/noiserate.

209F. Madricardo et al. / Estuarine, Coastal and Shelf Science 75 (2007) 205e213

After the image processing for noise reduction describedabove had been applied to all the data, we proceeded to themanual classification of the acoustic anomalies. The twomain morphological structures we were looking for are buriedpalaeocanals and palaeosurfaces. Palaeocanals were classifiedthrough their peculiar acoustic response. They generally showdipping acoustic reflectors interpretable as inclined channeldeposits due to the migration of channel meanders (interpreta-tion later confirmed by the core sedimentary sequences). Theother signals were classified as palaeosurfaces, with indica-tions about their form and their convexity stored in a databasefor further investigations.

In this paper, we present the study of one main buried geo-morphological structure identified in the acoustic profiles. Inparticular, in Fig. 4 we show three acoustic profiles whereone can recognize some buried geomorphological structuresthat were also object of the geological investigation.

In Fig. 4a, one can recognize an extended acoustic anom-aly, which is interpreted as the result of the active lateral ac-cretion through point bar migration of a palaeocanal. Thereflecting horizons correspond to changes in lithology anddensity in the sediment. The corresponding layers are inclinedin the structure migration direction.

In Fig. 4b, one has a survey profile of the same palaeocanaltaken 30 m to the east with respect to the one shown in Fig. 4a.In particular, the latter clearly shows the southern margin ofthe palaeocanal.

Table 1

Sequence of operators adopted in the optimised filtering procedure for image

processing

Step no. Description of operator

1 Nonlinear (median) filter, cell size: 3 � 3 pixels

2 Histogram equalisation

3 Power law amplification, exponent ¼ 3

4 Nonlinear (median) filter, cell size: 3 � 7 pixels

5 Pixel erosion, cell size: 3 � 5 (hexagonal)

6 Power law amplification, exponent ¼ 0.33

7 Pixel inflation, cell size: 5 � 3 (hexagonal)

8 Pixel erosion, cell size: 3 � 3 (hexagonal)

9 Power law amplification, exponent ¼ 1.5

10 Power law amplification, exponent ¼ 2, threshold level ¼ 70

The vertical lines indicate the cores, whose locations wereinspired by the outcome of the acoustic survey in the area. Infact, once the acoustic data analysis was performed, it was de-cided to extract the core SA4 in correspondence with the cen-tre of the acoustic anomaly of Fig. 4a (palaeocanal), to verifyits nature.

Regarding the positioning of SA5, it was located immedi-ately outside the anomaly as shown in Fig. 4b, in order toclearly distinguish the sedimentary characteristics given bythe presence or the absence of the acoustic anomaly.

Finally, in Fig. 4c one has a survey profile of a palaeocanalwhere it was decided to extract two cores, SA9 and SA8,respectively.

The coordinates of the initial and final point of each anom-aly were saved and a straight line connecting them was drawnon a map. This was done for all the 730 lines and these pro-cedures allowed the 2-D reconstruction of the path of a buriedpalaeocanal depicted in Fig. 5.

3.2. Geological data

The four cores were drilled inside (SA4, SA8, SA9) andoutside (SA5) the morphological structure identified by theacoustic survey depicted in Fig. 5.

The descriptions of the stratigraphic succession, sedimen-tary structures and organic content of sediments are summa-rized. The sedimentary sequences recovered in the cores areshown in Fig. 6 while in Table 2, the results of the 14C analysisfor three samples collected from the boreholes are listed. Allthese analyses are needed for the reconstruction of the envi-ronmental development of the area.

In the study area, the Holocene lagoonal deposits overlaythe Upper Pleistocene alluvial deposits. In the cores SA8and SA5, the top of the Pleistocene deposits of the alluvialplain is situated between �4.98 (SA8) and �4.88 m (SA5).In these cores, Pleistocene sediments are composed of massiveclay and fine sand layers that indicate repeated conditions ofhomogeneous sedimentation, interrupted by periods of higher-energy fluvial activity. In the upper part they are composed ofover-consolidated clayey and sandy silt; they present yellow-orange mottles and contain carbonate concretions indicating

Fig. 3. Example of a reflection profile before (a) and after (b) image processing.

210 F. Madricardo et al. / Estuarine, Coastal and Shelf Science 75 (2007) 205e213

SA4

SA5

SA8

LAGOON BOTTOM

(a)

(b)

(c)

ECHO

LATERAL ACCRETION

PALAEOCANAL

PALAEOCANAL

PALAEOCANAL

MIGRATION DIRECTION

SA9

Fig. 4. Reflection profiles of a palaeocanal in different points. The straight vertical lines indicate the location of the cores sampled. Horizontal and vertical scales

are 300 m and 10 ms (two-way time), respectively.

pedogenetic processes. In fact, micromorphological analysesdemonstrate that this horizon is characterized by iron oxideand calcareous redeposition inside soil pores. They are alluvialsediments deposed in the proximity of a watercourse andsuccessively exposed to subaerial alteration.

Over the Pleistocene sediments, one generally finds a layermade up of dark clay and silt with a thickness of few deci-metres, rich in organic content. During the Holocene marinetransgression, prior to the formation of the coastal line at the

present considered site, it is reasonable to hypothesize thata number of continental swamps could have formed. Theabove-mentioned layer can thus be interpreted as a result ofthe sedimentation of these basins. These sediments havebeen found in core SA5 between �4.88 and �4.21 m. Theyare highly bioturbated by galleries containing lagoon fauna be-longing to the above layers.

Above this level, the core SA5 stratigraphic sequence re-vealed lagoon sediment, whose bottom radiometric conventional

Fig. 5. Map of the acoustic anomalies identifying the path of a buried palaeocanal with the locations of the cores SA4, SA5, SA8, SA9.

211F. Madricardo et al. / Estuarine, Coastal and Shelf Science 75 (2007) 205e213

Fig. 6. Correlation of the stratigraphic sequences drilled by the four cores SA4, SA5, SA8, SA9. The correlation lines connect limits of sedimentological facies

represented by layers homogeneous in terms of palaeoenvironmental meaning and deposition age. The horizontal arrows indicate the depths reached by the acous-

tic signals, whereas the numbers are the radiometric ages (expressed in years BP) of samples collected at the corresponding depths.

age is 4710 � 40 years BP. These are mainly clayey or sandysilts and they contain marine mollusc shells deposited duringthe Flandrian transgression and the subsequent marine high-stand phase up to the present day (i.e. during the past5000 years). The foraminiferal association found in thesesediments reveals that the environment is an intermediate-or inner-lagoon influenced by freshwater inputs. The micro-morphological analysis suggests that these subtidal depositsare characterized by a high level of bioturbation and bythe consequent disorganization of the sediment. Organic mat-ter was observed to be diffused in very fine materials.

Such sedimentary sequence is modified when lagoon canalsor palaeocanals cut into both the lagoonal sediments them-selves and into the underlying alluvial sediment. This wasthe case, for example, with the palaeocanal delineated by theacoustic survey and later drilled by the cores SA4 and SA9.Deposits were observed to fill the canal starting from�9.44 m in SA9 and from �8.54 m in SA4. They are depositsof medium to fine silty laminated sand and sandy silt withfrequent millimetric sand layers. The micromorphologicalanalysis, moreover, revealed within these silty sediments thepresence of clearly differentiated thin horizontal layers of

Table 2

Conventional 14C ages obtained from organogenic material in sediments of cores SA4, SA5 and SA9. The symbols * and ** indicate that analyses were carried out

by Beta Analytic Inc. Laboratories (USA) and by the CEDAD Laboratories (Italy), respectively

Core name Sample Lab. no. Depth bottom Depth in 1897 Conventional age C13eC12 rate Material

(m) (m) (yrs BP) (&)

SA4 SA4-7,95 Beta-200503* 7.95 8.49 4500 � 50 �2.1 Shells

SA5 SA5-4,25 Beta-200507* 4.25 4.83 4710 � 40 �3.6 Shells

SA9 SA9-8,33 LTL1230A** 8.33 9.03 4563 � 45 þ1.1 � 0.5 Shells

212 F. Madricardo et al. / Estuarine, Coastal and Shelf Science 75 (2007) 205e213

sandy sediment and of fragments of lagoonal mollusc shells orvegetal remains. The formation of these layers is due to thesediment deposition, which is certainly linked to the differentenergy of the tidal phases. In these sediments the content ofCollema granosum is about 20% of the total Foraminiferaamount and indicates consistent freshwater inputs (Donniciand Serandrei-Barbero, 2005). One can hypothesize that it isa lagoon delta of a fluvial path. The shells at the bottom ofthe canal give us a chronological indication that is similarfor both samples: between 4500 � 50 and 4563 � 45 years BP.

Also at the bottom of core SA8, one finds silt with sandysilt layers and fine silty sand, which represent canal facies con-taining a typical foraminiferal association of outer lagoon en-vironment. These deposits, however, remain within the widthof lagoon sediments and, therefore, they do not cut into thePleistocene alluvial sediments. As a consequence, the top ofthe continental deposits altered by subaerial exposition ispreserved.

Above a depth ranging between �2 and �2.70 m, the sed-imentary facies, as we have already discussed in the case ofSA5, are typical of a shallow lagoon bottom. This indicatesthat the activity of the canal has come to an end.

4. Discussion

The geological data presented in the previous section con-firmed the results of the acoustic investigations. To test thestructure revealed by the acoustic survey, three of the fourcores discussed above (SA4, SA8, SA9), were collected insidethe region where the echosounder indicated the presence ofa buried canal. The fourth one (SA5) was sampled outsideof this region to allow for a contrastive analysis. The geolog-ical investigation confirmed the presence of the canal. The ra-diometric ages of the shells found at the bottom of lagoonaldeposits in cores SA4 and SA9 are in very good agreement,confirming the hypothesis that they lay at the bottom of thesame canal. Core SA5, coherently with acoustic data, didnot reveal the presence of canal sediments (see Fig. 6).

In Fig. 6, where the correlation of the sedimentary facies ofthe four cores is presented, we have reported with small hori-zontal arrows also the maximum depths reached by the acous-tic signal. One observes that the signal finally stops incorrespondence of an interface between a clayey silt layerand a medium-fine silty sand layer (as in SA4 and SA8).This can be explained considering that the attenuation coeffi-cient changes conspicuously in these cases due to the differentnature of dissipation in clay (viscous attenuation) or sand(solid friction) (see McCann and McCann, 1969; Hamilton,1980; Hamilton and Bachman, 1982). Presumably, this is thecase also for SA9 where the signal appears to have stoppedin the presence of a centimetric layer of sandy silt.

In order to generalize these interpretations, however, furtherinvestigations and comparison analyses between geologicaland acoustic data are necessary.

The general observation is that, with these settings, theechosounder does not always return the entire buried canalstructure.

The acoustic method, nevertheless, proved to be sufficient toindividuate morphological structures otherwise hard to detect.Moreover, none of the difficulties discussed above are insur-mountable. By decreasing the frequency of the echosounder;for example, its vertical penetration would doubtlessly beenhanced. In this case, though, its resolution would be reduced.Our choice of frequency was determined by the need to discrim-inate also objects of anthropogenic origin, which can be expect-edly found in the first few meters of the sediment. A greaterflexibility in the choice of the frequency would be, in principle,highly preferable. More flexible instruments, such as chirpsonar, however, have not yet been adapted for use in theseextreme environments. Ours was a first attempt to use acousticmethods in such shallow waters.

5. Conclusions

This was the first time that acoustic surveys were adoptedin an extensive and systematic way in such an extreme envi-ronment as the Venice lagoon. Adopting both geologic andacoustic research methods allowed identification of buriedmorphological structures. In particular, we were able to indi-viduate and to reconstruct the path of a palaeocanal. Radio-metric dating applied to the organic content at the bottom ofthe sedimentary filling of the canal indicates that the structureexisted and was active at least 4500 years BP. The reconstruc-tion was verified through four cores positioned as indicated bythe acoustic data. The stratigraphic study of these cores, to-gether with the radiometric micropalaeontological and micro-morphological analysis, allowed us to reconstruct thepalaeogeography of the area.

It is important to stress that, had we based our analysis juston the stratigraphy correlation of the several cores, it wouldnot have been possible to reconstruct the positioning and theevolution of the canal. Therefore, with the results presentedin this paper we demonstrate the effectiveness of this com-bined acoustic and geological research in extreme shallow wa-ter environments for improving our knowledge about theVenice lagoon and its ancient morphology.

Acknowledgements

The authors would like to thank Silvia Cavazzoni, RossanaSerandrei-Barbero, Giovanni Bosco Cannelli and Jane Frank-enfield for their helpful scientific suggestions and for carefullyreading the manuscript. The authors are very grateful to Giu-liano Sissot for his technical help during the measurementcampaigns, to Georg Umgiesser for his help with the tide cor-rections, to Barbara Bertani for her help with the GIS data-base, to Jaros1aw Tegowski and to the anonymous refereesfor their comments and suggestions. The archaeological dataare responsibility of the ‘‘Soprintendenza Archeologica peril Veneto’’ Ufficio NAUSICAA of Venice. This research wassupported technically and financially by the ‘‘Magistrato alleAcque di Venezia’’ (Venice Water Authority).

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References

Alberotanza, L., Serandrei-Barbero, R., Favero, V., 1977. I sedimenti olocenici

della laguna di Venezia (bacino settentrionale). Bollettino Societa Geolog-

ica Italiana 96, 243e269.

Ammerman, A.J., De Min, M., Housley, R., McClennen, C.E., 1995. More on

the origins of Venice. Antiquity 69, 501e510.

Bachman, R.T., 1985. Acoustic and physical property relationships in marine

sediment. Journal of the Acoustical Society of America 78, 616e621.

Bachman, R.T., 1989. Estimating velocity ratio in marine sediments. Journal

of the Acoustical Society of America 86, 2029e2032.

Brekhovskikh, L.M., Lysanov, Y., 2005. Fundamentals of Ocean Acoustics.

Springer, New York, 278 pp.

Bullock, P., Fedoroff, N., Jongerius, A., Stoops, G., Tursina, T., Babel, U.,

1985. Handbook for Soil Thin Section Description. Waine Research Pub-

lications, Wolverhampton, 150 pp.

Buogo, S., Canal, E., Cannelli, G.B., Cavazzoni, S., Donnici, S., Lezziero, A.,

2005. Geoarchaeology in the Lagoon of Venice: palaeoenvironmental

changes, ancient sea-level oscillation and geophysical surveys by acoustic

techniques. In: Fletcher, C.A., Spencer, T. (Eds.), Flooding and Environ-

mental Challenges for Venice and its Lagoon: the State of Knowledge.

Cambridge University Press, Cambridge, pp. 117e121.

Canal, E., Fersuoch, L., Spector, S., Zambon, G., 1989. Indagini archeologiche

a S. Lorenzo di Ammiana (Venezia). Archeologia Veneta 12, 71e96.

Canal, E., 1998. Testimonianze archeologiche nella Laguna di Venezia, Edi-

zioni del Vento. Venice, Italy, 91 pp.

Canal, E., Cavazzoni, S., 2000. Rilievi con ecosonda a ultrasuoni nella laguna

di Venezia, CNR-ISDGM Technical Report 239, pp. 1e28.

Canal, E., Fozzati, L., Lezziero, A., 2001a. Archeologia Subacquea e Geolo-

gia: Fattori Naturali e Fattori Antropici nella Conservazione del Patrimo-

nio Sommerso. Il Caso della Laguna di Venezia. In: Lollino, Giorgio

(Ed.), Final Proc. Geoben2001dGeological and Geotechnical Influences

in the Preservation of Historical and Cultural Heritage. Torino, Italy, pp.

325e332.

Canal, E., Fozzati, L., Lezziero, A., 2001b. Geoarchaeology in Venice Lagoon:

palaeoenvironmental changes and ancient sea level oscillation. In:

Ozhan, Erdal (Ed.), Final Proc. MEDCOAST 01dThe Fifth International

Conference on the Mediterranean Coastal Environment. Hammamet,

Tunisia, pp. 1141e1152.

Cavazzoni, S., 1977. Variazioni batimetriche e idrografiche nella laguna di

Venezia intercorse tra il 1933 e il 1971. Istituto Veneto di Scienze, Lettere

ed Arti, Rapporti e Studi 7, 1e17.

Donnici, S., Serandrei-Barbero, R., 2005. I foraminiferi di ambiente vallivo

della Laguna di Venezia. Lavori Societa Veneziana Scienze Naturali 30,

25e36.

Dorigo, W., 1983. Venezia Origini. Fondamenti, ipotesi, metodi, Electa,

Milano, Italy, 775 pp.

Favero, V., Serandrei-Barbero, R., 1981. Evoluzione paleoambientale della La-

guna di Venezia nell’area archeologica tra Burano e Canale S. Felice. Lav-

ori Societa Veneziana Scienze Naturali 6, 119e134.

Favero, V., Serandrei-Barbero, R., 1983. Oscillazioni del livello del mare ed

evoluzione paleoambientale della laguna di Venezia nell’area compresa

tra Torcello ed il margine lagunare. Lavori Societa Veneziana Scienze Nat-

urali 8, 83e102.

Favero, V., Heyvaert, F., Serandrei-Barbero, R., 1995. Motta S. Lorenzo: evo-

luzione dell’ambiente in un sito archeologico della Laguna di Venezia. Is-

tituto Veneto Scienze, Lettere ed Arti, Rapporti e Studi. 12, 183e218.

Gatto, P., Carbognin, L., 1981. The Lagoon of Venice: natural environment

trend and man-induced modification. Hydrological Sciences Bulletin 26,

379e391.

Guerzoni, S., Tagliapietra, D., 2006. Atlante della laguna: Venezia tra terra e

mare. Marsilio, Venezia, 242 pp.

Hampton, L., 1974. Physics of Sound in Marine Sediments. Springer, New

York, 567 pp.

Hamilton, E.L., 1980. Geoacoustic modeling of the sea floor. Journal of the

Acoustical Society of America 68, 1313e1340.

Hamilton, E.L., Bachman, R.T., 1982. Sound velocity and related properties of ma-

rine sediments. Journal of the Acoustical Society of America 71, 1891e1904.

Kinsler, L.E., Frey, A.R., Coppens, A.B., Sanders, J.V., 2000. Fundamentals of

Acoustics. John Wiley & Sons, New York, 548 pp.

Lezziero, A., Donnici, S., Serandrei-Barbero, R., 2005. Evoluzione paleoambien-

tale dell’area archeologica sommersa di S. Leonardo in Fossa Mala (Laguna

di Venezia). Geografia Fisica e Dinamica Quaternaria Suppl. 7, 201e210.

Loeblich, A.R., Tappan, H., 1987. Foraminiferal Genera and their Classifica-

tion. Van Nostrand Reinhold Company, New York, 970 pp.

Madricardo, F., Buogo, S., Calicchia, P., Cannelli, G.B., Donnici, S., Lezziero,

2006. Acoustical prospecting in extremely shallow water in the Venice

lagoon: first results and comparisons. In: Proceedings of 8th European

Conference on Underwater Acoustics. Tipografia Uni~ao, Faro, Portugal,

pp. 409e414.

McCann, C., McCann, D.M., 1969. The attenuation of compressional waves in

marine sediments. Geophysics 34, 882e892.

McClennen, C.E., Ammerman, A.J., Schock, S.G., 1997. Framework stratigra-

phy for the Lagoon of Venice, Italy: revealed in new seismic-reflection pro-

files and cores. Journal of Coastal Research 13, 745e759.

McClennen, C.E., Housley, R.A., 2006. Late-Holocene channel meander mi-

gration and mudflat accumulation rates, Lagoon of Venice, Italy. Journal

of Coastal Research 22, 930e945.

Serandrei-Barbero, R., Albani, A.D., Zecchetto, S., 1997. Palaeoenvironmental

significance of a benthic foraminifera fauna from an archaeological exca-

vation in the Lagoon of Venice, Italy. Palaeogeography, Palaeoclimatology,

Palaeoecology 136, 41e52.

Serandrei-Barbero, R., Donnici, S., Lezziero, A., 2002. Contributo alla cono-

scenza dell’Area Arsenale a Venezia: l’evoluzione del territorio negli ul-

timi 25.000 anni. Atti dell’Istituto Veneto di Scienze, Lettere ed Arti,

Classe di Scienze fisiche. matematiche e naturali 160, 363e383.

Serandrei-Barbero, R., Albani, A., Bonardi, M., 2004. Ancient and modern salt

marshes in the Lagoon of Venice. Palaeogeography, Paleaoclimatology,

Palaeoecology 202, 229e244.

Serandrei-Barbero, R., Bertoldi, R., Canali, G., Donnici, S., Lezziero, A.,

2005. Paleoclimatic record of the past 22,000 years in Venice (northern

Italy): Biostratigraphic evidence and Chronology. Quaternary International

140e141, 37e52.

Serandrei-Barbero, R., Albani, A., Donnici, S., Rizzetto, F., 2006. Past and re-

cent sedimentation rates in the Lagoon of Venice (Northern Italy). Estua-

rine, Coastal and Shelf Science 69, 255e269.

Tegowski, J., 2005. Acoustical Classification of the Bottom Sediments in the

Southern Baltic Sea. Quaternary International 130, 153e161.