Unexpected electrical resistivity readings on citadel mound.

As you know, we have been conducting electrical resistivity surveys on the high mound at the western edge of the Bronze Palace. In past seasons, we have used a different subsurface geophysical survey technique (magnetic gradiometry) on three separate occassions to try and map the citadel mound. The results have been poor, in part because the stratigraphy of the citadel mound is very complex. Each new occupational group at Ziyaret Tepe has disturbed the remains left by the previous occupants. In our case, the Medieval period villagers dug hundreds of pits into the citadel mound, storing grain and other commodities, and eventually filling the pits with their trash.

This season, I wanted to try a different technique on the citadel mound using electrical resistivity in the hopes that this technique, which in the past has provided us with more detailed maps than the magnetic surveys in the lower town, might provide interesting insight into the western half of the Bronze Palace.

Here are some of our early results from 2011.

Electrical resistivity survey on citadel mound, west of the Bronze Palace excavations.

Not exactly obvious to the untrained eye. In fact, it is not exactly obvious even to the trained eye what is going on hereĀ – this is a difficult dataset! The square represents a plan of a 20m by 20m grid, laid out with the grid point N1000 E1110 in the southwestern corner. Each box represents a measurement of resistance (in ohms) at a given point on the citadel mound. Each square (or pixel) is 50m by 50cm. In broad terms, the dark areas have a high resistance; the light areas have low resistance. The red squares are those with the highest resistance.

At Ziyaret Tepe, pits and mudbrick walls tend to retain moisture well, so they have less resistance to the passage of electricity. Remember water is a great conductor of electricity (which is why you don’t plug in your radio perched on the edge of the bathtub) so moisture content is an important part of our electrical resistivity maps. The soil inside and outside of the mudbrick walls tends, in general, to have slightly higher resistance, while compacted soils (like pathways and streets), cobbled surfaces, and large stones tend to have the highest resistance.

My interpretation of this map suggests that the Medieval pits have again badly damaged the underlying Assyrian walls, which should appear as linear light shaded features. There are hints of walls, but it is hard to point to a distinct plan. There are also vaguely circular features which may be the results of pits. All in all, it is a disappointing plan except for two single points, close together which gave extremely high resistance readings at the place where I have put the blue circles. The resistance here was not high enough to suggest a void or open space (such as a partially-filled well), but more like a very large stone, a coarse gravel fill, or a small, highly compacted surface. It is probably worth spending a few days digging here in order to identify the source of the anomalous readings.

This process, commonly used in the geophysical surveys, is called “ground-truthing”. If these places turn out to be interesting archaeological features, we can expand the area of our survey to find other, similar anomalies.

Digging starts on Tuesday, so perhaps we will know soon what these high resistance points represent.

About matney

Dr. Matney is Professor of Archaeology in the Department of Anthropology and Classical Studies at the University of Akron. He is the Director of the Ziyaret Tepe Archaeological Expedition.
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