Archaeologists Want Seismic View

'Non-Invasive, Non-Destructive'

At the height of the Maya civilization, the population in Central America had swelled to between 12 to 20 million people, many of whom lived in expansive cities with formal plazas, ceremonial altars, ball courts and towering pyramids.

Around 900 A.D., in the Late Classic era, the Maya civilization — with its unique hieroglyphic text and advanced knowledge of cosmology — vanished mysteriously.

This spring, in the dense tropical forests of Belize, geophysicists will conduct reflection and tomographic seismic surveys in an attempt to help archaeologists reconstruct the events that led to the mysterious disappearance of the Maya.

While archaeologists traditionally excavate on the scale of centimeters and meters, geophysicists look at things on the scale of hundreds of meters and kilometers. Using the seismic method, geophysicists can sample significantly larger areas, condensing timelines for archaeologists by highgrading tracts of potential acreage for further investigation.

Transferring technical expertise developed in the imaging of subsurface structures that contain oil and gas, the geophysicists will seismically map the interiors of Maya pyramids that may contain burial tombs of the ruling class or the "elite."

"The use of seismic is driven by a new ethic in archaeology," said Robert Stewart, professor of geophysics at the University of Calgary and head of CREWES, the Consortium for Research in Elastic Wave Exploration Seismology.

"Seismic is non-invasive, non-destructive," Stewart added. "Once you excavate, you've altered the provenance and the associations, and you may have even lost some microscopic evidence."

Encouraging Results

In 2000 and 2001, using the hammer seismic technique with three-component geophones, Stewart acquired high-resolution 2-D seismic data over two Maya pyramids and an adjacent plaza at the Ma'ax Na and the Chan Chich archaeological sites located in the Rio Bravo Conservation and Management Area (RBCMA).

Situated just east of the Guatemalan border, the privately owned RBCMA contains 260,000 acres, and represents about four percent of Belize's landmass.

"We can transmit seismic energy through these rubble-filled archaeological ruins," Stewart said. "Using this energy, we have been able to make both 2-D tomograhic slices and reflection images of these structures.

"The seismic survey geometry is similar to a VSP or a crosswell tomographic survey turned on its side."

High signal-to-noise first arrivals were recorded on all of the data. Processing of the first-breaking energy — via traveltime inversion or tomography — created interior images of the structures.

Image Caption

One person's hill is another person's buried treasure. This tropical rainforest at Upper Plaza, Chan Chich in Belize covers remnants of Maya civilization.
Photo courtesy of Robert Stewart

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At the height of the Maya civilization, the population in Central America had swelled to between 12 to 20 million people, many of whom lived in expansive cities with formal plazas, ceremonial altars, ball courts and towering pyramids.

Around 900 A.D., in the Late Classic era, the Maya civilization — with its unique hieroglyphic text and advanced knowledge of cosmology — vanished mysteriously.

This spring, in the dense tropical forests of Belize, geophysicists will conduct reflection and tomographic seismic surveys in an attempt to help archaeologists reconstruct the events that led to the mysterious disappearance of the Maya.

While archaeologists traditionally excavate on the scale of centimeters and meters, geophysicists look at things on the scale of hundreds of meters and kilometers. Using the seismic method, geophysicists can sample significantly larger areas, condensing timelines for archaeologists by highgrading tracts of potential acreage for further investigation.

Transferring technical expertise developed in the imaging of subsurface structures that contain oil and gas, the geophysicists will seismically map the interiors of Maya pyramids that may contain burial tombs of the ruling class or the "elite."

"The use of seismic is driven by a new ethic in archaeology," said Robert Stewart, professor of geophysics at the University of Calgary and head of CREWES, the Consortium for Research in Elastic Wave Exploration Seismology.

"Seismic is non-invasive, non-destructive," Stewart added. "Once you excavate, you've altered the provenance and the associations, and you may have even lost some microscopic evidence."

Encouraging Results

In 2000 and 2001, using the hammer seismic technique with three-component geophones, Stewart acquired high-resolution 2-D seismic data over two Maya pyramids and an adjacent plaza at the Ma'ax Na and the Chan Chich archaeological sites located in the Rio Bravo Conservation and Management Area (RBCMA).

Situated just east of the Guatemalan border, the privately owned RBCMA contains 260,000 acres, and represents about four percent of Belize's landmass.

"We can transmit seismic energy through these rubble-filled archaeological ruins," Stewart said. "Using this energy, we have been able to make both 2-D tomograhic slices and reflection images of these structures.

"The seismic survey geometry is similar to a VSP or a crosswell tomographic survey turned on its side."

High signal-to-noise first arrivals were recorded on all of the data. Processing of the first-breaking energy — via traveltime inversion or tomography — created interior images of the structures.

Constructed of local carbonate block and mortar, the pyramids were found to have surface velocities of 200-300meters/second and interior seismic velocities of about 500-700 meters/second. There is seismic evidence of anomalous, low velocity regions surrounded by the higher velocity areas within the cores of the pyramids.

One possible interpretation of such anomalies is the existence of open areas that could be burial chambers or tombs.

Encouraged by these results, Stewart is returning to northwestern Belize in May to conduct a third field season.

Non-Invasive Visits

Chan Chich and Ma'ax Na were built on the Eocene age limestone platform that comprises the Yucatan Peninsula. The karsted limestone platform — altered by erosion, slumping, and faulting — forms a series of escarpments, uplands and bajos (wetlands) in the Three Rivers Region, which straddles the Guatemalan-Belizian border.

Fourteen known major archaeological sites exist in the Three Rivers Region; only seven have been investigated. With the exception of Ma'ax Na, most sites have been plundered and bear the scars of looters' trenches.

Leslie Shaw of Bowdoin College in Brunswick, Maine, is the director of the Ma'ax Na Archaeological Project. Discovered in 1995 by researchers from the University of Texas at Austin, Ma'ax Na ("Monkey House" in Mayan) is large, approaching the size of Tikal in neighboring Guatemala.

Excavation of the site began in 1997. Non-invasive seismic data acquisition is being used to high-grade excavation sites at Ma'ax Na — the site contains 20 to 25 large, intact structures that have not been looted.

"We would rather limit the number of large structures that we excavate," Shaw said, "because excavation can destabilize a structure, and because it takes a lot of time and money to do it well.

"We could not identify the early construction phases of a structure without major excavation."

According to Shaw, Stewart is the first to use 2-D seismic imaging of Maya tombs — and his work has enabled Shaw and her peers to identify three possible construction phases inside a pyramid at Ma'ax Na.

The Maya were opportunistic builders who took advantage of pre-existing or earlier structures — pyramids increased in height with additional construction phases.

Shaw's enthusiasm for the seismic technique is echoed by Brett Houk, the director of the Chan Chich Archaeological Project. Chan Chich ("Little Bird" in Mayan) was discovered in 1938 by J.E. Thompson. The city was occupied by the Maya from about 770 B.C. to 900 A.D.

In 1997, the University of Texas at Austin began excavation at Chan Chich.

"There's a big movement in Belize not to destroy anything that's intact," Houk said. All archaeological work, including seismic data acquisition, is done under permit with the Belize Department of Archaeology. Archaeologists need compelling arguments to obtain excavation permits from the Belizian government.

Houk suggests that seismic imaging might just provide the subsurface picture needed to advance such compelling arguments, and is optimistic that the seismic method could provide an interior "floor plan," pinpointing the locations of the walls and floors, burial chambers, tombs and various phases of construction.

He is ready to go to the next step.

"I think it's really exciting," Houk said. "We now have to excavate to prove the seismic concept, to see if it works."

Evolution of a Process

Stewart's archaeological application of high-resolution, multicomponent seismic technology has evolved through two field seasons.

"We're still trying to figure out the correct shooting geometries and the right instruments," he said. Ideally, he would like to use a real time processing unit in the field, facilitating daily discussions between geophysicists and archeologists.

In 2000 and 2001, hammer seismic surveys were conducted around the base of A-15 Pyramid at Chan Chich (see accompanying graphics). A-15 is the largest structure bordering the Upper Plaza — it stands some 18 meters high and is about 40 meters by 40 meters at its base.

The geophones were planted along a contour line two meters above the base of the A-15 Pyramid.

Stewart experimented by varying the location of the source points:

  • In the 2000 survey, the geophones were planted, with a two-meter spacing, on one side of A-15, with the five source points on the opposite side of the pyramid. This configuration produced a geometry similar to that of a VSP on its side.
  • In the 2001 survey, the geophones were laid along the same two-meter high contour line at A-15, and the 27 sources were located at the same elevation as the receiver (geophone) line.

Also in 2001, a similar survey was conducted at a Ma'ax Na pyramid that was about 28 meters by 28 meters at the base and 15 meters high.

Both the 2000 and 2001 seismic surveys were acquired using three-component surface geophones, a 60-channel portable seismograph unit and — as a surface impact source — a hammer.

In 2000, Stewart, assisted by an 18-year-old student, expended a lot of muscle power hitting a sledgehammer 20 times at each source location.

Processing of the 2000 survey suggested that Stewart didn't need so much muscle power. Sufficiently enlightened, he switched to three hits with a mallet at each source location, resulting in a higher frequency content in the 2001 data set.

"We need really high frequencies for the small pyramids," he said, "and low frequencies for the larger structures."

Stewart's research indicates that the seismic method can image deeper into archaeological sites than other shallow remote-sensing or geophysical methods developed in the mining industry — magnatometry, thermal imaging, ground penetrating radar, resistivity and conductivity. This is not surprising, given that seismic is designed to image kilometers into the earth's subsurface.

Ground penetrating radar (GPR), according to Stewart, doesn't always work in wetlands or in soils that are conductive.

Shaw concurs.

"Wet soils in the tropics don't give great GPR readings," he said. "You need a level surface that's cleared to get good readings."

Into the Woods

At Ma'ax Na, the 15-meter-high pyramids are covered with dense tropical forest and vegetation — in fact, trees favor the limestone pyramids because they can establish deep root systems that aren't possible in the thin tropical soils.

Dragging GPR equipment through the dense jungle — up and over pyramids without getting it stuck — is a logistical challenge and an archaeologist's worst nightmare. The deployment of a string of geophones, in contrast, is simple and doesn't require clearing of vegetation.

Around 850 A.D., the social fabric of the Maya civilization began to unravel. The demise of the Maya may have occurred within a single generation or even over a period of a few years.

Archaeological evidence points to apparent upheaval, violence and warfare — perhaps brought on by an extended period of drought, famine or disease.

"What collapsed was not the Maya," Houk said, "but the ruling class of the Maya around 900 A.D."

Today, the descendants of the Maya live a subsistence lifestyle in the shadows of their ancestors' great cities. Archaeologists are hopeful that non-invasive seismic technology will play a key role in reconstructing the past and unlocking the mystery of the Maya.

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