This photograph shows Willis Monroe and our driver, Mehmet Tekin, helping with a geophysical surveying on the citadel mound two days ago. We are, in effect, trying to find the western edge of the Bronze Palace before we start the excavations. Our efforts in previous seasons have been entirely concentrated on the eastern part of the building, so we need to expand our explorations.
As I mentioned earlier, the Bronze Palace was built, in part, on a thick platform of mudbricks which served as a strong foundation for the building. One clue we have about the location of the western edge of the palace is a steep slope about a meter high and approximately 80m (=240 feet) west of our current excavation area. Since the mudbrick platform should erode more slowly than other materials, it makes sense that this steep erosional escarpment might just be the edge of the platform for the palace.
To test this hypothesis, we are trying to map the ancient subsurface features, like the walls of the palace, using electrical resistivity. This is a geophysical survey technique in which we pass an electrical current underground between two probes. A second set of probes measures the grounds’s resistance to the flow of electricity. Buried walls affect how easily an electrical current flows underground between two points and, if we collect readings systematically along our site grid, we can map them and “see” features below ground without excavating.
The frame that Willis and Mehmet are inserting into the ground holds two of the probes and a long yellow wire connects to two distant probes, some 50m away. These probes create the electrical field and measure the subsurface effects. The box on top of the frame collects the measurements; we have sophisticated mapping software on our field laptop that allows us to convert the resistance readings into a map of subsurface features.
As you can see in the photograph, there is nothing visible on the ground to guide our excavations, so we rely on subsurface geophysics to help us choose the best places to dig.
For any UAkron students who might be reading this post, I will be teaching my Archaeogeophysical Survey course again in the Spring of 2012. It’s a fun class and you never know where it might lead you.
2 replies on “Geophysical surveys begins.”
I was wondering, after reading the message about re-establishing the grid – does your datum point ever move? Since Turkey is pretty active in terms of earthquakes, how do you know that your datum point hasn’t moved?
Nice plug for the Geophysical Survey class!
Hi Liz. I really hadn’t thought about that. The Diyarbakir area is in an active fault zone. I guess if there were a serious earthquake in the immediate vicinity, we might have to recalculate the location of the site datum. Of course, much of the surveying done today uses satellite data and GPS units, so they are less immediately tied to a single physical iron rebar. In our case, we have a series of check points (more iron rebar) around the site that allow us to confirm the internal consistency of the grid using the total station. Unless an earthquake were to move the entire site a uniform direction and distance, we could, in theory, re-establish the grid by georeferencing our known points relative to one another. I sure hope we don’t get to that, though!