Above The Ground: Penetrating Radar Survey

One goal of our project is to map the entire ancient city at Ziyaret Tepe. Using traditional excavation techniques, this would take several lifetimes to complete. Instead of relying on excavation alone, we have embarked upon an aggressive campaign of subsurface geophysical mapping to map the Assyrian city without excavation. As a general category of techniques, geophysical surveys measure any of a number of properties of the earth such as magnetism, electrical conductivity or the ability to reflect radar pulses. Human constructions often appear as anomalous readings in these measurements and are clearly distinct from natural, or geologic, events.

At Ziyaret Tepe, we have employed magnetic gradiometry to map over 50% of the surface of the site in four seasons (1998, 1999, 2002 and 2003). Magnetic gradiometry is a passive technique, measuring the strength and direction of the earth's ambient magnetic field. Immediate subsurface anomalies are often caused by human activities. Hearths and other burnt installations are often strongly magnetic (owing to the realignment of iron particles in their clay structures during heating), while limestone walls are often very weak magnetically and contrast well with iron rich soils.

In 2004, we hope to start an additional geophysical survey measuring the variation in the electrical resistivity of the soil over a uniformly-spaced grid of electrical probes placed in the ground. With this technique, we pass a current between two probes and measure the resistance of the soil to the passage of electricity. Under appropriate conditions, anomalies such as pits (with less compact soil) are easily discernable, as are stone walls, which have increased resistance to the flow of electricity.

In future seasons, we also hope to add a ground-penetrating radar (GPR) survey. This is an active technique which passes radar pulses into the ground, measuring the reflected energy from subsurface features. Unlike magnetic gradiometry and electrical resistivity, GPR allows for differentiation of remains at different depths, lending itself to the study of subsurface stratigraphy.