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Comparison of different geophysical techniques in r elation to

archaeological data for settlement reconstruction – the case

study of Nauportus, Slovenia.

Branko MUŠIČ1 / Jana HORVAT2 / Franc DIMC3 1Department of Archaeology, Faculty of Arts, University of Ljubljana, Slovenia / 2Institute of Archaeology,

Scientific Research Centre ZRC, Slovenia / 3Faculty of Maritime Studies and Transportation, University of

Ljubljana, Slovenia

The vicus of Nauportus, situated on the northeastern edge of the territory of the colony of Aquileia,

functioned as a freight and reloading station during the second half of the 1st century BC and at the

beginning of the 1st century AD. Large archaeological campaigns in 1934 and 1936 revealed a ground plan

of a fortified settlement.

Precise spatial positioning of architectural remains was impossible without additional investigation. Thus, the

aim of applying geophysical survey methods was to ascertain the reliability of a ground plan based on old

archaeological excavations and to produce a more detailed settlement reconstruction.

Information needed for effective geophysical research design was obtained from revised archaeological

documentation and completed by additional information extracted from Ground Probing Radar profiles

carried out in the initial phase of geophysical survey by several randomly directed profiles, georeferenced by

applying an inexpensive, single frequency GPS system.

A detailed reconstruction of the settlement was possible using the results of the geophysical survey and the

assessment of the previous archaeological data. It was also possible to construct a 3D presentation of

preserved architectural remains, on the basis of qualitative and quantitative analyses of geophysical data

sets (resistivity, magnetometry, conductivity and GPR), and the complementarity of techniques applied.

Previous Archaeological Investigations

The vicus of Nauportus, situated on the northeastern edge of the territory of the colony of Aquileia, held a

key position: at the junction between land routes leading from northeastern Italy and Istria and water routes

leading eastwards to Pannonia. The settlement arose proximate to the sources of the river Ljubljanica, in the

area of present-day Vrhnika. The Early Roman settlement area lay at Dolge njive, delimited by the

meandering Ljubljanica river.

Several limited archaeological excavations were carried out at the site Dolge njive (Fig. 1). Small trenches

were excavated in 1884-1886. Walter Schmid carried out large campaigns in 1934 and 1936, which revealed

a half of the ground plan of a fortified settlement. The trenches were made also in 1969 by Iva Mikl Curk

(HORVAT 1990).

An early Roman trading post, that is, a market place surrounded by storehouses, was discovered there (Fig.

2). A defence wall with towers protected the entire complex (HORVAT 1990). A part of the river port (the

remains of a rectangular wooden quay) was situated north of the settlement (LOGAR 1986). The small

archaeological material indicates that the stronghold at Dolge njive was particularly active in the Augustan

period (27 BC–14 AD), when the Romans conquered the Alpine region, the Balkans, and the central

Danubian basin (MUŠIČ, HORVAT 2007).

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Fig. 1 - Location map. Geophysically explored Early Roman trading post at Vrhnika - Dolge njive, forming a part of Nauportus, is located on the eastern bank, inside the hook of the Ljubljanica riverbend.

Fig. 2 - Walter Schmid carried out extensive archaeological excavations at Dolge njive in 1934 and 1936. It is evident that Šmid trailed only the tops of the walls. The courses of the walls were repeatedly hypothesized, despite the fact that they were never excavated in full length (after: Horvat 1990, 51, fig. 9, map 1).

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The Geological Survey in Ljubljana surveyed a large part of the archaeological region at Dolge njive in 1969

prior to highway construction using a resistivity method with a Wenner probe array. High values of apparent

resistivity were discerned in the areas where the southern, eastern and western defence walls run, as well as

a paved tract along the exterior side of the eastern defence wall (Fig. 3).

Fig. 3 - The Geological Survey in Ljubljana surveyed Dolge njive using a resistivity method in 1969 for an archaeological potential assessment before beginning construction work of modern highway (after: HORVAT 1990, 51, fig. 9, map 1 and MIKL-CURK 1970; Archive of National Heritage Centre; Archive of Institute of Archaeology, Scientific Research Centre ZRC).

Geophysical Prospection – Research Design and Resul ts

Initial phase of geophysical prospection

Information needed for an effective geophysical research design was obtained, in addition to older

archaeological investigations, also by additional information extracted from Ground Penetrating Radar

profiles (GPR) carried out in the initial phase of geophysical survey by several randomly directed profiles,

georeferenced by applying an inexpensive, single frequency GPS system.

The idea behind the construction of an inexpensive single frequency GPS system (see. Figs. 4, 5 and 6) that

is simple to use arises from the need for flexibility and effectiveness of geophysical surveys in the initial

phase of geophysical exploration. A sufficient degree of flexibility can only be assured by the complete

autonomy of the team in the planning and executing of fieldwork through the positioning of identified

anomalous areas. This is the data layer which represents the basis for the planning of further systematic and

detailed geophysical research on identified sites through the application of the multi-method approach.

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GPS equipment is relatively cheap, easily portable, offers real-time fixes and thus makes kinematical

positioning possible (WANG et al., 2002). Neither does its application require specialist skills. This makes

GPS positioning technology well suited for autonomous geophysical prospection in preliminary work within

large-scale evaluation projects particularly in the regions where geodetic fixed points for terrestrial position

measurements are not accessible. The nominal accuracy of the GPS positioning can be greatly improved by

advanced signal processing and post processing, which means that a 30 cm accuracy of kinematical relative

positioning is attainable, which it is deemed sufficient. The goal is achieved on sites where GPS satellites

are not obscured by surrounding obstacles. Since obstacles also act as another source of error, called

multipath, the accuracy of 0.9 m is reached in comparatively good satellite signal reception conditions (Fig.

8). A better accuracy is expected also with introduction of additional sensors (RETSCHER, KEALY, 2006).

Fig. 4 - Single frequency GPS system Allstar (Canadian Marconi Company) used for positioning of GPR profiles in the initial phase.

Fig. 5 - Two single frequency GPS receivers were used: FlexPak (Novatel), as a rover GS20 (Leica) both fixed on a 200 MHz GPR antenna (A) and in operational mode (B). As a GPS reference station (DGPS) a SR20 (Leica) is applied.

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We were concerned solely with horizontal positioning, which is of primary importance, leaving the more

difficult problem of vertical positioning to a future project. In the autonomous mode with a single frequency

receiver, the position of a reference point is first measured as accurately as possible. A roving receiver (rover)

then continuously corrects its position outputs by calculating its pseudo range by using single pseudo range

differences (DGPS mode) obtained from a nearby monitoring static GNSS station on a known location, from

a network of such stations, or from geostationary satellites.

Fig. 6 - GPS test positioning compared with total station acquisitions: a GPS receiver (as on Fig. 4) and an automated total station TCRA (Leica). It should be noted that both positioning devices are susceptible to different sources of errors. Traces from both devices shows shape similarity. Deviations of the observed traces regularly exceed 2 m (data could not be compared simultaneously due to the different triggering) whereas in Figs. 7 and 8 the deviations exceed 2 m in 12% of fixes.

Kinematical sub-meter accuracy as determined by a single receiver was also studied. This requires time

triggering and/or event triggering of the logging process, and a synchronization of the process with position

output acquisition. In the kinematical acquisition mode, if there is insufficient time for the settlement of the

position outputs, the positioning device must still ensure sufficient continuous accuracy. When settlement of

geophysical results takes minutes, instead of seconds, semi-kinematical positioning is possible. This requires

the surveyor to return to a reference measurement point after a certain period of time.

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Fig. 7 - Tested GPS trace compared with another GPS results (as from Fig. 5). Comparison of the simultaneously taken fixes shows deviations between both GPS traces with a median of 0.7 m as observed on an entire site. Approximately 30% of the whole sky is obscured. Two details of traces are shown in Fig. 8 (A) and (B). One of the GPR/GPS directions (GPS fixes) selected on the basis of Schmid’s ground plan (after: Horvat 1990, 51, fig. 9, map 1).

Fig. 8 - Two runs were made for the comparison of traces of tested GPS (when moving) and a reference GPS receiver between various reference points. The traces show deviations on the line between points 2 and 3 (A) and between points 6 and 7 (B) (see Fig. 7). It should be noted that both positioning devices are prone and susceptible to the same sources of error, which in certain conditions may result in doubling the deviation from the geodetic true position.

Multi method geophysical approach

While the basic physical theory, survey principles and usage are the same for geological, geotechnical etc.

and archaeological purposes, the shallow depths and relatively limited volume of archaeological remains test

the functioning of geophysics in marginal conditions; such conditions are far less common in the field of

“conventional” geophysics. The appellation »high resolution (ultra)shallow geophysics«, which definitively

establishes the required methodology applicable for geophysical prospecting in archaeology, is the adequate

term for archaeological geophysics.

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Considering that it is often difficult to classify the signal to noise ratio for each of the various methods,

archaeological prospecting has adopted a multi-method approach – by taking advantage of the

complementarity of the various and independent methods - for the purpose of developing efficient research

strategies. This way the hazard of making an erroneous evaluation of the signal to noise ratio due to

insufficient knowledge of the archaeological and natural contexts is avoided.

The geophysical prospecting at Dolge njive in Vrhnika incorporated the application of the geoelectric

resistivity method with an electrode array Twin probes (Geoscan RM15) (Fig. 9), the magnetic method by

measuring changes in the gradient of the magnetic field density (Geometrics G-858), GPR sounding using

200 and 500 MHz antennas (GSSI SIR3000) (Figs. 10 – 14) and measurements of the electric conductivity

by way of electromagnetic induction (Geonics EM38) (Fig. 15).

Measurements of the magnetic susceptibility (Kappameter KT-5) of top soil and building material with

prevailing limestone fragments revealed minimal difference in susceptibility. Poor contrast in magnetic

susceptibility forecasted weak magnetic anomalies generated by building foundations made of quarried

limestone. Results of magnetic prospection didn’t contribute significantly to geophysically based

reconstruction of the settlement and are therefore neglected in this paper (Mušič, Horvat 2007).

The Geoelectric Resistivity method

The twin probes array method (Geoscan RM15) applied in this project is the most commonly used technique

for geoelectric mapping in archaeology (CLARK 1990). One pair of the current and potential electrodes (C1P1)

is practically infinitely distant from the other matching pair (C2P2). Such a mutual distance between both twin

probes prevents the orientation of the measuring twin probe from having much of an influence on the

measured values. In order to detect relatively small archaeological structures, it is important that they are

situated in the vicinity of a high gradient electrical field during measurements. In practice this denotes a high

lateral resolution for structures with high resistivity shallowly beneath the surface – of importance for

archaeological prospecting. This method is applied for geoelectric mapping as the values of the resistance

are recorded to the same depth, which is determined by the distance between the mobile twin probes (C1P1).

At a distance of 0.5 m, and with optimal humidity of the soil, the depth range measures 1.5 m at the most. In

addition to the distances between the mobile twin probes in the depth range, the seepage of the soil also has

significant impact. The depth range is usually less when there is a high level of humidity in the upper soil

layer; this is because most of the electrical current flows in the direction of higher electrical conductivity

shallowly beneath the surface. The geoelectric research at Dolge njive was carried out in stages; thus the

seepage of the soil, and hence also the depth range and the contrast in the results, were all variable,

however not to any degree that made a significant impact on the measurement results (see Fig. 9).

Geoelectric mapping was executed in a grid of 0.5 x 0.5 m (MUŠIČ, HORVAT 2007).

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Fig. 9 - Results of geoelectrical mapping using Twin probes array (Geoscan RM15) after filtering the raw data by the low pass filter filter. Areas selected for the Ground Penetrating Radar survey (A) and electrical conductivity survey (B) on the basis of resistivity results. (Source of aerial photograph: Public information of Slovenia, © Surveying and Mapping Authority of the Republic of Slovenia, DOF at a scale of 1:5000).

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The Ground Penetrating Radar method (GPR)

The GPR method is based on the transmitting of high frequency electromagnetic waves via a transmitting

aerial, or transmitter, directed at the ground and a recording of the times and amplitudes of the reflecting

waves registered by the receiving aerial, or receiver. Once the electromagnetic wave reaches the

electromagnetic limit, part of the energy reflects towards the surface and is registered by the receiver, while

another part continues to disseminate through the media to the next electromagnetic limit.

Measurements were carried out using the classic reflection measuring technique, where there is a short

distance between the transmitter and the receiver. The resolution is mostly dependent upon the wavelength.

The wavelength of electromagnetic waves from a 200 MHz antenna, as was used in the GPR investigations

at Dolge njive, measures 1.5 m in the air. In materials with a relative dielectric constant of 15, this

wavelength decreases to 0.52 m, and further down to 0.4 m with a dielectric of 25, etc. (CONYERS,

GOODMAN 1997). The suitability of using an antenna with a central frequency of 200 MHz and twice as

large a wavelength from a 400 MHz antenna, which is also most recommended for archaeological purposes,

is best confirmed by the archaeological evidence corresponding to the results of the GPR research (MUŠIČ,

HORVAT 2007).

GPR sounding was used to determine the depth and height of preservation and the spatial relationship of the

architectural elements in areas of the settlement, wherever the results from geoelectric mapping deemed it

advantageous to check (Fig. 10). The GPR method is the only type among the geophysical methods used for

geophysical trenching; and it enables a precise 3D portrayal as well as analyses of the measurement results.

Fig. 10 - Interpretation of GPR echoes based on a single profile (GSSI SIR3000, 200MHz antenna). GPR profile approximately perpendicular to storehouses walls (horrea) alignment (GPR 1). Single and double walls of storehouses are easily discernible. Profile GPR 2 shows echoes from horizontal reflector interpreted as a platform paved by stone slabs (temple?) and profile GPR 3 shows several distinct echoes from walls and a strong reflection of GPR waves from horizontal reflector (paved road).

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Nine regions were selected for GPR sounding on the basis of the results from the geoelectrical resistivity

method (Fig. 9). They were determined by the demand for additional information concerning the mutual

spatial relationships of the architectural elements, their depths and the level of preservation of the

architectural remains discovered using the resistivity method. All regions were measured using a 200 MHz

antenna.

The best known manner for presenting results is in so-called time slices, which are essentially time slices of

a series of parallel and usually equally distant radar profiles. Time slices together compose a diagram of

equal amplitudes of reflections in the same time range of returning waves. In the archaeological field this

generates a series of “ground plans” at arbitrary depths (Fig. 11 and 13).

Fig. 11 - GPR time slices in the area of the eastern defence wall with tower (see Fig. 9A: area G6). Depth range between approx. 60 (A) and 90 cm (C).

Fig. 12 - 3D view of GPR echoes in the area of the eastern defence wall with tower (see Fig. 9A: area G6).

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Fig. 13 - GPR time slices in the area G2 (see Fig. 9A). Inside the central structure two column bases are visible. Depth range between approx. 60 (A) and 90 cm (C).

Fig. 14 - 3D view of GPR echoes in the area G2 (see Fig. 9A).

The research presented in this contribution also generated the results in a 3D environment, thus providing

cross sections of the investigated soil mass in arbitrary directions, as well as detailed insight into the spatial

relationships of the architectural elements, their depths, widths and level of preservation. This procedure is

still particularly welcome for interactive interpretation in a 3D environment; in an archaeological context this

allows for the discernment of building phases (Fig. 12 and 14).

The Electrical Conductivity method

The apparent electrical conductivity was measured with an instrument (Geonics EM38) in vertical dipole

mode, whereby the longer side was set in the direction of the profiles. In this configuration the sensitivity of

the instrument is at its highest for depth, which is the same as the distance between the coils, that is, 1 m.

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The greatest depth range measured 1.5 m, which is the same as for the resistivity method. Measurements

were executed in a grid of 0.5 x 0.5 m

Within the framework of geophysical research at Dolge njive, two regions were investigated with electrical

conductivity measurements (Fig. 9B). These regions were chosen on the basis of the resistivity results.

Region K1 (Fig. 9B) was selected to check the efficiency of the electrical conductivity method in

distinguishing architectural remains with high resistivity, which is otherwise a weakness of this method. The

region K2 (Fig. 9B) checked the response of the defence ditch that was recognized from the results of the

resistivity method as a 7 m wide tract bearing low values and running parallel with the eastern wall. As

defence ditches are good collectors of water, and thus are also highly electrically conducive, they are an

ideal target for the electrical conductivity method (MUŠIČ, HORVAT 2007).

As anticipated, the results were much more conducive to archaeological interpretation in the area of the

defence ditch (Fig. 14). In general it holds true that this method is used for determining negative structures,

and furthermore, that the lateral resolution when near to structures with high resistivity (e.g. limestone

foundations) is much lower than in resistivity methods.

Fig. 15 - Results of conductivity measurements. The course of the defence ditch (area K2) is much more easily discernable on the conductivity results than resistivity results (see Fig. 9).

Geophysically based settlement reconstruction

With the help of geophysical survey, the architectural remains at Nauportus - Dolge njive were accurately

positioned, the mistakes on the old ground plan from 1934-1936 were corrected, and almost the whole site

(half of which was previously unknown) was researched. The completely new structures and a lot of new

details have been perceived. The new and old data combined allowed very good historical and

archaeological interpretation (MUŠIČ, HORVAT 2007).

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The desire for effective defence is manifested by the defence wall, towers, water-filled defence ditch, and the

river, which surrounded the site Dolge njive from all directions. An extensive square was located in the centre

of the stronghold, encircled by a colonnade and large storehouses. The storehouses, used both for storage

and trading, were built as rows of long narrow rooms (length from 20 to 27 m, width 6 m), which could be

completely open towards the square. The storehouses as well as a row of tabernae (southwestern part of

the site), all together covered one third of the settlement surface area (or 6.400 m2 of storage capacity). A

temple with an ambulatory stood in the southwestern corner of the square. It is of a type that was used in the

Roman period to worship ancient local deities in the region from Gaul to the eastern Alps, but outside Italy.

The results from the new prospecting also confirm the relatively short span of the prosperity of the site: there

are no traces of any larger building reconstructions (Fig. 16).

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Fig. 16 - Realistic 3D portrayal of the settlement remains on the basis of the combined interpretation of results from the application of several independent geophysical methods. A: view from the southwest, B: view from the northwest, C: view from the southeast.

Fig. 17 - Realistic 3D portrayal of the settlement remains on the basis of the combined interpretation of results from the application of different independent geophysical methods; on the background of an aerial photograph. View from the southwest. (Source of aerial photograph: Public information of Slovenia, © Surveying and Mapping Authority of the Republic of Slovenia, DOF at a scale of 1:5000).

The river port was situated north of the settlement. A paved road led through the northern gateway to the

bank of the Ljubljanica, where remains of a rectangular wooden pier stood in the river. The tract between the

defence wall and the river was paved (Fig. 16 and 17).

The plan of the entire settlement and the individual buildings (storehouses and tabernae) correspond with

examples found in Late Republican towns in northern Italy, as well as with the architectural remains of ports

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throughout the entire Roman Empire. It can be interpreted as a fortified trans-shipment post handling transit

transport and trade. The size of the settlement and the great size of the warehouses indicate an exceptional

amount of cargo. The stronghold must have played an important role in supplying the legions in the area of

the central Danube basin and the northern Balkans (HORVAT, MUŠIČ 2007; MUŠIČ, HORVAT 2007).

Fig. 18 - Ground plan of the architecture at Nauportus - Dolge njive on the basis of geophysical prospecting, archaeological excavations and reconstruction of the courses of the walls (after MUŠIČ, HORVAT 2007, 256, fig. 39).

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