Harbours

from the Roman Period to the Middle Ages

Central geophysical project

The aim of the central geophysical project is the prospection of archaeological sites situated on the transition zone between land and sea.

For the prospection of this amphibious zone, established geophysical methods (geomagnetics, geoelectrics or GPR-Ground Penetrating Radar) can be applied for the land and sea sites apart from the shore, though some logistical and physical restrictions may apply depending on the specifics of the location. Especially for the amphibious zone, a new approach of imaging the subsurface through seismic surface waves shall be developed.

Prospection sites are located in Iceland, at the German coast of North Sea and Baltic Sea, on the river Rhine in Bonn and in NW Turkey.

Geoelectrical measurements in Ainos
Seismic measurements in Elaia
Onshore geophysical prospection

Geophysical prospection of archaeological sites on land has reached a high technical level. By combining different sorts of geophysical sensors, multi-equipment carriers and positioning systems it is today possible to map even large (many-ha-scale) settlement areas with high spatial resolution. The backbone of the prospection is geomagnetic mapping using sensor arrays. Often it is complemented by GPR, geoelectrics and/or EMI (electromagnetic induction) measurements.

Established methods for onshore archaeological prospection
Offshore geophysical prospection

The use of well-known methods from archaeometry on land is limited in shallow water. Geophysical prospection for the amphibious zone is in a development phase regarding both scientific methodology as well as measurement technology. Therefore, established methods shall be applied from the beginning for the project as far as possible. Parallel to this, the methodology shall be scientifically and technically improved, including the new development of subsurface imaging with seismic surface waves.

Transfer of methods for offshore and on-ice prospection, planned marine reflection seismic system and system to acquire surface waves at the sea bottom
The role of the central geophysical project in the SPP

In order to minimize interpretational ambiguity, geophysical  methods shall be applied to the archaeological sites in the SPP preferably in combination. A cooperation with five site-specific archaeological SPP-projects has been established and my be extended during the SPP. At present there are plans to investigate 20 different locations.

These archaeological partner projects are:

  • The Rhine as a European transportation route
  • The Thracian harbour city Ainos
  • The Leiruvogur Harbor Project
  • Early medieval Baltic coast harbours
  • Medieval North Sea coast harbours 

The sites of the project yield a representative section of amphibious prospection questions. Regarding geology and morphology they can be divided into three categories that require different geophysical measuring approaches:

  • Areas that are currently silting up with lagoonal or swampy parts, also salt marshes
  • Border areas of open waters with coastal areas or beaches
  • Areas almost completely silted up 

Prospection targets are building and settlement structures as well as the determination of geological conditions of the settlements and the former shape of the landscape or the characterization of sedimentation.

Connections between the geophysical central project and its partner projects and the division into 3 categories (s. text)
New methodological development

The focus of the methodological and technical development is on the seismic method, which has played a minor role in archaeological prospection on land so far. In the amphibious zone the success of well-established measurement techniques such as GPR or geoelectrics is limited due to the high electrical conductivity of salty or brackish waters, which leads to low or even no penetration depth. The seismic method is not affected by this factor and thus would yield independent material parameters of the underground. These are basically velocities of seismic wave propagation or elastic constants (bulk modulus and shear modulus). In archaeological applications, seismics is always thought of as an additional method supplementing well-established methods, because both the measurements and the processing of seismic data are time-consuming.

In amphibious archaeological prospecting the use of seismic surface waves seems to be the most promising seismic approach. Advantages of surface waves are:

  • They are easy to generate.
  • They propagate on land as well as in the sea bottom and can thus be recorded continuously across the shore.
  • In shallow depths they have sufficiently high resolution for detecting archaeological targets.
  • Their propagation is not influenced by biogenic gas.
  • Imaging techniques for the archaeological interpretation of the measurements can be developed based on existent approaches.

Until now surface waves have been used mainly for the determination of shallow geological stratigraphy. This has been done along single linear profiles and rarely with dense areal coverage. However, in archaeological prospection a high lateral resolution of structures is required, which cannot be achieved by conventional methods, because the processing includes strong spatial smoothing. Nevertheless, the measured data themselves are of sufficiently high sensitivity for small-scale structures in the underground. This has been shown by test measurements in the ancient Lion’s harbour in Milet, Turkey. Therefore, it is necessary to make the information accessible, which in principle is available.

In the framework of the SPP, algorithms requiring low computing time will be developed in order to image the underground with surface waves.

Using conventional seismics the signals will be scattered by gas in the sediment almost completely. Therefore, buried archaeological objects cannot be detected. Using surface waves generated at the sea bottom instead, they are reflected by the archaeological object and thus it can be detected.

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