ImRa in archaeology

Toikka Ltd. Engineering participated in an archaeological survey with ImRa in Greece on an excavation site in 2010.

The main objective was to find signs of ancient settlement, such as tombs and remains of buildings, beneath the surface of ground.

In June 2010 an exploratory archaeological survey was carried out with the ImRa impulse radar in Thesprotia, northwestern Greece at one of the excavation sites of the Thesprotia expedition by the Finnish Institute at Athens. The site contained remains of bronze-age stone structures, among other things, a limestone wall, that having been partially excavated in 2008, had been buried again to preserve it. Trial excavations indicated the presence of more structures in the area.

The objective of the survey was mainly to evaluate the performance of the radar by measuring a known target in a realistic environment and secondarily to measure unknown portions of the site to map promising starting points for possible future excavations.

The survey area with slice positions marked on the ground with string.

Preparing to measure. The antenna is placed in a sledge with an odometer wheel at the back and a tow handle in front.

The site was on a small field at the bottom of a hill. The soil on the site was mainly a red loam mixed with irregular chunks of limestone and flint from an ancient landslide. The red soil, also called Terra Rossa, seems to strongly attenuate microwave radiation and is therefore a challenging target for a radar measurement. Indeed, a previous survey by another crew using a different radar had not produced interpretable results.

The potential targets at the site were expected to be buried only at a shallow depth, perhaps less than half a metre. Because of this, a relatively high frequency and therefore high resolution 1 GHz antenna was chosen, so that a reflection from a shallow target could be discerned from reflections of the ground surface. In favor of choosing a high-frequency antenna was also that the unlucky survey conducted earlier had used a low-frequency antenna with no results.

The measurement was done in parallel slices 50 cm apart by towing the antenna in a small sledge equipped with an odometer wheel. The measurement area, a 25 m by 25 m square, was covered in three separate sets of slices. A fourth set of slices was measured from a 250 cm wide region on one edge of the square because reflections from that area looked promising.

Surprisingly, the known target, the wall, could not be recognized from the images, but indications of other structures could. For example, a regularly shaped formation with parts positioned in right angles relative to each other appears in the unexcavated part of the field. In the fourth set of slices a series of adjacent circular shapes can be seen. It is possible that these structures are closer to the surface than the wall and therefore give a stronger signal. The measurements are cluttered with reflections from natural stones and all the stronger reflections constituting the regularly shaped features originate from close to the surface. Deeper reflections are weak and it is difficult to tell if they form continuous patterns.

Not many features can be discerned from single slices.

A composite image of the area. On the lower right part of the image continuous shapes possibly originating from buried stone walls can be seen.

As expected from the soil properties, the individual slices did not show much informative content. The signal was so weak that reflections could hardly be discerned from interference. However, when the slices were filtered and composited in a three-dimensional map, features appeared.

It is essential - in case measuremens are made in unfavorable conditions - that suitable image processing is used to extract meaningful information. By composition of multiple radar profiles with ImraLab, target features that span several slices can be recognized even when their reflections in individual slices are uninformative.

A plan view of the slice arrangement in ImraLab. Each thick line marks the position of one radar profile

ImraLab 3D -image

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