Archaeogeophysical survey was started at Ziyaret Tepe in 1998. The size of the ancient site - 32 hectares (79 acres) - makes complete excavation impossible. As one of the goals of the archaeological project at Ziyaret Tepe is to map the layout of the Late Assyrian regional center (882-610 BC), we decided to employ large-scale geophysical survey techniques to augment traditional excavation.Two early seasons (1998 and 1999) were devoted entirely to geophysical survey using magnetic gradiometry. This technique measures the strength and orientation of the earth's magnetic field. Small fluctuations in this field caused by features (walls, pits, and ovens) immediately below the surface of the ground are mapped using a sensitive hand-held magnetic gradiometer (see right).

When used over a large area, it is possible to map the ancient city that is buried less than two meters below ground. This method is fairly fast and our team can map ten 100 square meter survey areas in a single day.

The map at the left shows an area of the lower town mapped during the 2002 field season. The surface of the ground in this area is basically flat farmland. The dark spots on the map represent pits, while the bipolar (white and black) anomalies are kilns or other pyrotechnical installations. The broad linear feature running along the southern and western edges of the map are the mudbrick city wall, which is entirely buried.

By carefully studying these maps, and by making small test trenches to ground truth the results of the magnetic gradiometry maps, we have been able to accurately trace the city wall, locate several gates, large buildings, and major streets in the Lower Town.

Magnetic gradiometry survey was continued in 2002 and 2003 and we have now mapped about 75% of Ziyaret Tepe.

During the 2004 and 2006 field seasons, we expanded the range of archaeogeophysical techniques used at Ziyaret Tepe to include electrical resistivity survey. This technique uses a mobile frame with a portable resistance meter to measure the flow of electrical current through the soil. The resistance of soil to the flow of an electrical current is influenced by a number of factors, such as moisture content, salinity, and degree of compaction. At Ziyaret Tepe, we have had excellent results using electrical resistivity to map large mudbrick structures, due to the differential in moisture retention between the compact mudbrick and the looser surrounding soils.