This paper highlights some magnetic evidences revealed by a recent marine magnetic survey ran near the Margherita di Savoia shoreline, Puglia (Italy). The survey has been planned to probe one of the expected settlement site not perused yet. The accurate elaboration of high density total magnetic field intensity data shows some features which can be interpreted as
markers of harbor framework, at present time partially uprooted. A model of the bodies which cause the magnetic anomaly is also provided

Caratori Tontini, F.

Carmisciano, C.

Ciminale, M.

Cipriani, S.

Cocchi, L.

De Marte, M.

Gambetta, M.

Locritani, M.

Stefanelli, P.

Earth-prints Repository

submitted to Annals of Geophysics
Some magnetic evidences of the Ist century BC Salapia harbor
near the Margherita di Savoia shoreline, SE Italy
M. Gambetta,1∗ C. Carmisciano,1 M. Locritani1
L. Cocchi,1 F. Caratori Tontini,1 P. Stefanelli1
M. Ciminale,2 S. Cipriani,3 M. Demarte4
1 Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy
2 Dipartimento di Geologia e Geofisica, University of Bari, Italy
3 Provincia di La Spezia, Italy
4 Istituto Idrografico della Marina, Italy
ABSTRACT
This paper highlights some magnetic evidences revealed by a recent marine magnetic survey
ran near the Margherita di Savoia shoreline, Puglia (Italy). The survey has been planned to
probe one of the expected settlement site not perused yet. The accurate elaboration of high
density total magnetic field intensity data shows some features which can be interpreted as
markers of harbor framework, at present time partially uprooted. A model of the bodies which
cause the magnetic anomaly is also provided.
Key words: Marine magnetic surveys, marine archeology
2 M. Gambetta et al.
1 INTRODUCTION
Geophysical surveying provides a relatively fast, non invasive and low cost tool that succeeds in
obtaining different kinds of information on shallow subsurface features (Urbini et al. 2007) and
this distinctive feature become much more relevant in submerged settlement sites probing. The
magnetic survey was carried out in order to provide information over the site number 6 of the
Salapia archaeological site ensemble fig. 1, pointed out but not surveyed yet. The Salapia harbor
framework are, at present time, fully submerged due to the known sea level uplift (Mastronuzzi and
Sans 2002) and, newly, to the alteration of shoreline balance engendered by human activity and
notably by quarrying (Comune di Margherita di Savoia 2006). The relevance of Salapia harbor
is testified first by Strabone (Strabone) and recent topographyc studies (Marin 1968) figure out
the river connections used to goods shifting from inland to the Salapia shipping facilities. The
whole area was subject to major rejig (Vitruvio) and a new settlement was built up in the Ist
century BC by patronus M. Hostilius together with a fairway channel which linked the new town
of Salapia Romana to the above said shipping facilities (Volpe 1990). Aerial photography and
direct archaeological prospecting (Franchin Radcliffe 2006) uphold the historical reference.
2 SURVEY SETUP
The study area (fig. 1) has been probed with a profile network composed by 54 along-shore lines
and 10 tie lines, adding up 35484 m of magnetic profiling over a surface of 348.110m2. Surveying
operations occurred between June and July 2005. A Geometrics G880 caesium marine magne-
tometers has been operated to collect the scalar values of the Earth magnetic field at a sampling
rate of 1Hz. The a-magnetic boat, sensors and ancillary instruments (DGPS) were provided by
IMM technicians which operated the equipment following the assigned survey plan.
3 MAGNETIC DATA PROCESSING
In order to obtain a residual magnetic map with particular emphasis about shallow magnetic
sources a robust processing scheme of magnetic total field data was applied. A general process-
Some magnetic evidences of the Ist century BC... 3
ing flow includes manual editing of survey lines, despike, IGRF and time varying magnetic field
removal. (Luyendyk 1997) (Pozza et al. 2004) To get rid of short wavelength components due to
heavy seas conditions while surveying a low pass filter have been applied to the recorded data
before any further processing. The time dependent recording of the magnetic field have been pro-
vided by L’Aquila INGV magnetic observatory. The time varying Earth magnetic field exposed
only slow and low amplitude variation (22.2 nT peak-to-peak). The preprocessed data were then
leveled (Soul and Parson 1998) and a intermediate magnetic anomaly map was produced by min-
imum curvature interpolation (Briggs 1974). The leveled map was then microleveled (Ferraccioli
et al. 1998) to remove leveling error still present, achieving a clean final map, suitable for digi-
tal enhancement. This TMI (Total Magnetic field Intensity) anomaly map exhibits a strong 150N
trending noise which was removed by means of frequency domain sharp cosine directional filters.
The resulting map was then analytically draped (Cordell 1985) (Paterson et al. 1990) (Pilkington
and Thurston 2001) to a clearance of 1,5 m with a high-cut filtering to remove the known instabil-
ity (Ridsdill-Smith and Dentith 2000) of the downward continuation.
In order to achieve a better comprehension the the causative bodies of the anomalies B and D (fig 3)
the Analytic Signal depth solutions from magnetic profile of fig 3 has been calculated (Nabighian
1972) (MacLeod et al. 1993) using :
A (x, z) =
δM (x, z)
δx
− j δM (x, z)
δz
(1)
where A is the Analytic Signal, M is the TMI anomaly, j is the imaginary number, x and z are
the Cartesian coordinates for the horizontal and vertical direction perpendicular to the strike of the
the causative body. The horizontal and vertical derivatives have been calculated using FFT. The
ensemble of the solution was calculated using an operator window to detect peaks and calculate
depths, ranging from 1 to 100 m with expansion increment of 1 m. The profile was chose using the
calculated horizontal gradient of the TMI anomaly (not reported) which singles out the features
labeled B and D in figure 3.
4 M. Gambetta et al.
4 INTERPRETATION
A number of magnetic lineaments arise from the processed dataset (fig. 3). A general SW-NE
trend, roughly perpendicular to the shoreline, is recognizable. The maxima labeled A, B, C form
an ensemble that could be interpreted as the marker of man-made brick or stonework. The mag-
netic evidence labeled D could be seen as an uprooted piece of the above said structure tilted and
shifted northwards. If this reconstruction would be accepted the magnetic survey would figure
out the trace of some waterway as those reported by the historical references (Vitruvio) Figure 2
shows a dipping elongate stonework piece which is expected to be the survey target; as visible
on the right the framework can be uprooted, tilted and shifted as the magnetic maps seems to
assume. The magnetic lineaments appears not to be constrained by any topography as reported
by the draped dataset of figure 3 which underscores the significance of the features revealed by
the magnetic anomaly map. Moreover, the spatial coherence of the magnetic lineaments labeled E
and F and highlighted by the draped magnetic map (fig. 3) cannot be disregarded . These features
exhibit sharp and elongated anomalies which span over 200 m. The 3D analytic signal map (not
reported) shows almost no evidence of the trending features revealed by the draped RTP magnetic
anomaly map (fig. 3). This lack of signature suggests a very shallow placement of the bodies which
generate the magnetic anomalies. Figure 4 reports the solution of the Analytic Signal calculated
for elongated prismatic bodies with strike orthogonal to the profile. Since there are no evidence
of outcropping structures from the seabed the solution placed above the bathymetry have not been
considered. Red diamonds figure out the ensemble of solutions clustered with a half window size
of 10 m. The clustered solutions report shallow causative bodies with expected mean susceptibil-
ity of 0.0045± 9.5−5 SI. A conceivable overall interpretation of the results counts some masonry
related to the shipping facility and the plants likely exiting nearby the waterway. A quantitative
model, calculated using the above profile data, shows two tilted prismatic bodies of about 12 m
wide and 4 m high. These bodies figure out the docks structure and the estimated magnetic sus-
ceptibility suggest that they were built using local stone blocks bonded with mortar grit. The bulk
of the pier structure is responsible of the low amplitude but persistent magnetic anomalies.
Some magnetic evidences of the Ist century BC... 5
5 CONCLUSIONS
The survey singles out a number of magnetic anomalies that has been interpreted as the mark-
ers of residual structures belonging to the I century BC shipping facility of new town of Salapia
Romana. Even if the interpretation may be not unique, the persistence of the magnetic anomalies
cannot be disregarded. Pattern of the TMI anomaly map is similar to the results achieved by mag-
netic prospecting in analogous environment (Boyce et al. 2004). The evidences provided by this
study suggest a direct survey in order to confirm the presence of hypothesized shipping facility
framework.
ACKNOWLEDGMENTS
The authors are very grateful to the Italian Navy (Istituto Idrografico della Marina) for the logistic
support.
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Figure 1. Survey location
8 M. Gambetta et al.
Figure 2. Target of the geophysical survey
Some magnetic evidences of the Ist century BC... 9
Figure 3. Draped TMI anomaly map
10 M. Gambetta et al.
Figure 4. Analytic Signal solution calculated for elongated bodies


Some magnetic evidences of the Ist century BC Salapia harbor near the Margherita di Savoia shoreline, SE Italy

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Some magnetic evidences of the Ist century BC Salapia harbor near the Margherita di Savoia shoreline, SE Italy

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