When geophysicist Sirri Seren of Austria’s Central Institute for Meteorology and Geodynamics (ZAMG) goes looking for ancient and forgotten settlements, he uses what resembles an oversized lawn mower and an oddly shaped plow. Pushing his equipment at a walking pace, he can either measure the changes in the magnetic field as a function of depth or send radio pulses into the ground to get reflections from stones buried in the soil. The results, after computer analysis, can produce maps of towns, churches, or small cities, all without excavation. “I am the first person to see these buildings in hundreds of years,” says Seren. “I find that exciting.”
The ground-penetrating radar (GPR) and magnetic field surveying (MS) techniques that Seren uses were not originally developed for archaeology. In fact, GPR’s main purpose is for surveying construction sites, nondestructive testing of structures, and searching for water leaks and ice-flow patterns. MS is used in the mining, oil, and gas industries to look for new mineral deposits. But over the past 15 years, scientists at the Austrian Archaeological Institute (AAI) and the department of applied geophysics at ZAMG in Vienna have customized software and modified equipment to convert the two techniques into valuable archaeological tools to study Ephesus, close to the present-day coastal city of Selçuk in the İzmir Province of Turkey.
The long dig
Ephesus started as an early Greek colony that later became one of the largest cities and seaports in the Roman and Byzantine Empires. The city was famed for the Temple of Artemis, one of the seven wonders of the ancient world. The temple and the city were repeatedly damaged by earthquakes and invaders, but they were always rebuilt because of their strategic coastal location. Only in the 16th century, when the bay and harbor had become buried in silt, was Ephesus abandoned. Today the ruins are 5 km inland from the coast.
Ironically, the same silting that led to the city’s abandonment created advantages for archaeologists, who have been excavating the site for more than 100 years. As the coastline built up and the sea moved farther away, the residents moved closer to the sea, rebuilding their city at frequent intervals without disturbing or accidentally destroying other buried settlements, explains AAI director Sabine Ladstätter, who runs the Ephesus dig.
Only something like 15% of Ephesus’s estimated 10 km2 of ruins has been excavated, and the site’s large size presents challenges for archaeologists. In addition, says Ladstätter, “you don’t want to keep digging up another rich merchant’s mansion. You want to find something new.”
Enter GPR and MS. The detectors cost about $100 000 each. A programmer works on the software full time. A technician, two support staff, and a researcher assist Seren with fieldwork. Ladstätter and her colleagues provide advice on where to survey, and they organize excavations based on the sensor findings.
In the past 10 years, the team has mapped 8 km2 of the dig site and applied their techniques to many other ruins. The two surveying techniques are complementary, says Seren. GPR’s strength is in detecting solid objects such as stone walls, while MS can see things invisible to GPR, such as small bricks, pieces of wood, and variations in soil composition.
Where lab meets reality
In GPR, an antenna sends high-frequency radio pulses (100–1000 MHz) into the ground. When a pulse hits an object whose dielectric constant differs from that of its surroundings, the pulse is reflected back. A receiving antenna on the surface picks up the changes in the reflected signal. Adjusting the frequency and power output alters the depth and type of materials the equipment can recognize and the resolution of the resulting images. The higher the frequency, the better the resolution (on the scale of centimeters), but the less deep the pulses can penetrate. The depth is also limited by electrical conductivity; in clay or moist soils, the reach is just a few centimeters, whereas in granite and limestone, it’s meters.
In MS, up to four spatially separated sensors are used to pick up changes in the magnetic field caused by objects below the surface. Strong magnetic fields can easily overwhelm the sensors and bury the weak fluctuations, so no materials with magnetic properties are used on or near the detector. Depending on local conditions, the method typically surveys 50–140 cm underground.
Seren expects three-dimensional laser scanning, which can quickly create a digital surface map of a site, to be more commonly adopted soon as a tool for archaeological surveying. Five years ago the equipment cost $250 000; now it’s $30 000, he says. The technique helps document and re-create what the settlement would look like. More powerful and sensitive sensors for MS and GPR are also on the horizon, but the real improvements remain in enhancing the software.
Vegetation and steep contours can hinder surveying. Tourists, too, cause problems. “As soon as someone throws a can on the ground, it makes our life difficult,” Seren says. During his team’s recent survey of Stonehenge they found numerous cans from a music festival held in the 1960s.
Typically, the geophysicists use MS for large-scale surveys; images can be compiled within hours of collecting data. Based on the results, the archaeologists suggest areas to probe with GPR. Processing the data is at least five times faster with MS than GPR, says Seren. “The trick is to put the results in such a way it’s useful to archaeologists.”
For example, using the GPR and MS techniques, archaeologists can distinguish Byzantine structures from earlier Hellenistic and Roman ones by looking at the dimensions, organization, and alignment of the structures they detect. Recently the team discovered 1900-year-old water pipelines feeding homes in Ephesus. Seren and his colleagues also found a canal from the sea to the harbor and identified warehouses next to the canal. “The clue was in the windows,” says Seren. “They went down to the floor in order to get the cargo out quickly.” The surveys have also led to revised estimates of Ephesus’s population from 100 000 to as many as 400 000.
Last year Seren’s team visited Hadrianoupolis in Turkey’s Black Sea region. “Documents in the Vatican talk about a saint who lived on top of a pillar for 50 years,” says Seren. “Yet the existing excavations had found only one church. If it was a religious center, there would be more.” Over two weeks the team surveyed the area and found nothing until the last day, when they found two churches close together, then two more. Says Seren, “If you have five churches in a 5000-square-meter area, it must have been an important place.”