Transcript PowerPoint - University of Montana

Imaging Furniture, Graves, and Stone Rings at Depth:
Recent Magnetic and Radar Results from the Northern Rockies
Steve Sheriff
Professor of Geophysics, University of Montana
Collaborators include:
Doug MacDonald, Department of Anthropology, University of Montana
Elaine Hale, Yellowstone National Park
Robert & Virginia O’Boyle, Integrity Resources Archaeology, Potomac, MT
Garry Carlson, Gradient Geophysics, Missoula, MT
Total Field Magnetics
Subtle changes in subsurface magnetic properties create subtle changes
in Earth’s magnetic field
• measure Earth’s magnetic field at 1:500,000
• process data
• interpret the maps
• best cost-benefit tool
• follow up with radar
Yellowstone Lake – Total Magnetic Intensity ¼ hectare
broad signature is geologic; best to remove it
Interesting zone
Buried cable?
Map Algebra: TMI – Regional = Residual
Total Magnetic Intensity (TMI)
Regional (relatively deep sources)
Residual (for analysis)
• NE corner looks even more
interesting
• clip out the northeast corner
for further analysis
Northeast Corner with regional removed
• Strong, classic dipolar,
central anomaly
• Radial distribution of
surrounding highs
• Each anomaly has a
source: geological,
archaeological,
or historic
3D model and excavation results
• Central anomaly = furniture rock, base ~ 1 meter
• Radial distribution of surrounding anomalies & sources suggest
long term camping around that piece of furniture
Ground Penetrating Radar – echoes off reflectors
Transmit & receive radar waves
(200–1000 MHz)
Waves reflect off subsurface layers
& objects
Make profiles and maps of the
reflectors
In search of a Historic Grave - Yellowstone
Ground Penetrating Radar
In search of a Historic Grave - Yellowstone
GPR – across current pavement
• 61 transects
• 0.5 meter line spacing
• 250 MHz antenna
• .05 meter trace spacing
In search of a Historic Grave - Yellowstone
Ground Penetrating Radar
• 5 transects and one time slice
• Dashed line shows disrupted reflectors marking trench
• Next: interpolate horizontal slices
In search of a Historic Grave – Time Slices (map view)
• Horizontal slice of radar volume at 0.6 meters depth
• Lower edge of image follows current road edge
• Magenta lines mark probable edge of an older road flanked by trenches
In search of a Historic Grave – map view of radar
• 1.5 meters deep
• white box shows the most likely location of the historic grave
Stone Rings: Kevin Rim, MT & Kelly Forks, ID
Processed magnetic data
Robert O’Boyle photo and data
How deep could we bury the stone ring and still detect it?
• Current anomaly
• Mapped stones – a real mix of magnetizations
• Recalculate to 1 meter deep – easily detectable
‘BURIED’
1m
Buried stone rings at depth - Clearwater River, Idaho
Easternmost grid
Buried stone rings at depth - Clearwater River, Idaho
• High energy fluvial
deposits
• 1930’s CCC camp
• Recent construction
Buried stone rings at depth - Clearwater River, Idaho
• nice camp area
• at least 2 stages of road construction
• central anomalies are probable 1930’s
CCC camp
Left 1/3 is impacted by
recent construction
Zooming in: expect archaeological anomalies of 1’s – 10’s of nT
Extremely high amplitude anomalies from bits of metal, tent
stakes, survey pins, etc.
Pre-filtering, nothing
is very apparent
Processed TMI: we expect anomalies of 1’s – 10’s of nanoteslas
Circular features
Compare a radar time slice
• ~ 1 meter deep
• Roughly the same area
• GPR confirms the character of the magnetic anomalies
GPR Time Slices
Probable stone circles at
about 1 meter deep
Probable 1930’s road to
CCC camp
Combined Magnetics and GPR
Yellowstone National Park
Raw and Residual Magnetics
Raw data
Regional removed
Overlapping Ground Penetrating Radar
Despite rough ground conditions
(fallen logs, bunchgrass, and
sagebrush) we acquired GPR data
over part of the magnetic grid.
GPR data collected over 10 meters
of the magnetic grid.
500 MHz antenna
0.5 meter profile spacing
trace separation of 0.05 meters
3D: GPR Features from Fluvial Structures
Fluvial silts and sands below 0.90
meters, confirmed by auguring
Time slice at ~1 meter
Arcuate features result from
horizontal sections through the
fluvial structures.
Results from Test Units
•#1 - hearth (1,720 +/-40 B.P.) & evidence of obsidian tool manufacturing
•#2, 3, and 4 yielded only boulders. Each individual anomaly has the character of a boulder
yet their concentration and alignment was promising
•#5 - hearth at about 0.8 meters (2,920 +/- 40 B.P.)
•#6 - likely hearth dated at 3,090 +/- 40 B.P.)
Noninvasive methods, substantial results
Total Field Magnetics
Ground Penetrating Radar