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.