Mapping the Past a Step at a Time

How magnetic imaging is used to map the ancient city:

After a farmer’s field is rented, and sometimes his whole crop is purchased, the archeologists clear the plot for mapping.

A surveyor lays out a 132-by-132-foot grid, and ropes are laid on the ground outlining the square. A geophysicist carrying a magnetometer walks one length of the grid, then zigzags back and forth until the whole square has been covered.

Readings are obtained of “interruptions” to the earth’s magnetic field caused by the presence of structures underground.

The magnetometer is a small computer worn on the chest, attached by wires to a wooden frame bearing the cylinder-shaped sensors that take the readings. The device must be held steady about a foot above the ground and triggered to take a reading every 16 1/2 feet.

The instrument has trouble reading accurately deeper than 6.6 feet. The structures at Qantir are at depths ranging from 20 inches to 16.5 feet.