Archaeological Geophysics – a quick lookMagnetics, Radar, and Resistivity

www.umt.edu/geosciences

Steve SheriffProfessor of GeophysicsUniversity of Montana – Missoulawww.umt.edu/geosciences

Bar magnetic and iron filings Earth’s magnetic field

Total Field Magnetics

Magnetics exploits changes in subsurface magnetic properties:

• measure subtle changes in Earth’s magnetic field at the surface

• map those changes

• Interpret the results

The best all round tool for archaeological investigation

Use for large area

Use other tools on smaller areas outlined by magnetic anomalies

Ground Penetrating Radar – echoes off reflectors

Ground Penetrating Radar

GPR relies on radar waves reflecting off subsurface layers and objects

Transmit & receive radar waves (200 MHz - 1000 MHZ)

Make profiles and maps of the reflectors

Electrical Resistivity

• measures the ability of the subsurface to transmit electricity

• we put electrodes in the ground, connect them to a power source, and measure the result

Sand Hill Cemetery served the mining towns of Coloma and Garnet

Electrical Resistivity

Sand Hill Cemetery between the mining towns of Coloma and Garnet

Syscal Kid 24 DC resistivity switch

Electrodes & cabling

Congress would be proud: we found graves under tombstones!

Electrical resistivity over a suspected burial site near Coloma, MT

Cinnabar, the historic entrance to Yellowstone National Park

S.D. Sheriff, D.MacDonald, D.Dick, 2010, Decorrugation, Edge Detection, and Modeling of Total Field Magnetic Observations from a Historic Town Site, Yellowstone National Park, USA. Archaeological Prospection, V. 17, p.49-60.

Magnetics

yields maps and subsurface models

enhance and look for non-natural features

Inverting for the subsurface shape yields ‘best estimates’ for the distribution of magnetization causing the observations

Total Field Magnetic, Radar, and Archaeological Studies on the Shores of Yellowstone Lake, Yellowstone National Park, USA

Magnetic surveying results in maps and subsurface models

Radar yields 3D volumes with both profiles and time-slice maps

• TU 1 yielded a fire hearth dating to 1720±40 B.P. (Beta-265305), as well as abundant evidence of obsidian stone tool manufacture • TUs 2, 3, and 4 yielded only boulders. We excavated these, despite each individual anomaly having the character of a boulder with remanent magnetization, because their concentration and alignment was promising. In a nearby area with similar analysis one such buried boulder turned out to be a long-term bench for flaking and other cultural activities.

• At about 0.8 meters below ground surface, TU 5 contained a fire hearth dating to 2920±40 B.P. (Beta-265306).

• TU 6 contained a rock concentration (likely a hearth) dated at 3,100±40 B.P. (Beta-265307).

Total Field Magnetics

Radially distributed features around the center anomaly from an obsidian boulder at one meter

The boulder was a long term seat for flaking

The magnetic signature of the boulder has been attenuated to highlight the surrounding features

Radial magnetic highs around an obsidian boulder one meter below the surface (center anomaly) are almost certainly cultural features – the boulder was a long term seat for flaking

Northeastern Washington

Magnetics

Before:

After:rectilinear footprints are most likely cultural features

Where are the boundaries of this cemetery?

What was the layout of this 1800’s mining town?

Serendipity – 4” cast iron pipe!

Missoula

IDAHO - GPR

Here’s one I’m excited about:

~ 75 cm deep

~6-8 m diameters

The old road is shallower and not apparent on the surface

WWII Hospital Trenches in the Philippines

S.D. Sheriff, D.MacDonald, D.Dick, 2010, Decorrugation, Edge Detection, and Modeling of Total Field Magnetic Observations from a Historic Town Site, Yellowstone National Park, USA. Archaeological Prospection, V. 17, p.49-60.

S.D. Sheriff, D.MacDonald, 2010, Total Field Magnetic, Radar, and Archaeological Studies on the Shores of Yellowstone Lake, Yellowstone National Park, USA. International Society of Archaeological Prospection (ISAP), v. 23, April 2010, p.3-5.

S.D. Sheriff, 2010, Matched Filter Separation of Magnetic Anomalies Caused by Scattered Surface Debris at Archaeological Sites. Near Surface Geophysics, v. 8, #2, p. 145-150.

S.D. Sheriff and P.T. Doughty, 2009, Magnetic and Radar Investigations of Site 45CH703, Tumwater Canyon, Washington. Report (not refereed) prepared for Archaeological and historical Services, Eastern Washington University, 46 p.

S.D. Sheriff and G. Carlson, 2009, Total Field Magnetometry and Ground Penetrating Radar Investigations at Kelly Forks Work Center, Clearwater National Forest, Idaho. Report (not refereed) prepared for USFS Clearwater National Forest, 32 p.

S.D. Sheriff, 2009, Archaeological Scale Magnetic and Radar Investigations at Northwestern Yellowstone Lake, Yellowstone national Park, USA. Report (not refereed) presented to Yellowstone National Park Center for Resources, Yellowstone National Park, USA, 41 p.

S.D. Sheriff, 2009, Archaeological Scale magnetic, Electrical, and Radar Investigations at Boundary, Washington, LPOE, USA. Report (not refereed) prepared for Historical Research Associates, Inc., Missoula, MT, USA, 46 p.

Schmidt, R., Crossland, N., Ballas, M., McKeown, and Sheriff, S., 2008, Remote Sensing of Pineview Park Missoula Montana. Student Project Report (not refereed) completed for Missoula Parks & Recreation Department, Missoula, Montana.