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Imaging Furniture, Graves, and Stone Rings at Depth: Recent Magnetic and Radar Results from the Northern Rockies
Steve SheriffProfessor 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
• measure Earth’s magnetic field at 1:500,000
• process data
• interpret the maps
• best cost-benefit tool
• follow up with radar
Subtle changes in subsurface magnetic properties create subtle changes in Earth’s magnetic field
Yellowstone Lake – Total Magnetic Intensity ¼ hectarebroad signature is geologic; best to remove it
Buried cable?
Interesting zone
Map Algebra: TMI – Regional = ResidualTotal Magnetic Intensity (TMI) Regional (relatively deep sources)
• NE corner looks even more interesting
• clip out the northeast corner for further analysis
Residual (for analysis)
• Strong, classic dipolar, central anomaly
• Radial distribution of surrounding highs
• Each anomaly has a source: geological, archaeological, or historic
Northeast Corner with regional removed
• Central anomaly = furniture rock, base ~ 1 meter
• Radial distribution of surrounding anomalies & sources suggest long term camping around that piece of furniture
3D model and excavation results
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 GPR – across current pavement
• 61 transects
• 0.5 meter line spacing
• 250 MHz antenna
• .05 meter trace spacing
• 5 transects and one time slice
• Dashed line shows disrupted reflectors marking trench
• Next: interpolate horizontal slices
In search of a Historic Grave - Yellowstone Ground Penetrating Radar
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
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’ 1 m
Buried stone rings at depth - Clearwater River, Idaho
• High energy fluvial deposits
• 1930’s CCC camp
• Recent construction
Left 1/3 is impacted by recent construction
• nice camp area• at least 2 stages of road construction• central anomalies are probable 1930’s
CCC camp
Buried stone rings at depth - Clearwater River, Idaho
Zooming in: expect archaeological anomalies of 1’s – 10’s of nTExtremely high amplitude anomalies from bits of metal, tent stakes, survey pins, etc.
Pre-filtering, nothing is very apparent
Compare a radar time slice• ~ 1 meter deep• Roughly the same area• GPR confirms the character of the magnetic anomalies
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
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.
3D: GPR Features from 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.)