Integrated geophysical methods applied to ...

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Integrated geophysical methods applied to submerged archaeological remains detection Underwater archaeological sites of Campania investigation by mean of geophysical intrumentation and methods Francesco Giordano

Gaia Mattei

Faculty of Scienze e Tecnologie Università Parthenope Napoli, Italy [email protected]

Dept. Scienze per l’Ambiente Università Parthenope Napoli, Italy [email protected]

Abstract— This paper illustrates the results obtained by investigating two underwater archaeological sites of Campania by mean of marine geophysical methods. In particular, will be illustrated processing techniques of acoustic and magnetic data acquired in order to identify and measure the submerged archaeological remains. The three marine geophysical methods for archaeological surveys used in this study are: two acoustic methods (stratigraphic survey with DSeismic, morphological survey of the surface by mean of the side scan sonar) and the magnetic method by mean of the magnetic gradiometer. The reconstruction of dimension of the archaeological site and the evaluation of his physical proprieties are the principal results of the complex data processing, that will be described. Keywords: geophysical methods; underwater archaeology; geophysical data processing; GIS project; marine archaeological remains

I.

INTRODUCTION

The marine geophysical prospection is extremely useful for detecting submerged archaeological sites [1], [2], [7], [10], [13], [14], for defining their surface area, the nature of the seabed and sub-seabed [8], [9], for executing marine engineering works. Such surveys enable the research, mapping and geo-referencing of archaeological finds and structures by analyzing acoustic images of the seabed and its stratigraphy as well as analyzing magnetometric data. They also permit detection of traces of ancient coastlines and submerged beaches, while truncations of acoustic facies are indicators of prehistoric and historic marine transgressions and regressions which are useful for archaeological research. The purpose of this study is to obtain the maximum investigative efficiency, overlaying the data acquired from three components (2 acoustic, 1 magnetic). This reliable information are acquired at high resolution. The three hardware/software components used to investigate underwater archaeological sites are: sub-bottom profiler system for acoustic seabed stratigraphies; side scan sonar, system for acoustic detection of objects on the seabed; magnetometer system for magnetometric surveys. Archaeological targets are classified by means of the data processing, into: stratigraphic targets (T - SBP); morphological targets (T - SSS); magnetic targets (T - MG).

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This process performs a preliminary classification of AT (Archaeological Target), as a final result. Together, these techniques provide substantial morpho-bathymetric information (depths, elevations from seabed of archaeological targets), the geological characterization of the bottom substrata and the magnetic anomaly identification, with the purpose to evaluate the physical proprieties of archaeological targets. The integration of marine geophysical data into a single survey using advanced software technologies, provides complete information of the coastal environment [10]. This feature makes the geophysics one of the methodological basis for experimental research applied to the marine archaeological environment. II.

GEOPHYSICAL METHODS

The geophysical methods applied to underwater archeology represent a standard of experimental research. The development and management of data from these surveys is typically performed with GIS software [12]. the geophysical surveys have the fundamental peculiarity of relying on noninvasive methods. This enables to obtain indirect information of the seabed and sub-seabed without altering the surrounding environment. The three marine geophysical methods for archaeological surveys used in this study are: morphological investigation method of sea bottom by mean of the side scan sonar (SSS); stratigraphic investigation method of sea subbottom by mean of the Sub-Bottom Profiler (SBP) and magnetic method to measure the magnetic anomaly, by mean of the magnetometer (MG). The SSS is a high frequency acoustic instrument (200-500 kHz) that reconstructs the morphology of the seabed and objects placed on it as a function of the acoustic response (backscattering). The SSS is the main remote sensing tool for detecting objects on the sea floor. The different shapes of objects on the seabed, ensonified by lateral beams, are a convincing aid in identifying targets and evaluating their height from the seabed. A GeoAcoustics dualfrequency (114/410 kHz) SSS system (MOD259) data have been used in this research. The SSS have been used in conjunction with a Trimble DSM 232 receiver for towfish positioning.

SECTION 15. Natural science - physics

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Acquisition of SSS data have been carried out using the GeoAcoustics proprietary software (GeoPro). The SSS sonograms have been processed using Chesapeake Sonar Web Pro 3.16 software to create a real-time mosaic and allow for better interpretation. The archaeological targets identification is performed through GeoPro and DeepView, analysing the waterfalls. The mosaic then is imported into a GIS system to optimise the detail and to recognise targets through raster data processing tools [12]. SBP is an electroacoustic penetrative method at low frequency (100 - 5000Hz), that uses the contrast of the acoustic impedance between the various sedimentary layers of the subbottom, in order to reconstruct the evolution and acoustically characterize the composition. SBP data have been collected for seabed penetration of up to 30 m below seabed, or up to depths that is adequate to penetrate bedrock. The EG&G Uniboom system data and DSEISMIC acquisition system is used for this purpose. The resulting positional accuracies were ±0.6 m (horizontal) and approximately ±0.25 m (vertical). DSEISMIC [5] is a two-channel hardware/software platform developed by the Università Parthenope (Naples, Italy) in collaboration with the Università di Genova, within the Programma Nazionale di Ricerca in Antarctica (PNRA). It allows acquisition, real-time processing, and post-processing of seismic reflection data. It was developed for Windows (98, 2000, XP, Vista, 7) and can be combined with any electro-acoustic system operating at frequencies of up to 25 kHz. The collected data are stored in a compressed proprietary format (.dda) and can be converted to SEG-Y or ASCII formats. Any acquired seismogram is georeferenced and synchronized automatically. The interpreted data of the single seismic line is input into GIS in x, y, z data table format, where z represents either the top part of a geologic facies or of an archaeologically interesting target [12]. MG provides a measure of the Earth's Magnetic Field local area, and identify areas for maximum magnetic variation, associated with the presence of magnetic anomalies. The magnetic methods are very suitable in marine archaeological studies [1], [13]. Marine Magnetics SeaQuest data is used in this paper. This is a magnetometer platform that is designed to detect ferrous objects and targets in marine environments. SeaQuest contains two magnetic Overhauser sensors separate by a fixed and known distance. The Overhauser effect takes advantage of a quantum physics effect that applies to the hydrogen atom. This effect occurs when a special liquid (containing free, unpaired electrons) is combined with hydrogen atoms and then exposed to secondary polarization from a radio frequency (RF) magnetic field (i.e. generated from a RF source). RF magnetic fields are ideal for use in magnetic devices because they are “transparent” to the Earth’s “DC” magnetic field and the RF frequency is well out of the bandwidth of the precession signal (i.e. they do not contribute noise to the measuring system).

A. SSS image interpretation By carefully observing the sonogramms, is possible to identify targets of small size as the pillar in Figure 1 of an ancient coastal villa now submerged, or targets of extended dimensions as the canal walls to ancient Baianus Lacuus in Figure 2. Sonograms are carefully analyzed to identify the archaeological targets and characterize their nature and size. The backscatter signal in an area with rocky bottom and a zone of human constructions (pillar in Figure 1) can be analyzed to detect the difference. In particular in Figure 1, the target dimensions are evaluated: 2.8 m N-S and 3.4 m WE, with a height of 1.5 m. In Figure 2 the target dimensions are: 60 m NS and 200 m WE. The reconstruction of the morphology of the most important underwater archaeological sites is possible with the acoustic method SSS, with high accuracy in calculating the size of the target. In the case of Figure 1 for example, the SSS survey has allowed the delimitation of the arbour of Roman villa now submerged near a very touristic site and has prevented that this site was covered from the construction of a security barrier. B. SBP Stratigraphic image interpretation The reconstruction of archaeological target submerged from sediment is possible with the stratigraphic method SBP. Furthermore, the recognition of the material that makes up the target can be effectuated by means of the study of the acoustic response and the oscillogram of the central ping of the target (Figure 3 C and D). In the case of Figure 3, the pillar of the Roman villa submerged (described above in Figure 1) was identified by comparing the acoustic response of pillar with that of the surrounding rock. In particular, the oscillogram of the pillar target (C) is characteristic and very different of the oscillogram of the rock target (D). In fact its amplitude (y axis) is greater than the amplitude relating to the signal of the less reflective rocky debris.

The Overhauser sensor measures magnetic flux density in tesla (T), with an accuracy of 0.1 nT.. III.

RESULTS

In this section the results of geophysical data processing for studying the underwater archaeological sites are described.



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Figure 1. Side scan sonar: acoustic image of a pillar

Figure 2. Side scan sonar: acoustic image of a canal walls

C. Magnetic data interpretation To evaluate the magnetic anomalies of archaeological origin, the remanent magnetization of the archaeological targets is measured. The remanent magnetization of archaeological objects is particularly significant not only because of its large relative intensity, but because it is intimately associated with many enduring objects of ancient habitation, namely, baked clay which comprises bricks, tiles, pottery, kilns, hearths and similar features. This remanent magnetization otherwise called thermoremanent magnetization is created when the magnetite bearing clay is heated to a relatively high temperature and cooled in the presence of the earth’s magnetic field. Magnetic domains within each magnetite crystal are at first randomly oriented then move about during heating. Upon cooling, many domains align themselves with the ambient or earth’s field and thus parallel to each other creating a net magnetization fixed with respect to the object and parallel to the earth’s total field at the time of cooling [3], [4] magnetic field. The processing of data from the magnetometric survey allowed us to reconstruct the trend of the local Earth Magnetic Field. The diurnal time-varying correction was performed using the magnetic data acquired by a base station located at Capri Island, at 225m a.s.l., relatively far from sources of magnetic disturbance [13], integrated with the data measured by the Geomagnetic Observatory of L’Aquila (Italy) [13]. The density analysis of the Magnetic Field isolines provides the assessment of the main magnetic anomalies. The density surface (Figure 4) shows where magnetic lines features are concentrated (magnetic anomaly) [16].

cv = nl / m

(1)

Thanks to this calculation (1) archaeological targets with strong gradient of remanent magnetization are identified. Figure 4 shows the high value of density in the zone of Lacuus Baianus, as well as in the vicinity of the ruins of the Villa dei Pisoni , Roman baths and Ninfeo. There are no magnetic anomalies evident in correspondence of the Lacuus Baianus canal, because the materials of construction have a low remanent magnetization [3]. D. GIS project The phases of this research are supported by the use of a GIS project [12] structured in modules: the SSS module that manages both data from the survey planning phase and GeoTif elaboration data; the SBP module that manages survey planning data and processing data and related to the georeferenced acoustic targets identified on the seismic profiles; the GM module that manages both the data in the planning phase of the survey, that the magnetic data processed. The overlay of results allows the identification of the most significant targets (Archaeological Targets). The integration of all geophysical survey data makes GIS archaeological site complete and can get information resulting from the integration of deductive investigation. The overlay of the layers allows geophysicists to obtain the identification and classification of the target very reliable, useful for the subsequent archaeological underwater survey.

If cv is cell value, nl is number of magnetic lines and m is meter:



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Figure 3. In the upper part is visible the comparison between the acoustical backscattering of a handmade construction A (pillar) and a rocky bottom (B); in the lower part is shown a single oscillogram of pillar (C) and rocky bottom (D).

in Figure 3, and the small ones such as the pillar of Figure 2, not previously recognized. With the SBP, with a reasonably wide range of frequencies of the emitted signal, it is possible to investigate a few tens of meters below the seabed, highlighting the submerged archaeological structures. Through the study of the acoustical backscattering signal (Figure 3) it is possible to discriminate the structures built by man from natural ones. At the same time the SBP high resolution survey can easily identify regressions and transgressions sea, ancient shorelines, faults, subsidence, submarine landslides. It is also possible to investigate the nature of the sub-seabed, reconstructing the geological evolution of archaeological sites. This can be done by the analysis of the signal. The MG method is able to identify archaeological objects containing iron and magnetite: particularly in the magnetometric survey carried out in Baia (Naples), we have find a good approximation of the true dimension of Lacuus Baianus [13] in the Roman Empire (Figure 4). In fact, after a quick bradyseism the banks of Lacuus were submerged by the sea and by the layers of sediment, over the centuries. TABLE I.

TABLE OF GEOPHYSIC METHODS EFFIVIENCY

Figure 4. Density analysis of the Earth Magnetic Field contour lines

IV.

CONCLUSION

In this paper, the satisfactory results obtained by the use of three non-invasive geophysical methods most appropriate in underwater archeology were shown. The results of the geophysical surveys are essential for the planning of direct archaeological investigations. The Geophysic offers three very effective methods for the identification of archaeological elements determining the extension of the site. The use of SSS has allowed to recognize the large archaeological targets as for the access canal to Lacuus Baianus


Method

Characteristics

SSS

Frequency 410 kHz

SBP

Frequency0 – 5 kHz

MG

Gradiometers or magnetometers sensitivity of 0.1 nT


Use encouraged in presence of: Outcroppin g bodies, small size objects Objects covered by sediments

Use discouraged in presence of:

Target classification

Objects covered by sediments

T - SSS

Small size objects

T - SBP

High susceptive targets expected

Low susceptive targets expected

T - MG

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In conclusion, the results showed that the combination of SSS, SBP, and GM methods can better distinguish the archaeological target of interest from other artificial and natural objects (Table 1). The overlay of data in a single GIS project has allowed the spatial correlation of the results and the overlay of targets SSS, SBP, and GM for the detection of the archaeological remains (Archeaological Targets).

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[9]

ACKNOWLEDGMENT A special thanks is due to our collaborators Alberto Giordano, Luigi De Luca, and Francesco Peluso, who made a significant contribution to the success of the research. REFERENCES [1]

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