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 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

• 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

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’ 1 m

Easternmost grid

Buried stone rings at depth - Clearwater River, Idaho

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

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 GPRYellowstone 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

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.)

Total Field Magnetics

Ground Penetrating Radar

Noninvasive methods, substantial results